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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Great attention has been paid to reduce the use of conventional chemical fertilizers harming living beings through food chain supplements from the soil environment. Therefore, it is necessary to develop alternative sustainable fertilizers to enhance soil sustainability and agriculture productivity. Biofertilizers are the substance that contains microorganisms (bacteria, algae, and fungi) living or latent cells that can enrich the soil quality with nitrogen, phosphorous, potassium, organic matter, etc. They are a cost-effective, biodegradable, and renewable source of plant nutrients/supplements to improve the soil-health properties. Biofertilizers emerge as an attractive alternative to chemical fertilizers, and as a promising cost-effective technology for eco-friendly agriculture and a sustainable environment that holds microorganisms which enhance the soil nutrients' solubility leading a raise in its fertility, stimulates crop growth and healthy food safety. This book provides in-depth knowledge about history and fundamentals to advances biofertilizers, including latest reviews, challenges, and future perspectives. It covers fabrication approaches, and various types of biofertilizers and their applications in agriculture, environment, forestry and industrial sectors. Also, organic farming, quality control, quality assurance, food safety and case-studies of biofertilizers are briefly discussed. Biofertilizers' physical properties, affecting factors, impact, and industry profiles in the market are well addressed. This book is an essential guide for farmers, agrochemists, environmental engineers, scientists, students, and faculty who would like to understand the science behind the sustainable fertilizers, soil chemistry and agroecology.
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Table of Contents
Cover
Title page
Copyright
Preface
1 Biofertilizer Utilization in Forestry
1.1 Introduction
1.2 Mechanisms of Actions of Biofertilizers
1.3 Factors Influencing the Outcome of Forestry-Related Biofertilizer Applications
1.4 Applications of Biofertilizers in Forestry
1.5 Conclusion and Future Prospects
References
2 Impact of Biofertilizers on Horticultural Crops
2.1 Introduction
2.2 Microbial Strains Used in Biofertilizers
2.3 Impact of Biofertilizer Application on Horticultural Crops
2.4 Future Perspectives and Challenges Ahead
2.5 Conclusion
References
3 N2 Fixation in Biofertilizers
3.1 Introduction
3.2 Biofertilizers
3.3 Biofertilizer: Transporter Constituents
3.4 Mechanism of Actions of Biofertilizers
3.5 Biochemistry of Manufacture of Biofertilizer
3.6 Benefits of Biofertilizer Over Biochemical Fertilizers
3.7 Variances Among Organic and Biofertilizer
3.8 Types of Biofertilizers
3.9 Microorganisms Utilized to Make Biofertilizer
3.10 Microorganism in Nitrogen Fixation
3.11 Phosphorus Solubilizing Microbes
3.12 Conclusion and Future Prospect
Acknowledgments
Abbreviations
References
4 Organic Farming by Biofertilizers
4.1 Introduction
4.2 Biofertilizers
4.3 Classification of Biofertilizers
4.4 Organic Farming
4.5 Traditional Agriculture vs. Organic and Inorganic Farming
4.6 Reasons for Doing Organic Farming
4.7 Advantage of Organic Farming
4.8 Disadvantages of Organic Farming
4.9 Conclusion
Acknowledgement
References
5 Phosphorus Solubilizing Microorganisms
5.1 Phosphorus Pollution
5.2 Phosphate Solubilization
5.3 Microbial Mechanisms of Phosphate Solubilization
5.4 Phosphate-Solubilizing Bacteria
5.5 Phosphate-Solubilizing Fungi
5.6 Bacteria-Fungi Consortium for Phosphate Solubilization
5.7 Conclusions
References
6 Exophytical and Endophytical Interactions of Plants and Microbial Activities
6.1 Introduction
6.2 Beneficial Interactions
6.3 Pathogenic (Harmful) Interactions
6.4 Conclusion
References
7 Biofertilizer Formulations
7.1 Introduction
7.2 Biofertilizer Formulations
7.3 Types of Formulations
7.4 Stickers
7.5 Additives
7.6 Packaging
7.7 Conclusion
References
8 Scoping the Use of Transgenic Microorganisms as Potential Biofertilizers for Sustainable Agriculture and Environmental Safety
8.1 Introduction
8.2 Role of Nitrogen in Plant Growth and Development
8.3 Importance of Phosphorus
8.4 Significance of Potassium (K)
8.5 Biofertilizers Used in Agriculture
8.6 Role of Biotechnology in Agricultural Sector
8.7 Conclusion
Acknowledgements
References
9 Biofertilizer Utilization in Agricultural Sector
9.1 Introduction
9.2 Application of Biofertilizer as Bioaugmentation Agent for Bioremediation of Heavily Polluted Soil
9.3 Advantages of Biofertilizer in Comparison With Synthetic Fertilizer
9.4 Specific Examples of a Biofertilizer for Crop Improvement in Agricultural Sector
9.5 Management of Biotic and Abiotic Stress
9.6 Combinatory Effect of Biofertilizer With Other Substance and Their Effect on Crops
9.7 Conclusion and Recommendation to Knowledge
References
10 Azospirillum: A Salient Source for Sustainable Agriculture
10.1 Introduction
10.2 Azospirillum and Induction of Stimulatory Effects for Promoting Plant Growth
10.3 Applications in Various Fields
10.4 Current Status
10.5 Challenges in Large-Scale Commercial Applications of Azospirillum Inoculants
10.6 Programs Employed for Enhanced Applications of Azospirillum Inoculants
10.7 Conclusion and Future Prospects
References
11 Actinomycetes: Implications and Prospects in Sustainable Agriculture
11.1 Introduction
11.2 Role in Maintaining Soil Fertility
11.3 Role in Maintaining Soil Ecology
11.4 Role as Biocontrol Agents
11.5 Role as Plant Stress Busters
11.6 Conclusion
11.7 Future Perspectives
References
12 Influence of Growth Pattern of Cyanobacterial Species on Biofertilizer Production
12.1 Introduction
12.2 Habit and Habitat of Cyanobacteria
12.3 Morphology and Mode of Reproduction
12.4 Role of a Fertilizer in Plant Growth
12.5 Cyanobacteria as Biofertilizer
12.6 Production of Cyanobacteria
12.7 Methods for
In Vitro
Culture of Cyanobacteria
12.8 Methods for Gene Transfer into Cyanobacteria
12.9 Conclusion and Future Prospects
12.10 Abbreviations
References
13 Biofertilizers Application in Agriculture: A Viable Option to Chemical Fertilizers
13.1 Introduction
13.2 Chemical Fertilizer
13.3 Biofertilizers
13.4 Conclusion
13.5 Abbreviations
References
14 Quality Control of Biofertilizers
14.1 Introduction
14.2 Biofertilizer Requirement and Supply
14.3 Process of Biofertilizer Quality Control
14.4 Requirement of Quality Control
14.5 Standards for Biofertilizers Quality Control
14.6 Methods for Quality Testing
14.7 Conclusion
Acknowledgement
References
15 Biofertilizers: Characteristic Features and Applications
15.1 Introduction
15.2 Types of Biofertilizers
15.3 Characteristic Features and Applications of Biofertilizers
15.4 Phosphate Solubilizing Bacteria (PSB) and Fungus (PSF)
15.5 Effect of Biofertilizer on Various Plants (Experimental Design)
15.6 Screening of Microbes for Biofertilizer
15.7 Limitations of Biofertilizers
15.8 Success of Biofertilizer
15.9 Debottlenecking
15.10 Optimization of Biofertilizer
15.11 Concomitant of Biofertilizer
15.12 New Approach
15.13 Conclusion and Future Prospects
References
16 Fabrication Approaches for Biofertilizers
16.1 Introduction
16.2 Biofertilizers
16.3 Types of Biofertilizers
16.4 Preparation Approaches for Biofertilizers
16.5 Methods of Biofertilizer Formulation
16.6 Application Modes for Biofertilizers
16.7 Factors Affecting the Preparation of Biofertilizers
16.8 Beneficial Effects of Biofertilizers
16.9 Challenges and Limitations of Biofertilizers
16.10 Future Prospects
16.11 Conclusion
References
17 Biofertilizers From Waste
17.1 Introduction
17.2 Waste Sources
17.3 Technologies for Waste Treatment
17.4 Main Applications of Microalgae Biofertilizers
17.5 Conclusion and Recommendations
References
18 Biofertilizers Industry Profiles in Market
18.1 Biofertilizers and Biofertilizer Technology
18.2 Limitations in Usage of Biofertilizers
18.3 Biofertilizer Market Segments
18.4 Biofertilizers Market Drivers in India
18.5 Present Scenario of Biofertilizer Market
18.6 Key Players of Biofertilizers in Indian Market
18.7 Problems in Promotion of Biofertilizer
18.8 Popular Marketed Biofertilizers in Indian Market
18.9 Recent Trends in Biofertilizer: Liquid Biofertilizer
18.10 Conclusion and Future Scope
References
19 Case Study on Biofertilizer Utilization in African Continents
19.1 Introduction
19.2 Specific Examples of Biofertilizer for Crop Improvement, Environmental Bioremediation, and Their Advantages and Challenges in Africa
19.3 Conclusion and Future Recommendations
References
20 Biofertilizers: Prospects and Challenges for Future
20.1 Introduction
20.2 Definition
20.3 Advances in Biofertilizer
20.4 Preparation of Biofertilizer
20.5 The Carrier Materials
20.6 Production System of Biofertilizer
20.7 Mechanism of Growth-Promoting Activity of Biofertilizers
20.8 Advantages and Limitations
20.9 Future Aspects
20.10 Conclusion
References
21 Biofertilizers: Past, Present, and Future
21.1 Introduction
21.2 Biofertilizer: A Brief History
21.3 Biofertilizer Classification
21.4 Different Paradigms of Biofertilizers
21.5 Biofertilizers: Current Status
21.6 Biofertilizers: Future Paradigm
21.7 Conclusion
References
22 Algal Biofertilizer
22.1 Introduction
22.2 Algae and Algal Biofertilizers
22.3 Techniques of Application of Algal Biofertilizer
22.4 Cultivation of Algae and Production of Algal Biofertilizer
22.5 Conclusion
References
Index
Also of Interest
End User License Agreement
List of Illustrations
Chapter 2
Figure 2.1 Mechanism of crop growth and yield enhancement induced by beneficial ...
Figure 2.2 Mechanism of biofortification of crops induced by beneficial microbes...
Figure 2.3 Mechanism of crop tolerance against plant pathogens (bacterial, funga...
Figure 2.4 Mechanism of crop tolerance against insect pests and weeds as induced...
Figure 2.5 Mechanism of crop tolerance against abiotic stress as induced by bene...
Chapter 3
Figure 3.1 Different microorganism as biofertilizers.
Chapter 4
Figure 4.1 Multifunctional areas in which biofertilizer is used extensively and ...
Figure 4.2 Nitrogen convert to ammonia which used by plants and nitrifying bacte...
Chapter 6
Figure 6.1 Plants react differently to varying environmental pressures and alter...
Figure 6.2 (a) Shows a spread-out infection by TMV on the leaf of N. tabacum cv....
Chapter 7
Figure 7.1 Schematic of possible mechanisms adopted by growth-promoting substanc...
Figure 7.2 Types of direct mechanisms shown by growth-promoting microorganisms [...
Figure 7.3 Possible indirect mechanisms of PGPR [44].
Figure 7.4 Overview of complete formulation process.
Figure 7.5 Factors affecting the efficacy of inoculant formulation [23].
Figure 7.6 Types of carrier formulation materials depending upon the composition...
Figure 7.7 Mycorrhizae-based formulations can be prepared by the above-mentioned...
Chapter 8
Figure 8.1 Plant growth stimulating mechanisms regulated by microorganisms.
Figure 8.2 Microbes utilized as biofertilizers.
Figure 8.3 Enhanced root development with the application of Azospirillum inocul...
Figure 8.4 Decreased levels of nitrogen content in soil planted with Azospirillu...
Figure 8.5 Effect of genetically modified Azospirillum brasilense strains on nat...
Figure 8.6 Movement of rhizobium nearer to the plant roots. MCP (Methyl acceptin...
Figure 8.7 Comparing the impact of wild-type and its GM Sinorhizobium meliloti s...
Figure 8.8 Comparing the effects of Sinorhizobium meliloti wild-type and GM stra...
Chapter 10
Figure 10.1 Biphasic attachment of Azospirillum brasilense to plant root surface...
Chapter 12
Figure 12.1 Types of fertilizers and their derivatives.
Figure 12.2 Culturing of cyanobacteria through tank method.
Chapter 13
Figure 13.1 Schematic diagram of biofertilizer manufactures.
Figure 13.2 Properties of biofertilizer exhibited for plant growth.
Figure 13.3 Steps for vermicompost formation that used as biofertilizers.
Chapter 14
Figure 14.1 Global presentation of biofertilizer market.
Figure 14.2 Process for quality control of biofertilizers.
Figure 14.3 Main factors affecting the quality of the inoculants from production...
Chapter 17
Figure 17.1 Different sources of wastes. Adapted from Insam et al. [8].
Figure 17.2 Scheme of a conventional waste treatment process. (A) Pre-treatment ...
Figure 17.3 Simplified schematic representation of the main biological phenomena...
Figure 17.4 Process flow diagram of the main steps in waste recovery for microal...
Figure 17.5 Potential applications of microalgae biofertilizers.
Chapter 18
Figure 18.1 Different biofertilizer product in Indian market.
Chapter 22
Figure 22.1 Effects of biofertilizers on the physicochemical properties of soil....
Figure 22.2 Potential functions of cyanobacteria in sustainable agriculture and ...
Figure 22.3 Development of sustainable agriculture practices by utilization of b...
Figure 22.4 Influence of liquid extract of Stoechospermum marginatum on the tota...
Figure 22.5 Influence of liquid extract of Stoechospermum marginatum on the leaf...
Figure 22.6 Influence of liquid extracts of Stoechospermum marginatum on fruit w...
Figure 22.7 Influence of liquid extract of Stoechospermum marginatum on the frui...
Figure 22.8 Effect of culture medium containing Chlorella grown for 3, 6, 9, and...
Figure 22.9 Effect of culture medium containing Chlorella grown for 3, 6, 9, and...
Figure 22.10 Mass culture of biofertilizer and the steps. (Reprinted with permis...
Figure 22.11 Flow diagrams and system boundaries of the wastewater treatment alt...
Figure 22.12 Marine algal bioactives and methods of extraction. (Reprinted with ...
Figure 22.13 Schematic diagram of ultrasound-assisted extraction for the prepara...
Figure 22.14 Schematic diagram of supercritical fluid extraction for the prepara...
Figure 22.15 Schematic diagram of pressurized liquid extraction for the preparat...
Tables
Chapter 2
Table 2.1 Microbial strains used in biofertilizers.
Table 2.2 Challenges and recommendations to improve the efficiency of biofertili...
Chapter 3
Table 3.1 The different types of microorganisms utilized for different types of ...
Chapter 4
Table 4.1 Collection of biofertilizers quantity (tones) in different states of I...
Table 4.2 Based on the three types of biofertilizers and their major function, t...
Table 4.3 The production of yeast extract mannitol medium with their quantity in...
Table 4.4 Tabulated factors directly affect the functioning of cyanobacterial bi...
Table 4.5 Tabulated factors and their influences directly affect the functioning...
Chapter 5
Table 5.1 Various organic acids produced by phosphate-solubilizing bacteria*.
Table 5.2 Different types of bacteria solubilizing phosphorus.
Table 5.3 Different types of fungi solubilizing phosphorus.
Table 5.4 Synergistic effects of arbuscular mycorrhizal fungi and phosphate-solu...
Chapter 6
Table 6.1 The impact of mycorrhizal fungi on crops [2]. (Open Access)
Table 6.2 Rhizobacteria that promote plant growth and the nutrients they make av...
Table 6.3 Examples of oomycetes, the plants they affect, as well as their mode o...
Table 6.4 Ranking of the 10 significant viruses, listed my order of importance [...
Chapter 7
Table 7.1 Comparison of benefits and limitations of biofertilizers [19].
Table 7.2 An overview of plant inoculation using different strains.
Table 7.3 The study of algal strains in different formulations.
Table 7.4 Effect of fungal strains on targeted crops.
Table 7.5 Features of a good bioformulation.
Table 7.6 Advantages and constraints of alginate-based materials.
Chapter 8
Table 8.1 Comparing the effects of organic and inorganic fertilizers on plant gr...
Table 8.2 Potentiality of transgenic rhizobium strains in nodule occupancy in co...
Table 8.3 Population density of genetically modified and control R. leguminosaru...
Chapter 11
Table 11.1 Actinomycetes with soil bioremediation properties.
Chapter 12
Table 12.1 Side effects of applying synthetic fertilizer.
Table 12.2 List of bacteria capable of nitrogen fixation.
Table 12.3 Specific conditions for the culture of cyanobacteria.
Chapter 13
Table 13.1 Various forms of chemical fertilizers used in crops production [15].
Table 13.2 The details of sales of chemical fertilizers during two years 2012–20...
Table 13.3 Sales of urea and N or K fertilizers shown for year of 2010–2012–2013...
Chapter 14
Table 14.1 Year-wise demand and supply chart for biofertilizers.
Chapter 17
Table 17.1 Average composition of some waste sources.
Table 17.2 Metabolisms involved in waste treatment processes from the microalgae...
Chapter 18
Table 18.1 Table depicting all Indian production of N- and P2 O5-based fertilize...
Table 18.2 Depicting all Indian consumption of N-, P2 O5-, and K2O-based fertili...
Table 18.3 Type of biofertilizers used for different types of crops (data.govt.i...
Table 18.4 List of prices of different biofertilizers.
Guide
Cover
Table of Contents
Title page
Copyright
Preface
Begin Reading
Index
Also of Interest
End User License Agreement
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])Phillip Carmical ([email protected])
Biofertilizers
Study and Impact
Edited by
Inamuddin, Mohd Imran Ahamed
Rajender Boddula
Mashallah Rezakazemi
This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
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Library of Congress Cataloging-in-Publication Data
ISBN 9781119724674
Cover image: Wikimedia Commons
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
Great attention has been paid to reduce the use of conventional chemical fertilizers harming living beings through food chain supplements from the soil environment. Therefore, it is necessary to develop alternative sustainable fertilizers to enhance soil sustainability and agriculture productivity. Biofertilizers are the substance that contains microorganisms (bacteria, algae, and fungi) living or latent cells that can enrich the soil quality with nitrogen, phosphorous, potassium, organic matter, etc. They area cost-effective, biodegradable, and renewable source of plant nutrients/ supplements to improve the soil-health properties. Biofertilizers emerge as an attractive alternative to chemical fertilizers and as a promising cost-effective technology for eco-friendly agriculture and a sustainable environment that holds microorganisms which enhance the soil nutrients’ solubility leading a raise in its fertility and stimulate crop growth and healthy food safety.
This book provides in-depth knowledge about history and fundamentals to advances biofertilizers, including latest reviews, challenges, and future perspectives. It covers fabrication approaches and various types of biofertilizers and their applications in agriculture, environment, forestry, and industrial sectors. Also, organic farming, quality control, quality assurance, food safety, and case studies of biofertilizers are briefly discussed. Biofertilizers’ physical properties, affecting factors, impact, and industry profiles in the market are well addressed. This book is an essential guide for farmers, agrochemists, environmental engineers, scientists, students, and faculty who would like to understand the science behind the sustainable fertilizers, soil chemistry, and agroecology, etc.
Chapter 1 focuses on the various action mechanisms observed in microorganisms, those that drive effective biofertilizer functions for forestry-related utilization. Besides, the chapter discusses the factors influencing the success of forestry-related biofertilizer applications as well as the current use and prospects of biofertilizers in the forestry sector.
Chapter 2 highlights the impact of applying biofertilizers on horticultural crops including the possible mechanisms, leading to improved crop growth and stress tolerance. Possible challenges of biofertilizer application and recommended solutions to these problems are also discussed to ensure the efficient use of biofertilizers in the horticulture industry.
Chapter 3 discusses various microorganisms which as act as biofertilizers and also the nitrogen-fixing bacteria including different symbiotic and asymbiotic nitrogen-fixing microbes and other substitutes for easy making of biofertilizers. The major focus is given to innovative methods, for example, growing of microorganisms, accumulation of microorganisms, and conveniences for distribution, applying, and framing of microorganisms for moving from greenhouse and laboratory to field test.
Chapter 4 highlights the usefulness of organic manure in biofarming. Various types of agrochemicals have spoiled our life, environment, and ecosystem. This chapter provides a detailed discussion about how organic manure can save the life of our earth and how it is better than agrochemicals.
Chapter 5 reviews the scientific literature on environmental phosphorus pollution and mechanisms of phosphate solubilization through intact bacteria and fungi or their enzymes. Moreover, inoculation methodologies, factors affecting the inoculum efficiency, and applications of single or multiple species as promising biofertilizer components for sustainable and ecological farming practices are summarized.
Chapter 6 reviews plant-microbe associations occurring both exo- and endophytically on different plant species. The beneficial and pathogenic outcomes of these interactions are discussed, highlighting the microorganisms and the plants involved. Furthermore, the importance of research of these interrelationships is considered concerning use in agriculture for the development of agricultural agents.
Chapter 7 discusses the different formulation technologies of biofertilizers used to mitigate the harmful effects of chemical fertilizers. The complete formulation process is discussed, highlighting the significance of each step, i.e., types of selected microbes, choice of suitable carrier, and addition of sticking materials while unifying the biofertilizer formulation.
Chapter 8 describes the scope of exploiting efficient transgenic microorganisms produced by genetic engineering strategy as potential biofertilizers to enhance the yield of crops through the sustainable farming approach. Furthermore, environment-friendly benefits of utilizing various types of microorganisms alternative to chemical fertilizers in improving soil fertility of agricultural lands are also emphasized.
Chapter 9 intends to provide detailed information on the application of biofertilizer as a sustainable biotechnological tool that could lead to an increase in agricultural production. Detailed information is provided on the modes of action of these biofertilizers while specific examples of cases where biofertilizer has been utilized for improving an increase in agricultural production are also discussed.
Chapter 10 discusses the various characteristic properties, utility, and challenges of free-living nitrogen-fixing bacteria of the genus Azospirillum as a commercial inoculant, aimed to enhance the nitrogen availability in the soil for sustainable agriculture. Mechanistic routes aiding in nitrogen fixation by the bacteria are comprehensively elaborated.
Chapter 11 discusses the beneficial role of actinomycetes in the field of sustainable agriculture. Its unique ability to mitigate soil ecosystem and to promote plant growth by producing important agro-active substances is highlighted. Also, the role of actinomycetes to serve as a potential and efficient source of biofertilizer is discussed.
Chapter 12 discusses the influence of growth conditions and other parameters including morphological patterns of cyanobacterial species, found in biofertilizer quality and nutrients richness.
Chapter 13 provides the positive influences of biofertilizers’ application on agricultural sector via improving the productivity and yields of the crop. Further, it discusses the role of biofertilizers’ production and promotion, with a viable option to chemical fertilizers that have minimized the productivity of the crop with negative impacts on soil, water bodies, or environment components.
Chapter 14 provides detail informationabout the necessity of quality control of biofertilizers. Quality supervision is crucial and should be achieved constantly to manage the microbial products in support of the clients. The rules applied for calculating the quality are restricted to the concentration and viability of the microbes.
Chapter 15 focuses on the significance of biofertilizers with their microbial composition having an edge over chemical fertilizers. The emphasis is on recent advances in preparation, mechanism, growth promotion, carrier materials, and production system of biofertilizers. The chapter also discusses the prospects of biofertilizers along with its advantages and limitations.
Chapter 16 provides an in-depth understanding of biofertilizers and the various approaches available for their preparation. The chapter ends with some prospects and recommendations needed for further improvements in the development, preparation, and application of biofertilizers to achieve green, cleaner, and sustainable food production.
Chapter 17 discusses the use of biofertilizers based on waste recycling as a potential substitute for chemical fertilizers. Initially, an overview of biofertilizers is presented. Furthermore, the main sources of organic waste are discussed, as well as the appropriate treatment processes. Finally, emerging technologies and the main applications of biofertilizers are presented.
Chapter 18 clearly describes the know-how of biofertilizer technology, its segments, and a brief description of the types of biofertilizer available in the market. The highlight of the chapter is the recent trend in biofertilizer technology: liquid biofertilizer, its merits over conventional fertilizers, and future potentials.
Chapter 19 discusses the current situation on the application of biofertilizer in Africa and their mechanism of action. Different types of biofertilizer that have been introduced are also highlighted. Moreover, specific examples are cited where biofertilizer has led to an increase in agricultural production.
Chapter 20 focuses on various types of biofertilizers, their properties, significance, preparation, production, uses, and outcome. It also focuses on experimental designing on screening and selection of microbes and their optimization as biofertilizers. The chapter also deals with the success of biofertilizers, their limitations, and new approaches to overcome constraints.
Chapter 21 discusses the use of microorganisms in the form of biological fertilizers. It also discusses the characteristics and features of different formulations in which the biofertilizers are used extensively. The chapter highlights the development and recent advances in biological fertilizer and its performance.
Chapter 22 covers the biodiversity of algae and biochemical constituents of algal biofertilizers with the effects on plant growth and yield. The state-of-the-art techniques for the mass cultivation of algae are also part of the discussion. This chapter also focuses on novel strategies for the mass production of algal biofertilizers.
1Biofertilizer Utilization in Forestry
Wendy Ying Ying Liu* and Ranjetta Poobathy
School of Biological Sciences, Quest International University, Perak, Malaysia
Abstract
The forest biomes provide crucial ecosystem services which include acting as carbon sinks, providing habitats for biodiversity, preventing soil erosion, mitigating climate change, and producing important resources such as timber, fuel, and bioproducts. However, due to various human activities, the productivity of forests has greatly reduced over time. In order to manage nutrient deficiencies and phytopathogens, chemical products are utilized in forest nurseries, plantation forests, and natural stands. However, this often leads to nutrient losses via leaching, gaseous losses, and other detrimental effects. Exploitation of biofertilizers in the bid to reduce reliance on chemicals could promote the growth and development of tree species and ultimately increase forest productivity in a more sustainable manner. Most biofertilizer utilizations are focused in the agriculture and horticulture sectors with less emphasis in forestry, with the exception of mycorrhizae. It is imperative to recognize the various mechanisms of action of biofertilizers (e.g., facilitation of nutrient uptake, phytohormone modulation, and phytoprotection) to fully exploit the potential of biofertilizer in promoting the ecosystem services of forest biomes. Hence, this chapter explores the mechanisms of action of effective microorganisms in biofertilizers, factors influencing the effectiveness of biofertilizer application, and applications of biofertilizers in the forestry sector.
Keywords: Biofertilizer, forestry, plant growth promoting microorganisms, biological nitrogen fixation, mycorrhizae, phytohormones, biocontrol, sustainability
1.1 Introduction
The forest biomes are globally important as they cover 30% of Earth’s terrestrial surface with more than three trillion trees at an estimated size of four billion hectares [1–3]. The forest ecosystem services include, but are not limited to, acting as carbon sinks, providing biodiversity, protecting soil quality, and producing a wide array of resources such as wood, timber, biomass, and coal [4, 5]. The forest ecosystems are widely distributed worldwide, in boreal, temperate, and tropical regions [6]. For some forest ecosystems, their distributions are highly correlated to land use and soil characteristics, whereby nutrient-poor soils are allocated for forests while high fertility soils are utilized for agriculture crops and grasslands [6]. The productivity of forests has been on a decline due to accelerated growth of human and livestock populations, forest fires and undiscerning exploitation of forest products [7]. These human activities have led to continuous soil erosion causing the forest soils to experience deficiencies of essential nutrients [8].
Chemical fertilizer usage in forestry, from forest nurseries to plantations and natural stands, ranges from zero to very minimal in comparison to its usage in the agriculture sector [9]. In the field, fertilizers are typically applied only once, or at maximum, a few times over a rotation that takes 25 to 30 years [10–12]. The application of chemical fertilizer, when required, allows management of nutrient deficiencies in marginal soils, increased productivity of planted forests and/or maintenance of sustainable soil nutrients after successive rotations [10, 11]. However, the use of chemical fertilizers has its drawbacks. Firstly, excessive applications in the field could lead to nutrient losses via leaching, gaseous losses, and reduced beneficial forest microbiomes [13]. Also, it is arduous to fertilize forest soils due to the remoteness, low accessibility, and low fertility of many forests [6]. Hence, it is more viable to exploit the potential of biofertilizers to reduce the reliance on chemical fertilizers to promote the growth and development of tree species and ultimately promote sustainable forest productivity.
Biofertilizers are products containing beneficial microorganisms, mainly bacteria and fungi, which can inhabit the rhizosphere and/or plant interiors and subsequently promote plant growth via application onto seeds, plants, and/or soil [14, 15]. Such microorganisms, also known as plant growth promoting microorganisms (PGPMs), can directly increase plant growth by facilitating nutrient acquisition in plants and modulating phytohormones while indirectly promote plant growth by decreasing inhibitory effects of phytopathogens while making the rhizosphere more favorable for plants and other beneficial microorganisms [16–18]. The utilization of biofertilizer in agriculture and horticulture is more extensive and widespread while the utilization of biofertilizers in forestry is still under investigation or restricted to forest nurseries, with most studies focusing only on mycorrhizae [19, 20]. It is imperative to explore other beneficial microorganisms and their mechanisms of actions to fully utilize the potential of biofertilizer in promoting growth of tree species, be it in forest nurseries, plantations, and/or natural stands, in order to fully harness the ecosystem services of forest biomes.
1.2 Mechanisms of Actions of Biofertilizers
Biofertilizers could improve plant health and growth, soil nutrient, and fertility status in forest biomes [21]. Nonetheless, the efficacy of the biofertilizers could be greatly affected by various external factors including soil characteristics, tree species, and native microbiome composition [22]. Hence, it is crucial to understand the mechanisms of actions of PGPMs [inclusive of the commonly used term, plant growth promoting rhizobacteria (PGPR)] utilized in biofertilizer so as to employ them under appropriate circumstances. In reality, many PGPMs are likely to employ more than a single mechanism, either simultaneously or at different times under different conditions [14, 17]. The mechanisms of action of PGPMs comprise of aiding nutrient uptake [e.g., nitrogen (N), phosphorus (P), potassium (K), iron (Fe), zinc (Zn), and sulfur (S), modulation of phytohormones (e.g., abscisic acid, cytokinin, ethylene, indole acetic acid (IAA), and gibberellin (GA)] and biocontrol ability to confer phytoprotection [17, 23–25].
1.2.1 Facilitation of N Acquisition
Nitrogen is vital for plant growth and development as it is a major component of nucleic acids, membrane lipid, amino acids, proteins, chlorophylls, and enzymes [18, 26]. However, N is a key limiting nutrient in forest biomes, especially in forest ecosystems that are developed on marginal soils [27–29]. Despite N abundance in the atmosphere as N2, most plants are incapable of utilizing it and are highly dependent on soil N bioavailability, in organic and inorganic N forms, to sustain their growth. However, N2 can be converted into inorganic forms, such as ammonium and nitrate, by the aid of N2-fixing microorganisms via a highly energy intensive biological N2 fixation (BNF) process catalyzed by nitrogenase [23, 30]. Nitrogen-fixing microorganisms in forest biomes can be categorized into two categories: (i) symbiotic N2 fixers (e.g., rhizobia in association with leguminous plants and Frankia spp. in association with actinorhizal plants) and (ii) non-symbiotic N2 fixers (in free-living, associative or endophytic forms) [17, 23]. Nevertheless, symbiotic N2 fixation contributes to majority of the amount of fixed N required by plants in comparison to non-symbiotic N2 fixation [17]. Most literature have referred to symbiotic N2 fixation as symbioses with the development of root nodules while others have also included associative N2 fixation as symbiotic as it fits the definition of symbiosis which is beneficial association between two different organisms [31]. Nonetheless, the former definition will be used in this review.
1.2.1.1 Mutualistic N2 Fixation
Studies have shown that the legume-rhizobia interaction is a main contributor to the amount of fixed N2 in many forest biomes [32]. Many, albeit not all, tree legume species are able to fix atmospheric N2 in association with rhizobia, a group of gram-negative bacteria that are capable of forming N2 nodules on mainly leguminous roots, under N-deficient conditions [33]. Rhizobia can penetrate leguminous root tissues either via hair infection, crack entry or epidermal entry [34]. Most legumes employ the root hair infection strategy where flavonoids secreted by the plants will signal for rhizobia to secrete signal molecules (Nod factors) that binds to root hair cell receptors and then results in the curling of root hair, bacterial entry into the cells of the root hair, division of cortical cells, followed by the formation and ramification of an infection thread in developing nodule primordia [35, 36]. Subsequently, the bacteria will differentiate morphologically to form bacteroids and are eventually removed from the infection thread to form symbiosomes via the synthesis of nitrogenase [35–37]. Ammonium produced in the nodules will be transported to plant cells where it will be assimilated into amino acids for the plant’s use via the glutamine synthetase (GS)/glutamate synthase (GOGAT) pathway [38]. While rhizobia provide N source for plant growth, the legumes provide protection and photosynthates for the rhizobia as a carbon source [39].
Another common N2-fixation symbiosis in forest biomes is the interaction between actinorhizal plants and the actinomycete Frankia via nodule formation [40, 41]. Many actinorhizal trees and shrubs can form mutualisms with mycorrhizae and tripartite symbiosis (actinorhizal plant-Frankia-mycorrhiza), thus providing them an edge to grow in soils with poor nutrient availability [42, 43]. The strategy employed by Frankia in infecting their host plants will depend on the host and Frankia species involved. Nonetheless, they are similar to those employed by rhizobia, which are hair infection, crack entry, or epidermal entry. For actinorhizal plants that belong in the order Fagales, their intracellular infection progress through via root hairs while for those in the Rosales, the intracellular entry of Frankia is through the roots [43]. Nodules produced by actinorhizal plants have indeterminate growth and are able to fix a substantial amount of N2 that is equivalent to those produced by legumes [43, 44].
1.2.1.2 Non-Symbiotic N2 Fixation
Many free-living heterotrophic diazotrophs in the rhizosphere, such as Azotobacter and Klebsiella, are also capable of fixing atmospheric N2, albeit without direct interaction with any other organisms [45–47]. Hence, these bacteria have to source for their own energy to carry out the highly energy-intensive BNF process. They can usually oxidize organic molecules released through decomposition or the aid of other organisms while some have chemolithotrophic abilities in utilizing inorganic compounds instead in order to obtain their energy sources [48]. It has been reported that the free-living BNF rates are strongly correlated to soil organic matter contents, with higher rates in woody residues and surface organic layers compared to mineral soil [48, 49]. Also, as oxygen inhibits nitrogenase activities, these bacteria will have to be able to act as anaerobes or microaerophiles during the N2 fixation [50, 51]. Thus, with these restrictions, free-living N2 fixation, in comparison with symbiotic N2 fixation, will not be able to contribute greatly to BNF in most of the ecosystems [48, 49]. However, it was also reported that free-living N2 fixers fix substantial amounts of N in rain forests [52, 53]. In addition, studies have shown that leguminous trees in mature tropical forests do not fix as much N due to N deposition [54].
Meanwhile, other bacteria, such as Azospirillum and Herbaspirillum, can form endophytic and/or associate mutualisms with various varieties of plants [17, 55]. This manner of N2 fixation is similar to symbiotic N2 fixation without the formation of specialized structures whereby these diazotrophs receive reduced carbon and other nutrients from the plants while they provide fixed nitrogen for the plants to use [56, 57].
1.2.2 Facilitation of P Acquisition
Despite N being generally recognized as the most limiting nutrient in many forest ecosystems, P is also another major limiting nutrient [58, 59]. Phosphorus is vital for plants as a major component of energy source (ATP) to perform various metabolic processes such as signal transduction, macromolecule synthesis, respiration, and photosynthesis [30, 60]. It has been widely reported that high N deposition that is not matched by equivalent increase in the P inputs could cause nutritional imbalances which eventually reduce forest growth and productivity [61–63]. Demand for P has been shown to increase as growth promotion takes place following the additions of N as plant P concentrations and N:P ratios decrease [64, 65]. Also, there is evidence suggesting that P limitation has detrimentally affected the rate of N2 fixation in tropical forests that mainly consist of non-leguminous tree species [66, 67].
Phosphorus can be found abundantly in most soil (as organic and inorganic forms) but the amount of accessible P for plants to utilize is low, as >90% of soil P are insoluble and precipitated (e.g., phosphotriesters and aluminium phosphate) [30, 68]. Plants are only able to access soluble P sources such as monobasic (H2PO4) and dibasic ions [30, 140]. Although chemical fertilizer provides sufficient soluble inorganic P, most of them are immobilized quickly after its application, thus rendering them unavailable to plants [17]. Hence, a sustainable alternative would be to employ microorganisms to aid in P solubilization and mineralization. These microorganisms can be divided in two major groups, which are phosphate solubilizing microorganisms (PSMs) and mycorrhizal fungi.
1.2.2.1 Phosphate Solubilizing Microorganisms
PSMs have been shown to effectively increase the accessibility of available P to plants via solubilization and mineralization of complex P compounds [69, 70]. There are a few strategies that can be employed by PSMs to solubilize insoluble P forms, of which the secretion of organic acids (OA) with low molecular weight, such as acetic, citric, and gluconic acids, is recognized as a main way P is solubilized [17, 71, 72]. The OA secreted will either chelate the mineral ions or lower the cell’s pH level in order to acid-ify these microorganisms and their environments, resulting in the P-ion being released from P-mineral via the swap of H+ for Ca2+ instead [73]. The efficacy of the solubilization process is highly reliant on on the type, concentration, and quality of OAs released into the soil, with the quality of OA being more crucial [74]. The ability of the PSMs to secrete various OAs simultaneously may enhance the P solubilization process further [71].
Some studies have also recommended that the transformation of insoluble P to soluble forms of P could take place without the production of OA [75, 76]. This was reported by Altomare et al. [77] whereby Trichoderma harzianum strain T-22 was able to solubilize P, potentially due to chelation and reduction processes, along with providing biocontrol to the plant without the acidification process. Besides OA production, inorganic acids secreted by chemoautotrophs (e.g., nitric and sulfuric acids) have been shown to solubilize P by converting tricalcium phosphate into mono- and dibasic phosphates [78, 79].
Phosphate-solubilizing microorganisms can mineralize organic phosphorus via the secretion of phosphatases and phytases that catalyze the hydrolysis of phosphoric esters [72]. Acid phosphates, commonly found in fungi, have been proposed to be the main mechanism in the organic P mineralization [80]. Meanwhile, phytase releases P from organic materials that are stored as phytate [72]. Although plants generally are not capable in acquiring P directly from phytate, PSMs in the rhizosphere could alleviate the effects due to the plant’s inability to do so [81]. A single PSM strain could carry out both phosphate solubilization and mineralization [17]. Examples of bacterial genera capable of solubilizing and mineralizing P include Azotobacter, Bacillus, Bradyrhizobium, Burkholderia, Enterobacter, Erwinia, Mezorhizobium, Rhizobium, Microbacterium, Paenibacillus, Pseudomonas, and many others [82]. Meanwhile, fungal PSMs include those from the genera Aspergillus, Cladosporium, Penicillium, Rhizoctonia, Rhizopus, Sclerotium, Trichoderma, and others [82, 83].
1.2.2.2 Mycorrhizas
Mycorrhizal fungi can establish symbiotic relationships, either obligately or facultatively, with many plants, where these mycobionts are reliant on their host plants for photosynthates and energy while they contribute various benefits to their hosts [84]. This group of microorganisms is one of the most commonly studied PGPMs used in biofertilizer for forestry purposes [85]. The fungi can increase surface areas for better access to procure nutrients, in particular insoluble phosphorus sources by extending their mycelia from root surfaces into the soil [86]. Besides that mycorrhizal fungi are also able to improve quality and aeration of soil, reduce susceptibility of the plants to phytopathogens, and promote tolerance of host plants to environmental stresses [84, 87, 88].
Mycorrhiza can colonize their host plant’s root tissues either via intracellular or extracellular mechanisms. Hence, they can be divided into two main categories, which are endomycorrhiza and ectomycorrhiza, respectively. Endomycorrhizas can be further categorized as arbuscular, arbutoid, ericoid, monotropoid, and orchid mycorrhizaes [88, 89]. The most common mycorrhizal associations are arbuscular mycorrhizal fungi (AMF) which are obligate mycobionts (generally, Glomeromycetes) that can form mutualism with more than 80% of vascular plants [90]. Vesicles and arbuscules are specialized structures of AMF that assist in mobilizing and making P available to their host plants and also obtaining mineral and water sources from further areas in the soil [3, 85]. However, as AMF are obligate in nature and cannot be grown as pure cultures in isolation from their hosts, these make AMF biofertilizer production in large scale a very difficult task [91].
Although ectomycorrhizal fungi (ECMF) are extensively distributed in many ecosystems, they are only affiliated with only 3% of the vascular plant families, particularly woody trees (e.g., birch, beech, myrtle, pine, and willow) [85]. Nonetheless, as they are effective in symbiotic association with many important woody trees, they are crucial in forestry and ecosystem management. Species of ECMF (>20,000 species) are mainly members of the phyla Ascomycota and occasionally Basidiomycota [92, 93]. ECMF are unable to penetrate the cell walls of their host plants but they form an intercellular interface, the Hartig network, comprising extremely branched hyphae to form a lattice between epidermal and cortical root cells to allow metabolic exchange between both fungi and the root [85]. The thick hyphal sheath, termed as the mantle, formed by ECMF are connected to extraradical mycelia that extend into the soil to allow minerals and water to be mobilized, absorbed and translocated to the hosts, often aiding their survival during adverse conditions [94, 95].
As of late 1950s, ECMF were investigated and utilized as biofertilizer to promote plant growth as they are able to assist in plant acquisition of P, N, water, and other minerals in forestry [96]. The application of these fungi is more common in nurseries whereby tree seedlings are inoculated in the nursery before planting in field in order to ensure healthy fungi system. Successful ECMF association is highly dependent on the suitable selection of plant-host species [85]. Mycorrhizal helper bacteria (MHB), when used with ECMF, have been shown to assist in the establishment and effectiveness of the mycorrhizal symbioses [97] by stimulating mycelial growth, increase the interaction and surface areas for root-mycorrhizae colonization via phytohormone and flavonoid production, and mitigate the detrimental effect of environmental stress on mycelia [85, 98, 99].
1.2.3 Potassium Solubilization
Potassium is an integral macronutrient for enzyme activation, osmotic balance, phloem transport, photosynthesis, and protein synthesis in plants [100, 101]. The concentration of soluble K is generally minimal in soil as most K occur in the forms of silicate minerals and insoluble rocks [102]. The soil is the main source of K for plants and its accessibility in soil is reliant on the dynamics and content of K in the biomes [100]. Besides that, K limitation may detrimentally affect the soil microbial community resulting in inefficient nutrient cycling in the forest biomes [103, 104].
It has been suggested that the K dynamics and distribution in plant tissues, soils, and water in the forest biomes could be strongly affected by biotic factors [105]. The availability of K could also be further affected by stressors such as timber harvesting, forest fire, and intensification of land usage [106]. Many forests of which tree species have been continuously harvested for fuel, fertilizers, and others may have seen a decrease in substantial amounts of K available in the ecosystem, which may have decelerated the K availability for the growth and development of various tree species [107, 108]. Also, the addition of other major limiting nutrients (N and P) through atmospheric deposition, without increasing the amount of K, may be detrimental to the ecology of the forest biomes [105].
Limitations of K in the soil could be addressed by using potassium-solubilizing microorganisms (KSMs) which produce OAs (e.g., formic, malic, oxalic, and tartaric acids) to release K from various insoluble minerals which include illite, micas, and orthoclases [100]. The OAs can convert K into soluble forms via direct solubilization of rock K or excretion of chelated silicon. These acids increase the solubilization of K compounds via provision of protons and formation of complexes with metal ions, such as Ca2+, present in the soil [109]. Examples of genera of KSMs include Arthrobacter, Acidithiobacillus, Bacillus, Burkholderia, Paenibacillus, and others [110, 111].
1.2.4 Production of Siderophores
Iron is an important trace element for plant growth as it acts as a major component of redox enzymes that aids in oxygen transport, cellular proliferation, chlorophyll synthesis, and nucleic acid synthesis [112]. Despite being the fourth most copious element on earth, Fe cannot be readily assimilated by plants and microorganisms as it occurs predominantly as Fe3+ in nature and tends to form highly insoluble oxides and hydroxides under aerobic conditions, particularly in calcareous soils [113]. Though iron is not a limiting factor in many forest biomes, it has reported to be a limiting factor in some mangrove forests [114–116].
Under Fe-limiting conditions, microorganisms can acquire Fe via the secretion of siderophores, which are low-molecular mass iron chelators (~400–1,500 Dalton) with high affinity for Fe3+ ions [17, 117]. Siderophores will bind to Fe3+ ions and form complexes that are then returned to the cytosol where the reduction of Fe3+ takes place, followed by the secretion of Fe2+ into the cell through a gating machinery, making Fe accessible to the microorganism [118]. Siderophores are divided into three major families, which are catecholates (e.g., enterobactin), carboxylates (e.g., rhizobactin), and hydroxamates (e.g., ferrioxamine B) based on their functional groups [119]. Thus far, there are 270 types of siderophores that have been structurally characterized out of more than 500 known siderophores [120].
Generally, plants may employ two strategies to sequester iron: (i) lowering of pH levels in the rhizosphere, trailed by reduction of Fe3+ ions by ferric-chelate reductase, allowing the root cells to successively uptake of Fe2+; and/or (ii) secretion of phytosiderophores for iron solubilization, binding, and subsequent transport into root cells [121]. Yet, these strategies are unable to provide sufficient Fe for the plants under Fe-deficient conditions, especially in calcareous and alkaline soils [121]. Hence, a better alternative would be to assimilate Fe from microorganisms. Plants can assimilate Fe from these bacterial or fungal siderophores by two strategies which are (i) transfer of their Fe-siderophore complexes to plant roots for Fe reduction to take place and (ii) direct Fe chelation followed by ligand exchange reaction with phytosiderophores to release Fe2+ [23, 122].
Although the main function of the siderophores is to scavenge for Fe, they are also able to form complexes with other metals (e.g., aluminium, cadmium, copper, and nickel) present in the rhizosphere allowing them to be utilized by the microorganisms [123]. This, in turn, could help to ameliorate the abiotic stresses caused onto plants by toxic concentrations of heavy metals in the soil. Besides improving plant growth directly, siderophore production could also indirectly increase plant growth by inhibiting the growth of phytopathogens by binding to most Fe3+ in the root area and reducing Fe availability for them to proliferate [124]. This was proposed as an effective biocontrol mechanism as the siderophores produced by PGPMs have higher affinity for Fe3+ compared to fungal pathogens, thus out-competing them for available Fe [17].
1.2.5 Modulation of Phytohormones
Phytohormones are organic substances that, at low concentrations (lesser than 1 mM), have the ability to boost, impede or modify the developmental activities of plants, particularly in their response to the environment [30, 125]. When exposed to environmental stresses (e.g., low pH, drought, salinity, and high temperature), plants usually try to regulate the levels of endogenous plant hormones so as to reduce the detrimental effects posed by these stresses [17, 126]. Certain PGPMs are capable of in vitro phytohormone production and could modulate the levels of phytohormones to reach equilibrium, thus aiding in regulating plants’ responses to environmental stresses [17, 127]. Common phytohormones that are produced by PGPMs comprise abscisic acid, cytokinin, IAA, and GA [128, 129].
IAA has various functions such as to promote cell division and differentiation, enhance germination rates and percentages of seeds, initiate and increase the expansion of roots, regulate plants’ responses to stress, produce metabolites, and negatively affect photosynthesis process [17, 30, 130]. It has been reported that many PGPMs are capable of releasing IAA as secondary metabolites in a slow and continuous manner, subsequently changing the auxin pool and affecting the plants’ physiological functions [131, 132]. The biosynthesis of IAA by PGPMs could involve L-tryptophan–dependent or independent pathways. Tryptophan, an amino acid that occurs at plant root regions, is the main precursor to synthesize IAA [133]. In addition, tryptophan can indirectly increase IAA production by inhibiting the formation of anthranilate, a precursor that inhibits IAA biosynthesis, via negative feedback regulation on the amount of anthranilate synthase [23, 134]. The indole-3-pyruvic acid pathway is the most frequent IAA biosysnthesis pathway utilized by PGPMs, followed by the indole-3-acetamide pathway [30, 131].
GAs are diterpenoid phytohormones that are crucial in various processes including stem elongation, leaf expansion, seed germination, floral induction, breaking of dormancy in seeds and tubers, and augmentation in size and number of fruits [128, 135]. Generally, GAs are produced endogenously by plants to regulate plant growth and development, with GA1, GA3, and GA4 being the three most common types of GAs that promote plant growth and shoot elongation [136]. There are three regulatory processes that control the synthesis of GA which are biosynthesis, reversible conjugation, and catabolism [130, 137].
Ethylene can affect plant growth by stimulating root initiation, hindering root elongation, accelerating the ripening of fruits, reducing wilting, improving seed germination, and triggering the production of other phytohormones [30]. When exposed to stress conditions, plants will secrete ethylene at higher concentrations to overcome the stresses [138, 139]. An essential precursor required for ethylene production is 1-aminocyclopropane-1-carboxylic acid (ACC) [17, 139]. The production of ACC can be stimulated by the presence of both endogenous and bacterial IAA [17]. However, high concentrations of ethylene can cause defoliation and alter cellular processes that will eventually decrease plant growth [23, 30, 138, 140]. The production of ACC deaminase by PGPMs can reduce the negative impacts of high concentrations of ethylene on plant growth. The beneficial microorganisms, via ACC deaminase, are able to hydrolyze ACC into ɑ-ketobutyrate and NH3 which are then utilized as carbon and nitrogen sources, thus reducing the ethylene levels [139].
1.2.6 Phytoprotection
Despite being used to a much lesser extent in forestry compared to agriculture, pesticides are mostly applied in forest nurseries and planted forests as they are the most cost-effective manner to manage insect pests, diseases, and weeds [141]. However, continual application of pesticides can bring about environmental damage to forest biomes (e.g., contamination of ground water, reduction in soil fertility, and disruption in ecosystem dynamics) [30]. Hence, the use of PGPMs as biocontrol agents in forests could be a more sustainable approach to control pests. The biocontrol activities include antibiosis nutrient competition, hydrogen cyanide (HCN) and lytic enzyme production, and induced systemic resistance (ISR) [17, 30].
One of the most effective biocontrol mechanisms by PGPMs is antibiosis to suppress the proliferation of phytopathogens (generally fungi). Some of the identified antimicrobial compounds responsible for phytopathogen inhibition include 2,4-diacetylphloroglucinol (DAPG), ecomycins, pyrrolnitrin, pyoluteorin, oomycin A, amphisin, phenazine, and others [142]. However, overdependence on these antimicrobial compounds could lead to the development of resistance in phytopathogens [30]. Thus, selection of strains that are capable of producing more than a single antimicrobial compound as well as synthesizing HCN is preferred to address this [17].
The synthesis of HCN, a volatile secondary metabolite with low molecular weight, can confer selective advantages on the producer, despite the lack of a role in their growth and primary metabolism. Production of HCN could inhibit the synthesis of cytochrome-c oxidase and some metalloenzymes in the pathogens, consequently preventing electron transport and disrupting energy supply to the cells [131, 143]. However, the utilization of HCN on its own does not render significant biocontrol activity, but instead, it works synergistically with antimicrobial compounds produced in antibiosis [17]. It was also reported that HCN could aid in nutrient acquisition by chelating metal ions and boosting nutrient availability to the plants [144]. PGPMs could also exert their biocontrol ability by producing lytic enzymes, such as β-glucanase, chitinase, dehydrogenase, phosphatises, and proteases that are capable of directly inhibiting fungal pathogens by affecting the structural integrity of their cell walls [145]. β-glucanase and chitinase are able to degrade fungal cell walls that are normally made up of β-1,4-N-acetyl-glucoseamine and chitin. Besides lytic enzyme production, PGPMs could also suppress phytopathogen growth via competition as they
