**2. Main factors influencing the decomposition of crop residues and green manures in tropical environments**

The decomposition of crop residues or green manures in the soil is a complex process, which is the result of the interaction between different factors (biotic and abiotic) specific of each environment. However, the main abiotic factors that drive this process are related to their influence on soil organisms, since the decomposition is essentially a biological process [1].

Initially, the decomposition consists in the physical fragmentation process of the organic residues into smaller particles, which is a process performed by components of the soil macro‐, meso‐, and microfauna. Physical fragmentation of residues provides an increase in surface area, facilitating microbial colonization and subsequent hydrolysis by microbial extracellular enzymes. Thus, complex polymers are degraded into monomeric compounds and ions, which can be absorbed by microbial cells or plants.

The factors that can affect the direction and magnitude of the decomposition process are the nutrient content and biochemical composition of the crop residues or green manure added to the soil, the nature and abundance of the present microbial communities, Soil moisture, temperature, aeration, pH, the carbon/nutrient ratios of the soil organic matter (SOM) [2, 3], and the presence or absence of inhibitor substances.

In this part, we discuss the main factors involved in decomposition and its peculiarities in tropical environments.

#### **2.1. C/N, C/P, and C/S ratios in crop residues and green manures**

The use of green manures and crop residues provides various conditioning effects to the soil; however, the main objectives of this practice in low-fertile tropical soils are increasing soil cation exchange capacity (CEC) and provide nutrients for the plants. Thus, the nutrient contents present in these plants (mainly N, P, and S) are one of the first characteristics to be observed.

However, N, P, and S contents in the residue do not necessarily mean that they will be released synchronously with the plant needs during the decomposition process. After the decomposi‐ tion, monomeric compounds and ions can be absorbed by microorganisms present in the soil, which use them as energy supply or metabolic precursors. After these requirements are satisfied, excess ions may be released into the soil solution and be available for the plants.

### **2.2. Mineralization and immobilization**

The soils in most of these tropical environments have high acidity and aluminum toxicity, and are rich in oxides and poor in nutrients. Therefore, the use of lime and fertilizers accounts for a large part of the agricultural production cost. Thus, to increase the environmental and economical sustainability of these environments, it is important to make rational use of fertilizers and find viable alternatives to maintain a good physical, chemical, and biological

We highlight the use of green manures and crop residues as practices that can help maintain or increase the productivity capacity of the soils, since they act as conditioners of the physical, chemical, and biological characteristics. Since the ancient Greeks, Romans, and Chinese, the humanity wisely used fresh organic matter as green manures in order to maintain the land productivity, and even today, this practice has been used with the same purpose. Meanwhile, crop residues were "a problem" for many years in agriculture. The removal and/or burning of residues were common practiced used in order to accelerate its degradation in conventional tillage. The expansion of no‐till system, from the 1970s, presented several benefits by conserv‐ ing and managing residues of annual crops, especially at medium and long term. The culti‐ vation of perennial crops also presented major recent changes, such as management of weeds between plants, not using fire in renewing and especially the use of processed residues.

In this chapter, we present some of the benefits of management of green manure and crop residues, mainly the nutrient supply potential for crops of economic interest. Initially, we discuss the factors that most influence the fresh organic matter decomposition and nutrient mineralization in tropical areas. Subsequently, we discuss the concept of green manure, its management, the amount of nutrients potentially accumulated and their advantages and disadvantages. In the last part, we present examples from the main crops grown in the tropics. Regarding the annual crops, we focused on legumes with greater economic impact, since they are the first crops to be planted in rotation or succession managements. Regarding perennial crops, we present the contribution of the main crop residues and the processing of sugarcane,

**2. Main factors influencing the decomposition of crop residues and green**

The decomposition of crop residues or green manures in the soil is a complex process, which is the result of the interaction between different factors (biotic and abiotic) specific of each environment. However, the main abiotic factors that drive this process are related to their influence on soil organisms, since the decomposition is essentially a biological process [1]. Initially, the decomposition consists in the physical fragmentation process of the organic residues into smaller particles, which is a process performed by components of the soil macro‐, meso‐, and microfauna. Physical fragmentation of residues provides an increase in surface area, facilitating microbial colonization and subsequent hydrolysis by microbial extracellular enzymes. Thus, complex polymers are degraded into monomeric compounds and ions, which

soil characteristics.

52 Organic Fertilizers - From Basic Concepts to Applied Outcomes

coffee, and eucalyptus.

**manures in tropical environments**

can be absorbed by microbial cells or plants.

Extracellular enzymes released by the fauna and soil microorganisms during the decomposi‐ tion release part of the P, S and N initially linked to organic compounds in the fresh organic matter (crop residues or green manures). Extracellular phosphatases are responsible for the mineralization of P in organic compounds (C–O–P links) to phosphate ions, HPO4 2- , and H2PO4 - (prevalent in tropical soils). Sulfatases are responsible for breaking estersulphates (C– O–S links) to SO4 2- ions, and urease converts urea into NH4 + .

After organic compounds fragmentation, amino acids containing C–N and C–S bonds, amino sugars, and nucleic acids are absorbed by microorganisms to attend their energy, C, N and S demand. Then, intracellular hydrolases convert C-N and C-S bonds into NH4 + and SO4 2- for subsequent internal use. Mineralization is the sequence of reactions that converts the nutrient from organic to inorganic form, resulting from microbial decomposition [4]. However, in practice, mineralization is when nutrients are released in the soil solution during the decom‐ position process. Thus, we say that nutrients were immobilized in microbial biomass and are unavailable for absorption by the plants when the N and S are mineralized into the microbial cell and/or when the NH4 + , SO4 2- and H2PO4 ions from soil solution are carried to microbial cytoplasm and subsequently used as precursors for synthesis of other compounds.

The microbial cell demand for energy, nutrients, and C does not occur at the same proportion and is different for each microbial community. The C/N ratio of bacteria range between 4 and 5, while for fungal, it is about 15. But, since fungal biomass is often twofold larger than bacterial in most soils, it is assumed that the C/N ratio of the total biomass of the soil is approximately 8 [5].

Each microbial community in the soil has a different C/N, C/P, and C/S demand, as well as C utilization efficiency. Thus, an average C/N, C/P and C/S are also assumed to the fresh organic matter (crop residues or green manures), from which the nutrient mineralization or immobi‐ lization will be defined. The average C/N in plant residues is 20–30, when the C/N > 30, the N is immobilized in the microbial biomass and when the C/N < 20, the N mineralization is favored. The average C/P in the plant matter is approximately 200–300. When C/P>300, P immobilization is greater than mineralization, the opposite occurring when C/P<200. The average C/S in plant residues is 200, with C/S>200 promoting greater S immobilization and C/ S<200 favoring S mineralization.

The C/N, C/P, and C/S of the crop residues and green manures added to the soil must be known, since they present major influence on the mineralization/immobilization processes. Legumes have higher levels of N in their tissues compared to non‐legume species, due to the biological nitrogen (N2) fixation (BNF). Studies have shown the effectiveness of the use of these species as green manures to meet N plant demands [6, 7]. However, this is not the case for P demand, which is barely attended by use of such cover crops [8–10].

The immobilization and mineralization process does not depend only on the C/N, C/P, and C/ S in the residues, since they are also greatly influenced by these ratios in the SOM compounds. Organic compounds from decomposed residues interact with the SOM during the decompo‐ sition process. SOM decomposition may increase when organic residues are added to the soil. A theory for this fact is that the soil microorganisms degrade easily degradable organic compounds present in SOM to acquire energy, C or N (and possibly other nutrients) co‐ metabolically with the residues added to the soil [11–13]. This effect is known as "priming effect" [14] and can be positive when it accelerates SOM decomposition or negative when it slows down SOM decomposition. Therefore, N, P and S contents in the residues and SOM are important, as well as the availability of these nutrients in the soil solution, which influences mineralization and immobilization processes, being an important tool in the management of residue decomposition.

However, the decomposition of residues and SOM would release other nutrients like Ca, Mg, K and trace elements, which are subjected to the same principles of the mineralization and immobilization explained above. K is a very abundant element in plant tissues, however it is not part of biomolecules, being released very easily and at high rates [15]. Studies have shown that legumes used as green manures have been effective in supplying K for plants [6, 7]. Besides N, they also show higher levels of K due to their branched and deep root systems, allowing nutrient cycling [16].

Tropical soils are generally acid and with low natural fertility, presenting restrictions to the crops because their high Al3+ availability in the soil solution and the low P availability for plants, due to the formation of irreversible Tropical soils are generally acid and with low natural fertility, presenting restrictions to the crops because their high Al3+ content and low P availability for plants, especially due to the formation of irreversible bindings with Fe and Al oxyhydroxides. In this context, the high production of organic acids by the decomposition of residues and green manures causes a competition for P adsorption sites on the soil, promoting a greater P availability in the soil solution [17]. This process occurs due to: *i*) H+ and Al3+ sorption on the surface of the organic material; *ii*) Al3+ complexation with organic acids; *iii*) competition with phosphate by binding sites, decreasing P adsorption. Moreover, in tropical environments, the P organic forms from residues are essential for P availability to plants [18]

### **2.3. Other factors influencing the decomposition**

in most soils, it is assumed that the C/N ratio of the total biomass of the soil is approximately

Each microbial community in the soil has a different C/N, C/P, and C/S demand, as well as C utilization efficiency. Thus, an average C/N, C/P and C/S are also assumed to the fresh organic matter (crop residues or green manures), from which the nutrient mineralization or immobi‐ lization will be defined. The average C/N in plant residues is 20–30, when the C/N > 30, the N is immobilized in the microbial biomass and when the C/N < 20, the N mineralization is favored. The average C/P in the plant matter is approximately 200–300. When C/P>300, P immobilization is greater than mineralization, the opposite occurring when C/P<200. The average C/S in plant residues is 200, with C/S>200 promoting greater S immobilization and C/

The C/N, C/P, and C/S of the crop residues and green manures added to the soil must be known, since they present major influence on the mineralization/immobilization processes. Legumes have higher levels of N in their tissues compared to non‐legume species, due to the biological nitrogen (N2) fixation (BNF). Studies have shown the effectiveness of the use of these species as green manures to meet N plant demands [6, 7]. However, this is not the case for P demand,

The immobilization and mineralization process does not depend only on the C/N, C/P, and C/ S in the residues, since they are also greatly influenced by these ratios in the SOM compounds. Organic compounds from decomposed residues interact with the SOM during the decompo‐ sition process. SOM decomposition may increase when organic residues are added to the soil. A theory for this fact is that the soil microorganisms degrade easily degradable organic compounds present in SOM to acquire energy, C or N (and possibly other nutrients) co‐ metabolically with the residues added to the soil [11–13]. This effect is known as "priming effect" [14] and can be positive when it accelerates SOM decomposition or negative when it slows down SOM decomposition. Therefore, N, P and S contents in the residues and SOM are important, as well as the availability of these nutrients in the soil solution, which influences mineralization and immobilization processes, being an important tool in the management of

However, the decomposition of residues and SOM would release other nutrients like Ca, Mg, K and trace elements, which are subjected to the same principles of the mineralization and immobilization explained above. K is a very abundant element in plant tissues, however it is not part of biomolecules, being released very easily and at high rates [15]. Studies have shown that legumes used as green manures have been effective in supplying K for plants [6, 7]. Besides N, they also show higher levels of K due to their branched and deep root systems, allowing

Tropical soils are generally acid and with low natural fertility, presenting restrictions to the crops because their high Al3+ availability in the soil solution and the low P availability for plants, due to the formation of irreversible Tropical soils are generally acid and with low natural fertility, presenting restrictions to the crops because their high Al3+ content and low P availability for plants, especially due to the formation of irreversible bindings with Fe and Al

8 [5].

S<200 favoring S mineralization.

54 Organic Fertilizers - From Basic Concepts to Applied Outcomes

residue decomposition.

nutrient cycling [16].

which is barely attended by use of such cover crops [8–10].

#### *2.3.1. Biochemical composition of green manures and crop residues*

Biochemical composition influences plant residue decomposition and microbial communities in the soil. Plant residues consist basically of the same components, but the proportions can vary between species, plants of the same species, organs of the same plant, and crop conditions [19].

Green manure residues quality is dependent on the species used (N2‐fixing species have usually lower C/N compared with non‐legume species), nutrient content and the age of the crop used as green manure, which affect the size, fiber content, lignin content and C/N ratio [20].

In general, the compounds present in the plant cell cytoplasm and walls are waxes and pigments (1%), amino acids, nucleotides and sugars (5%), starch (2–20%), proteins (5–7%), hemicellulose (15–20%) celulose (4–50%), lignin (8–20%) and secondary compounds (2–30%) [21]. Most of these components are present in the primary and secondary cell walls of plant cells. The primary wall is formed basically by cellulose and hemicellulose. After the primary wall growth cease, the secondary walls begin to form, which has the lignin as the main component and gives resistance to the cell wall [22]. Phenolic compounds are secondary metabolites such as polyphenol and lignin, which have no direct function in the plant growth and development [22]. Additionally, non‐structural carbohydrates, such as free sugars, starch, and arabinose, may affect the decomposition of materials in the soil [23].

Some studies report that soluble carbohydrate materials are readily decomposed, as well as the components rich in N, establishing the initial decomposition rate of the crop residue [23– 25].

The decomposition of some organic compounds of wild pine trees (*Pinus sylvestris*), for example, can be explained through a system comprising two phases [26]. In phase 1, nutrients found in higher concentrations such as N, P and S favor the mass loss of non-lignified organic compounds. In general, organic compounds from simple structures (labile) tend to be used more efficiently compared to those more complex (polymerized) or associated with other compounds (e.g., lignin–cellulose) [27–29]. In phase 2, lignin content increases during residue decomposition [30, 31], remaining most of the more lignified material. Plant degradation is determined by the reduction of the lignin concentration, which is negatively affected by high N concentrations and positively affected by high cellulose concentrations.

The negative influence of high N concentrations on lignin degradation may be due to the ligninolytics enzymes suppression at high levels of NH4 + and N organic compounds of low molecular weight. This repression can be explained by: *i*) the N can change the decomposing microorganisms competition, including those able to degrade lignin [32]; *ii*) high NH4 + levels reduce the production of ligninolytics enzymes [33, 34]; *iii*) the amino compounds condensate with polyphenols, forming toxic compounds or inhibitors [35, 36].

On the other hand, the positive effect of lignin degradation by high cellulose content occurs because lignin has very stable bonds, which require energy to break. Thus, a co-metabolism with more labile (easy degraded) compounds is necessary and positively influences the residue decomposition [37]. The SOM also influences this process by providing nutrients and more labile compounds, which can supply the most immediate forms of energy to the microorgan‐ isms, enabling them to degrade some recalcitrant compounds of the residues, which direct affect the energy supply to the microorganisms along the decomposition.

Consequently, the values of some biochemical fractions of the residues can serve as decom‐ position rate indicators. The fractions commonly used are the water‐soluble extractives and extractives soluble in neutral organic solvents, which are secondary components and are not part of the cell structure. The holocellulose fraction, which consists of cellulose and hemicel‐ lulose, is constituent of structural components as well as the lignin fraction. Thus, the propor‐ tion of these fractions in crop residues and green manures, as well as the C/N, lignin/N and polyphenols/N, directly affect the decomposition of crop residues [38, 39]. This information can help managing the residues in order to synchronize nutrients release to plants.

### *2.3.2. Temperature and humidity*

Temperature and humidity are factors that directly affect the microbial activity, more precisely the microbial enzyme complex [40, 41]. Temperature and humidity are factors that directly affect the microbial activity, more precisely the microbial enzyme complex [40, 41]. Residue decomposition rate correlates positively with temperature and water availability within a broad range [42].The enzymatic activity increases with an increase temperature or humidity up to a plateau, from which temperature and humidity can limit decomposition. Thus, the weather strongly affects the residue decomposition rate [43].

Moist tropical soils, which have Moist tropical soils, which have high average temperature and humidity throughout the year, have faster decomposition rates than temperate soils. Thus, in the tropical environments, the temperature and humidity present less restriction to decom‐ position, which depends primarily on the quality of the residues and the SOM [44].

It has been shown that chemically complex litterfall have stronger responses to temperature compared to less chemically complex litterfall [45]. Moisture is essential for the reactions that occur in the soil, and water is required for all hydrolytic reactions, affecting the extracellular enzymes activity and the diffusion coefficients [46]. Soil water content of 60% of its total porous space is assumed to be the optimum for the decomposition rate of aerobic soil microorganisms [47].

Natural cycles of wetting and drying in the soil are important modulators of decomposition rates [48]. Some agricultural practices can assist water management in order to achieve higher rates of decomposition, such as: (*i*) irrigation; (*ii*) drainage furrows installation in the field to remove excess water; (*iii*) maintenance of residues on the soil surface to increase water infiltration and decrease evaporation and *iv*) synchronization of residue incorporation with the rainy season based on historical and forecast rainfall [49].

Excess moisture can also cause anaerobic conditions that will negatively influence plant residues decomposition rates. Anaerobic environments are not favorable to fungi and actino‐ mycetes, and only some bacteria are able to perform anaerobic digestion, decreasing decom‐ position rates.

### *2.3.3. Microbial communities in the soil*

The negative influence of high N concentrations on lignin degradation may be due to the

molecular weight. This repression can be explained by: *i*) the N can change the decomposing

reduce the production of ligninolytics enzymes [33, 34]; *iii*) the amino compounds condensate

On the other hand, the positive effect of lignin degradation by high cellulose content occurs because lignin has very stable bonds, which require energy to break. Thus, a co-metabolism with more labile (easy degraded) compounds is necessary and positively influences the residue decomposition [37]. The SOM also influences this process by providing nutrients and more labile compounds, which can supply the most immediate forms of energy to the microorgan‐ isms, enabling them to degrade some recalcitrant compounds of the residues, which direct

Consequently, the values of some biochemical fractions of the residues can serve as decom‐ position rate indicators. The fractions commonly used are the water‐soluble extractives and extractives soluble in neutral organic solvents, which are secondary components and are not part of the cell structure. The holocellulose fraction, which consists of cellulose and hemicel‐ lulose, is constituent of structural components as well as the lignin fraction. Thus, the propor‐ tion of these fractions in crop residues and green manures, as well as the C/N, lignin/N and polyphenols/N, directly affect the decomposition of crop residues [38, 39]. This information

Temperature and humidity are factors that directly affect the microbial activity, more precisely the microbial enzyme complex [40, 41]. Temperature and humidity are factors that directly affect the microbial activity, more precisely the microbial enzyme complex [40, 41]. Residue decomposition rate correlates positively with temperature and water availability within a broad range [42].The enzymatic activity increases with an increase temperature or humidity up to a plateau, from which temperature and humidity can limit decomposition. Thus, the

Moist tropical soils, which have Moist tropical soils, which have high average temperature and humidity throughout the year, have faster decomposition rates than temperate soils. Thus, in the tropical environments, the temperature and humidity present less restriction to decom‐

It has been shown that chemically complex litterfall have stronger responses to temperature compared to less chemically complex litterfall [45]. Moisture is essential for the reactions that occur in the soil, and water is required for all hydrolytic reactions, affecting the extracellular enzymes activity and the diffusion coefficients [46]. Soil water content of 60% of its total porous space is assumed to be the optimum for the decomposition rate of aerobic soil microorganisms

position, which depends primarily on the quality of the residues and the SOM [44].

can help managing the residues in order to synchronize nutrients release to plants.

weather strongly affects the residue decomposition rate [43].

microorganisms competition, including those able to degrade lignin [32]; *ii*) high NH4

+

and N organic compounds of low

+ levels

ligninolytics enzymes suppression at high levels of NH4

56 Organic Fertilizers - From Basic Concepts to Applied Outcomes

*2.3.2. Temperature and humidity*

[47].

with polyphenols, forming toxic compounds or inhibitors [35, 36].

affect the energy supply to the microorganisms along the decomposition.

Different organisms work in the residues within the soil. The soil microbial community is diverse and shows an uneven distribution along the soil profile and throughout the microen‐ vironments [50].

There is a change in the biochemical composition of the residues during the decomposition process, what drives a microbial succession. Simpler compounds are used as a growth substrate for a large number of microorganisms that have short life‐span, which are called r‐ strategists or copiotrophs. In the later, degradation stages occur the metabolism of more complex compounds, in which some microorganisms break components more slowly and are called oligotrophic or k-strategists [4].

Among the r‐strategist and k‐strategist microorganisms there are autotrophic and heterotro‐ phic bacteria, fungi, and actinomycetes. Moreover, studies have reported that the presence of N influences the microbial community diversity during decomposition.

Changes in the microbial community are reported to happen within150 days of incubation of eucalyptus residues due to changes in the residue biochemical composition [51]. Moreover, these same authors found that, with the application of N, the change in the microbial com‐ munity occurred within the 25 days of incubation, remaining constant to the end of the experiment, suggesting that the chemical differences of residues can be minimized when the C/N of the residues are closer to the C/N of the soil microorganisms [52, 53].

High bacteria and low fungi biomasses were observed for 8 years in soils with eucalyptus receiving applications of N [54]. Moreover, an increase in the Gram+/Gram-bacteria ratio was observed in an experiment with incubation of residues (leaves) of *P. massoniana* and *M. macclurei* in coniferous forest soils [55]. A study of *Pinus sylvestris* L. forests grown for 50 years in northern Sweden, report that, during two decades with nitrogen fertilization, there were reductions in the ectomycorrhizal fungi community, showing that the effects of N go beyond simple enzymatic removal of fungi and may comprise factors that are still unclear regarding the decrease in residue decomposition with presence of N [52].

Another important factor during residue decomposition is the soil pH, which directly affects the type, density, and activity of bacteria, fungi, and actinomycetes. The residue decomposition rate is higher in soils with neutral pH than in more acidic soils such as tropical soils. However, liming acidic soils promotes accelerated decomposition of the residues.

The construction and maintenance of microbial diversity in the soil favor higher rates of decomposition [49]. Some agronomic practices are recommended for this purpose, such as: (*i*) regular application of organic residues or the use of biochemically complex green manures associated with those biochemically simple and easy decomposing, which supports the greater diversity of microbial communities in the soil; (*ii*) maintenance of soil cover, which promotes energy (via root exudates) for free‐living and symbiotic microorganisms and produces extracellular enzymes in addition to the enzymes released by plant roots [56].
