**4. The process model — Litter decomposition**

Litter decomposition is a so-called support process of ecosystems, necessary to the mainte‐ nance of several ecosystem processes [36]. Due to the implications and to the importance of litter decomposition process in local scale, such as in the productivity of agricultural areas [37], and also in global scale, such as in the global carbon cycling [38], studies involving biodiversity and litter decomposition are increasingly frequent [39-43]. The central goal of these studies is to understand how the factors that drive litter decomposition interact and what are the consequences of biodiversity decrease on this process.

Litter decomposition is the process by which organic matter is progressively broken in smaller parts, since all organic molecules have been mineralized into their primary constituents: water, carbon dioxide and mineral elements [44]. Litter decomposition process is regulated by three main factors: the physico-chemical environment, the quality of the material to be decomposed and the edaphic biota [44-46].

Biodiversity and Ecosystem Functioning: a Conceptual Model of Leaf Litter Decomposition http://dx.doi.org/10.5772/57396 39

There are three aspects in the studies of BEF relationship that have to be distinguished. The first aspect is the evaluation of which would be the appropriate metrics for biodiversity and for ecosystem functioning. This evaluation involves both methodological and mechanistic considerations, but certainly may affect the results of BEF relationship evaluation, for nonbiological reasons. Therefore, one has to be especially careful to distinguish actual mechanistic relationships from tautological correlations, disconnected from biology itself. The second aspect of BEF studies is the evaluation of BEF relationship hypotheses, which necessarily involve regression models that must be tested against null hypothesis of no relationship, or

These two first aspects of BEF studies have been often confused, and may lead to unwarranted conclusions. For example, studies on the relationship of phylogenetic diversity in relation to ecosystem functioning [12], have shown that phylogenetic diversity is more relevant to predict ecosystem functioning, at least when evaluated by primary productivity, than crude species richness or diversity. The authors argue for evolutionary reasons for this pattern, and further suggest that thus one should favor phylogenetic diversity rather than species diversity, as a metric to evaluate BEF. This should not, however, be interpreted as a test of the BEF relation‐

The third aspect of BEF studies, which is the one that we proposed to illustrate in this chapter, regards the explanations for the actual relationship between biodiversity and ecosystem functioning. Disregarding the metrics used to depict biodiversity and the shape of the relationship between biodiversity and ecosystem function, the question why does ecosystem function vary with biodiversity remains. The mechanistic processes by which these two community parameters relate are then described below, using decomposition as the ecosystem function and the variety of species that compose the litter, the so-called litter mixture, as the

Litter decomposition is a so-called support process of ecosystems, necessary to the mainte‐ nance of several ecosystem processes [36]. Due to the implications and to the importance of litter decomposition process in local scale, such as in the productivity of agricultural areas [37], and also in global scale, such as in the global carbon cycling [38], studies involving biodiversity and litter decomposition are increasingly frequent [39-43]. The central goal of these studies is to understand how the factors that drive litter decomposition interact and what are the

Litter decomposition is the process by which organic matter is progressively broken in smaller parts, since all organic molecules have been mineralized into their primary constituents: water, carbon dioxide and mineral elements [44]. Litter decomposition process is regulated by three main factors: the physico-chemical environment, the quality of the material to be decomposed

spurious relationship due exclusively to methodological or mathematical issues.

ship, but rather a methodological refinement, prior to an actual hypotheses testing.

biodiversity metrics.

38 Biodiversity - The Dynamic Balance of the Planet

and the edaphic biota [44-46].

**4. The process model — Litter decomposition**

consequences of biodiversity decrease on this process.

**Figure 1.** Relationship between functional and phylogenetic diversities in a community, considering the existence of a phylogenetic signal to the functional characteristics in question. PD: Phylogenetic diversity. Closely related species are more similar (symbols differ only in color) than distantly related species (symbols differ in shape). Adapted from [15].

The physico-chemical environment is related to climate, especially humidity and temperature [45, 47]. The climate may indirectly affect litter decomposition, changing litter characteristics, or directly, controlling the activity of decomposing organisms. Plant chemical composition may result from soil formation and from nutrient cycling, and both are regulated by the climate [47]. Therefore, nutrient mineralization may be more accelerated in hot and humid climates, resulting in higher nutrient concentration in litter, increasing its degradability and decompo‐ sition [48]. In addition, the direct effect of physico-chemical environment on decomposition occurs from favoring the activity of decomposer organisms by the higher temperature and humidity, increasing the rates of litter decomposition and nutrient release [48, 49].

The second main factor regulating litter decomposition is the quality of organic matter from litter, which is frequently associated to leaf degradability [46]. There is not a unanimity regarding a valid index of litter degradability, although certain nutrient concentrations have been usually associated to higher quality [50]. High nitrogen (N) and phosphorus (P) concen‐ trations, as well as high proportions of easily degradable carbon compounds, such as sugars, have been associated to better litter quality [50], which translates into higher decomposition rates. Contrarily, less degradable carbon compounds (such as lignin), decrease degradability and litter quality, because these compounds require higher energy from decomposers to break the organic matter [46].

The edaphic biota is the third factor regulating decomposition and comprises a plethora of organisms, ranging from bacteria to insects. These organisms remove, mix, break and digest the organic matter, metabolizing litter constituents, mineralizing and making nutrients available to plants [51]. Even though all components of the edaphic biota may perform important roles on litter decomposition, the main decomposing agents are fungi and bacteria, being responsible by nutrient mineralization. Fungi may colonize recently fallen leaves, building up a net of hyphae that allows them the transference of carbon and nitrogen from litter to soil. Bacteria, on other hand, are the main responsible by nitrogen mineralization and availability, which make them extremely significant for the cycling of this nutrient through the soil and to the plants [52]. Several bacteria not only degrade the organic matter, but are central in nitrogen transformations, in a complex of chemical reactions of oxidation and reduction, fixing nitrogen from the air, transforming nitrites into nitrates and back to nitrites, ammonia and returning it to the air. The activity of decomposers, both fungi and bacteria, is affected by the action of detritivore arthropods, which break the litter by its ingestion and digestion, increasing the litter area available to decomposers and facilitating litter decompo‐ sition by microorganisms [53].

by different decomposer species, which possess distinct nutritional needs, or different abilities to exploit resources originated from a varied plant species. The relationship between an increasing environmental heterogeneity affecting positively the species richness of the communities is a well-established pattern [57], and may be based on the amount or variety of

Biodiversity and Ecosystem Functioning: a Conceptual Model of Leaf Litter Decomposition

http://dx.doi.org/10.5772/57396

41

Theoretically, it would be possible to predict litter decomposition rates from the proportional sum of the decomposition rates observed in each plant species composing the litter mixture. However, this expected outcome does not necessarily occurs [58, 59], due to interactions between the species composing the litter mixture. When the expected prediction, decomposi‐ tion rates of the litter mixture corresponds to the sum of decomposition rates of each species composing it, we say that an additive effect is occurring. However, when leaves from two or more plant species are mixed, decomposition rates of the litter mixture may not correspond to that estimated from the decomposition rates of each plant species alone, due to synergistic and antagonistic effects among species composing the litter mixture [59]. These effects may occur because nutrients may be transferred from one plant species to another one [60], altering the expected effects of diversity of litter mixture on leaf litter decomposition [26]. There are evidences that a nutrient transference from a nutrient-rich species (with a lower carbon/ nitrogen ratio) to a nutrient-poor species may increase litter mixture decomposition when compared to the sum of decomposition rates of each species considered alone [56], resulting in a synergistic effect. On the other hand, the presence of some secondary plant metabolites, such as polyphenols, may decrease, revert, or even compensate synergistic mechanisms that would be occurring simultaneously. This effect would decrease the decomposition rate of litter mixture, when compared to the sum of decomposition rates observed in each of the species composing the mixture, causing the antagonistic effect [51, 59]. Thus, plant diversity that composes the litter mixture would not always have the expected positive effect on decompo‐ sition. This is so mostly because the number of species in litter [40], the environment in which the litter is decomposing [61], the origin of leaves [62], amongst other aspects, may alter the

resources that may allow the coexistence of potentially competitive species.

response of decomposition to plant species diversity of the litter mixture.

**control**

extremely abundant and their effect on decomposition is more consistent [49, 63].

Moreover, the effect of litter mixture biodiversity on decomposition rates is deeply related to the edaphic biota, its activity, abundance and composition [56]. The edaphic biota may modulate the decomposition process, mainly in tropical ecosystems, where arthropods are

**5. Effect of the edaphic biota on decomposition — Top-down/bottom-up**

The edaphic organisms organize in an intricate trophic web, and the diversity of the litter mixture would alter the interactions among these organisms and therefore the effects of this interaction on litter decomposition. The knowledge regarding trophic webs related to the decomposition process is very weak, especially in tropical biomes. This lack of knowledge is caused primarily by the weakness of taxonomic and biological knowledge [64, 65], both

The diversity of plants whose leaves compose the litter, the litter mixture, may affect the decomposition process through different pathways (Figure 2). According the general findings of the relationship between biodiversity and ecosystem functioning, it is expected a positive relationship between litter mixture diversity (biodiversity) and litter decomposition (ecosys‐ tem process). Nevertheless, there are possible pathways that may conduct to the absence of such relationship (the null hypothesis), or even to a negative relationship. The importance of the flowchart depicted in Figure 2 is to generate hypotheses to explain this diversity of possible outcomes in the BEF relationship, allowing the posterior designing of experiments to test these hypotheses.

In general, the main mechanism whereby litter mixture diversity affects litter decomposition it is via resource heterogeneity [54]. In Figure 2, litter taxonomic diversity may be understood in three ways: different species richness and/or compositions, varied functional groups or phylogenetic diversity. We discussed these metrics previously in this chapter, highlighting the advantages and disadvantages of each of them, but disregarding the metrics used, we considered that an increase of each of them would result in higher litter heterogeneity.

A more diverse litter mixture would present a more varied resource supply, allowing the occurrence of a higher abundance and richness of detritivore and decomposer organisms [54]. A higher abundance and/or species richness of decomposers and detritivores would increase litter decomposition.

The heterogeneity promoted by the increase of species composing the litter mixture may occur both due to the physical and chemical plant characteristics. Leaves with certain physical characteristics, such as lower hardness and lignin content, are correlated to higher decompo‐ sition rates [55]. Chemical aspects, such as higher carbon and nitrogen concentrations, are frequently correlated to higher decomposition rates [26, 50, 55, 56], even though this pattern is not fully established [51]. According to our flowchart, there would be a positive relationship between environmental heterogeneity and the possibility of the litter mixture being explored by different decomposer species, which possess distinct nutritional needs, or different abilities to exploit resources originated from a varied plant species. The relationship between an increasing environmental heterogeneity affecting positively the species richness of the communities is a well-established pattern [57], and may be based on the amount or variety of resources that may allow the coexistence of potentially competitive species.

The edaphic biota is the third factor regulating decomposition and comprises a plethora of organisms, ranging from bacteria to insects. These organisms remove, mix, break and digest the organic matter, metabolizing litter constituents, mineralizing and making nutrients available to plants [51]. Even though all components of the edaphic biota may perform important roles on litter decomposition, the main decomposing agents are fungi and bacteria, being responsible by nutrient mineralization. Fungi may colonize recently fallen leaves, building up a net of hyphae that allows them the transference of carbon and nitrogen from litter to soil. Bacteria, on other hand, are the main responsible by nitrogen mineralization and availability, which make them extremely significant for the cycling of this nutrient through the soil and to the plants [52]. Several bacteria not only degrade the organic matter, but are central in nitrogen transformations, in a complex of chemical reactions of oxidation and reduction, fixing nitrogen from the air, transforming nitrites into nitrates and back to nitrites, ammonia and returning it to the air. The activity of decomposers, both fungi and bacteria, is affected by the action of detritivore arthropods, which break the litter by its ingestion and digestion, increasing the litter area available to decomposers and facilitating litter decompo‐

The diversity of plants whose leaves compose the litter, the litter mixture, may affect the decomposition process through different pathways (Figure 2). According the general findings of the relationship between biodiversity and ecosystem functioning, it is expected a positive relationship between litter mixture diversity (biodiversity) and litter decomposition (ecosys‐ tem process). Nevertheless, there are possible pathways that may conduct to the absence of such relationship (the null hypothesis), or even to a negative relationship. The importance of the flowchart depicted in Figure 2 is to generate hypotheses to explain this diversity of possible outcomes in the BEF relationship, allowing the posterior designing of experiments to test these

In general, the main mechanism whereby litter mixture diversity affects litter decomposition it is via resource heterogeneity [54]. In Figure 2, litter taxonomic diversity may be understood in three ways: different species richness and/or compositions, varied functional groups or phylogenetic diversity. We discussed these metrics previously in this chapter, highlighting the advantages and disadvantages of each of them, but disregarding the metrics used, we considered that an increase of each of them would result in higher litter heterogeneity.

A more diverse litter mixture would present a more varied resource supply, allowing the occurrence of a higher abundance and richness of detritivore and decomposer organisms [54]. A higher abundance and/or species richness of decomposers and detritivores would increase

The heterogeneity promoted by the increase of species composing the litter mixture may occur both due to the physical and chemical plant characteristics. Leaves with certain physical characteristics, such as lower hardness and lignin content, are correlated to higher decompo‐ sition rates [55]. Chemical aspects, such as higher carbon and nitrogen concentrations, are frequently correlated to higher decomposition rates [26, 50, 55, 56], even though this pattern is not fully established [51]. According to our flowchart, there would be a positive relationship between environmental heterogeneity and the possibility of the litter mixture being explored

sition by microorganisms [53].

40 Biodiversity - The Dynamic Balance of the Planet

hypotheses.

litter decomposition.

Theoretically, it would be possible to predict litter decomposition rates from the proportional sum of the decomposition rates observed in each plant species composing the litter mixture. However, this expected outcome does not necessarily occurs [58, 59], due to interactions between the species composing the litter mixture. When the expected prediction, decomposi‐ tion rates of the litter mixture corresponds to the sum of decomposition rates of each species composing it, we say that an additive effect is occurring. However, when leaves from two or more plant species are mixed, decomposition rates of the litter mixture may not correspond to that estimated from the decomposition rates of each plant species alone, due to synergistic and antagonistic effects among species composing the litter mixture [59]. These effects may occur because nutrients may be transferred from one plant species to another one [60], altering the expected effects of diversity of litter mixture on leaf litter decomposition [26]. There are evidences that a nutrient transference from a nutrient-rich species (with a lower carbon/ nitrogen ratio) to a nutrient-poor species may increase litter mixture decomposition when compared to the sum of decomposition rates of each species considered alone [56], resulting in a synergistic effect. On the other hand, the presence of some secondary plant metabolites, such as polyphenols, may decrease, revert, or even compensate synergistic mechanisms that would be occurring simultaneously. This effect would decrease the decomposition rate of litter mixture, when compared to the sum of decomposition rates observed in each of the species composing the mixture, causing the antagonistic effect [51, 59]. Thus, plant diversity that composes the litter mixture would not always have the expected positive effect on decompo‐ sition. This is so mostly because the number of species in litter [40], the environment in which the litter is decomposing [61], the origin of leaves [62], amongst other aspects, may alter the response of decomposition to plant species diversity of the litter mixture.

Moreover, the effect of litter mixture biodiversity on decomposition rates is deeply related to the edaphic biota, its activity, abundance and composition [56]. The edaphic biota may modulate the decomposition process, mainly in tropical ecosystems, where arthropods are extremely abundant and their effect on decomposition is more consistent [49, 63].
