Carbon quality rather than stoichiometry controls litter decomposition in a tropical rain forest

Abstract

Summary1. Ecological stoichiometry predicts important control of the relative abundance of the key elements carbon (C), nitrogen (N) and phosphorus (P) on trophic interactions. In a nutrient-poor Amazonian lowland rain forest of French Guiana, we tested the hypothesis that decomposers exploit stoichiometrically diverse plant litter more efficiently, resulting in faster litter decomposition compared to litter with a uniform stoichiometry. 2. In a field experiment in the presence or absence of soil macrofauna, we measured litter mass loss, and N and P dynamics from all possible combinations of leaf litter from four common tree species which were distinctly separated along a C:N and along a N:P gradient. 3. Mean litter mass remaining after 204 days of field exposure varied between 25.2% and 71.3% among litter treatments. Fauna increased litter mass loss by 18%, N loss by 21% and P loss by 14%. Litter species richness had no effect on litter mass loss or nutrient dynamics. In contrast, litter mass and nutrient losses increased with increasing stoichiometric dissimilarity of litter mixtures in presence of fauna, suggesting faster decomposition of a stoichiometrically more heterogeneous litter. 4. However, the effect of stoichiometric dissimilarity was smaller than the strong C quality related litter composition effect and disappeared in the absence of fauna. Increasing proportions of litter that is relatively rich in accessible C compounds (non-structural carbohydrates, phenolics) and relatively poor in recalcitrant C (condensed tannins, lignin), correlated best with litter mass loss irrespective of fauna presence. No correlation was found for any of the nutrient related litter quality parameters and decomposition. 5. Synthesis. Our results suggest that Amazonian decomposer communities studied here are primarily limited by energy, and only secondarily by litter stoichiometry. Tropical tree species might thus influence decomposers and detritivores by the production of litter of specific C quality with potentially important feedback effects on ecosystem nutrient dynamics and availability.

C, N and P fertilization in an Amazonian rainforest supports stoichiometric dissimilarity as a driver of litter diversity effects on decomposition

“…For example, Hättenschwiler & Bracht Jørgensen [16] showed that C quality of litter mixtures explained more variation in mixture decomposition than stoichiometric dissimilarity. Moreover, when fauna was excluded in our study, we could not confirm the positive correlation between stoichiometric dissimilarity of litter mixtures and mixture effects on decomposition.…”

“…However, owing to limiting data, the importance of stoichiometric heterogeneity in mixtures of different litter species for decomposers and for litter decomposition is at present difficult to evaluate. Moreover, the positive correlation between stoichiometric dissimilarity of litter mixtures and their decomposition in the study by Hättenschwiler & Bracht Jørgensen [16] was relatively weak compared with the positive effect of the availability of labile C in litter mixtures. It is reasonable to expect that the strength of the effects of C : N : P dissimilarity on litter mixture decomposition may depend on the overall availability (including external resources) of any of the three elements considered.…”

“…Although decomposition is largely driven by microorganisms, fauna can contribute significantly [17], particularly in tropical rainforests [3], and litter-feeding fauna has to deal with similar stoichiometric constraints as microbial decomposers do [18]. Because fauna is more mobile than microorganisms, they can more easily switch between individual leaves of different plant species which might explain a stronger impact of stoichiometric dissimilarity of litter mixtures on mixture decomposition when fauna is present [16]. However, owing to limiting data, the importance of stoichiometric heterogeneity in mixtures of different litter species for decomposers and for litter decomposition is at present difficult to evaluate.…”

“…According to the theoretical prediction developed above, Hättenschwiler & Bracht Jørgensen [16] reported increasing litter mass loss with increasing dissimilarity in C : N : P stoichiometry of litter mixtures of tropical tree species. Interestingly, the positive correlation between stoichiometric dissimilarity and litter mass loss was observed in the presence of soil macrofauna, but not when fauna was excluded.…”

“…Consequently, the divergence in litter stoichiometry of component species should be a strong predictor of diversity effects on litter mixture decomposition [15]. However, only very few studies explicitly tested the effect of stoichiometric, or more generally, functional divergence of litter mixtures on decomposition [8], and only one of them created a gradient of litter mixture C : N : P stoichiometry [16].…”