Decomposition and stabilization of the organic matter in an old-growth tropical riparian forest: effects of soil properties and vegetation structure

Abstract

Background: Nutrient cycling in tropical forests has large importance for primary productivity, and decomposition of litterfall is a major process influencing nutrient balance in forest soils. Although large-scale factors strongly influence decomposition patterns, small-scale factors can have major influences, especially in old-growth forests that have high structural complexity and strong plant-soil correlations. Methods: We evaluated decomposition rates and stabilization of soil organic matter using the Tea Bag Index in an old-growth riparian forest in southeastern Brazil. We buried 50 pairs of green and red tea at two distances from the watercourse to evaluate the effects of forest structure and soil properties on decomposition processes. Forest structure and soil properties were described using Principal Components Analysis (PCA). The main axes for each analysis were considered predictors of decomposition processes using a structural equations model.Results: Decomposition rates presented a large variation among tea bags and were positively correlated with forest structure, as characterized by higher basal area, larger trees, and tree density. Higher decomposition rates were probably correlated with higher litter production and microbial activity. On the other hand, stabilization factor was related mainly to soil chemical properties, with higher values with increased soil fertility as indicated by the PCA axes. Further analyses evaluated the effects of clay content, soil moisture, soil organic matter, soil base saturation, and soil fertility as predictors of the stabilization factor. These predictors were highly correlated, but clay content was the best predictor, explaining 79% of the variation among plots. Conclusions: These results showed that this old-growth forest presented high heterogeneity in both forest structure and soil properties at small spatial scales, that influenced decomposition processes and contributed to small-scale variation in nutrient cycling. Heterogeneity in ecological processes can contribute to the resilience of old-growth forests, strengthening ecosystem functions such as nutrient cycling and carbon fixation, and highlighting the importance of restoration strategies focused in the recovery of ecosystem processes