Combined effects of soil fertility and vegetation structure on early decomposition of organic matter in a tropical riparian zone

Abstract

Monitoring forests undergoing restoration is important to evaluate not only the development of trees, but also the recovery of ecosystem functions. The decomposition of the organic matter is an important ecological process, and nutrient cycling in ecosystems is a good indicator of forest ecosystem functions because it responds to physical, chemical, and biological characteristics of the vegetation and the soil. We hypothesized that both soil fertility and vegetation structure have direct and positive effects on decomposition processes. We used the Tea Bag Index (Keuskamp et al., 2013) to evaluate early decomposition of the organic matter in three land use categories, pasture, riparian forest undergoing restoration, and riparian forest remnant. Each category extended along a gradient of soil nutrients, enabling to jointly evaluate the effects of soil nutrients and vegetation structure on the decomposition process, using structural equations models to also describe the covariance between soil nutrients and vegetation. The decomposition rates (k) were negatively related to vegetation development, possibly due to less light in the forest floor, and less activity of the decomposer community. On the other hand, soil fertility had direct positive effects on k, possibly with higher microbial activity due to higher nutrient availability. The stabilization factor (S) was not influenced by soil nutrients or vegetation structure. However, in the riparian forest remnant plots, S increased with soil base saturation, indicating an interaction between the effects of soil fertility and land use category. Therefore, the decomposition process in this system responded to soil fertility and vegetation structure, but the magnitude of these effects varied with land use. The monitoring of this ecosystem function to evaluate ecosystem recovery should include simultaneously both soil and vegetation variables.