Exogenous nutrient manipulations alter endogenous extractability of carbohydrates in decomposing foliar litters under a typical mixed forest of subtropics

Abstract

Litter decomposition is a major driver of nutrient availability and carbon (C) cycling in forest ecosystems, and may be susceptible to perturbation by exogenous nutrient inputs from anthropogenic activity. The role of resource limitation in this process is unclear, although some models suggest the macronutrients nitrogen (N) and phosphorus (P) stimulate early-stage decomposition while availability of labile C restricts later-stage decomposition. We studied this interplay in a subtropical mixed forest in southern China through in situ incubations of litter samples over a 540-day period. Litter samples were amended with labile C (+C, as sucrose), N (+N, as NH4NO3), P (+P, as NaH2PO4), all three inputs (+CNP), or no inputs (CK). Litter mass and nutrient content were measured at 90-day intervals, while extractable carbohydrate profiles of the litter samples were assayed at 90, 270, and 450days of incubation. The +P and +CNP treatments showed greater reductions in litter mass, as well as a larger P pool throughout the incubation. The +N treatment had a larger N pool, but otherwise did not differ from the no input control CK. The concentration of accessible carbohydrate fractions remained constant or increased from 90 to 450days of incubation, while less accessible fractions thought to be lignocellulose and hemi-cellulose decreased during that period. Total extractable carbohydrates decreased in the +P treatment, but was not significantly different among other treatments. Our results suggest that mixed forest systems in southern China are likely to be minimally perturbed in the short term by exogenous N addition, and that decomposition activity is not regulated by labile C availability over the time period studied. Moreover, increased C inputs due to climate change-induced changes in litter deposition and root exudation will likely have a smaller impact on subtropical forest management than anthropogenic disturbances such as P fertilization.