Alterations of early-stage decomposition of leaves and absorptive roots by deposition of nitrogen and phosphorus have contrasting mechanisms

Nitrogen (N) deposition may inhibit decomposition by decreasing microbial growth and activity (microbial mechanism) and/or increasing the binding of inorganic N ions to the acid-unhydrolyzable residue (AUR) in decomposing litter (chemical mechanism). How increased phosphorus (P) deposition affects decomposition and mediates the effect of N on this process, however, remains unclear. We used a nutrient-manipulative experiment to examine the individual and interactive effects of N and P on the decomposition of two fast-cycling organs (leaves vs. absorptive roots) in Pinus massoniana and Schima superba forests in subtropical China. The carbon fractions and microbial enzymatic activities in decomposing leaves and roots were also determined to identify the underlying mechanisms. Adding only N negatively affected the decomposition of absorptive roots but had no or a positive effect on foliar decomposition. Adding only P increased foliar decomposition but did not affect root decomposition. Adding both N and P had little effect on decomposition, except for a positive effect on P. massoniana leaves. The nutrient additions did not affect the amount of carbon remaining in the leaves, and adding only N increased the residual AUR content in the roots. Adding only P decreased the activity of acid phosphatase (AP) in the leaves. Foliar decomposition was correlated with microbial enzymatic activity but not AUR content. Root decomposition was correlated more strongly with AUR content than microbial enzymatic activity. The increased decomposition but decreased AP activity of the leaves with P addition suggests a stronger P limitation on the decomposition of leaves than roots in these subtropical forests. The stimulating effect of P on foliar decomposition may be due to a microbial mechanism, and a chemical mechanism may dominate the inhibitory effect of N on root decomposition. Our findings facilitate the mechanistic understanding of the effects of nutrient deposition on the decomposition of leaves and absorptive roots, and highlight the role of P deposition in mediating the effect of N on decomposition.