Decomposition of leaf mixtures and absorptive-root mixtures synchronously changes with deposition of nitrogen and phosphorus

Abstract

Leaves and absorptive roots, the plant tissues that acquire resources, decompose quite differently due to their distinct morphology, chemistry, and decomposition microenvironments. Root litters of different species often decompose in mixtures in natural forests or mixed plantations, similar to leaf litters. However, the mechanisms behind leaf mixture decomposition and root mixture decomposition and their responses to nutrient deposition remain poorly understood. We used a nutrient-addition manipulative experiment to investigate the individual and interactive effects of nitrogen (N) and phosphorus (P) on leaf mixture decomposition and absorptive-root mixture decomposition of Pinus massoniana and Schima superba forests in subtropical China. Four treatments were employed: control (CTRL), +N (120 kg N ha−1y−1), +P (40 kg P ha−1y−1), and +NP (120 kg N and 40 kg P ha−1y−1). Both the leaves and roots of P. massoniana and S. superba decomposed faster when in mixtures (observed) than in isolation (expected) under all treatments. The synergistic effects were decreased with +N and +NP relative to the CTRL for the leaf mixtures, but only with +NP for the root mixtures. The differences (observed - expected) of the extracellular enzymatic activities (EEAs) of microorganisms were decreased with +N in the leaf mixtures relative to the CTRL, and the differences (observed - expected) of acid unhydrolyzable residue (AUR) loss were decreased with +NP in the root mixtures. The differences (observed - expected) of mass loss were positively correlated with microbial EEAs differences for the leaf mixtures and with AUR loss difference for the root mixtures. Our results indicate that the decomposition of both the leaf mixtures and the root mixtures had synergistic effects. The decreased synergistic interactions under nutrient addition were associated with microbial EEAs for the leaf mixtures and with AUR loss for the root mixtures. These findings highlight the contrasting mechanisms underlying the decomposition of above-ground mixtures and below-ground mixtures, and imply a slow cycling of carbon and nutrients under atmospheric nutrient deposition.