Stoichiometric controls on carbon, nitrogen, and phosphorus dynamics in decomposing litter

Authors: Manzoni S, JA Trofymow, RB Jackson, A Porporato

The mineralization of nitrogen and phosphorus from plant residues provides an important input of inorganic nutrients to the soil, which can be taken up by plants. The dynamics of nutrient mineralization or immobilization during decomposition are controlled by different biological and physical factors. Decomposers sequester carbon (C) and nutrients from organic substrates and exchange inorganic nutrients with the environment to maintain their stoichiometric balance. Additionally, physical losses of organic compounds from leaching and other processes may alter the nutrient content of litter. In this work, we extend a stoichiometric model of litter nitrogen mineralization to include 1) phosphorus mineralization, 2) physical losses of organic nutrients, and 3) chemical heterogeneity of litter substrates. The enhanced model provides analytical mineralization curves for nitrogen (N) and phosphorus (P) as well as critical litter carbon-to-nutrient ratios (the carbon-to-nutrient ratios below which net nutrient release occurs) as a function of the elemental composition of the decomposers, their carbon-use efficiency, and the rate of physical loss of organic compounds. The model is used to infer the critical litter C-to-nutrient ratios from observed N and P dynamics in ~2,600 litterbag samplings from 21 decomposition datasets spanning artic to tropical ecosystems. At the beginning of decomposition N and P tend to be immobilized in boreal and temperate climates (i.e., both C:N and C:P critical ratios are lower than the initial ratios), while in tropical areas N is generally released and P may be either immobilized or released, regardless of the typically low P concentrations. The critical C-to-nutrient ratios we observed were found to increase with initial litter C-to-nutrient ratios, indicating that decomposers adapt to low-nutrient conditions by reducing their carbon-use efficiency. This stoichiometric control on nutrient dynamics appears ubiquitous across climatic regions and ecosystems, although other biological and physical processes also play important roles in litter decomposition. In tropical humid conditions, we found high critical C:P ratios likely due to high leaching and low decomposer P concentrations. In general, the compound effect of stoichiometric constraints and physical losses explain most of the variability in critical C-to-nutrient ratios and dynamics of nutrient immobilization and release at the global scale.
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