Nutrient limitation across tropical forests inferred from stoichiometry of nutrient losses
Nutrient losses from ecosystems can be a important indicator of the nutrient limitation status of plants. For example, previous analyses have shown that many mature tropical forests worldwide lose large quantities of bioavailable N at levels that are difficult to reconcile simply with N limitation. Early studies of litter nutrient return and more recent multi-element fertilization studies suggest a more complex picture of nutrient limitation in tropical forests emphasizing N in some places and widespread P limitation and significant limitation by K and other cations in others, particularly in old weathered environments. While fertilization studies are few and far between, NPP estimates are global in extent and surface water is easy to measure offering the advantage of large spatial and temporal coverage in examination of nutrient constraints to plant growth. Here we investigate the strength of nitrogen, phosphorus and cation limitation by expanding and evaluating a theoretical model that scales plant uptake to ecosystem nutrient retention using MODIS NPP, litter stoichiometry, and original measures of hydrologic nutrient losses from tropical forests spanning Earth’s major geologic provinces ranging from young volcanic to tectonically quiescent cratonic shield areas that vary greatly in rock nutrient supply.
Our study areas span a large range of global tropical NPP. Across these forests, we find widespread evidence for large bioavailable N losses, confirming that previous findings apply to a broader set of geographic and geologic settings. Mineral N dominated over organic forms in all but some lowland and black water systems draining white sand areas. Solving for biological turnover of N, P, and K we find widespread evidence for tight retention of P and N relative to K in dissolved losses relative to steady state internal flows from NPP. We accounted for weathering influences on P and K that are not under direct biological control by analyzing relationships with silica and other cations across a parent material and weathering sequence spanning tens of thousands to 1.8 billion years old. Overall, while we find broad evidence for high geographic diversity in patterns N and P retention relative to internal fluxes, mean N:P of losses converged globally on N:P of detrital fluxes suggesting biological control of nutrient loss stoichiometry. Our analysis also offers geographically broad support for K conservation at the ecosystem level: K losses and N:K and P:K ratios were relatively conserved across forest types, NPP, and geomorphic settings.