Temporal Pattern and Metabolic Mechanisms of Ecosystem Carbon Fluxes Under Decadal‐Long Nitrogen Enrichment in an Alpine Grassland

Due to widespread nitrogen limitation, reactive nitrogen input is expected to stimulate vegetation carbon fixation, thus potentially offsetting the decomposition-induced soil carbon losses. However, this projection is largely based on short-term measurements. The crucial question is whether such responses persist over time. Combining a decade-long nitrogen-addition experiment in an alpine grassland with biochemical and plant metabolomics analyses, we showed that a decade of nitrogen addition elicited only a transient increase in net ecosystem productivity (NEP; balance between ecosystem carbon uptake and release); the stimulating effect diminished over time. This phenomenon was likely associated with sustained phosphorus limitation, as indicated by increased plant nitrogen:phosphorus ratio and reduced metabolites involved in phosphorus-related metabolic pathways. The temporal decline in NEP response was reversed with phosphorus supplementation, providing experimental evidence of aggravated phosphorus limitation under long-term nitrogen loadings. Further analysis demonstrated that plant metabolic traits surpassed the classical traits in mediating productivity response to long-term nitrogen:phosphorus imbalance; the metabolites in the Calvin-Benson cycle and pentose phosphate pathway of the dominant species were important predictors of variations in productivity under nutrient additions. Taken together, our findings imply that prior short-term measurements may overestimate the nitrogen-triggered ecosystem carbon sink and provide metabolic insights into the nitrogen effects on primary productivity.