Data from:[Temporal patterns and metabolomic pathways of grassland ecosystem carbon sequestration under a decadal-long nitrogen enrichment]

Due to widespread nitrogen (N) limitation, reactive N input is expected to stimulate vegetation carbon fixation, thus potentially offsetting decomposition-induced soil carbon losses. However, this projection is largely based on short-term measurements. The crucial question for ecosystem carbon sequestration is whether such responses are sustainable over time. Here, we show that in an N-limited alpine steppe, a decade-long N addition only elicited a transient increase in net ecosystem productivity (NEP); the stimulating effect of N on NEP diminished over time. This phenomenon is likely associated with N-induced phosphorus (P) limitation, as indicated by increases in plant and soil N:P ratios and soil phosphatase activity. The temporal decline in NEP response was reversed with P supplementation, providing experimental evidence of aggravated P limitation under long-term N loadings. Plant metabolomic analysis further revealed that metabolites in the Calvin-Benson cycle and pentose phosphate pathway were particularly sensitive to nutrient changes in the leaves of the dominant species, Leymus secalinus, offering deeper mechanistic insights into the N-induced ecosystem P limitation. Taken together, our findings imply that the N-triggered enhancements in ecosystem carbon sink may be less than previously suggested.