Large variations are observed concerning the responses of ecosystem production and carbon (C) fluxes to nitrogen (N) enrichment. Traditionally, temperature and moisture are regarded as the major controllers over the response direction and magnitude. The N-phosphorous (P) stoichiometry regulates plant and soil organism growth and metabolisms, thus affecting many ecosystem C processes. To date, it remains unclear how the N-induced shift in plant N-P stoichiometry affects ecosystem production and C fluxes. We conducted a field manipulative experiment with eight N addition levels in a Tibetan alpine steppe, and assessed the influences of N on aboveground net primary production (ANPP), gross ecosystem productivity (GEP), ecosystem respiration (ER) and net ecosystem exchange (NEE), and further determined the relative contributions of various biotic and abiotic factors to the changes in these parameters using linear mixed-effects models.
Results/Conclusions
Our results showed that, ANPP, GEP, ER and NEE all exhibited nonlinear and saturated response patterns with increasing N additions. There was a positive relationship between the N-induced changes of ANPP (ΔANPP) and plant N:P ratio (ΔN:P), while the ΔGEP, ΔER and ΔNEE exhibited quadratic correlations with ΔN:P. Further analysis by linear mixed-effects models indicated that the plant N-P stoichiometry played a dominate role in shaping these C exchange processes. By contrast, edaphic variables (soil temperature, moisture, and inorganic N and P concentrations) were relatively weaker in predicting the changes of ecosystem production and C fluxes induced by N additions. Given that N-P stoichiometry has significant impacts on C cycles, these findings highlight that the human-induced ecosystem N-P imbalance should be incorporated into Earth System Models for better predicting the trajectory of ecosystem C balance under changing environment.