Atmospheric nitrogen (N) deposition has been increasing dramatically since the Industrial Revolution via fertilization and fossil fuel combustion. Such N enrichment has a profound impact on community structure and primary productivity of terrestrial ecosystems and is related to carbon cycling. However, the mechanistic effect of N enrichment on carbon cycling remains unclear. A manipulative experiment with eight levels of N addition (0, 1, 2, 4, 8, 16, 32, 64 gN m-2 yr-1) began in 2003 to examine responses of ecosystem carbon processes to N addition in a temperate steppe of Inner Mongolia, China. Response variables including soil respiration (SR, the second largest flux of terrestrial ecosystems), aboveground biomass of forb (AGBforb) and grass (AGBgrass), belowground biomass (BGB), soil moisture (SM) and temperature, and soil nutrient content were measured during the growing seasons of 2015 and 2016 (three measurements each year). Repeated-measures analysis of variance was employed to test the effects of N addition on response variables using mixed models. The relationship between SR and N addition was fitted by linear and nonlinear models. Acceptance of the optimal model was based upon Akaike Information Criterion (AIC). Multiple regression was used to examine the relationships of SR with other response variables.
Results/Conclusions
Across all measurements, SR was significantly affected by N addition (F7,144 = 3.81, P < 0.001), measurement time (F5,144 = 156.70, P < 0.001), and their interaction (F35,144 = 2.14, P < 0.001) in this study. SR decreased by 8.6%, 14.1%, 11.5%, 18.7%, 12.6%, 16.4%, and 22.7% under N addition of 1, 2, 4, 8, 16, 32, and 64 gN m-2 yr-1, respectively (all P < 0.001). The optimal model for the SR-N addition relationship was a logarithmic model (lowest AIC = -46.79), suggesting a nonlinear response of SR to N addition. The combination of SM, AGBforb, and BGB accounted for 57.1% of changes in SR (F3,188 = 85.62, P < 0.001). SM, AGBforb, and BGB contributed to 82.2%, 14.6%, and 3.2% of this explanation, respectively, implying that N addition impact on SR was mainly mediated by SM in the temperate steppe. The reduction in SM was attributed to the increased evapotranspiration induced by the enhancement of plant growth under N addition. Our findings highlight the predominant role of soil water availability in regulating soil carbon emission under N enrichment facilitating the understanding of terrestrial ecosystem carbon cycling in response to atmospheric N deposition in the future.