Effects Of Water And Nitrogen Additions On Ecosystem Carbon Exchanges In A Grazing Ecosystem Of Inner Mongolia Typical Steppe

Grazing is one of the major measures for grassland management in Inner Mongolia grassland. However, overgrazing has greatly depletedsoil nutrients and soil water, which in turn results in the extensive degradation of these grassland ecosystems and consequently reduces their potential of carbon(C) sequestration. Therefore, the C sequestration in Inner Mongolia grasslands may be co-limited by the availabilities of both water and nitrogen (N). Because C and N are essential elements for plant growth and development and their cycling are closely related, ecosystem C cycling should be greatly affected by soil N availability. Furthermore, because the availability of N is usually mediated by soil moisture, the combination of soil water and soil N plays an important role in influencing grassland C cycling.Based on a long-term grazing experiment initiated by Inner Mongolia Grassland Ecosystem Research Station (IMGERS) in a typical steppe ecosystem, we conducted a water and N addition experiment, to examine the responses of ecosystem C exchanges to water addition and N addition under different stocking rates in2012and 2013. Firstly, we examined the effects of different stocking rates on ecosystem C exchanges. Secondly, we examined how the effects of grazing on C exchanges were mediated by water and N addition. The key findings are as follows:(1) Grazing decreased NEE, ecosystem respiration (ER) and gross primary production (GEP), but no significant difference was found among treatments with different stocking rates.(2) Grazing greatly changed the proportion of two dominant plant species. With the increase of stocking rate the fraction of Stipa grandis significantly increased while that of Leymus chinensis decreased significantly.(3) Topography played an important role in influencing NEE. The NEEs differed substantially between flat and slope areas, and NEEs in the slope plots were significantly greater than those in flat plots, but no significant difference was found on GEPs.(4) Water and N additions enhanced the carbon exchanges in this ecosystem. Compared with control treatment, NEE, ER and GEP increased28%,9%and18%respectively after water and nitrogen addition in flat plots. Similarly, the NEE, ER and GEP increased2%,14%and21%in slope plots.(5) Net ecosystem carbon exchange (NEE) followed an unimodal pattern during the growing season with a peak value achieved at the beginning of August in both2012and2013. This is consistent with the seasonal patterns of precipitation and temperature.(6) Nitrogen addition and the combination of both water and N added significantly increased aboveground net primary production (ANPP). However, the additions of both water and nitrogen did not change the proportion of two dominant plant species.(7) Ecosystem exchanges indicated by NEE, ER and GEP positively related to soil temperature but had no relationship with soil moisture.Our study had important implications for understanding the effects of grazing on grassland ecosystems, developing a better grazing policy and restoring the degraded grasslands.