Non-linear effects of temperature and moisture on gross N transformation rates in an Inner Mongolian grassland

Understanding how soil nitrogen (N) turnover responds to temperature and soil moisture alterations is important for accurately predicting responses of soil N availability and plant productivity to global climate change. However, few studies had explored such questions under natural conditions. We investigated effects of temperature and moisture on soil gross N turnover with an annual scale field experiment in an Inner Mongolian grassland and a laboratory incubation experiment. In the field experiment, on the annual scale both gross ammonification and nitrification rates showed a hump-shaped response to temperature, with maximum rates occurring around 10 degrees C and 5 degrees C, respectively. Gross ammonification rates did not respond to soil moisture, and gross nitrification rates decreased first and then increasing with soil moisture, which the threshold was 35% water holding capacity (WHC). Totally, soil temperature explained more variation than moisture in gross ammonification (0.30 VS 0.04) and nitrification (0.24 VS 0.14) rates. On the seasonal scale, both soil temperature and moisture showed generally significant effects on gross N turnover. Grazing had no effects on soil temperature and moisture sensitivity of gross N turnover on the annual scale, but increased the sensitivity of gross ammonification to soil moisture during the spring freeze-thaw period. In the laboratory incubation experiment, gross ammonification and nitrification rates increased with the increase of temperature, although there was no difference between 20 degrees C and 30 degrees C. Gross ammonification rates reached a maximum at 50% WHC, while gross nitrification rates did not respond to soil moisture until 100% WHC. Soil temperature explained more variation than moisture in gross ammonification (0.71 VS 0.01) and nitrification (0.37 VS 0.25) rates. Our results imply that climate warming may have greater impact on gross nitrogen turnover compared to changes in rainfall, however it remains great uncertainty due to the low explanation of soil temperature and moisture.