Non-Linear Effects of Temperature and Moisture on Gross N Transformation Rates in an Inner Mongolian Grassland

Abstract

Understanding gross N turnover in response to changes in temperature and soil moisture associated with global climate change is important for accurately predicting potential changes in N availability and productivity. We investigated gross N turnover in an Inner Mongolian grassland and in a laboratory parameterization experiment. On an annual scale, field data on gross ammonification and nitrification showed a hump-shaped response to temperature with maximum rates occurring around 10°C and 5°C, respectively. The relationship between gross ammonification and temperature is similar to the annual scale in the non-freezing-thawing period, but opposite in the freezing-thawing period. In addition, the relationship between gross nitrification and temperature in non-freezing-thawing period is opposite to the annual scale. Field rates of gross nitrification on an annual scale decreased with soil moisture at low levels of soil moisture and increased at high levels of soil moisture with the threshold of 35% water holding capacity (WHC), while the non-freezing-thawing period showed a completely opposite trend. There was no significant relationship between soil moisture and gross ammonification, but the trend was opposite in freezing-thawing period and non-freezing-thawing period. Gross ammonification and nitrification rates in the laboratory incubation experiment increased between 0 and at 20°C with no further increase at 30°C. Gross ammonification rates reached a maximum at 50% WHC, while soil moisture surprisingly showed no significant effect on gross nitrification until 100% WHC. Temperature explained 71% and 37% of the variation in GA and GN, respectively, while moisture explained 1% and 25%. On the seasonal scale in the field, both soil temperature and moisture showed generally significant effects on gross N turnover, however the effects were very different among seasons. Soil temperature explained more variation in gross ammonification (0.30 VS 0.04) and nitrification (0.24 VS 0.14) than moisture on an annual scale. Our results show that direct temperature and moisture effects on gross ammonification and nitrification as observed in laboratory incubation experiments are often masked by indirect effects and other environmental controls under field conditions at the annual scale, with the occurrence of freeze thaw events being of critical importance for gross N turnover.

Keywords: Climate change, gross ammonification, gross nitrification, N cycling, precipitation, steppe