No response of soil N mineralization to experimental warming in a northern middle-high latitude agro-ecosystem

The effect of warming on soil nitrogen (N) pools and turnover in agro-ecosystems has rarely been investigated, particularly in middle-high latitudes where the temperature is predicted to increase more than that in low latitudes. In this study, we determined the dynamics of soil inorganic N pools and the net N mineralization in an agro-ecosystem from 2013 to 2015 in a field warming experiment in Northeast China. The objectives were to examine the effects of warming on soil N turnover and determine how the effects differ in the growing versus the non-growing seasons. We hypothesized that experimental warming would increase N mineralization and the inorganic N pools, and the effects of warming would be greater in the non-growing season than those in the growing season. In contrast to these predictions, the soil N mineralization and inorganic N pools in the soil solution were not affected by experimental warming either within or across year and season. The nitrification rate was 0.14 mg kg^-1 d^-1 higher, but the ammonification rate was 0.17 mg kg^-1 d^-1 lower in the growing season than that in the non-growing season. When averaged across all sources of variation, the net mineralization rate of the soil N was 0.09 mg kg^-1 d^-1 in both the control and warming treatments, while the total inorganic N in the soil solution was 34 and 41 mg kg^-1, respectively. Both the nitrification and ammonification processes were regulated by soil moisture and/or root biomass, which was not affected by warming. These results suggest that soil N turnover and availability will be less sensitive to future global warming in this middle-high latitude agro-ecosystem.