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Soil N transformation mechanisms can effectively conserve N in soil under saturated conditions compared to unsaturated conditions in subtropical China

Zhang, Yushu, Ding, Hong, Zheng, Xiangzhou, Cai, Zucong, Misselbrook, Tom, Carswell, Alison, Müller, Christoph, Zhang, Jinbo
Biology and fertility of soils 2018 v.54 no.4 pp. 495-507
ammonium, ammonium nitrogen, climate, denitrification, field experimentation, leaching, mathematical models, mineralization, monsoon season, nitrate nitrogen, nitrates, nitrification, nitrogen, nitrogen fertilizers, phosphorus, potassium, potassium fertilizers, rice, risk, risk reduction, runoff, saturated conditions, soil, soil sampling, stable isotopes, straw, tobacco, unsaturated conditions, water holding capacity, water management, China
The connection between moisture and nitrogen (N) transformation in soils is key to understanding N losses, particularly nitrate (NO₃⁻) losses, and also provides a theoretical framework for appropriate water management in agricultural systems. Thus, we designed this study to provide a process-based background for management decision. We collected soil samples from the long-term field experiment in subtropical China, which was designed to examine tobacco and rice rotations under a subtropical monsoon climate. The field experiment was established in 2008 with four treatments: (1) no fertilization as control; (2) N, phosphorus (P), and potassium (K) fertilizers applied at recommended rates; (3) N fertilizers applied at rates 50% higher than the recommended amounts and P and K fertilizers applied at recommended rates; and (4) N, P, and K fertilizers applied at recommended rates with straw incorporated (NPKS). Soil samples were collected during the unsaturated tobacco-cropping season and saturated rice-cropping season and were incubated at 60% water holding capacity and under saturated conditions, respectively. Two ¹⁵N tracing treatments (¹⁵NH₄NO₃ and NH₄¹⁵NO₃) and a numerical modeling method were used to quantify N transformations and gross N dynamics. Autotrophic nitrification was stimulated by N fertilizer both under unsaturated and saturated conditions. The rate of NO₃⁻ consumption (via immobilization and denitrification) increased under the NPKS treatment under saturated conditions. Secondly, the rates of processes associated with ammonium (NH₄⁺) cycling, including mineralization of organic N, NH₄⁺ immobilization, and dissimilatory NO₃⁻ reduction to NH₄⁺, were all increased under saturated conditions relative to unsaturated conditions, except for autotrophic nitrification. Consequently, NO₃⁻-N and NH₄⁺-N concentrations were significantly lower under saturated conditions relative to unsaturated conditions, which resulted in reduced risks of N losses via runoff or leaching. Our results suggest that under saturated conditions, there is a soil N conservation mechanism which alleviates the potential risk of N losses by runoff or leaching.