Effects of long-term no tillage treatment on gross soil N transformations in black soil in Northeast China

No tillage (NT) is a common conservation tillage practice used to enhance soil C sequestration. However, to date, the understanding regarding the impact of NT on soil nitrogen (N) transformations remains limited. Here, we used N-15 tracing to investigate the effects of 14 years of NT on the gross N transformation rates in black soil in China. We also evaluated the risk of N loss from soil in NT systems compared to that treated with conventional tillage (CT). The gross N mineralization rates in the top 5 cm of NT-treated soil was more than three times higher than the rates in the 5-15 cm layer (P < 0.01), which was significantly higher than those in the same layer of CT soils (P < 0.05). However, the total gross NH4+ immobilization rates were almost negligible in NT soil than in CT soil, which led to significantly higher net mineralization rates. The highest gross autotrophic nitrification rates were observed in the top 5 cm of NT soil, which was significantly higher than those in CT (P < 0.05). Heterotrophic nitrification rates were very low in NT soil, and ranged from 0.02-0.11 mg N.kg(-1) d(-1), which we're significantly lower than those in CT (average rate of 0.80 mg N.kg-1 d(-1)). The ratio of autotrophic nitrification to NH4+ immobilization, and the N2O emission rate were significantly higher in NT than in CT. Soil N2O emission rates were positively correlated with autotrophic nitrification rates. Thus, long-term NT treatment was more likely than CT treatment to increase the risk of NO3- leaching and N2O emission.