Dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in soil nitrogen conservation in neutral and alkaline but not acidic rice soil

In the rice soils, the alternate wetting and drying conditions and the leakage of O-2 from rice roots provides a favorable environment for many N transformation processes, such as DNRA, nitrification, mineralization, and denitrification. Since nitrification is an inevitable process providing available NO3 (-) for DNRA and other NO3 (-) loss pathways, it is logical to hypothesize that a relationship exists between nitrification and DNRA. Thus, we quantified the specific gross N transformation rates occurring simultaneously to investigate the correlation among the N transformation rates in the rice soil. In this study, three rice soils with different pH values were selected. The combination of N-15 tracing experiment and numerical modeling method was performed to quantify the specific gross N transformation rates (e.g., DNRA), to analyze the correlation between DNRA and the other N transformations, and to estimate the functional role of DNRA comparing to other N transformation in the rice soil. The results showed that there were two NH4 (+) production pathways, organic N mineralization and DNRA, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, while the DNRA process was negligible in acidic soil (pH 4.7). The mineralization rate in the acidic soil (2.69 mg kg(-1)d(-1)) was significantly higher than that in neutral and alkaline soils. The DNRA rate was 0.48 and 1.09 mg kg(-1)d(-1), which was almost equal to the mineralization rate, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, offsetting efficiently the effect of low mineralization on N availability in ecosystems. The DNRA rate linearly increased with the increase in gross nitrification rate (p < 0.05). This significant coupling relationship between DNRA and nitrification favors N conservation and availability for paddy growth in the rice ecosystem. DNRA plays an important role in soil N conservation and availability in neutral and alkaline but not acidic rice soil.