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.

Biochar is commonly used to improve acidic soil and reduce nitrogen loss. However, the impact of biochar on soil nitrogen retention, especially at varying pH levels, is not fully understood. Soil samples were obtained from an acidic red soil citrus orchard. The soil pH was adjusted using CaO, with five levels (4.0, 5.1, 5.8, 6.6, and 7.2), and two biochar doses (0% and 1%) were applied. The study used ¹⁵N-Tracer and Ntrace to investigate biochar’s influence on soil nitrogen retention at different pH levels. The results showed that soil amendment with biochar improved gross mineralization rates (TM) and gross NH₄⁺ immobilization rates (TI), except at pH 4.0 for TI. Biochar enhanced heterotrophic nitrification (O_Nrec) within pH 4.0–7.4, with a threshold for autotrophic nitrification (O_NH4) at pH 6.4. The findings revealed biochar’s positive effect on soil nitrogen retention within pH 4.5–6.4. Biochar had a greater impact on TI than TM and inhibited O_NH4, potentially enhancing nitrogen retention in this pH range. These results highlight the significance of considering biochar incorporation for improving nitrogen use efficiency and reducing NO₃⁻-N loss in subtropical pomelo orchards.