Globally, mineral nitrogen (N) losses as nitrate leaching (NL) are a substantial portion of applied fertilizer and cause surface and sub-surface water contamination. To precisely measure NL and its interlink parameters, biochar soil amendment was tested in this study. Three treatments—biochar (BC), without biochar (WB) with ¹⁵N urea (300 kg/ha), and control (no fertilization)—were tested in soil-filled lysimeters (circular PVC (Polyvinyl Chloride) tank of 30 cm diameter and 35 cm height) equipped with moisture content sensors and weighing assembly for the consecutive two cropping of Brassica Camprestis Var. Chinensis. The ¹⁵N-urea in the first season and the poultry manure in the second season were applied, but the fate of the ¹⁵N was examined in leachate, dry matter, and soil. As compared to WB, BC significantly decreased mineral N leaching, including nitrate levels (35%), increased electrical conductivity (68.5%), and water availability (20% inches per foot), while there was a non-significant increase in biomass per plant (2.84%), evapotranspiration (8.33%), dry matter (6.89%), and a decrease in mean leachate volume (7.63%). Moreover, BC accumulated values were higher than WB, as N uptake (38%), water use efficiency (12.24%), maximum fresh weight (11.4%), and soil N retained (185%) after cropping. The soil pH, the bulk density, and the total nitrogen were changed but presented non-significant differences. Therefore, biochar can increase soil N retention and available water to improve water use efficiency and decrease potential N leaching.

To evaluate the effects of application of biochar and straw return for consecutive years on N₂O emissions and crop yields in North China, a three-year field experiment of applying biochar and straw following a ten-year application was conducted in a wheat–maize rotation system. Four treatments were set up, including F (NPK fertilizer only); FB (NPK fertilizer + 9.0 t·ha⁻¹ biochar); FS (NPK fertilizer + straw); and FSB ((NPK fertilizer + 9.0 t·ha⁻¹ biochar combined with straw). The results showed that compared with the F treatment, the FB treatment significantly reduced soil N₂O emissions by 20.2%, while the FS and FSB treatments increased it by 23.7% and 41.4%, respectively. The FB treatment reduced soil N₂O emissions by 15.1% in the wheat season and 23.2% in the maize season, respectively. The FS and FSB treatments increased the N₂O emissions by 20.7% and 36.7% in the wheat season, respectively, and by 25.5% and 44.2% in the maize season, respectively. In the wheat season, the soil water content (SWC), NO₃⁻-N content and pH were the main influencing factors of the soil N₂O emissions. In the maize season, SWC and NO₃⁻-N content were the main influencing factors. In addition, the FB, FS and FSB treatments increased the crop yield by 4.99%, 8.40% and 10.25% compared with the F treatment, respectively. In conclusion, consecutive application of biochar can significantly reduce N₂O emissions and improve crop yield. Although FS and FSB treatments can also improve the crop yield, they are not beneficial to suppressing N₂O emissions. Therefore, the successive application of biochar is an effective measure to reduce N₂O emissions and maintain crop yield.

Agricultural soils are hotspots of nitrous oxide (N₂O) emissions, where carbon substrates act as a critical factor influencing microbial community composition. However, how carbon availability modulates microbial denitrifying pathways and further influences N₂O emissions remains poorly understood. Here, we conducted anaerobic incubations to investigate North China vegetable soil N₂O production and consumption in response to varied glucose concentrations (0, 0.5 (Glu_0.5), 1.0 (Glu_1.0), and 2.0 (Glu_2.0) g C kg⁻¹ d.w. of soil). In this study, the δ¹⁵N^SP/δ¹⁸O mapping approach (δ¹⁵N^SP/δ¹⁸O MAP) and acetylene inhibition technique (AIT) were used to quantify the residual N₂O ratio (r_N2O) and the relative contributions of bacterial (f_BD) and fungal (f_FD) denitrification to N₂O production. The results showed that increasing glucose concentrations significantly increased CO₂ and N₂O emissions, with peak fluxes observed at Glu_2.0 on day 1 (116.22 ± 2.80 mg CO₂-C kg⁻¹ and 1.08 ± 0.02 mg N₂O-N kg⁻¹). Concurrently, δ¹⁵N^SP was also significantly elevated (p < 0.001), indicating enhanced f_FD, which was further corroborated by positive correlations between f_FD and glucose concentration (r = 0.48–0.56, p < 0.001). Nevertheless, bacterial denitrification (BD) still dominated N₂O production throughout the incubation period, except on day 1 in Glu_1.0 and Glu_2.0 of Case 2. Bland–Altman analysis with 95% limits of agreement (LoA) demonstrated strong agreement between the MAP and AIT in r_N2O estimation, particularly under Glu_2.0. All the above revealed glucose-induced denitrifying microbial shifts from BD to fungal denitrification (FD), which consequently modulated N₂O emissions and promoted incomplete denitrification. These findings collectively demonstrate that in vegetable cropping systems, rational carbon management strategies can promote N₂O reduction to N₂, thereby achieving effective N₂O mitigation.