Acid-modified biochar combined with reduced nitrogen fertilizer improves soil organic carbon and cotton yield in...

Saline-alkali soils commonly face the dual challenges of low stock and activity in soil organic carbon (SOC), and long-term excessive nitrogen (N) fertilizer application further exacerbates soil quality degradation. Acid-modified biochar offers a promising approach for improving the fertility of saline-alkali soils, yet the effects of its combined application with N fertilizer on SOC fractions and soil quality remain insufficiently studies.

Therefore, we conducted a two-year field experiment to comprehensively assess the effects of acid-modified biochar combined with N fertilizer application on soil quality index (SQI), microbial nutrient limitation and microbial carbon use efficiency (CUE), SOC fractions and yield. A randomized complete block experiment with five treatments was established: no chemical N fertilizer and no acid-modified biochar (CK), 300 kg N ha^-1 (N1), 450 kg N ha^-1 (Recommended N application rate, N2), 300 kg N ha^-1 + acid-modified biochar (BN1), and 450 kg N ha^-1 + acid-modified biochar (BN2).

The BN1 and BN2 treatments effectively lowered soil pH by 3.7%-5.6% and increased the contents of available phosphorus (Av-P), available potassium (Av-K), nitrate nitrogen (NO₃ ^--N), ammonium nitrogen (NH₄ ⁺-N), and microbial biomass nitrogen (MBN). These improvements collectively resulted in a SQI that was 4.0-5.1 times higher than that in the N1 and N2 treatments. Furthermore, the BN1 and BN2 treatments reversed the suppression of soil enzyme activities induced by sole N application, enhanced microbial CUE by 71.7%-105.2%, and mitigated microbial C limitation. Compared with N2, BN1 and BN2 treatments increased SOC by 35.5%-51.2%, easily oxidizable carbon (EOC) by 1.2-2.7 folds, and microbial biomass carbon (MBC) by 34.4%-46.1%. The cotton yield under BN treatments exceeded that of N2 by 10.1%-15.5% in two years. No significant differences were observed between BN1 and BN2 in terms of SQI, microbial CUE, and yield. Path analysis confirmed that fertilization-induced alterations in soil properties indirectly promoted soil enzyme activities and microbial CUE, which not only facilitated SOC accumulation but also directly boosted yield.

Overall, 20% reduction of conventional N fertilizer combined with acid-modified biochar was more effective at improving soil quality and reducing microbial C limitation, enhance microbial CUE, and foster the accumulation of SOC, providing guidance for sustainable development in saline-alkali soil.