Improving Rice Root Development and Soil Health in Saline Soils: A Biochar and Microbial-Inoculated Biochar with Nitrogen Approach

This study investigated the combined effects of microbial-inoculated biochar and nitrogen (N) on rice growth and soil properties under saline conditions. A randomized complete block design with three replications was employed to evaluate three factors: (i) salinity level (non-saline, S0; saline, 0.4% NaCl, S1), (ii) biochar type (20 t/ha BC, BF, BB, and BFB), and (iii) nitrogen application rate (60 and 120 kg ha−1). Soil physicochemical and biological properties, along with rice root development, were assessed. Salinity significantly reduced soil organic matter (OM) by 9%, nitrate nitrogen (NO3−-N) by 16%, ammonium nitrogen (NH4+-N) by 8.18%, and available phosphorus (AP) by 6.81%. Soil enzyme activities, including catalase (CAT), acid phosphatase (ACP), polyphenol oxidase (POX), and β-D-glucosidase (BG), decreased by 32.69%, 29%, 39.18%, and 19.44%, respectively, resulting in suppressed root growth compared with non-saline conditions. The combined treatment of microbial biochar (BFB) and N at 120 kg ha−1 (BFB + N120) markedly improved saline soil quality and rice root performance by maintaining a favorable K+/Na+ balance in roots. Specifically, BFB+N120 increased OM by 145% and 120% compared with N120 and BC alone, respectively, and enhanced NO3−-N, NH4+-N, and soil enzyme activities (CAT, ACP, POX, and BG). These improvements were strongly associated with enhanced root development. Under saline conditions, BFB+N120 increased root dry mass by 429% and 1185.71%, and root length by 63% and 83%, compared with N120 and BC alone, respectively, in the cultivar Jing Liang You 534. Overall, the results demonstrate that microbial-modified biochar combined with nitrogen fertilizer mitigates salt-induced soil degradation by improving physicochemical and biological properties, thereby enhancing nutrient availability, ionic homeostasis, and root growth. This study provides mechanistic insights into the combined role of microbial biochar and nitrogen in the remediation of saline soils.