Increased soil salinity significantly inhibits crop production worldwide, and biochar may alleviate salt stress. In the present study, the application of biochar significantly increased the biomass of rice under salt stress treatment. The analysis of soil properties demonstrated that biochar application significantly decreased electrical conductivity and soluble Na⁺ and Cl⁻ contents in the soil under salt stress. In addition, biochar application increased the soil cation exchange capacity, soil organic matter, humic acid, total nitrogen, and total phosphorus contents in the soil, suggesting that biochar improved the soil nutrient conditions. The application of biochar further increased the abundance of soil bacteria and changed the bacterial community structure under salt stress. Proteobacteria, Chloroflexi, and Acidobacteria were the top three phyla in bacterial abundance. Biochar increased Proteobacteria abundance and decreased Chloroflexi abundance, which were considered to be eutrophic bacteria and oligotrophic bacteria, respectively. Redundancy analysis showed that soil bacterial communities were mainly affected by soil pH and EC (p < 0.05). In conclusion, the application of biochar alleviated salt stress in rice via modifying soil properties and regulating the bacterial abundance and community structure.

Soil acidification is a significant form of agricultural soil degradation, which is accelerated by irrational fertilizer application. Sweetpotato and wheat rotation has emerged as an important rotation system and an effective strategy to optimize nutrient cycling and enhance soil fertility in hilly areas, which is also a good option to improve soil acidification and raise soil quality. Studying the effects of different fertilization regimes on soil acidification provides crucial data for managing it effectively. An eight-year field experiment explored seven fertilizer treatments: without fertilization (CK), phosphorus (P) and potassium (K) fertilization (PK), nitrogen (N) and K fertilization (NK), NP fertilization (NP), NP with K chloride fertilization (NPK1), NP with K sulfate fertilization (NPK2), and NPK combined with organic fertilization (NPKM). This study focused on the soil acidity, buffering capacity, and related indicators. After eight years of continuous fertilization in the sweetpotato–wheat rotation, all the treatments accelerated the soil acidification. Notably, N fertilization reduced the soil pH by 1.30–1.84, whereas N-deficient soil showed minimal change. Organic fertilizer addition resulted in the slowest pH reduction among the N treatments. Both N-deficient (PK) and organic fertilizer addition (NPKM) significantly increased the soil cation exchange capacity (CEC) by 8.83% and 6.55%, respectively, compared to CK. Similar trends were observed for the soil-buffering capacity (pHBC). NPK2 increased the soil K⁺ content more effectively than NPK1. NPKM reduced the sodium and magnesium content compared to CK, with the highest magnesium content among the treatments at 1.60 cmol·kg⁻¹. Regression tree analysis identified the N input and soil magnesium and calcium content as the primary factors influencing the pHBC changes. Structural equation modeling showed that the soil pH is mainly influenced by the soil ammonium N content and pHBC, with coefficients of −0.28 and 0.29, respectively. Changes in the soil pH in the sweetpotato–wheat rotation were primarily associated with the pHBC and N input, where the CEC content emerged as the main factor, modulated by magnesium and calcium. Long-term organic fertilization enhances the soil pHBC and CEC, slowing the magnesium reduction and mitigating soil acidification in agricultural settings.