Biochar application enhances soil quality by improving soil physical structure under particular water and salt conditions in arid region of Northwest China

• Deficit and brackish water irrigation significantly reduce soil quality index. • Biochar enhances SQI by improving soil physical structure under full irrigation. • Biochar shows positive effects on bacterial community structure. Exploring the suitability of biochar for improving soil quality under different water and salt conditions is important for maintaining soil health and productivity in the arid regions of Northwestern China. We compared the effects of biochar application practices on soil physical, chemical and biological properties under different irrigation and water salinity levels in a two-year field experiment in a mulched and drip-irrigated maize field in Gansu province, China. Eight treatments in total included the combination of two biochar addition rates of 0 t ha −1 (B0) and 60 t ha −1 (B1), two irrigation levels of full (W1) and deficit irrigation (W2; W2=1/2 W1) and two water salinity levels of fresh water (S0, 0.71 g L −1 ) and brackish water (S1, 4.00 g L −1 ). The minimum dataset method was used to calculate the soil quality index (SQI) under different treatments. Deficit and brackish water irrigation significantly reduced SQI by 3.80-9.80% through reducing some soil physical, chemical and biological properties. Biochar application significantly increased the SQI by 6.13 and 10.40% under full irrigation with fresh and brackish water, respectively. Biochar addition enhanced the relative abundance of beneficial bacteria (e.g., Proteobacteria, Patescibacteria) in the soil in all water-salt treatments. The partial least squares path model showed that biochar application significantly enhanced the SQI mainly by improving soil aggregation and pore structure under particular water-salt conditions. This research provides an important basis for utilizing biochar to improve soil quality in arid regions of Northwest China under various water-salt conditions.