Straw and biochar amendments enhance cotton nitrogen uptake and yield by regulating ammonia-oxidizing microbial communities in saline water-irrigated fields

Soil salinization severely restricts cotton nitrogen uptake and yield in arid saline drip irrigation fields by inhibiting nitrification and disrupting ammonia-oxidizing microbial communities. This study set four treatments (fresh water, FW; saline water, SW; saline water + straw, SWST; saline water + biochar, SWBC) to explore how straw and biochar regulate cotton nitrogen uptake, yield and ammonia-oxidizing microbial communities. Results showed SW reduced cotton nitrogen uptake by 49.80% and yield by 27.92% vs. FW, alongside 15.01% lower soil potential nitrification rate (PNR), decreased nitrate nitrogen (NO 3 - -N), reduced ammonia-oxidizing microbial abundance and altered community structure. SWST and SWBC effectively mitigated these adverse effects, increasing nitrogen uptake by 37.53% and 32.82%, and yield by 11.77% and 7.75% respectively, with straw outperforming biochar. Partial least squares-structural equation model (PLS-SEM, GOF=0.70) confirmed straw/biochar enhanced cotton nitrogen uptake and yield by reducing soil salinity, reshaping ammonia-oxidizing microbial communities and improving nitrification, with ammonia-oxidizing bacteria (AOB) as the key nitrification-driving microorganism. Their divergent effects derived from distinct regulatory pathways: straw optimized AOB and Comammox clade A communities to boost nitrification significantly, while biochar mainly enriched low-efficiency Comammox clade B, leading to limited nitrification improvement. This study elucidates the superior yield-increasing effect of straw over biochar and its microbial-driven nitrification regulation mechanism, providing a scientific basis for rational saline water utilization and efficient nitrogen management in arid cotton fields. • Straw and biochar increase nitrification in saline-water-irrigated cotton soils. • Straw increases AOB and Comammox-clade-A gene abundance under saline irrigation. • Biochar increases AOA, AOB, and Comammox-clade-B gene abundance. • Straw outperforms biochar in enhancing cotton N uptake and yield. • Soil salinity, moisture, and pH drive ammonia-oxidizing microbial communities.