Biochar Amendment Combined with Straw Mulching Increases Winter Wheat Yield by Optimizing Soil Water-Salt Condition under Saline Irrigation

The freshwater deficit is the major constraint to winter wheat production. Saline water irrigation could alleviate wheat water stress while increasing the risk of soil salinization, which would result in wheat yield reduction due to additional salt stress. The objective of the present study is to explore the effect of a straw-returning mode to promote winter wheat production under saline water irrigation. A field experiment was conducted during the winter wheat growing seasons of 2017–2018 and 2018–2019. Four returning modes were set, based on an equivalent carbon input: straw mulching (SM), biochar amendment (BA), straw mulching combined with biochar amendment (SM+BA), and the control without straw-returning (CK), along with three salinity levels of irrigation water: 0.47 dS m−1 (I0, freshwater), 3.25 dS m−1 (I1), and 6.75 dS m−1 (I2). Saline water irrigation alone triggered soil salt accumulation and reduced the wheat grain yield by 9.43––18.19%. Returning straw to fields increased soil organic carbon content by 16.41––52.21% and decreased soil bulk density by 0.69––1.46%. The highest increase in wheat grain yield (16.60––21.80%) was always obtained when using treatment SM+BA, due to the increased soil moisture content (3.15––12.31%) and lower salt levels (24.79––44.29%) compared to CK. The results of the present study established that SM+BA provided better soil water–salt conditions and nutrient environment for winter wheat growth than a single treatment. Thus, the combined application of SM and BA was shown to be a proper mitigating strategy to cope with the adverse effects of saline irrigation on winter wheat production and to promote the sustainable use of saline water irrigation.