Biochar to Reduce Fertilizer Use and Soil Salinity for Crop Production in the Yellow River Delta

To test the hypothesis that by alleviating the salt stress in salt-affected soil, biochar could maintain crop yields even if fertilizer use is reduced by 25% in the Yellow River Delta (YRD). A field trial was conducted to assess the effect of biochar use alone (3, 6, and 12 t ha−1) and in combination with reduced fertilization (25% reduction) on alleviating salt stress, enhancing nutrient supply, and increasing crop yields in wheat–maize rotation. Porous biochar at 12 t ha−1 dose significantly decreased the bulk density of saline soil and increased its saturated hydraulic conductivity (Ks) and water content at wheat and maize harvest over the control (CK). Being rich in K+ (493.9 mmol kg−1), the biochar reduced sodium adsorption ratio (SAR) and Cl−/√SO42− at wheat harvest by 50% and 73%, respectively, and helped the uptake of K+ by crops over Na+, resulting in a higher K/Na ratio of grains in treatments as compared to the control. Similar trends were found when biochar (12 t ha−1) was applied together with 75% of conventional fertilization (CF: 375 kg ha−1). This combined biochar and fertilizers increased soil NH4+-N, Olsen-P, nutrient supply, and crop yields compared to 75% CF. Excessive Na+ and soil compaction limited crop yields in YRD. Biochar amendment reduced soil bulk density and increased saturated hydraulic conductivity (Ks). They, in turn, enhanced salt leaching and made salt compositions more favorable to crop growth. Compared with 75% CF, co-application of 6–12 t ha−1 biochar and 75% CF increased crop yields.