Biochar affects soil water content but not soybean yield in a sandy southeastern U.S. soil

Drought is the greatest abiotic cause of soybean [ Glycine max (L.) Merr.] yield loss in rainfed systems in the United States. Preplant incorporation of powdery biochar to soil cultivated with soybean in these water‐stressed regions presents the potential to increase volumetric soil water content (VSWC) between rainfall events and ameliorate the impact of intermittent water stress. However, VSWC and the above‐ and belowground response of soybean to biochar amendment are not well understood. This 2‐yr field study (2014–2015) evaluated the effectiveness of incorporating powdery (325 μm) biochar to 0.2‐m soil depth to increase soil water storage in a coarse‐textured soil in the southeastern United States. Soybean yield, aboveground biomass accumulation, root morphology, and VSWC in the soil profile to 1‐m depth were assessed under three biochar rates (0, 2.5, and 10 Mg ha –1 ) and two water treatments (rainfed and irrigated). In 2014, application of 10 Mg biochar ha –1 increased the cumulative soil water storage in the top 1‐m soil depth. Biochar application did not affect soybean yield or root morphology. Biochar only elicited changes in fresh weight of pods during reproductive stages, which indicates biochar's limited benefit to the water status of this shoot component. Irrigation increased yield by 28% compared with the rainfed treatment. Our study provides evidence that field application of biochar to soybean may be a sustainable practice to sequester recalcitrant C in the soil when biochar is available to producers, but its benefits on protecting soybean yield from water stress might depend on other factors such as soil texture, rainfall amount, and frequency.