Enhanced nutrient retention following biochar application linked to in situ biochar surface functionalization through a common mechanism across variable management conditions

Biochar is a proven measure to increase soil carbon sequestration. Soil management practices used for sustainable management of corn-soy crop systems including cover crops or organic fertilization may influence biochar aging with long-term impacts on surface-mediated processes including nutrient retention and co-stabilization of non-biochar derived organic carbon (OC). Biochar was applied to two plot-scale field experiments under rainfed and irrigated water management in 2022 at a rate of 70 Mg ha^-1 while a third field experiment, established in 2023 on 16 acres of farmland, used a rate of 20 Mg ha^-1 with mineral or biosolid fertilizer. Soil was sampled six months after application and analyzed for OC and nutrients. Biochar particles were picked from topsoil samples and characterized using X-Ray Photoelectron spectroscopy (XPS). Soil OC stocks indicated biochar-OC recovery between 36-100%. Enhanced nitrate retention was observed within the first year after biochar addition for both application rates, with an increase of 31 kg ha^-1 under rainfed conditions and 11 kg ha^-1 for irrigated conditions at high application rate. Similar trends were observed under lower biochar application rate with enhanced nitrate retention between 2.2 and 6.9 kg ha^-1. Analysis of biochar particles using XPS revealed extensive changes to biochar surface chemistry across sites and management. Speciation complexity and enrichment of iron and nitrogen increased after field-aging and was enhanced by greater in-season soil moisture under irrigation or during higher-rainfall sampling year, suggesting that management practices can be leveraged to promote biochar surface aging processes to improve soil nutrient retention.