Combined application of vitamin C industrial fermentation by-product and dicyandiamide (DCD) modifies nitrogen fertilizer utilization and nitrous oxide (N2O) emission in acid soil

Organic carbon input has a profound impact on the soil material cycle. Residue after evaporation (RAE) of fermentation broth from the vitamin C industry, which mainly consists of low-molecular-weight organic acids, effectively regulates soil microbial communities and fertility. Nevertheless, there is a lack of assessment regarding RAE’s influence on nitrogen fertilizer use and nitrous oxide (N 2 O) emissions. Here, we investigated the effect of RAE on nitrogen agronomic efficiency (NAE), partial factor productivity (PFP n ), and N 2 O release, while also assessing the combined effects of the nitrification inhibitor dicyandiamide (DCD). RAE enhanced maize ( Zea mays L.) nitrogen uptake and increased leaf nitrogen content during both heading and maturity stages. RAE and RAE + DCD increased grain harvest by 6.13–7.11 % and 14.31–14.67 %, respectively. NAE and PFP n increased by 15.45–20.63 % and 5.63–7.53 % with RAE alone, while combined with DCD increased by 36.70–37.93 % and 13.77–14.11 %. Importantly, RAE and DCD had a synergistic effect in enhancing NAE and PFP n (DCD increased NAE and PFP n by 12.24–15.01 % and 4.64–5.75 %). RAE input increased soil organic carbon, stimulated soil microbial activity, and promoted the copies of soil nitrification and denitrification functional genes (AOB- amoA , narG , nirS ), temporarily leading to higher N 2 O fluxes. However, RAE also increased nitrogen fixation gene ( nifH ) copies and soil total nitrogen content. Enhancing the genetic capacity and microbial activity represents a potential driver for RAE-mediated regulation of soil nitrogen turnover. Combined application of DCD and RAE optimizes nitrogen fertilizer utilization and crop production. Our findings provide novel insights into how water-soluble organic carbon promotes agricultural productivity. • Reuse of vitamin C industrial waste—fermentation residues after evaporation (RAE). • The combination of RAE and dicyandiamide (DCD) significantly increased maize yield. • RAE and DCD had a synergistic effect on improving nitrogen use efficiency. • The impact of water-soluble organic carbon on nitrous oxide is rapid and transient.