The combined application of controlled-release urea and fulvic acid improved the soil nutrient supply and maize yield

Controlled-release urea (CRU) and fulvic acid (FA) could improve nitrogen (N) use efficiency (NUE) and crop yield. This study aimed to reveal the synergistic interactions between CRU and FA on NUE, soil nutrient supply, and maize (Zea mays L.) yield. Here, we used no fertilizer for the control; low- or full-strength regular urea with or without FA (urea (U), U2/3, U+FA, U2/3+FA); and low or full-strength CRU with or without FA (CRU, CRU2/3, CRU+FA, CRU2/3+FA). Compared with U treatment, the maize yield, NUE and net returns of CRU and CRU+FA treatments were significantly increased, by 6.4%, 37.1%, and 11.5%, and 14.2%, 33.8%, and 24.7%, respectively; SPAD values were significantly increased after CRU, CRU+FA treatment, by 10.4% and 13.6%, respectively, at the 12-leaf collar (V12) stage. After CRU treatment, the NO3-N content of was significantly increased by 14-28%; meanwhile, CRU+FA treatment increased significantly the NO3-N content by 15-71% at the V12 and silking stages. When ammonia volatilization was maximal after U+FA treatment, the soil urease activity, nitrification, denitrification, and respiration levels were significantly improved by 25, 262, 370, and 428%, respectively. Based on these results, CRU+FA can be suggested as a new eco-friendly fertilizer that could improve maize yield.