[Effects of slow-release urea combined with common urea application layered in different soil depths on soil nitrogen, enzyme activity, and maize yield].

We examined the effects of a combination of slow-release urea (PCU) and common urea (PU) applied at different soil depths (0-30 cm soil layer) on inorganic nitrogen content, enzyme activity, and crop yield during two years (2017-2018) in a field experiment. There were eight treatments: CK (without N fertilizer); PU(common urea applied at 5-10 cm deep soil layer); PU(common urea applied at 5-10 cm deep soil layer, 60% seed fertilizer + 40% topdressing); PU(20% common urea at 5-10 cm soil depth, 30% common urea at 15-20 cm soil depth, 50% common urea at 25-30 cm soil depth); PCU(20% total nitrogen application rate at 5-10 cm soil depth, 30% total nitrogen application rate at 15-20 cm soil depth, 50% total nitrogen application rate at 25-30 cm soil depth), the N fertilizer at 5-10 cm was common urea, but, at 15-20 and 25-30 cm, it was a combination of PCU and PU at ratios of 3:7 and 3:7; PCU was as PCU but the ratio of PCU and PU was 5:5 at 15-20 cm and 5:5 at 25-30 cm; in PCU, the ratio of PCU and PU was 3:7 at 15-20 cm and 5:5 at 25-30 cm; in PCU, the ratio of PCU and PU was 5:5 at 15-20 cm and 3:7 at 25-30 cm. The results showed that PU could meet nitrogen demand at the 0-10 cm layer in the early growth stage compared with CK. PU and PU could meet nitrogen demand for 10-30 cm soil layer in the early stage of maize development. The combined application of slow release urea and common urea could meet nitrogen demand for the whole growth period of maize. In the filling and maturing period, combined application of slow release and common urea significantly increased not only NO-N, NH-N, and alkali-hydrolyzed nitrogen contents but also urease and protease activities in the 10-20 cm and 20-30 cm soil layers compared with PU-PU. Compared with PU, maize yield increased by 2.3%-24.6% and 1.3%-16.5% in the PCU-PCU treatments in 2017 and 2018, respectively. PCU had the highest yield, with 13899 and 12439 kg·hm, respectively. Therefore, the combined application of slow-release and common urea at different soil layers could meet nitrogen demand in the early growth stage of maize and increase the content of inorganic nitrogen and enzyme activities in the 10-30 cm soil layers in the later growth period, which promoted the growth and increased the yield of maize. Among all the treatments PCU treatment was the most effective.