Objective The paper studied the nitrogen release characteristics in water and soil of sulfur and resin coated urea (SRCU) specially made for winter wheat and summer maize, and the fitness of their release with the nitrogen requirement of crop during growth periods, and the combination ratio of the SRCU with common urea, aiming to provide reference for the modifying of the nitrogen release characteristics and improving utilization efficiency of the SRCU.

Methods The reseach includes experiments of nutrient release characteristies in both water and soil, and the experiment of the best combination ratio of the SRCU with common ureas. The release periods of SRCU in water for maize and wheat was two (SRCU2) and three months (SRCU3), respectively. In the experiment of nitrogen release in water, 10.00 g of SRCU was weighed in a 100 mesh nylon bag (pore size 0.15 mm), and the bags were loaded inside 200 mL of deionized water and incubated at 25℃. The N content in the water was determined regularly until the total release rate to 80%. In the soil bury experiment, 20.00 g SRCU was weighed inside a nylon bag and buried into 20 cm deep soil at the time of wheat and maize sowing. The bags were taken out of soil regularly for weighting, and the difference in weight of neighboring test was used to calculate the daily and cumulative release of nitrogen from SRCU. Fertilizer combination experiment included seven treatments: no nitrogen application (CK), twice application of common urea as basal and topdressing (FP), once basal application of common urea (100%U), once basal application and 100%, 70%, 50% and 30% of SRCU, respectively, combined with urea nitrogen. At harvest of wheat and maize, plant and grain were sampled for measuring N contents, yield and yield components. The nitrogen apparent efficiency and total income were also estimated.

Results 1) The cumulative release rates for SRCU2 and SRCU3 were 9.29% and 7.09% in water at the first day, respectively, and the nitrogen cumulative release rates in 28 days were 63.45% and 43.86%, respectively; The nitrogen cumulative release rate of SRCU2 was 89.77% at 64 days and that of SRCU3 was 85.70% at 90 days. 2) The nitrogen cumulative release rates in soil buried SRCU2 were 8.96%, 25.28%, 9.54%, 21.77%, and 15.82% at maize seedling (1–14 days), jointing (14–42 days), big trumping (42–56 days), silking (56–70 days) and grain filling stage (70–94 days), respectively. The relative high nitrogen daily average release rate was appeared at jointing stage of maize, and the highest nitrogen daily average release rate occured at silking stage. The periodical release rate in soil buried SRCU3 was 21.54% during overwintering period of winter wheat (1–128 days), 7.60% at reviving stage (128–170 days), 26.96% at jointing stage (170–201 days), 26.24% from flowering to harvest (201–239 days). The average daily release rates of nitrogen were relatively high at jointing and gain filling stages. 3) As compared with FP, the maize yield significantly increased with 100%SRCU and 70%SRCU treatments while wheat yield significantly increased only for 70%SRCU treatment. The economic incomes for maize were increased 1107.82, 1348.70 and 1065.80 yuan/hm² under 100%, 70% and 50% SRCU treatments whease the income of the wheat increased by 1609.98 and 683.71 yuan/hm² with 70% and 50% of SRCU treatments, respectively. The nitrogen appearent use efficiency of 70% SRCU in maize and wheat increased by 8.97 and 7.10 percentage point, respectively.

Conclusions The nitrogen release rate of SRCU used for wheat and maize meets the industry standards but the release rate in soil should be adjusted to crop growth. The SRCU3 does no provide enough nutrients for wheat reviving and jointing stages while SRCU2 needs more nutrients during after maize flowering. Our results suggest that combination of 70% of SRCU with 30% common urea is suitable for once basal application in both maize and wheat.