Preparation of dual controlled release urea with nitrogen inhibitor and resine coating and the effects on soil nitrogen supply capacity and wheat yield

Objectives Coating and adding inhibitors are commonly used to prepare slow-release and controlled-release fertilizers. We tried to prepare new fertilizers with more controllable nitrogen release and transformation by applying these two methods at the same time and study the application effect of dual controlled release urea with nitrogen inhibitor and resine coating on wheat.

Methods The tested particle size of urea was 2.5–3.5 mm and coating material was resin, and the used inhibitors were urease inhibitor (HQ) and nitrification inhibitor (DCD) in the preparation. According to the inhibitor used, four kinds of new resin coated urea were prepared: uncoated (CU), HQ coating (CRU1), DCD coating (CRU2) and (HQ+DCD) combined coating (CRU3). The microstructure of four kinds of coated urea was observed by scanning electron microscope (SEM). The slow release characteristics of nutrient and inhibitors were determined using hydrostatic release test. Two field experiments were carried out separately in Weifang and Tai'an of Shandong Province with the same N, P and K application rates and the same fertilization methods. Soil samples were collected at seedling, jointing, flowering, filling and maturity stages of winter wheat to determine the content of available N. The yield and yield components at the mature stage of winter wheat were investigated.

Results 1) In the microscope images of 500× and 1000×, the film layers of all the four prepared slow-release urea were complete, uniform, smooth and dense. Some micropores could be seen on the surface of the film, which worked as the channel for urea and inhibitor to release out. The inhibitor layer tightly covered urea particle without gap, and was completely surrounded by the resin coating layer, realizing the control of the release of urea and inhibitor at the same time. 2) The combination of coating and inhibitor addition effectively controlled urea dissolution. When loaded in still water, the initial N release rates of CU, CRU1, CRU2 and CRU3 were 7.59%, 1.96%, 2.12% and 0.89%, respectively; the release periods of urea of CU, CRU1, CRU2 and CRU3 were 42, 56, 56 and 56 days, respectively; the release period of HQ in CRU1 was 28 days, that of DCD in CRU2 was 14 days, and those of HQ and DCD in CRU3 were 42 and 14 days, respectively. As the longer N and inhibitor release period in CRU3 than CRU1 and CRU2, CRU3 had longer sustained release effect. 3) Compared with the large granule urea control (U), the soil NH₄⁺-N concentrations of the four coated urea treatments were maintained at appropriate low levels at the seedling stage, and were significantly increased after flowering stage, which ensured the continuous supply of N to wheat; while the soil NO₃^–-N contents during the whole growth period of wheat were reduced significantly, which decreased the possibility of N leaching loss. CRU1 and CRU3 treatments could maintain the soil urease activity at a low level during the whole growth period of wheat, CRU2 and CRU3 treatments could inhibit the transformation of soil NH₄⁺-N to NO₃^–-N, and significantly reduce the apparent nitrification rate of soil NH₄⁺-N. Compared with CU, CRU1, CRU2 and CRU3 treatments significantly increased the yield of wheat by 23.38%, 23.13% and 38.79%, respectively in Weifang, and by 6.36%, 9.52% and 28.57% respectively in Tai'an.

Conclusions Double coating large granule urea with inhibitor and resin in turn could form a complete and uniform film layer, and some clear micropores still exist on the layer surface allowing the release of urea and inhibitor. Coated urea containing both urease and nitrification inhibitor (CUR3) has better controlling effect on soil ammonium and nitrate concentration during the whole growth stage of winter wheat, so obtains higher yield effect than the coated urea containing either urease or nitrification inhibitor.