Long-acting mechanisms of concentrated urea application — High urea concentrations are biological inhibitors

The concentrated application of urea (CAU; including band and hole placements) promotes cereal crop (e.g., rice and summer maize) yields and nitrogen (N) use efficiency by prolonging the retention of N fertilizer in the soil and reducing N loss. However, the long-acting mechanisms of CAU are not well understood. This study investigated the interactions between N transformation and soil microbes in the special micro-region around the fertilization point with the CAU, using a simulation experiment with two types of soil (silty loam and sandy loam). The soil samples were subjected to six N treatments: 1000, 3000, 5000, 10,000, 30,000 mg N kg−1, and dicyandiamide with 1000 mg N kg−1; no urea treatment was a control. Soil bacterial community compositions (examined using MiSeq sequencing), the abundance of ammonia-oxidizing microbes (quantified using real-time PCR), including ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), and the soil chemical properties were assessed at 20 and 60 days after urea application. The high soil ammonium concentration caused by CAU affected the soil bacterial community by changing the soil chemical properties. Soil ammonium concentration and pH were the main factors affecting microbial communities and amoA gene abundance of AOA and AOB. The 10,000 and 30,000 mg N kg−1 dosages for the silty loam soil and 30,000 mg N kg−1 dosage for the sandy loam soil in this simulation experiment significantly (p < 0.05) reduced bacterial α-diversity, amoA gene abundance of AOA and AOB, and inhibited nitrification throughout the culture period. However, the inhibitory effects of high soil N application concentration on nitrification weakened as the culture time increased. Overall, CAU acted as a biological nitrification inhibitor by maintaining high soil ammonium concentration, which decreases activity of AOA and AOB amoaA genes for a long time (>60 days).