Abstract:Application of nitrogen (N) fertilizer is one of the best ways to increase winter wheat (Triticum aestivum L.) grain yield and quality. However, incorrect application of N fertilizer, especially excessive N application, has been raising serious environmental and economic problems. Urea is the most popular N fertilizer used for wheat in China. In order to investigate the effects of urea application rate on the quantity and activity of soil microbes and enzymes in high-yielding wheat fields, split plot experimental designs with three replications were conducted at the Experimental Station of Henan Agricultural University, Zhengzhou, Henan, with different N treatments for two years. Two wheat cultivars, Lankaoaizao8 and Yumai49-198, and four urea treatments, T1, T2, T3 and T4, were used. T1, T2, T3 and T4 received 0, 391, 586 kg/hm2 and 782 kg/hm2 urea, respectively. The urea rates in T2, T3 and T4 were split with 50% of the total amount at presowing and the remaining 50% at jointing stage. Plots were 3 m wide and 7 m long. Each treatment received same amount of P (P2O5, 150 kg/hm2) and K (K2O, 150 kg/hm2) before sowing. Soil in the 0-20 cm rhizosphere was sampled for each treatment at turning greening stage, jointing stage, heading stage and inflorescence, respectively. All the soil close to the wheat root in 0-20 cm depths were mixed to make one sample and placed in plastic bag, then transported to the laboratory for subsequent analysis. A part of each soil sample was sifted with a 1-mm sieve for the analysis of microbial quantity, the rest was air-dried for the determination of soil enzyme activity. Results showed that the total numbers of microbes reached the maximum value at jointing and heading stage, and the urea application rate had significant effect on the quantity of microorganisms and activity of enzymes in wheat field. Significant differences among urea treatments were observed as well. Change of microorganisms population in rhizosphere soil of both cultivars showed a single-peaked curve with the increase of urea application rate. The maximum population of microbes in rhizosphere soil was recorded with T2 in Yumai49198 and with T3 in Lankaoaizao8. At all growth stages, the enzyme activity of protease and catalase increased with applied urea rate until 586 kg/hm2. Larger amounts of urea decreased their activities. Urease activity increased with increasing urea rate and maximum activity was observed with T4. The results suggested that optimal urea application rate would lead to increase the quantity and activity of microbes and enzymes activity in soil.