Effects of nitrogen application rates on inorganic nitrogen and microbial nitrogen-transformation functional genes in wheat rhizosphere soil of North China Plain.

Understanding the response of inorganic nitrogen (N) and microbial N transformation functional genes in the rhizosphere soil of wheat to N application rates can provide theoretical basis for the N optimizing in wheat production. Based on a long-term field experiment with five N application levels [0 (N0), 120 (N120), 180 (N180), 240 (N240), 360 (N360) kg·hm^-2] conducted on the North China Plain with two wheat varieties, Huayu 198 (HY198) and Xinong 979 (XN979), we investigated the effects of N application rates on inorganic N content and the abundance of microbial functional genes (<i>amoA</i>, <i>nirK</i>, <i>nirS</i>, <i>nosZ</i>) related to nitrification and denitrification in rhizosphere soil. The results showed that continuous N application for eight years increased soil ammonium and nitrate content, both of which increased significantly with increasing N application rates by 54.9%-274.2% and 110.3%-614.3% respectively compared to N0. At the same N application, the microbial functional gene abundances of <i>amoA</i>, <i>nirK</i>, and <i>nirS</i> in rhizosphere soil of XN 979 was significantly higher than that of HY 198, while the abundance of <i>nosZ</i> was significantly lower for HY 198. For the nitrification functional genes, the <i>amoA</i> gene abundance of XN 979 was lowest at N120, decreasing by 28.8%, whereas the lowest value for HY 198 occurred at N180, decreasing by 46.9% compared to N0. The <i>amoA</i> gene abundance of ammonia-oxidizing bacteria (AOB) in both cultivars increased significantly with increasing N levels, showing growth rates of 127.4%-362.7% for HY198 and 26.5%-417.9% for XN 979 compare with N0. For denitrification functional genes, the <i>nirK</i>, <i>nirS</i>, and <i>nosZ</i> gene abundances were highest for XN 979 at N240, while in HY 198, the <i>nirK</i> gene abundance was significantly lower at N360 compared to other N levels, and the lowest values for <i>nirS</i> and <i>nosZ</i> occurred at N180. There was significant positive correlation between AOB <i>amoA</i> gene abundance and NH₄⁺-N and NO₃^--N contents, indicating that AOB was more sensitive to N application. Overall, long-term N application significantly enhanced rhizosphere soil NH₄⁺-N and NO₃^--N contents, which in turn regulated the abundance of microbial functional genes in nitrification and denitrification.