Response of soil organic nitrogen components to nitrogen application rate in a wheat-maize rotation cropland

Objectives The composition and availability of soil organic nitrogen (SON) play key roles in soil N retention and supply. Studying the effects of N fertilizer application rate on soil organic N components could provide a reference for rational management of N fertilizer.

Methods A field experiment was conducted in Wenxian, Henan Province, and lasted for three winter wheat seasons and two summer maize seasons. The N fertilizer application rates on wheat were 300 kg/hm² (N300), 225 kg/hm² (N225), 195 kg/hm² (N195), 165 kg/hm² (N165), and 0 kg/hm² (N0). Wheat yield and N uptake were investigated. Before sowing and after wheat harvest, soil samples were collected to determine total N and organic N content.

Results The minimum N application rate (N165) recorded a stable yield while N195 met the N demand of winter wheat. Acid-soluble nitrogen (TNex) is the main component of SON, accounting for 59.06%−92.26% of total N. TNex was composed of three components, amino sugar nitrogen (ASN), ammonium nitrogen (ANN), and amino acid nitrogen (AAN), which accounted for 18.40%−46.62%, 8.91%−34.40%, and 13.43%−28.52% of TNex, respectively. The reduction of the N application rate led to an increase and decrease in ANN and ASN contents. The results of redundancy analysis indicated the relationship among soil organic N components and ASN before sowing, ANN before sowing, AAN before sowing, and N fertilizer applying rate. Their proportional contribution to the variation in SON was 19.11%, 18.19%, 9.80%, and 17.45%, respectively. The results of the structural equation analysis indicated that the N fertilizer application rate affected the transformation relationship among SON components and their contribution to soil N supply. Under medium and low N application rates, AAN was the key component of soil N supply. The contribution of AAN to the N uptake of winter wheat was 19.11%. ASN was the key component of soil N supply under a high N application rate, and the contribution of ASN to N uptake of winter wheat was 8.65%.

Conclusions Acid-soluble N is the main organic N component in Wenxian soil. N application rate affects soil organic N component contents and transformation relationship. Low N application rates decrease soil total N, reducing the proportion of acid-soluble nitrogen. High N application rates can maintain the stability of acid-soluble N. Among the acid-soluble N, low N application rates increase the proportion of ANN and decrease the proportion of ASN. Under medium and low N application rates, AAN is the key component of soil N supply. Under high N application rates, ASN is the key component of soil N supply.