The fate of N released from the fixed NH4+ pool in response to different straw application doses
Zhuqing Xia, Qiang Ma, Wantai Yu, Yanan Wang, Mengmeng Zhu, Xinhui Zhang, Yun Gao, Siyu An, Shuailin Li
Abstract
Ammonium (NH4+) can be fixed by clay minerals and subsequently released, and this process plays a vital role in promoting soil nitrogen retention and supply in agriculture. However, the process of fixed NH4+ release and its driving forces remain unclear, especially under inputs of exogenous organic matter. In this study, the fixed NH4+ pool of an Udoll (based on the USDA Soil Taxonomy System) in Northeast China was labelled using a 15N tracer under chloroform-fumigated conditions, and then the labelled soil was mixed with unlabelled (unfumigated) soil at four levels of straw incorporation: S0 (without straw), S4 (4 t ha−1), S8 (8 t ha−1), and S12 (12 t ha−1). By monitoring the tracer, we found that 20 %–24 % of the labelled fixed NH4+ was released at the end of the incubation period (288 d), and the release of fixed NH4+ was significantly affected by the straw addition rate. Generally, S4 promoted the accumulated release and fixation amounts, but S8 and S12 showed inhibitory effects compared with the S0 treatment. Moreover, microbial immobilization played a predominant role in the release of fixed NH4+ during stage 1 (0.5–72 d), while both nitrification and immobilization played important roles in promoting fixed NH4+ release during stage 2 (72–288 d). In addition, more than 47 % of the labelled fixed NH4+ released was incorporated into the nonmicrobial organic nitrogen pool after microbial turnover, reducing N loss and prolonging the N supply, and this proportion increased under increasing rates of straw application. This study clarifies the fate of fixed NH4+ derived N under different rates of straw addition, which is critical for our understanding of soil N cycling and to optimize straw application in agricultural soils.