Transformation of fertilizer nitrogen in fluvo-aquic soils with different textures and its influencing factors

Aims The transformation of the various fractions of soil organic nitrogen (N) plays a pivotal role in soil N retention and in supplying N for crop growth. Soil texture essentially determines the soil's physical, chemical and biological properties, which affect the soil organic N content and distribution. However, little is known about the incorporation and allocation of fertilizer N among different soil organic N fractions and the fate of fertilizer N in soils with different textures. Methods This study was conducted in a long-term experiment (began in 1990) including three soils of different texture (sandy soil, sandy clay loam and loamy clay) in a winter wheat-summer maize cropping system. A N-15-labelling microplot field experiment was carried out to investigate the distribution and seasonal dynamics of fertilizer N among different soil organic N fractions from October 2017 to May 2018. Results The residual amount of fertilizer N in different textural soils were loamy clay > sandy clay loam > sandy soil. More than 74% of the residual fertilizer N existed in the form of acid hydrolyzable nitrogen (AHN). The levels of hydrolyzable ammonium N (HAN) and hydrolyzable unknown N (UHN) derived from fertilizer decreased gradually during the wheat growing season, and the amino acid-N-15 (AAN-N-15) and amino sugar-N-15 (ASN-N-15) levels decreased first and then increased. The average value of ASN-N-15 in loamy clay and sandy clay loam was 119% and 58% higher than that in sandy soil; the concentrations of AAN-N-15 and HAN-N-15 and the N use efficiency were found to be highest in sandy clay loam. On average, the content of acid insoluble N (NHN) derived from fertilizer in loamy clay was 2.4 and 1.9 times those in sandy soil and sandy clay loam, respectively. The structural equation modeling (SEM) revealed that the different concentrations of dissolved organic nitrogen in the fluvo-aquic soils with different textures directly induced changes in the distribution and seasonal dynamics of fertilizer N in the HAN, ASN and UHN pools, further influencing fertilizer N storage in the soil and supply for wheat. Conclusions Loamy clay had the highest N fertilizer retention capacity of all tested soil types. Sandy clay loam provided more labile fertilizer-derived organic nitrogen and a better environment for roots, thereby increasing the N use efficiency of wheat. The fertilizer N residue in the soils and the fertilizer N supply for wheat were closely related to the concentrations of HAN-N-15, ASN-N-15 and UHN-N-15 and were affected by the dissolved organic N contents of the soils with different textures.