The Transformation Dynamics and Homogeneity of Different N Fractions in Compost following Glucose Addition

The application of compost to soil is a common fertilization practice for improving soil quality and crop growth. The isotopic labeling technique is mostly used to investigate the contribution of compost N to crop uptake. However, compost N includes various N fractions and labeling dissimilarity, which may cause bias when calculating the compost N contribution to plants. Therefore, the labeling dynamics of different N fractions in compost and the homogenous labeling time point should be clarified. Given the 15 N-labeling in chemical fertilizer and the carbon source, i.e., glucose, the compost N pools were divided into active N (mineral N, soluble organic N [SON], microbial biomass N [MBN]), stable N (hot-water extractable organic N [HWDON]), and recalcitrant N. The atom percentage excess (APE) of different N in compost notably varied at the beginning of incubation, ranging from 0–3.7%. After the addition of glucose, biological N immobilization was promoted (13.7% and 28.8% for MBN and HWDON, respectively) and promoted the transformation among available N pools. Adding distinct doses of glucose at three stages to 15 N-labeled compost resulted in diverse microbial responses, thereby redistributing exogenous N in each fraction ( 15 NH 4 + -N went into SO 15 N from day 15 to day 30 and increased by 5.1%; SO 15 N entered MB 15 N and HWDO 15 N during day 30 to day 45 and increased by 5.7% and 5.2%, respectively). On day 45, homogeneous 15 N-labeled compost was achieved, which was 2.4% for 15 N APE for all N fractions. Overall, the quantitative data for the transformation of N fractions in compost at distinct stages provides a scientific basis for compost labeling trials, in order to identify the time point at which compost N-labeling is homogeneous, which is necessary and meaningful to reduce the bias of the contribution rate of compost-N to plants.