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Influence of nitrogen addition on the changes in nitrogen and carbon fractions in soil profiles of wheat fields

Increasing inputs of nitrogen (N) fertilizer greatly affect the functionality of farmland carbon (C) and N cycling. Soil contains various C and N fractions that have diverse chemical and physical characteristics, and they can be used as sensitive evaluation indicators for the change in soil C and N content; however, N application's effects on the deep soil layer N and C fractions of the North China Plain remain unclear. Therefore, we investigated the changes in content, percentage, and sensitivity of N and C fractions under four N fertilization application rates in the upper 200 cm soil layer since 2012. N and C fractions in soil layers respond differently to N enrichment. For grain yield, soil organic carbon (SOC), and total nitrogen (TN) contents, the sensitivity index (SI) of N fractions was the highest at N applications of 180 and 240 kg ha−1. SOC and TN are most active in the 0–20 cm soil layer. The SI of most C fractions in the topsoil layer were not the highest, and the SI of the N fractions was higher in the middle soil layer. Obvious leaching during the application of 300 kg ha−1 of N fertilizer was indicated by the nitrate content. Under the experimental conditions, the 180 and 240 kg N ha−1 applications proved to be the best for stabilization of C and N and improved crop productivity.