Long-term application of sulfur-coated urea: From soil fertility improvement to increased acidification risk

Abstract

Sulfur-coated urea (SCU) is widely used in agriculture due to its cost-effectiveness, synchronized nitrogen (N) release with crop uptake, and enhanced sulfur (S) availability. However, most studies have primarily focused on the short-term yield-enhancing mechanisms of SCU, while its long-term impact on soil quality remains unclear. To address this gap, the effects of SCU on wheat yield and microbial functional profiles were investigated using a 15-year, 30-season wheat-maize rotation experiment and metagenomic analysis. Compared with urea (U), SCU application resulted in a short-term increase in wheat yield, followed by a sharp decline over the years. This decline may be linked to a significant decrease of 0.49 units in soil pH relative to U. Comparing the relative abundance of genes in U and SCU, it was found that SCU reduced the relative abundance of genes related to carbon (C) fixation and nitrification by 11.40–16.87 % and 24.14–50.02 %, respectively, while increasing the abundance of genes related to C degradation and sulfite oxidation by 8.34–11.75 % and 16.29–41.96 %, respectively. Variance partitioning analysis showed that biotic and abiotic factors synergistically explained 55.1–58.1 % of the variation in gene abundance. Structural equation modeling revealed that long-term SCU application had a significant positive effect on yield growth, but declining soil pH emerged as the primary limiting factor. In summary, while SCU enhances wheat yield and regulates soil fertility in the short term, its prolonged use increases the risk of acidification and long-term yield decline.