title: Effects of different nitrogen fertilizer treatments on soil microbial functional genes
design: Sulfur-coated urea (SCU) is widely used in agriculture due to its cost-effectiveness, synchronized nitrogen release with crop uptake, and enhanced sulfur 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. In contrast to urea (U), the long-term application of SCU was found to significantly increase soil total nitrogen (TN) and soil organic matter (SOM), leading to a notable rise in average wheat yield. However, this effect was not sustained, as 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 notable 13.56% reduction in soil pH relative to the original levels. Metagenomic analysis revealed that SCU reduced the relative abundance of genes involved in carbon (C) fixation, organic nitrogen (N) metabolism, and nitrification, while the abundance of genes related to C degradation was significantly enhanced. Interestingly, SCU also affected genes associated with sulfite oxidation (soeA, soeB, soeC), which contributes to the availability of absorbable SO4(16-) for plant growth. However, this process could also exacerbate the risk of soil acidification. Structural equation modeling indicated that long-term SCU application has a significant positive impact on yield growth, but soil pH is the most significant limiting factor. In summary, while the application of SCU plays a crucial role in increasing wheat yield and regulating soil fertility, its long-term use not only increases the risk of soil acidification but also leads to a significant decline in yield over time.