Abstract:

Soil aggregates are the fundamental building blocks of soil structure. Developing agronomic practices to enhance soil aggregate structure and understanding the mechanisms behind them are crucial for sustaining soil health. However, the long-term effects of synthetic N fertilizer and green manure (GM) on soil aggregate stability remain elusive. For this, a long-term wheat cropping field experiment was conducted in the arid region of northwestern China. The experiment considered two factors: green manure and N fertilizer rates. Green manure was applied at two levels: with GM application (W-GM) and summer fallow without GM (W). The synthetic N fertilizer was also tested at two levels: 180 kg ha⁻¹ of synthetic N fertilizer (N1) and no synthetic N fertilizer (N0). Results showed that GM application boosted the formation of large macroaggregates while reducing microaggregates. In contrast, the application of synthetic N fertilizer decreased the abundance of large macroaggregates, but increased microaggregates. Compared to treatments without fertilizer (WN0) or with just synthetic N fertilizer or GM alone (WN0-GM, WN1), the combination of synthetic N fertilizer and GM (WN1-GM) increased large macroaggregates, decreased microaggregate fractions, thereby enhancing soil aggregate stability. GM lowered soil pH in all fractions, while synthetic N fertilizer raised soil pH in small macroaggregates and microaggregates. Green manure and synthetic N fertilizer increased soil organic carbon (SOC), total nitrogen (TN), nitrate nitrogen (${\mathit{NO}}_3^{-}$N), soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) in all aggregate fractions. Collectively, WN1-GM effectively reduced soil pH, with enhancing SOC, TN, ${\mathit{NO}}_3^{-}$N, SMBC and SMBN, compared to WN1. Additionally, GM positively influenced the bacterial community structure. WN1-GM had the most significant effect, enhancing the relative abundance of selected soil bacterial phyla, including Proteobacteria, Actinobacteriota, and Bacteroidota. Finally, the structure equation model illustrated that soil aggregate stability was mainly affected by soil pH, SOC, TN, SMBN, and bacterial community composition. Therefore, WN1-GM is a promising management strategy for improving soil aggregate structure.