Combined effects of nitrogen fertilizer and straw application on aggregate distribution and aggregate-associated organic carbon stability in an alkaline sandy loam soil

Research on aggregate-associated organic carbon (AOC) stability has increased, but its response to nitrogen (N) and straw (S) application after nitrogen reduction in alkaline sandy loam soil remains unclear. A 2-year field study and a short-term incubation experiment were performed to investigate the combined effect of the N + S application on aggregate distribution and stability, and AOC content and mineralization in eastern China. The study involved three N amounts (75, 150 and 300 kg N ha(-1)) with or without straw amendments under continuous cotton-barley rotation. The N + S application promoted the formation of larger macroaggregates (8-2 mm), with larger mean weight diameter and geometric mean diameter than with N application alone. Aggregate stability increased significantly with increasing rate of N application. The amount of N had less effect on AOC content in the 0-20-cm layer and cumulative mineralization of AOC in both layers compared with the N + S application. The addition of straw in promoting AOC, however, was more evident in the 20-40-cm layer. Compared with N fertilizer alone, applying N + S increased the cumulative mineralization of AOC by 27.7% in the 0-20-cm layer and by 80.9% in the 20-40-cm layer for different rates of N and aggregate sizes. In addition, at 0-20-cm depth, there was less C mineralization in macroaggregates than in microaggregates, indicating that macroaggregates showed better protection of AOC in that layer. Our study suggests that N application alone had a large effect on aggregate stability, whereas combined N + S application had a large effect on AOC content and its mineralization in the soil studied.