Long‐term effects of straw mulching coupled with N application on soil organic carbon sequestration and soil aggregation in a winter wheat monoculture system

Straw mulching can be affected by N fertilization rate with regard to improving soil organic carbon (SOC) lability, which modifies the net accumulation of SOC and soil aggregation. We conducted a 14‐yr field experiment to determine how straw mulching coupled with N application rates affects crop yield, SOC sequestration as a net gain of SOC, and soil aggregation in a winter wheat (Triticum aestivum L.) monoculture system. Six combinations of two cultivation practices, conventional cultivation (CC) and straw mulching (SM), and three N application rates (0, 120, and 240 kg N ha–1) were compared. Results revealed that SM did not affect wheat yield throughout the 14 yr of cultivation, but increased the SOC stock, SOC lability, and the carbon management index (CMI) in surface soil (0–20 cm). Instead, N application, N120 and N240, increased the wheat production almost equally, and followed a trend for increasing SOC stock: N240 > N120 > N0; however, a different trend for increasing SOC lability and soil CMI (N120 > N240 > N0). Soil macro‐aggregation was increased by SM but decreased with increasing N application. Principal component analysis (PCA) indicated that organic C input was the key to improving SOC sequestration, SOC lability, and soil macro‐aggregation rather than N input, especially high N input (N240) reduced SOC lability and soil aggregation in soil. Consequently, medium N application (N120) may be expected to couple with straw direct‐returning for improving soil productivity and quality in this agro‐system.