Effects of straw returning on saline soil aggregate stability and its carbon, nitrogen contents

Soil aggregate is the physical foundation of soil fertility and straw returning is widely recognized as an effective method to facilitate soil aggregate formation and stabilization. However, it is still unclear what effects of straw returning might have on soil aggregate composition and its carbon (C) as well as nitrogen (N) distribution in saline soils. In this study, taking the typical salinized fluvo-aquic soil of the Yellow River Delta as research object, we analyzed the water-stable aggregate composition, stability and the organic carbon (SOC), total nitrogen (TN) content of each aggregate in soils under three salinization degrees (light, moderate, highly) and after 3-year straw returning. Results showed that the proportions of 0.25-2 mm and 0.053-0.25 mm aggregates in highly-salinized soil were significantly lower than that in light-salinized soil and moderate-salinized soil (p < 0.05). The soil total salt content was negatively correlated with the contribution rates of 0.25-2 mm aggregates to SOC and TN, while it was negatively correlated with the contribution rates of 0.053-0.25 mm aggregates to TN. Straw returning induced a 47.6%, 39.7% and 54.0% increase in mean weight diameter (MWD), geometric mean diameter (GMD) and the content of macro-aggregates (R0.25) respectively for light-salinized soil, and a 31.0%, 31.9% and 31.4% increase in the corresponding indexes for moderate-salinized soil, respectively. For bulk soil, straw returning increased SOC and TN contents by 20.2% and 20.0% respectively for light-salinized soil, and increased SOC content by 35.2% for moderate-salinized soil. For SOC content in soil water-stable aggregates, straw returning for light-salinized soil increased SOC content by 22.7%, 29.1% and 21.9% in <0.053 mm, 0.053-0.25 mm and >2 mm aggregates, respectively, while increased that by 15.8%, 16.1%, 56.1% and 36.8% respectively for moderate-salinized soil with increasing particle size of soil aggregates. For SOC distribution in soil aggregates, the proportion of SOC in <0.053 mm aggregates decreased by 18.3% and 21.5% for light- and moderate-salinized soils, respectively, after 3-year straw returning; 0.053-0.25 mm aggregates of light-salinized soil had the highest increase (by 33.2%) in the contribution rate to SOC, while 0.25-2 mm aggregates of moderate-salinized soil had the highest increase (by 58.7%) in that among all aggregates. For TN content in soil water-stable aggregates, straw returning for light-salinized soil induced a 18.4% and 21.2% decrease in TN content in >2 mm aggregates and 0.25-2 mm aggregates respectively, and led to a 28.8% increase in TN content in 0.053-0.25 mm micro-aggregates. However, straw returning in moderate-salinized soil significantly decreased by 62.1% of the TN content in 0.053-0.25 mm micro-aggregates and increased by 1.1 times that in <0.053 mm aggregates. For TN distribution in soil aggregates, the contribution rate of <0.053 mm aggregates to TN decreased by 43.9% and that of 0.053-0.25 mm aggregates increased by 33.6% for light-salinized soil, while the contribution rate of <0.053 mm aggregates to TN increased by 0.9 times and that of 0.053-0.25 mm aggregates decreased by 50.9% for moderate-salinized soil. Overall, straw returning significantly improved the soil aggregate stability under light- and moderate-salinized soils, but it showed different effects on soil aggregate C, N distribution in different salinized soils, which might be attributed to the distinct formation mechanisms of soil aggregate under different salt contents.