Organic amendments effects on soil aggregation and carbon sequestration in saline‐alkaline croplands in China

Saline-alkaline soils are wide-spread and approximately 6.5% of the world's land area is reported as being saline and sodic. Increasing soil organic carbon (SOC) concentration and aggregate stability in saline-alkaline soils has been challenging due to the deleterious effects of soil salinity and sodicity. To address this issue, a 2-year filed experiment was conducted to evaluate the effects of biochar (BC), farmyard manure (FM), corn straw (CS), and sheep manure (SM) in combination of chemical fertilizer (CK) on soil aggregation and C sequestration in mildly and moderately alkaline soils in Hetao Irrigation District, China. For mildly alkaline soil, the proportion of macroaggregates (>0.25 mm) was significantly higher under FM, CS, and SM (34.6%-51.2%) compared with CK (13.2%). The mean weight diameter (MWD) and geometric mean diameter (GMD) of water-stable aggregates significantly increased under FM, CS, and SM by an average of 172.7% and 58.7% over CK. In addition, FM, CS, and SM enhanced the distribution of organic carbon (OC) in macroaggregates (>0.25 mm) and accelerated the accumulation of particulate organic matter within macroaggregates. Treatment BC enhanced OC distribution in microaggregates (0.053-0.25 mm) and exhibited a 7.40 g kg(-1) SOC content, followed by SM and FM which exhibited SOC contents of 7.10 and 6.76 g kg(-1). For moderately alkaline soil, data obtained in this study showed a slight influence from organic amendments on soil aggregates and C sequestration, suggesting that the moderately alkaline soil was not as sensitive to organic amendments as the mildly alkaline soil. Overall, the results of this study demonstrated that FM and SM combined with chemical fertilizer were the top organic amendments to improve soil aggregation and C sequestration in the mildly alkaline soil but not the moderately alkaline soil over the short term.