Carbon sequestration under different organic amendments in saline-alkaline soils

Organic amendments are a recyclable resource and can improve soil quality and carbon (C) sequestration, however, the difference in their effects on soil aggregate formation and C sequestration in saline-alkaline soils are less studied. We examined the effects of organic amendments to saline rice paddy fields with 4 treatments: (i) basal paddy soil (CK) lacking amendments (ii) organic fertilizer (OF) treatment at 910 kg C ha−1 from soybean litter and beans (iii) rice straw (RS) treatment at 3600 kg C ha−1 and (iv) rice straw-derived biochar (RS-biochar) treatment at 3600 kg C ha−1. All organic amendments significantly increased the soil organic C (SOC) stock in the 0–30 cm soil layer compared to CK. The SOC stock in the biochar treatment increased by 6.2 and 40.4% over the OF and RS amended treatments, respectively. This was most likely due to the greater levels of aromatic C in RS-biochar at 1.22 and 1.24 times greater than for the other organic amendments, respectively. Soil aggregates of 2–8 mm at 0–15 cm in the OF and RS amended soils were 38.3 and 58.2%, respectively, higher than that from the RS-biochar treatment. The mean weight diameter (MWD) in the OF and RS treatments were 35.4% and 45.8% higher than for the RS-biochar, respectively. MWD was significantly negatively correlated with soil exchangeable Na (p < 0.05). This indicated OF and RS were better than RS-biochar in improving soil aggregate stability. In addition, the C management index values for RS were 21.6 and 45.8% greater than for OF and RS-biochar in the 0–15 cm soil layer, respectively. These data indicated that saline soil aggregate formation and C sequestration improved after organic amendments application, which caused the exchangeable Na to decrease and affected its distribution in aggregates, and changed the composition of C functional groups.