Effects of subsurface organic ameliorant combined with film mulching on saline soil organic and inorganic carbon in Hetao Irrigation District

Soil organic carbon (SOC) and inorganic carbon (SIC) are important carbon pools involved in the global carbon cycle. Subsurface (10−30 cm) organic ameliorant (OM) combined with film mulching (PM) is an effective measure to optimize the physical structure and regulate water and salt movement of saline soil in arid areas. However, the distribution of SOC and SIC in the 0–60 cm soil layer and their relationship with soil physicochemical properties remain unclear. This study was based on a 6-year micro-field experiment of saline soil at the Yichang Experiment Station, which is located in the Hetao irrigation area of Inner Mongolia. Four treatments were set: conventional control (CK), OM, PM, and OM+PM. The levels of SOC, SIC, total carbon (TC), and soil physicochemical property indexes (soil moisture, salinity, pH, and total nitrogen) in the 0−60 cm (0−20 cm, 20−40 cm, and 40−60 cm) soil layer after the harvest of Helianthus annuus during 2019–2020 were measured, and the variation characteristics and influencing factors of TC, SOC, and SIC were analyzed. The results showed that the TC content in the 0−60 cm soil layer and SOC in the 0−40 cm soil layer were mainly affected by OM treatment compared with PM treatment (P<0.01). The SIC content in the 0−40 cm soil layer was affected by OM treatment (P<0.001), PM treatment (P<0.05, except for the 20−40 cm soil layer in 2019), and their interaction (P<0.001); however, the 40−60 cm soil layer was mainly affected by OM treatment (P<0.05). Compared to CK and PM treatments, OM and OM+PM treatments significantly increased SOC content in the 0−40 cm (0−20 cm and 20−40 cm) soil layer by 31.9%−195.6% (P<0.05), and significantly increased SOC content in the 40−60 cm soil layer by 33.7%−49.4% (P<0.05) only in 2020, but significantly decreased SIC content in the 0−40 cm (0−20 cm and 20−40 cm) by 9.9%−35.0% (P<0.05). Based on the changes in SOC and SIC, compared with CK treatment, OM+PM treatment significantly increased TC content in the 20−60 cm (20−40 cm and 40−60 cm) soil layer in 2019 by 10.4%−39.4% (P<0.05), and the TC content of the 0−20 cm layer in 2020 was significantly increased by 13.0% (P<0.05). The regression analysis results further indicated that the dominant factor of the total carbon pool changed from SIC to SOC with the OM+PM treatment. The results of redundancy analysis showed that soil physicochemical properties were the main factors affecting soil TC, SOC, and SIC (explaining 60.7%−91.9% of the variation), and total nitrogen and pH were the main factors affecting soil TC, SOC, and SIC in the 0−40 cm layer, whereas soil TC, SOC, and SIC in the 40−60 cm layer were mainly affected by salinity and pH. Correlation analysis showed that changes in SOC and SIC were completely opposite. Soil organic carbon was positively correlated with total nitrogen and negatively correlated with salinity and pH (P<0.01). Soil inorganic carbon was negatively correlated with total nitrogen and positively correlated with pH (P<0.01). Therefore, OM combined with PM (OM+PM) could compensate for the loss of SIC and realize carbon accumulation by increasing SOC, which is an effective strategy to increase the carbon sequestration potential of saline soil in this region.