Long-Term Manure Application Increases Organic and Inorganic Carbon Accumulation in Saline-Sodic Soil

Organic manure application is considered an effective strategy for enhancing soil fertility and increasing carbon sequestration in saline-sodic soil. The contribution of manure-derived soil organic carbon (SOC) under long-term application remains unclear, as do its concurrent effects on SOC and soil inorganic carbon (SIC) pools. Empirical evidence addressing both aspects is scarce. This study was based on a long-term field experiment established in saline-sodic soil, with four manure application durations (14, 19, 25, and 30 years) and an unfertilized control, each with three replicates. Soil samples were collected from a 0–200 cm profile to investigate the vertical distributions of SOC, permanganate-oxidizable organic carbon (POXC), δ13C values, SIC and its carbonate (CO32−) and bicarbonate (HCO3−) fractions. The effects of manure application on the sources of SOC and the fractions of SIC were investigated using one-way analysis of variance (ANOVA) and the widely applied binary equations. The results revealed that long-term manure application significantly increased the SOC, POXC, SIC, and HCO3− contents, whereas the CO32− content decreased. The effects were most notable within the 0–60 cm soil layer. The δ13C values of SOC increased significantly with increasing fertilization duration, which indicates increased incorporation of exogenous carbon. The results of a binary mixing model revealed that after 30 years of manure application, manure-derived carbon contributed 31.64% (7.44 g kg−1) to SOC, maize-derived carbon contributed 22.42% (5.27 g kg−1) to SOC, and the remaining fraction originated from native SOC. Vertically, the SOC content decreased with soil depth, whereas the SIC content fluctuated and became dominant below 60 cm. Long-term manure application altered both SOC sources and SIC dynamics, thereby promoting the accumulation and redistribution of carbon within the soil profile. Overall, long-term manure application increased both organic and inorganic carbon content. These findings highlight the importance of organic fertilization as a sustainable management practice for increasing soil carbon storage and mitigating land degradation in saline-sodic agroecosystems.