Impacts of Land Use and Salinization on Soil Inorganic and Organic Carbon in the Middle-lower Yellow River Delta

Soil inorganic carbon (SIC) is an important reservoir of carbon in arid, semi-arid and semi-humid regions. However, knowledge is incomplete on the dynamics of SIC and its relationship with soil organic carbon (SOC) under different land uses in the semi-humid region, particularly in coastal zones impacted by soil salinization. We collected 170 soil samples from 34 profiles across various land use types (maize-wheat, cotton, paddy and reed) in the middle-lower Yellow River Delta (YRD), China. We measured soil pH, electrical conductivity (EC), water-soluble salts, and SOC and SIC contents. Our results showed significant differences in both SOC and SIC among land uses. The dry cropland (maize-wheat and cotton soils) had significantly higher SOC and SIC densities (4.71 and 15.46 kg C m−2, respectively) than the paddy fields (3.28 and 14.09 kg C m−2, respectively) in the 0-100 cm layer. Comparing with paddy soils, reed soils contained significantly higher SOC (4.68 kg C m−2) and similar SIC (15.02 kg C m−2). There was a significantly positive correlation between SOC and SIC densities over 0-100 cm soil depth in dry cropland, but a negative relationship in the paddy field. On average, SOC and SIC densities under maize-wheat cropping were 15% and 4% lower, respectively, in the salt-affected soils of the middle-lower YRD than that in the upper YRD. This study indicated that land use types had great influences on both SOC and SIC and the SIC-SOC relationship, and salinization had adverse effects on soil carbon storage in the YRD.