To reveal the effect of irrigation with sediment laden water from the Yellow River on soil organic carbon (SOC) distribution in profile (0-100 cm), Ningxia Irrigation Zone was selected as the study area to carry out the research. Totally, 40 soil profiles of five soil types (Irrigation-silted soils, Fluvo-aquic soils, Sierozem soil, Aeolian sandy soil and Alluvial soils) were collected, including 34 cropland profiles with the irrigated durations ranged from 10 to 2200 years and 6 natural soil profiles never irrigated and cultivated, to analyze profile distribution patterns of SOC content and density under different durations of irrigation and cultivation. The natural soil profiles were used as control profiles and their irrigation duration were set to zero. Pearson correlation coefficients were calculated to determine relationships between SOC density and irrigation durations and soil types. SOC content and density were subjected to analysis of variance (ANOVA) to test difference of irrigation effects from irrigation years and soil types at 5% probability level, respectively. SOC sequestration rate was also estimated by comparing irrigated soils with control ones.
Results indicated that SOC content of irrigated cropland showed a decreasing trend with the deepening of soil layers in profile. SOC content of surface layer (0-20 cm) affected mainly by irrigation and cultivation increased more rapidly than those of deeper layers affected mainly by soil parent material. When the irrigated duration was about 50 years, the SOC content showed the most significant difference among the five soil types. The irrigation-silted soil layer formed by irrigation and cultivation activities, usually thicker than 50 cm, were found in those areas with such activities of 50 years over. The depth influenced by irrigation and the increment of SOC density both are different due to the difference of irrigation durations. SOC density was significantly related with irrigation durations and soil types, the correlation coefficients were 0.63 (P < 0.01) and 0.74 (P < 0.01), respectively. SOC density in the profile depth were significant different (P < 0.05) among varied irrigation durations and soil types. SOC density increment was significantly and positively correlated with irrigation durations shorter than 50 years. On average, the SOC density increment of the cropland soils in depth of 0-20, 20-30, 30-60 and 60-100 cm were 14.69, 4.52, 9.84 and 9.52 Mg/hm² more than the corresponding soil layers of their control profiles. SOC density increment of irrigation-silted soils formed under the long-term alternate activities of irrigation, manure addition, silting and cultivation, was more than other four soil types in Ningxia Irrigation Zone.
The linear relationships between SOC density increments and irrigation durations inferred that average SOC sequestration rate reached 0.53 MgC · hm^-2 · a^-1 from the five soil types in Ningxia Irrigation Zone. The SOC sequestration rate varied among soil types.
Results discussion indicated, by stimulating plant and root growth as well as soil biotic activity,irrigation could potentially enhance SOC sequestration of cultivated land in the arid and semi-arid area. In addition, irrigation with sediment laden Yellow River water via sediment with organic matter silting, make the irrigation-silted soil layer thicker at the top of profile, and SOC increased accordingly. Overall, irrigation by the Yellow River water plays an important role in increasing SOC density.