Irrigation with sediment-laden river water affects the soil texture and composition of organic matter fractions in arid and semi-arid areas of Northwest China

Soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial ecosystem, and is an effective mean of enhancing crop production in the irrigated area of the arid and semi-arid regions. Understanding long-term changes in composition of SOM under irrigation from sediment-laden Yellow River water is essential to manage sustainability issues in the agro-ecosystem. A total of 45 soils, including 39 of irrigated fields differing in irrigation history and 6 of non-cultivated and non-irrigated natural fields as the control, were sampled at 0-20 cm depth. The soil was analyzed for total SOC content, partitioning of light (LFOM) and heavy (HFOM) fraction organic matter based on NaI solution density of 1.7 g m3 and for particle size distribution. Compared to the non-irrigated and non-cultivated control soil, the LFOM and HFOM in irrigated soils increased with the duration of irrigation though SOM existed dominantly as the heavy fraction. The soils irrigated for <50 years have overall lesser LFOM and HFOM and more sand compared to those with >50 years irrigation. Further, a positive relationship existed between the fine particle and the SOM or its fractions (negative relation with coarse particle) suggesting either SOM accumulated as fine particles or the fine mineral particles better preserved SOM. In addition, field soil moisture at the time of sampling during October 2009 correlated with HFOM (p < 0.001) and LFOM (p < 0.01). The study suggested that the long-term irrigation with water diverted from Yellow River increased fine particle, SOC and the light and heavy fraction in the Ningxia Irrigation Zone.