Effects of vegetation on soil aggregate stability and organic carbon sequestration in the Ningxia Loess Hilly Region of northwest China

Abstract

A close correlation exists between soil aggregation and soil organic carbon sequestration. Soil organic matter is the main material needed in the formation of soil aggregates and soil aggregates provide a place for soil organic matter to sequester carbon. The effects of different revegetation types on the aggregation of soil particles, the stability of soil aggregates, as well as carbon and fractions in soil aggregates were studied in a loess hilly-gully area; soil organic carbon and fraction's micro-response to different revegetation types are discussed. The results show the distribution of soil aggregates has a "V" distribution under a variety of revegetation and restoration measures. Small (< 0. 25 mm) and large (> 5. 0 mm) particle sizes form the most aggregates. The next largest aggregate size classes were 2-5 mm and 0. 25-1. 0 mm, while the 1-2 mm size class had the fewest aggregates. The mean weight diameter (MWD) of cropland is approximately 1.4. After land is revegetated the MWD increased to 1. 9-3. 1 in the soil indicating soil aggregate stability and porosity increased significantly. The aggregate stability of an area of natural grassland and also in a stand of 25-year-old Caragana korshinskii Kom. was higher than aggregate stability in a stand of 15-year-old C. korshinskii Kom. We measured various soil conditions under different revegetation types and found differences between the 0-20 cm and 20-40 cm soil layers. The soil organic carbon content was 7. 4 and 17. 7 g/kg; the microbial carbon content was 50. 3 and 664. 7 mg/kg; the carbon content of humus was 0. 9 and 2. 5 g/kg; the content of humic acid carbon was 0. 2 and 0. 6 g/kg; and the fulvic acid was 0. 6 and 1. 9 g/kg for the 0-20 cm and 20-40 cm soil layers, respectively. The organic carbon and the fraction in 0-20 cm soil layer were higher than measurements in 20-40 cm soil layer. The content of organic carbon, microbial carbon, humus carbon and fulvic acid carbon in the soil are the lowest in cropland; these contents under other revegetation types were 1. 1-2. 3 times, 2. 0-8. 4 times, 1. 0-2. 0 times and 1. 2-2. 4 times higher than those of cropland soils, respectively. Most of the soil organic carbon content was concentrated on intermediate sized aggregates (2-5 mm, 1-2 mm, 0. 25-1. 0 mm). The result of stepwise regression analysis show the higher the content of soil organic carbon was in the small and large particles (0. 25-1. 0 mm and 2-5 mm diameter aggregate, respectively) the more stable the water-stable aggregates were. The findings suggest vegetation restoration improved the soil organic carbon aggregate content, and in terms of forms of soil carbon, soil microbial biomass carbon and fulvic acid carbon, their content in the soil was sensitive to changes in aggregate size, while humic acid carbon was stable regardless of aggregate size.