Impact of soil management on organic carbon content and aggregate stability

Soil cultivation influences organic carbon storage and soil structures. To evaluate the impact of different soil-management practices on soil organic carbon (SOC) pools and aggregate stability in black soils, SOC in whole soil, various sizeaggregates, and density-separated fractions from three long-term experiments (20 years) was examined. The three soil-management systems were grassland (GL), bare land (BL), and croplands. The croplands had two treatments: nitrogen and phosphorus fertilizer application (NP) and NP together with organic manure (NPM). The SOC in the 0- to 10-cm layer decreased in the order NPM > GL > NP > BL and also declined with the soil depth. The SOC of GL increased by 9.7% as compared to NP after 20 years of natural vegetation restoration. The SOC of NPM increased by 11% over NP after 13 years of organic manure application. The percentages of water-stable aggregate (> 0.25 mm) (WSA(> 0.25mm)) decreased in the order GL > BL > NPM > NP in the top 0- to 20-cm horizon. WSA(> 2mm) the most important fraction for carbon (C) storage in GL and NPM, accounted for 33 and 45% of the whole soil for GL in the depths of 0- 10 and 10-20 em, respectively, and 25 and 18% for NPM in the same soil layers. A significant positive correlation was found between the C stored in WSA(> 2mm) and total SOC (r = 0.8 1, P < 0.05) and between the mean weight diameters (MWD) of aggregates and total SOC (r = 0.78, P < 0.05). Water-stable aggregate(0.25-2mm) was the largest fraction of WSA(> 0.25) , ranging from 54 to 72% for the 0- to10-cm layer and 46 to 71 % for the 10- to 20-cm layer; thus these aggregates would play a major role in soil sustainability as well as the resistance to soil erosion. The organic carbon (OC) of heavy fraction (HF) accounted for 94-99% of the SOC in the WSA(0.25-2mm), whereas free particulate organic matter (fPOM) and occluded particulate organic matter (oPOM) contributed a minor fraction of the OC in the WSA(0.25-2mm), suggesting that C sequestration in HF could enhance the stability of aggregates and C pools in black soil.