Soil organic carbon stocks in soil aggregates under different land use systems in Nepal

We investigated the soil organic carbon (SOC) associated with various aggregate size fractions in soil profiles under different land uses. Bulk soil samples were collected from incremental soil depths (0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm) from sites with the four dominant land use types [forest, grazing land, irrigated rice in level terraces (Khet) and upland maize-millet in sloping terraces (Bari)] of the Mardi watershed (area 144 km(2)), Nepal. The bulk soil was separated into five aggregate size fractions and the associated SOC contents were determined. Soil physical parameters necessary for estimating the soil SOC stock such as bulk density, stone and gravel content, and SOC content, were also measured for each soil depth. The SOC stock (mean+/-SE, kg C m(-2)) in the topsoil (0-10 cm) was higher in grazing land soil (3.4+/-0.1) compared to forest soil (1.4+/-0.2) and cultivated soil [Bari(2.0+/-0.2) and Khet (1.2+/-0.2)]. Forest and grazing lands had similar SOC contents, but the higher content of gravel and stone in forest soil resulted in a lower estimate of the SOC stock per unit area. The total SOC stock in the soil profile (to 1 m depth) over the entire watershed was estimated to be 721470 TC (tonnes of carbon). Its distribution was 52, 30, 11 and 7% in forestland, Bari, grazing land and Khet, respectively. The estimated depth wise distribution of SOC stock for 1 m soil depth in the entire watershed was 28, 22, 28 and 22% in the 0-10, 10-20, 20-40, and >40 cm soil depths, respectively. There was a net loss of SOC stock (0-40 cm soil depth) of 29%, due to internal trading of land uses in the period from 1978 to 1996. Macro aggregates (>1 mm) were found to be the dominant size in Bari and grazing land, whereas in forest and Khet soil micro aggregates (<1 mm) dominated. Micro aggregates of size <0.25 mm had a higher SOC concentration than aggregates of 0.25-0.5 mm, regardless of the depth or land uses and they may therefore contribute to soil C sequestration.