Effects of different returning farmland to forestlands on accumulation of soil organic carbon and nitrogen in typical karst area of southwestern China

To investigate the influences of different returning farmland to forestlands on accumulation of soil organic carbon and total nitrogen in karst area, soil organic carbon (SOC) and total nitrogen (TN) concentrations and density under five different land use types, including typical slope cropland (Ⅰ), abandoned cropland (Ⅱ), returned farmland to plantations of Dodonaea viscose (Ⅲ), Vernicia fordii (Ⅳ) and Ailanthus altissima (Ⅴ), were studied in Huajiang Gorge Area of Guizhou Province of southwestern China. The results showed that: 1) SOC and TN concentrations in the surface layer inⅣ andⅤhad no significant differences (P 0.05). However, they were significantly higher than those in Ⅰ,ⅡandⅢ(P 0.01); 2) SOC and TN concentrations and density decreased with the increasing of soil depth. The percentages of SOC and TN density in 0 -20 cm layer with regard to the total profile were 36.84% - 46.01% and 35.63% - 44.50%, respectively. This phenomenon showed obvious table cohesion; 3) Soil C/ N ratio was between 9.79 -13.59, generally at low levels. Meanwhile, C/ N ratio decreased with the increasing of soil depth among all the land use types except slope farmland; 4) In all the five land use types, SOC and TN concentrations showed extremely significant positive correlations(P 0.01) with alkaline hydrolysis nitrogen, total phosphorus and available potassium, while negative correlations with pH. Vegetation types and human activities were the key factors affecting SOC and TN concentrations. SOC and TN concentrations and density both increased after returning farmland to forest and grassland, which were beneficial to the accumulation of soil carbon and nitrogen pool.