Differences of soil organic carbon storage in surface soil aggregates between sloping farmland and sloping forestland in black soil region.

A typical sloping field in the black soil region of northeastern China was selected to analyze characteristics of soil aggregate composition and soil organic carbon of different size fractions under three different soil surface conditions, namely, sloping farmland, sloping forestland of larch (Larix gmelinii) converted from cultivated land, and sloping forestland of Mongolian pine (Pinus sylvestris var. mongolica). The sampling sites were situated at upper slope, upper to middle slope, middle slope, middle to lower slope, and lower slope positions. After air drying, the soil samples were sieved to 2-5 mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm and 0.25 mm size classes and then their proportions were calculated. Soil organic carbon content was measured by vario TOC instrument manufactured by Elementar Company, and then the soil organic carbon storage of each aggregate size class and total aggregates was calculated. Results showed that: the transportation and deposition processes of soil aggregates in farmland were different from those in forestland mainly because of tillage effects. Compared with forestland, the proportion of soil aggregates 2 mm in farmland was significantly higher, especially in the lower slope position with a proportion of 70.30%. For all three types of sloping lands, soil organic carbon content of aggregates increased with their diameter decreasing. The most pronounced difference of soil organic carbon content was found in Mongolian pine forestland (25.57-142.60 g/kg), which remained highest in all positions compared with sloping farmland (22.58-30.06 g/kg) and sloping larch forestland (21.58-66.53 g/kg). Soil organic carbon storage showed a trend of increasing from upper to lower slope position in forestland; in the same slope position, it was the highest in the Mongolian pine forestland, followed by larch forestland, and sloping farmland in the last place. The results confirm the existence of tillage erosion in sloping land of this area as well as its effects on the redistribution of soil organic carbon. It also suggests that converting cultivated land to forest plays an important role as a type of measure to reverse ecological destruction and maintain higher productivity of black soil in the long run.