Distribution of soil particulate organic carbon fractions as affected by tillage practices in dry farmland of the Loess Plateau of central Gansu Province

As a vital indicator of soil quality, soil organic carbon and its fractions play an essential role in soil productive capacity and crop yield, while may be affected by soil tillage methods in dry farmland areas. Organic carbon is a key component of soil because it carries many functions in agro-ecosystem. A study was carried out to investigate the effects of different tillage and straw application patterns on the distribution of soil particulate organic carbon fractions under spring wheat-pea rotation by using the density fraction methodNaI:(1.70±0.02) g·cm^-3. Four particulate fractions of soil total organic carbon (STOC), free particulate organic carbon (FPOC), occluded particulate organic carbon (OPOC), particulate organic carbon (POC) and mineral-associated organic carbon (MOC) were obtained. The study involved a 17-year local field experiment at the Rainfed Agricultural Experimental Station of Gansu Agricultural University, Dingxi, Gansu Province, China (35°28'N, 104°44'E). The experiment included four treatments, which were conventional tillage (T), no-tillage (NT), no-tillage with straw incorporation (NTS) and conventional tillage with straw mulching (TS) arranged in a complete randomized block design with three replications. The soil samples were taken at four different soil depths (0-5 cm, 5-10 cm, 10-20 cm and 20-40 cm) per plot. The results showed that the dominant fraction of STOC for each soil layer was POC (the ratio range was 54.02%-76.78%) in four treatments, and the main component of POC was OPOC, suggesting that the effect of physical protection was the crucial role for soil carbon sequestration and fixation in the area. The contents of STOC, FPOC, OPOC and POC were decreased with increasing soil layers, MOC content, however, was increased with increasing soil layers. FPOC/STOC, OPOC/STOC and POC/STOC were decreased with increasing soil layer, MOC/STOC was increased with increasing soil layers. In 0-40 cm soil depth, compared with treatment T, the mean values of STOC, FPOC, OPOC, POC and MOC in NT, TS and NTS treatments were greater, and NTS treatment exhibited the greatest effect. The same trend was represented for FPOC/STOC and POC/STOC. No tillage system represented significantly enhance effect on contents of FPOC, OPOC, POC and MOC in 0-20 cm soil depths, but the straw retention system showed significantly boosting effect on contents of STOC, FPOC, OPOC, POC and MOC in 0-40 cm soil depths, and F test values of straw retention were greater than that of tillage system, thereby the effects of straw retention were greater than that of tillage system. As a whole, NTS may be an ideal enhancer of farmland productivity in the semi-arid soil ecosystem through enhancing soil organic carbon pool which resulted in the maintenance of higher nutrient content, and subsequently helping in contributing sustainable agricultural development in the Loess Plateau of central Gansu Province.