Effect of long-term conservation tillage on total organic carbon and readily oxidizable organic carbon in loess soils

The Loess Plateau is characterized by severe soil erosion. Soil organic carbon is an essential element of soil quality, including soil tilth, soil fertility, soil aggregate stability and soil erodability. Soil organic carbon formation, change and depletion are very slow processes. Labile fractions of soil organic carbon are more sensitive to agricultural operations than total organic carbon. Tillage is a main factor affecing soil organic carbon in agricultural soils. Although a lot of work has been done on soil organic carbon on the Loess Plateau, most have actually focused only on total organic carbon. Relatively less research has been done on changes in the fractions of soil organic carbon over the long-term in response to tillage practices. Hence in this paper, the changes in the fractions of soil organic carbon due to long-term tillage practices in loess soils were investigated. Total organic carbon (TOC) and readily oxidizable organic carbon (ROOC) of soils from a 12-year experiment with different tillage practices were analyzed in this paper. The long-term experiment was commenced in 2001 in Dingxi City, Gansu Province, which is a typical semiarid rainfed area in the western Loess Plateau. The experiment included conventional tillage (T) and 5 conservation tillage practices no tillage with straw mulching (NTS), conventional tillage with straw incorporation (TS), no tillage (NT), no tillage with plastic film mulching (NTP) and conventional tillage with plastic film mulching (TP). The results showed that soil TOC content decreased with increasing soil depth, and this was especially true for soil depths within 10~30 cm. At the start of the experiment, differences among treatments were insignificant for soil depths of 0~5 cm, 5~10 cm and 10~30 cm. However, these differences became more significant with time. The order of the average content of TOC and ROOC in the 0~30 cm soil depth for the different treatments was NTS > TS > NTP > NT > T > TP. Compare with T, the average ranges of TOC and ROOC under NT, NTS, NTP and TS increased respectively by 1.2%~7.2% and 5.3%~16.6%. However, those of TP dropped by 4.3% and 2.7%, respectively. Both no tillage and straw mulching increased TOC and ROOC contents, and the combinative use (NTS) provided the optimum conditions for improving soil organic carbon. TP was the worst treatment in terms of soil TOC content. Compared with 2002, the average contents of TOC and ROOC under NTS increased respectively by 9.5% and 42.9%, 13.2% and 67.6%, 21.5% and 71.5%, 1.1% and 15.9%, 2.7% and 12.6% in 2004, 2006, 2008, 2010 and 2012. ROOC was much more sensitive to the tillage practices than TOC. Therefore no tillage used in conjunction with straw returning was critical for improving both TOC and ROOC in loess soils. It was possible to use ROOC as an early indicator for changes in soil organic carbon in loess soil of the western Loess Plateau. However, further research was needed to confirm the reliability of the use of ROOC for thus purpose.