Effects of straw incorporation on the stratification of the soil organic C, total N and C:N ratio in a semiarid region of China

The soil degradation caused by conventional tillage in rain-fed areas of northwest China is known to reduce crop yields because of major losses of soil organic carbon and nutrients. To evaluate the effects of straw incorporation on soil organic carbon (SOC) and total nitrogen (STN) sequestration capacity in loessial soil, we investigated the effects of straw incorporation on SOC, STN and crop yield in semiarid areas of southern Ningxia for a 4-year period (2007–2010). Four treatments were tested: (i) no straw incorporation (NA); (ii) incorporation of maize straw at a low rate of 4.5 Mg ha−1 yr−1 (LA); (iii) incorporation of maize straw at a medium rate of 9.0 Mg ha−1 yr−1 (MA); and (iv) incorporation of maize straw at a high rate of 13.5 Mg ha−1 yr−1 (HA). In the final year (2010), the results showed that the mean soil bulk density in the 0–60 cm depth had decreased with high, middle, and low straw incorporation rate treatment compared with no straw incorporation treatment (NA) by 3.7% (P < 0.05), 3.4% (P < 0.05), and 2.0%, respectively. Straw incorporation increased the concentration and storage of SOC and STN, which were ranked in order of increasing straw incorporation: high/middle straw incorporation rate treatment > low straw incorporation rate treatment > no straw incorporation treatment. The mean soil C:N ratio was significantly higher with straw incorporation, i.e., 6.9% higher than no straw incorporation treatment. Straw incorporation significantly (P < 0.05) increased the stratification ratio of SOC, STN, and soil C:N ratio from the surface (0–10 cm) to all depths compared with NA, i.e., the stratification ratio of SOC at the 0–10:20–40 cm depth increased with HA, MA and LA by 11.3% (P < 0.05), 10.7% (P < 0.05), and 4.4%, respectively, compared with no straw incorporation treatment (NA).