Depth of straw incorporation significantly alters crop yield, soil organic carbon and total nitrogen in the North China Plain

Appropriate straw incorporation has ample agronomic and environmental benefits in the North China Plain, but most studies are limited to straw mulching or incorporation on the soil surface. To determine the effect of depth of straw incorporation on crop yield, soil organic carbon (SOC) and total nitrogen (TN), a 3-year field experiment was established with the following four treatments: (i) no straw incorporation (CK); (ii) straw incorporation at 15 cm (S15); (iii) straw incorporation at 25 cm (S25) and (iv) straw incorporation at 40 cm (S40). The results showed that straw incorporation significantly increased SOC, TN and C:N ratio (P < 0.05). Compared with CK, substantial increases (P < 0.05) in SOC and TN were observed at the 0-20 cm layer under S15, at all depths under S25 and at the 40-60 cm depth under S40. Under S25 (20-40 cm) in particular, the SOC and TN levels considerably (P < 0.05) exceeded those under CK (8.83˜17.26% of SOC and 6.28˜10.25% of TN), S15 (3.66˜9.25% of SOC and 2.13˜4.70% of TN) and S40 (4.74˜8.23% of SOC and 3.21%˜4.27% of TN). The average SOC and TN at 0-60 cm under S25 was also significantly higher (P < 0.05) than those under S15 and S40. Straw incorporation also increased the C:N ratio and the response of C:N ratio to straw incorporation differed by the depth of straw incorporation and the soil layer. Compared with S15 and S40, S25 had a significant effect on the C:N ratio at each soil layer. Both S25 and S40 decreased the stratification ratio (SR) of SOC, TN and C:N ratio and promoted a uniform distribution of SOC and TN within the 0-60 cm depth. Compared with CK, straw incorporation also significantly increased (P < 0.05) crop yields, especially under S25, by 10.83% in 2014, 13.25% in 2015 and 7.68% in 2016. The grain yields of wheat and maize were positively correlated with SOC and TN (0-20 cm and 20-40 cm). Thus, in the North China Plain, straw incorporation at the 25 cm depth would boost soil fertility and crop yield.