Effects of tillage and straw mulching on soil total nitrogen and its components in north China plain

Abstract: Growing concerns about environmental conservation and sustainable intensification have raised interests in enhancing soil nitrogen pools. To estimate the effects of different tillage systems on soil nitrogen pools, a field experiment initiated from 2008 with 4 tillage systems (plow tillage without residue retention, PT; plow tillage with residue retention, PTS; no-till with residue retention, NTS; rotary tillage with residue retention, RTS) was conducted in a winter wheat - summer maize double cropping system at China Agricultural Experimental Station in Wuqiao on the North China Plain (NCP). After 7 years of experiment, soil total nitrogen (TN), particulate organic matter nitrogen (POM-N) and mineral-associated organic matter nitrogen (MAOM-N) were determined from 5 soil depths increments. This study observed that concentrations of TN under NTS and RTS were significantly higher than that under PTS in the 0-5 and 5-10 cm soil layers (25.05% and 23.39% higher in the 0-5 cm layer, and 8.16% and 19.60% higher in the 5-10 cm layer, respectively), but trends reversed in the 10-20 and 20-30 cm soil layers (7.72% and 6.66% lower in the 10-20 cm layer, and 26.76% and 27.20% lower in the 20-30 cm layer, respectively). Furthermore, no significant differences in total nitrogen stocks of either 0-30 or 0-50 cm soil layer were observed among tillage treatments with residue retention, but all of them were higher than that of PT treatment. POM-N showed the highest sensitivity among soil total nitrogen and its fractions. Concentration of POM-N under NTS increased by 51.79% in the 0-5 cm topsoil when compared with PTS, but decreased by 18.35% and 53.11% in the 10-20 and 20-30 cm subsoils, respectively. The proportion of POM-N in TN under NTS was also higher than that under PTS in the 0-5 cm layer, but lower in the 5-10 and 20-30 cm layers. Similarly, RTS increased both POM-N concentration and the proportion of POM-N in TN in the 0-5 cm layer when compared with PTS, but decreased the two in the 20-30 cm layer. However, when compared with PT, treatments with residue retention increased both POM-N concentrations and the proportions of POM-N in TN in the upper layers of soil. Concentration of MAOM-N showed a trend similar to TN concentration on the conditions with residue retention. As a result of the accumulation of soil organic materials in the topsoil, NTS and RTS showed relatively higher values of stratification ratio of soil total nitrogen and its fractions as compared with PTS and PT. Compared with PT, NTS and RTS significantly increased concentrations of TN, POM-N and MAOM-N in the 0-10 cm soil layer synchronously. However, PTS only increased TN and POM-N concentrations in the 0-20 cm soil layer compared with PT. In addition, a significantly positive correlation between variation of POM-N and variation of MAOM-N has been observed under NTS and RTS, but not under PTS, when compared with PT. When adopting plow tillage, soil nitrogen increased by residue retention was mainly distributed with the particulate fractions, rarely transferred to the mineral-associated fractions, which might accelerate the depletion of soil nitrogen, and then increase environmental risks.