Plant carbon inputs drive the stratification of soil organic carbon, microbial biomass and mineralizable nitrogen after reducing soil tillage

The impact of tillage on the stocks of soil organic carbon (SOC), soil total nitrogen (STN), microbial biomass carbon (MBC), and on nitrogen mineralization rates (NMR) remains a subject of debate. In a field experiment with loamy soil and a temperate climate, we compared three cropping systems and two tillage treatments (conventional and shallow tillage). Bulk density, SOC, STN and MBC were measured in three soil layers (0–6, 6–20 and 20–30 cm) and at four dates (1.5, 3.5, 5.5 and 7.5 years after the shallow tillage was implemented). NMR was measured in laboratory incubations at two dates (1.5 and 7.5 years). SOC stocks under shallow tillage increased at a rate of 0.25 t ha −1 yr −1 in the upper soil layer, but decreased at a rate of −0.20 t ha −1 yr −1 in the deeper layer, whereas stocks under conventional tillage remained almost stable in the three layers. The same pattern was observed for STN and MBC. Similarly, NMR under shallow tillage increased significantly in the upper layer (+35%), while decreasing in the deeper layer (−25%). However, neither cropping systems nor tillage treatments affected the total SOC, STN and MBC stocks or NMR when integrated over the entire profile (∼0–30 cm). The rates of change in SOC, STN, MBC and NMR in the three soil layers were highly correlated with estimated plant carbon inputs based on crop yields, but not with tillage intensity. These results challenge the hypothesis of a higher physical protection of the soil organic matter with reduced tillage.