Influence of agricultural management practices on soil organic carbon stock and distribution in topsoil and subsoil as revealed by a mid-term trial

Cover crops and no–tillage are agricultural practices used to improve soil organic carbon (OC) sequestration, although most studies are often limited to examining only the topsoil. In this study the influence of different cropping systems (CONV – integrated management without cover crop and conventional tillage, ORG – organic management with cover and temporary intercropping crops and conventional tillage, and NOTILL – integrated management with cover crops and no–tillage) was evaluated on OC quantity and distribution in topsoil (0–20 cm), midsoil (20–40 cm), and subsoil (40–60 cm) of a 10–year wheat–maize rotation trial. A physical– chemical fractionation was performed to isolate OC among labile (water soluble and particulate OC, WEOC and POC, respectively), stable (OC in sand–size and silt– and clay–size aggregates, SSA and SCA, respectively), and resistant (NaClO oxidation) pools. Further, soil samples were characterised for 13C and 15N natural abundance, phenols and glomalin (GRSP) contents, and microbial activity. The soil OC stock in the 0–60 cm depth was similar for CONV, ORG and NOTILL. This was attributed to enhanced mineralisation processes promoted by the addition of N–rich fresh legume cover crop residues in ORG and NOTILL soils that exceeded the rate of OC stabilisation. However, specific contributions of the functional OC pools to the total stock and along the soil depth intervals occurred. For ORG and NOTILL, the implementation with cover crops favoured the development of a stable macrostructure and the accumulation of OC in SSAs, whereas CONV system mainly accumulated OC in SCAs. When the system was implemented with both cover crops and no–tillage, as for NOTILL, almost half of the total soil OC stock was stored in the topsoil, mostly as POC and associated with SSAs. Regardless of agricultural management, 53–68 % of the total OC stock was found in the layers below the topsoil.