Soil profile carbon and nutrient stocks under long-term conventional and organic crop and alfalfa-crop rotations and re-established grassland

Soil carbon stocks are useful indicators of both C sequestration capacity and sustainability of agricultural systems. Yet, most investigations have only studied the effects of agricultural management on soil carbon in surface layers (< 0.3 m). Current soil organic carbon (SOC), total soil nitrogen (TN) and plant available phosphorus (POlsen) to a depth of 1.2 m was measured at two long-term (9 and 18 years) farming systems experiments in southern Manitoba, Canada. Both experiments compared an annual-crop rotation, an alfalfa (Medicago sativa L.)/crop rotation and re-established perennial grassland. At one site the two cropping systems were managed conventionally as well as in adherence to organic farming guidelines, but without manure additions. Due to higher net primary productivity and higher carbon inputs, particularly below ground, SOC stocks (0–120 cm) were 21–65 t C ha−1 higher under the re-established grassland than cropping systems at the clay soil site after 18 years, but not at the site with sandy loam soil after 9 years. On the clay soil, 30–40% of the additional C in the soil profile under the re-established grassland was found below 30 cm indicating the capacity of deep plant roots to sequester C in the sub-soil. Using alfalfa cut for hay in crop rotations did not increase SOC or N stocks compared to annual crop rotations, but plant-available P concentrations were depleted, especially under organic management. SOC was 25–30 t C ha−1 lower under organic than conventionally managed cropping systems, due to lower inputs of plant C (0.8 t C ha−1 yr−1) over the life of the experiment. This highlights that without additional C inputs organic management can reduce SOC compared to conventional cropping systems unless C inputs are maintained which may require manure or compost additions.