Farm Table says:
What is the problem?
Soil carbon stocks are useful indicators of both carbon sequestration capacity and sustainability of agricultural systems. Yet, most investigations have only studied the effects of agricultural management on soil carbon in surface layers.
What did the research involve?
In the two long-term (9 and 18 years) farming systems experiments in southern Manitoba, Canada, current soil organic carbon (SOC), total soil nitrogen (TN) and plant available phosphorus to a depth of 1.2m was measure.
Both experiments compared an annual-crop rotation, and alfalfa/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.
What were the key findings?
Due to higher net primary productivity and higher carbon inputs, particularly below ground, SOC stocks were higher under the re-established grassland than the 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 30cm 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 lower under organic than conventionally managed cropping systems, due to lower inputs of plant Cover the life of the experiment.
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.