In addition to potentially mitigating the greenhouse gas effect via carbon sequestration, building soil organic matter increases soil fertility and reduces nutrient runoff, increases water holding capacity and resistance to drought, and is positively associated with many measures of soil health. Management practices which increase organic inputs to soil, such as the incorporation of cover crops, can build soil organic matter. Recent work has shown that belowground plant inputs contribute more to soil organic carbon content than aboveground inputs. Using in situ pulse labeling of a cereal rye cover crop we showed that rhizodeposits contributed 30% of soil carbon inputs in a corn bioenergy system. We therefore predicted that different cover crop species with varying root profiles and root to shoot biomass ratios would have different effects on soil organic matter. To determine whether cover crops from different plant functional groups vary in their effect on soil organic matter, we performed a meta-analysis with 384 comparisons.
We found that while cover crop functional group was a significant predictor of soil organic matter, other management factors were more important such as the functional group of the cash crop, nitrogen fertilization practice, and tillage practice. Across all studies, soils in the cover crop treatment showed an 11% increase in the soil carbon pool. The greatest cover crop effects were observed in systems with no nitrogen fertilization where soils in the cover crop treatment showed a 19% increase in soil carbon and in systems with a forb cash crop where the soil carbon pool was 23% larger in cover crop soils. Our results suggest that the inclusion of any cover crop in rotation will build soil organic matter, an important component of soil health.