Consequences of intensive rotational grazing on soil carbon versus nitrogen

Management intensive rotational grazing (MIG) concentrates large numbers of animals for short periods of time into small paddocks rotated across fields. MIG is perceived as a management strategy with the potential for positive impacts on primary production, pasture quality, and soil organic carbon (SOC) sequestration. However, few quantitative analyses have substantiated these views or concomitantly evaluated how MIG might alter soil nitrogen cycling and nitrous oxide (N₂O) emissions, particularly in the context of the temporal and spatial variability of MIG systems. Our study examines linkages among MIG, SOC accumulation, and N₂O emissions during the transition from conventional to organic dairy operations in the New England region of the US, comparing MIG fields to fields managed for hay. We combine field measurements at three locations across the region with modeling using the Denitrification and Decomposition (DNDC) model at both field and regional scales. We found MIG systems may have the potential to increase SOC, but these gains may be offset by higher soil N and N₂O emissions in MIG soils. Realizing the promise of MIG systems to increase SOC may therefore have the unintended consequence of making N more susceptible to loss, suggesting the need for more detailed spatial study of factors such as the forms, quantities, and timing of N inputs and losses. Limited spatially and temporally-resolved data for key factors like manure inputs limit the capacity to use models to explore mechanisms that might drive observed behaviors, such as the strong decrease in soil C:N ratios in MIG versus hayed fields.