Effects of Rotational Grazing on the Dynamics of Soil Carbon and Nitrogen on Commercial Dairy Farms

D. H. Lynch¹, R. C. Martin¹, and V. Rodd²

Abstract
To assess the potential for soil organic carbon (SOC) sequestration through management intensive grazing (MIG) of pastures, four sites in humid temperate Atlantic Canada (including 3 NS dairy farms plus AAFC Nappan), which provided comparisons between medium to long-term (10y to 50y) MIG versus cropped or hayed land management were selected.

Above-ground litter and soil (to 85cm depth) was sampled. Soil bulk density, C, N and macro-organic matter content and microbial biomass C and labile C and N (28 d; 25°C) were determined. Profile mineral N content was measured bi-weekly from fall 2001 to spring 2003.

Mineral N at 0-30cm consistently accounted for most of the soil inorganic N. Grazing reduced surface (0-25cm) soil bulk density and altered the distribution of SOC and N, but had no consistent effect on organic matter quality (macro-organic matter; mineralizable C and N, microbial biomass C / SOC). When expressed per volume (Mg C ha-1), differences in total SOC between management regimes were small and difficult to detect even when litter (up to 10% of SOC) was included. When expressed on an equivalent soil mass basis, however, SOC stocks were greater under pasture than hayland and cropland by 12.9 and 8.4 Mg C ha-1 after 30y and 50y respectively.

Management intensive grazing in humid temperate regions appears to promote significant SOC deposition compared to haying or cropping regimes. Gains in sequestered C, however, may be difficult to detect in the short term and unless accompanying changes in soil bulk density are taken into account.

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Author Locations and Affiliations
(1) Deptartment of Plant and Animal Sciences, Nova Scotia Agricultural College,Truro, Nova Scotia
(2) Agriculture and Agri-Food Canada, Crops and Livestock Research Centre, Nappan, Nova Scotia


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