Does pasture rejuvenation by sod-seeding with non-bloat legumes affect greenhouse gas emissions?

Pasture rejuvenation through sod-seeding (i.e., the use a zero-till drill to place the seed directly into the soil) with non-bloat legumes, such as cicer milkvetch (Astragalus Cicer L.) or sainfoin (Onobrychis vicifolia Scop.), has emerged as a favored strategy for ranchers in the western Canadian prairies due to it being both time- and cost-effective. Meanwhile, this management also makes alterations to soil carbon and nitrogen cycling as well as to the diet of cattle grazing the pastures. Yet, our understanding on how these changes influence the greenhouse gas (GHG) budgets of the pastures remains limited. The goal of this research was to assess the impact of rejuvenating pastures by sod-seeding cicer milkvetch or sainfoin into a depleted meadow bromegrass-alfalfa mixed pasture on GHG emissions and their contribution to the total GHG footprint of the pasture system. To achieve this goal, field studies were conducted in east-central Saskatchewan, Canada to: (1) evaluate and compare GHG emissions between the rejuvenated pastures and the depleted pasture, and (2) quantify GHG emissions from dung and urine patches deposited by cattle grazing these pastures. Finally, the legume option resulting in the lowest GHG footprint was identified by integrating the GHG data with enteric methane (CH4) production by the cattle. \nThe results indicate pasture rejuvenation through sod-seeding had only a minimal impact on soil-derived GHG emissions relative to the control (depleted) pasture, likely due to the limited soil disturbance associated with this method. At the paddock-scale, annual GHG emissions averaged 10.11 Mg CO2-C ha-1, 2.54 kg CH4-C ha-1 (uptake), and 0.18 kg N2O-N ha-1, with no significant differences among pasture types. However, changes in grazing diet resulting from the pasture rejuvenation were found to affect urine patch N2O emissions, with the highest cumulative N2O emissions associated with beef cattle grazing on the depleted pastures sod-seeded with cicer milkvetch. Surprisingly, this did not reflect differences in the available N content of the urines, suggesting a potential link to the presence of secondary metabolites such as hippuric acid. Notably, the dung and urine patches yielded distinctly different N2O emission factors, averaging 0.03% and 0.26%, respectively. Within the pastures, landscape position emerged as a dominant regulatory factor for CO2 and CH4 emissions—with the lower slope positions exhibiting the highest CO2 emissions and the lowest CH4 uptake, likely due to denser vegetative cover in these areas due to an accumulation of soil moisture. Such landscape-scale patterns remained unaffected by dung/urine deposition. Partial C footprints for the pastures (based on non-CO2 emissions from the soil, cattle excreta, and enteric CH4) were developed and it was determined that there were no significant differences between the rejuvenated and depleted pastures. Averaged across pastures, the C footprint was 970 kg CO2eq ha-1 yr-1, with enteric CH4 being the largest contributor to the footprint. Insights gained from this study will be valuable for ranchers and policymakers in developing sustainable pasture management strategies.

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