Market, policy and climate changes as drivers of ecological and biogeochemical shifts in dairy ecosystems

Background/Question/Methods

Market, policy and climate changes drive shifts in the structure and function of dairy ecosystems. Steady, strong organic market growth results in transitions away from conventional, spatially-separated, feed-grain, and inorganic nitrogen (N)-based dairy ecosystem structure. The organic dairy ecosystems replacing them are instead characterized by new pastures, grazing, and increased utilization of organic N. These systems further diverged with passage of the Organic Pasture Rule in 2010, which stipulates baseline grazing levels. Together these market and policy changes increased in new pasture establishment and changed pastures species from perennial C3 to annual C4. All of these changes are playing out on a landscape of increased hydroclimatic variability in the Northeast, which motivates the question: how will the components of this coupled social-ecological system interact to alter biogeochemical functioning in dairy ecosystems? We explored this question with a field trial of establishing perennial C3 and annual C4 pastures exposed to grazing and extreme precipitation events on certified organic land at the Russell E. Larson Experimental Research Farm, Rock Springs, PA. Plant growth rates, N uptake, NH₃, N₂O, CO₂emissions, and soil N pools were monitored in conjunction with August, September and October 2012 simulations.

Results/Conclusions

An extraordinarily hot, dry June and July hindered C4 annual establishment in June, but did not limit C3 perennial establishment in March. This handicapped C4 resilience to grazing and extreme precipitation in August, September and October. Greenhouse gas emissions declined through the season with decreasing temperatures. Preliminary results include higher N2O emissions from C4 annual pastures in late summer coincident with lower growth rates and lower CO2 emissions. Increasing late season soil moisture and decreasing soil temperature was also coincident with the highest immediate post-graze N2O emissions in October, but lowest CO2 emissions. Initial evaluation of results suggests greater phenologic vulnerability of C4 annuals outweighed the greater drought tolerance of established C4s and resulted in comparatively greater resilience of C3 perennials to grazing and extreme precipitation even in a droughty establishment year.