Agriculture, which covers ~37% of the terrestrial Earth surface, is a major source for greenhouse gas (GHG) release to the atmosphere. Integrated crop-livestock systems (ICLS), which incorporate grazing livestock on crops and crop residues, forage cover crops, or phase farming, can reduce GHG emissions through appropriate fertilizer and manure management, as well as through offsetting GHG emissions through landscape net primary productivity. ICLS also better mimic the structure and functions of natural ecosystems by reducing external input supplies and by increasing crop diversity to boost production and ecosystem services.
In April 2018, an eddy covariance system was set up to measure seasonal and spatial variations in CO2 and CH4 fluxes and their feedbacks to the farm C footprint of an integrated dairy farm in Prairie du Sac, Wisconsin. Spatial and temporal variations in CO2 and CH4 dynamics were estimated using footprint modeling to detect differences in gas exchange by crop species and pasture. The farm carbon budget included seasonal variations in barn emissions. Crop harvest, tillage, manure applications and storage, feed and livestock inventories, as well as milk exports were monitored on a monthly basis.
Results/Conclusions
The farm was a small carbon sink for 2018, despite large GHG emissions from manure storage and barnyard emissions, especially during summer. Manure emissions accounted for ~38 % (in CO2e, CO2100-yr equivalent) of total farm emissions. The farm landscape was a net carbon sink, but C uptake was mainly driven by corn, pasture and forest productivity, whereas soybeans decreased the carbon sink potential of the farm. The greatest GHG emissions at the farm occurred following manure applications and again following harvest, due to an increase in soil respiration rates by ~14% from nutrient inputs and crop residues. This effect was greatly increased when average daily air temperatures were high. Daily CH4 and CO2 emissions at the barn amounted to 10600 kg of CO2e per cow, which for the whole barn was ~25% of annual net ecosystem production of the vegetation (for 800 ha of farm land). Milk production and export was higher outside of summer months (87 versus 82 liters of milk per cow), thus offsetting C release from plant respiration in fall months through C export. This study highlights the beneficial effects of ICLS on mitigating dairy C emissions. Studying the seasonal GHG budget could inform farm management to help offset farm barnyard and manure emissions by increasing plant productivity.