Soil nitrogen and carbon impacts of raising chickens on pasture

Abstract:

Chicken is the most consumed meat in the US, and production continues to intensify rapidly around the world. Chicken manure from confined feeding operations is typically applied in its raw form to nearby croplands, resulting in hotspots of soil nitrous oxide (N₂O) emissions. Pasture-raised chicken is an alternative to industrial production and is growing in popularity with rising consumer demand for more humanely raised protein sources. In this agricultural model, manure is deposited directly onto grassland soils where it is thought to increase pools of soil carbon and nitrogen. The fate of manure nitrogen from pasture-raised chicken production remains poorly understood.

We conducted a controlled, replicated experiment on a permaculture farm in Charlottesville, Virginia (Timbercreek Organics) in which small chicken coops (10 ft x 12 ft) were moved daily in a pasture. We measured manure deposition rates, soil inorganic nitrogen pools, soil moisture, and soil N₂O and CO₂ emissions. Measurements were made for the 28-day pasture life of three separate flocks of chickens in the spring, summer, and fall. Each flock consisted of approximately 200-300 chickens occupying three to five coops (~65 chickens/coop). Measurements were also made in paired ungrazed control plots.

Manure deposition rates were similar across flocks and averaged 1.5 kg_drywt ha^-1 during the spring grazing event and 4.0 kg_drywt ha^-1 during the summer and fall grazing events. Manure deposition was relatively constant over the four weeks pasture-lifetime of the chickens. Compared to control plots, grazed areas exhibited higher soil N₂O and CO₂ fluxes. The magnitude of these fluxes diminished significantly over the four-week span. Soil gas fluxes significantly increased following rainfall events. For a given rainfall event, higher fluxes were observed from transects that were grazed more recently. Soil gaseous reactive nitrogen losses were less in this pasture system compared to cultivated field amended with raw chicken manure. These results suggest that pasture manure management may have a smaller impact on gaseous reactive nitrogen pollution.