Nitrogen dynamics of pasture-raised chickens in a permaculture farm

Background/Question/Methods

Chicken is the most consumed meat in the US, and production continues to intensify rapidly around the world. The intensive production of chicken and the grains used to feed them is a major contributor to reactive nitrogen, including nitrous oxide (N₂O, a potent greenhouse gas), ammonia (NH₃, a contributor to acidification and coastal dead zones), and nitrogen oxides (NO_x, a precursor to smog). Chicken manure from confined feeding operations is typically applied in its raw form to nearby croplands, resulting in manure nutrient hotspots. Pasture-raised chicken is an alternative to industrial production and is growing in popularity with rising consumer demand for more natural and humanely raised protein sources. In this agricultural model, manure is deposited directly onto grassland soils. 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 the manure deposition rates, soil inorganic N pools, and soil gas fluxes of N₂O, NH₃, and NO_x. 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. Atmospheric NH₃ concentrations were monitored throughout the farm using Radiello passive samplers and a novel laser system.

Results/Conclusions

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, NH₃, and NO_x 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. Hotspots of elevated atmospheric NH₃ concentrations were detected above fields that with active chicken grazing, and were reduced by approximately half when measured one month following grazing. 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.