Can biochar reduce nitrogen pollution from poultry manure? Assessing biochar’s biogeochemical fate and policy opportunities in the Chesapeake Bay Watershed

Background/Question/Methods

Intensification of animal agriculture has profound impacts on global and local nitrogen (N) biogeochemistry of nitrogen. In the Chesapeake Bay watershed, manure from intensive poultry production is a primary contributor of N pollution. Our research aims to identify management strategies that maximize benefits of poultry manure as an agricultural resource while minimizing N impacts. We hypothesize that amending crop soils with manure-derived biochar reduces reactive N losses by improving soil conditions and acting as a slow-release fertilizer. We tested this hypothesis using a two-year field study replicated on cornfields with sandy soil (Georgetown, DE) and silty-loam soil (State College, PA) within the Chesapeake Bay watershed. At each site, N was applied at locally recommended rates (225kgNha^-1 in DE, 150kgNha^-1 in PA) in the forms of raw manure, composted manure, biochar, and urea. Over the course of two growing seasons, we measured soil nitrous oxide (N₂O) and nitrogen oxides emissions, dinitrogen fluxes, nitrate and ammonium leaching, soil inorganic N concentrations, crop yields, aboveground biomass N, soil physical properties, and organic N fractions. We also assessed policy drivers to identify barriers and opportunities for widespread adoption of biochar as a manure N mitigation strategy. 

Results/Conclusions

Biochar significantly reduced reactive N losses from cornfields at both sites. Soil N₂O emissions were higher from the silty-loam soils. At both sites, manure amendment produced the highest soil N₂O emissions (5.6±0.3kgN-N₂Oha^-1 at DE, 7.6±0.5kgN-N₂Oha^-1 at PA), and calculated emissions factors (EFs) ranged from 2.1% (sandy) to 4.3% (silty-loam). Biochar amendment significantly reduced soil N₂O emissions (1.9±0.1kgN-N₂Oha^-1 at DE, 2.8±0.1kgN-N₂Oha^-1 at PA) with EFs of 0.23 (sandy soils) and 0.67 (silty-loam soils. Profiles of soil inorganic N concentrations to 1 m revealed rapid vertical transport of raw manure compared to biochar, which tended to retain more inorganic N in the surface soils. Lysimeters installed below the rooting depth (70cm) showed that biochar reduced N leaching losses by half compared to raw manure amendment. Biochar amendment increased plant biomass by 15 to 30%, or by 4.2 ± 0.6Mgha^-1, compared to  manure amendment. We also found that biochar amendment increased the amount of soil organic N stored in physically protected fractions. The struggle to create a comprehensive policy addressing nitrogen pollution in the Bay is highly contentious. However, our findings suggest that given biochar's success as a low-impact fertilizer, it can be a useful tool in implementing a market-based regional nutrient trading credit program to reach pollution-reduction goals.