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