Biochar-enhanced bioreactors for agricultural nitrogen mitigation

Excess nitrogen from agricultural, urban, and wastewater sources is a major contributor to eutrophication and
water quality degradation, necessitating effective mitigation strategies. This study evaluates the effectiveness of
woodchip-biochar bioreactors in reducing nitrogen loads in agricultural drainage ditches. Biochar, a highly
porous and recalcitrant form of charcoal, was incorporated alongside woodchips due to its known capacity to
enhance microbial activity and nutrient retention. Three field-scale bioreactors, composed of a 50:50 volumetric
mix of woodchips and biochar, were installed in experimental ditches, with three control ditches left untreated.
Over an 18-month monitoring period, nitrate concentrations in bioreactor pore water were reduced by an
average of 87 % compared to control ditches, suggesting that denitrification - a microbial process converting
nitrate to nitrogen gas in low-oxygen conditions - played a dominant role. Biochar amendment enhanced microbial
habitat, improved pH buffering, and increased nutrient retention, fostering conditions favorable for
denitrification. Prokaryotic amplicon sequencing revealed a distinct microbial community structure in biochar-
amended bioreactors, with enrichment of denitrifying taxa and elevated functional potential for nitrogen
removal. While transient increases in ammonia and dissolved organic carbon were observed post-installation,
these effects did not extend beyond the bioreactor pore water and diminished over time. These findings underscore
the denitrifying potential of bioreactors in general, while highlighting the value of biochar as a strategic
enhancement to traditional woodchip systems, supporting their adoption as a scalable, cost-effective strategy for
reducing nitrogen pollution in agricultural watersheds.