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.