Dynamic biochar effects on soil nitrous oxide emissions and underlying microbial processes during the maize growing season

Biochar application is a promising approach to decrease nitrous oxide (N₂O) emissions from agricultural soils, yet little is known about how biochar affects N₂O-producing and consuming microbial processes under changing field conditions. We conducted a field study to assess if growing season patterns in soil N₂O emissions were correlated with the underlying microbial processes of nitrification and denitrification. We measured soil N₂O emissions, potential nitrification and denitrification rates, and the abundance of key soil nitrogen (N)-cycling functional genes in an intensive maize production field trial in Illinois, USA that included the following four treatments: Control (unamended), Biochar (100 Mg ha⁻¹), Nitrogen (269 kg N ha⁻¹ as Urea Ammonium Nitrate fertilizer), and Biochar + Nitrogen (100 Mg ha⁻¹ and 269 kg N ha⁻¹, respectively). Biochar increased potential nitrification rates when soil ammonium concentrations were high following fertilizer application, thus enhancing N₂O emissions in the Biochar + Nitrogen treatment early in the season which were likely nitrification-associated. However, over the full growing season, biochar application reduced cumulative N₂O emissions in Biochar + Nitrogen plots to levels similar to the unamended Control. This could be attributed to biochar suppression of potential denitrification throughout the growing season. The treatments amended with biochar avoided large pulses of N₂O emissions following intense rain events in the mid-season, while also sustaining lower N₂O emissions in the late-season. Our study demonstrates that biochar can have dynamic effects on soil N₂O emissions and the underlying microbial processes that depend on changing edaphic conditions, such as soil inorganic nitrogen availability and moisture, over the growing season.