Potential for Biochar to Mitigate N2O Emissions Varies Across Functional Locations and with Changes to Soil Physical and Chemical Properties in a California Wine Grape System.

Biochar has been proposed as a carbon negative soil amendment that can reduce CO2-e through a number of mechanisms, one being the mitigation of N2O emissions. We tested the efficacy of a high temperature (~950°C) walnut shell (WS) and moderate temperature (~550°C) pine chip (PC) biochar applied at 10t/ha in reducing N2O and CO2 emissions in a Californian commercial wine grape system. Our primary focus was to quantify N2O emissions in this system with the intent to elucidate mechanisms that drive emissions at a field scale. We identified two functional locations, berm and inter-row, along with four key axillary drivers of N2O fluxes; mineral N content (NO3-/NH4+), dissolved organic carbon (DOC), water filled pore space (WFPS), and soil pH. N2O emissions, and axillary soil measurements were monitored daily or bi-daily during management and precipitation events and otherwise weekly. To determine if there were broader changes to soil structure and hydrology that may be influencing GHG emissions we measured soil aggregation and performed water retention curves annually. In the berm location there were no significant differences in cumulative N2O or CO2 fluxes after one year. In contrast both biochar treatments had substantially higher N2O emissions in the inter-row location. Of the axillary soil measurements, only pH appeared to be consistently affected by biochar amendment. In both locations, biochar treatments had routinely higher soil pH, with the greatest increase seen in the inter-row WS treatment. Mean weight diameter was significantly higher in the inter-row PC treatment. As a whole our data show that biochar addition to temperate, perennial cropping systems may not mitigate N2O emissions to the extent found in lab studies or flooded systems and the direct mechanisms of altered N2O emissions remain unclear. We found emissions to vary depending on functional location and here attribute changes to interactions with cover crop residue and soil aggregation.