Biochar does not mitigate field-scale N2O emissions in a Northern California vineyard: An assessment across two years

Biochar amendment to soil has been proposed as a mechanism to mitigate climate change through an array of mechanisms; one being the mitigation of soil nitrous oxide (N2O) emissions. Yet the extent and mechanisms through which this may be achieved in temperate agroecosystems is uncertain. We used a pine chip biochar produced at a moderate temperature (550 °C, PC biochar) and a walnut shell biochar produced at a higher temperature (900 °C, WS biochar). Biochar was applied at 10 Mg ha−1 to a working commercial wine grape system in North-Central California. The effects of biochar were assessed over two years at two distinct functional locations: the berm and row, which differed in N application and irrigation. N2O emissions and ancillary soil properties (NH4+, NO3, water filled pore space (WFPS), and pH) were closely monitored following management and precipitation events. Soil bulk density, cover crop yield and soil C and N were measured annually to address longer term changes in cropping system and soil properties. In the PC biochar treatment, annual cumulative N2O emissions were significantly higher than the control treatment each year (p < 0.05); 4.14 ± 1.14 kg N2O-N ha−1 yr−1 versus 2.00 ± 0.66 kg N2O-N ha−1 yr−1 in year one, and 4.24 ± 0.74 kg N2O-N ha−1 yr−1 versus 1.60 ± 0.28 kg N2O-N ha−1 yr−1 in year two. Emissions of N2O in the WS biochar treatment were also higher than the control each year, but differences were not significant. The effect of biochar on N2O emissions was more pronounced in the row location where annual emissions were significantly higher than the control in one and both years for the WS and PC biochars, respectively (p < 0.05). In the PC biochar treatment, we observed increased N2O emissions at both functional locations, however increases were more pronounced in the row location where they were in part attributable to increased cover crop N inputs. Differences between treatments in NH4+, NO3− and WFPS were mostly not significant. The WS biochar significantly raised soil pH relative to the control (p < 0.05), however in the berm location only, and increased soil pH in this treatment did not correspond to changes in N2O emissions. Since neither biochar amendment reduced N2O emissions, our results demonstrate the need to evaluate N2O emissions at a cropping system scale (e.g. encompassing changes in N inputs and cycling) and in systems where nitrification processes may dominate emissions.