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 (N 2O) 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. N 2O emissions and ancillary soil properties (NH 4+, NO 3, 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 N 2O emissions were significantly higher than the control treatment each year ( p<0.05); 4.141.14 kg N 2O-N ha -1 yr -1 versus 2.000.66 kg N 2O-N ha -1 yr -1 in year one, and 4.240.74 kg N 2O-N ha -1 yr -1 versus 1.600.28 kg N 2O-N ha -1 yr -1 in year two. Emissions of N 2O in the WS biochar treatment were also higher than the control each year, but differences were not significant. The effect of biochar on N 2O 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 N 2O 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 NH 4+, NO 3- 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 N 2O emissions. Since neither biochar amendment reduced N 2O emissions, our results demonstrate the need to evaluate N 2O emissions at a cropping system scale (e.g. encompassing changes in N inputs and cycling) and in systems where nitrification processes may dominate emissions.
