We investigated the effect of biochar type on plant performance and soil nitrogen (N) transformations in mesocosms representing an organic lettuce ( Lactuca sativa) production system. Five biochar materials were added to a silt loam soil: Douglas fir wood pyrolyzed at 410°C (W410), Douglas fir wood pyrolyzed at 510°C (W510), pine chip pyrolyzed at 550°C (PC), hogwaste wood pyrolyzed between 600 and 700°C (SWC), and walnut shell gasified at 900°C (WS). Soil pH and cation exchange capacity were significantly increased by WS biochar only. Gross mineralization increased in response to biochar materials with high H/C ratio (i.e., W410, W510, and SWC), which can be favorable for organic farming systems challenged by insufficient N mineralization during plant growth. Net nitrification was increased by W510, PC, and WS without correlating with the abundance of ammonia oxidizing gene ( amoA). Increases in N transformation rates did not translate into increases in plant productivity or leaf N content. WS biochar increased the abundance of amoA and nitrite reductase gene ( nirK), while SWC biochar decreased the abundance of amoA and nitrous oxide gene ( nosZ). Decreases in N 2O emissions were only observed in soil amended with W510 for 3 days out of the 42-day growing season without affecting total cumulative N 2O fluxes. This suggests that effects of biochar on decreasing N 2O emissions may be transient, which compromise biochar's potential to be used as a N 2O mitigation strategy in organic systems. Overall, our results confirm that different biochar materials can distinctively affect soil properties and N turnover.
