Biochar alters nitrogen transformations but has minimal effects on nitrous oxide emissions in an organically managed lettuce mesocosm

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 A degrees C (W410), Douglas fir wood pyrolyzed at 510 A degrees C (W510), pine chip pyrolyzed at 550 A degrees C (PC), hogwaste wood pyrolyzed between 600 and 700 A degrees C (SWC), and walnut shell gasified at 900 A degrees 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 N2O emissions were only observed in soil amended with W510 for 3 days out of the 42-day growing season without affecting total cumulative N2O fluxes. This suggests that effects of biochar on decreasing N2O emissions may be transient, which compromise biochar's potential to be used as a N2O mitigation strategy in organic systems. Overall, our results confirm that different biochar materials can distinctively affect soil properties and N turnover.