Size matters: Fine biochar application mitigates N2O emissions during extreme drying and rewetting events in arable soils

Drying and rewetting (D/W) causes substantial stress to soil microbial communities, with important consequences for soil carbon (C) and nitrogen (N) dynamics. The impacts of biochar addition on these effects are underexplored. Fine biochar increases soil pH and enhances adsorption of labile ammonium (NH₄⁺) released during repeated D/W cycles due to large surface area. We therefore hypothesised that application of fine biochar would decrease D/W-induced soil N₂O emissions. Arable soils were prepared as (i) unamended controls, (ii) soils limed to replicate biochar pH effects, and (iii) soils amended at 1% of the dry soil weight with two particle-size fractions of biochar (<1.4mm “fine” and >3mm “coarse” pellets) produced from wheat straw and anaerobic digestate feedstocks. These soils were subjected to different frequencies of D/W cycles; 0, 1 or 4 cycles during a 58-day period. Ammonium nitrate fertilizer was applied at the start and after 45 days of incubation.

In the early stages of the incubation, lime and biochar addition both increased soil N₂O emissions relative to the controls. However, fine digestate biochar reduced cumulative N₂O emissions by 12.9% in the soil subjected to 0-cycles of D/W compared with non-amended control soils. Addition of lime to induce the same pH change as the biochar additions tended to decrease N₂O emissions, suggesting that the reduction in N₂O was partly mediated by a pH increase. Increasing D/W frequency elevated N₂O emissions across the treatments except for both particle size wheat straw biochar amended soils, where N₂O emissions were not altered by D/W frequency. Nonetheless, comparing N₂O emissions at highest D/W frequency across treatments, the N₂O released from soil amended with fine wheat straw biochar was the lowest. Lime and biochar addition decreased NH₄⁺ concentration in soil by 19 – 55.5% compared to control soils. This reduction in NH₄⁺ concentration suggest a pH-induced stimulation of nitrification with minimal N₂O release. Overall, application of fine biochar mitigates soil N₂O emissions, even during extreme D/W scenarios that may become increasingly frequent with climate change, and should therefore be considered a promising management practice for N₂O emissions reduction in arable soils.