Lower post-irrigation nitrous oxide pulses on diversified organic strawberry farms

Abstract Diversified organic farming systems are promoted for the ecosystem services they provide, but their potential to mitigate nitrous oxide (N₂O) emissions has not been evaluated in strawberry production, where emissions are delivered largely in short pulses following irrigation. We measured N₂O fluxes from automated soil chambers before, during, and after a single irrigation event on 48 focal plots across 34 certified organic strawberry farms on California’s Central Coast, over two growing seasons. Farms ranged from simplified to diversified. We indexed that contrast by whole-farm above-ground plant diversity (Shannon H' = 0 to 2.73); in practice it reflects differences in crop rotation, non-crop vegetation, fertility source, and weed management. Median focal plot emissions were 5.9 µg N₂O–N m⁻ 2 h −1 on diversified farms and 22.4 on simplified farms (Wilcoxon rank-sum W = 406, p = 0.003). Diversified farms emitted 17 to 26% of the rate on simplified farms, depending on the measure of central tendency (arithmetic mean, trimmed mean, or median). The gap widened through the irrigation cycle: mean emissions on simplified farms rose from 36.3 to 100.1 µg N₂O–N m⁻ 2 h −1 between the pre- and post-irrigation windows, while diversified plots fell from 12.4 to 6.2. Soil nitrate and total nitrogen did not differ between the two groups, and the contrast did not attenuate when we entered any of 19 measured soil properties as a covariate. The only covariate that partly absorbed it, estimated fruit yield, plausibly lies on the causal path. Diversified strawberry farms may therefore mitigate N₂O emissions, with the reduction concentrated in the post-irrigation window.