Cover crop diversity and summer drought increase off-season N2O emissions from Finnish agricultural soil

The use of cover crops in agriculture is one of the climate-smart practices that have multiple benefits, such as increasing SOC, reducing N losses, and increasing biodiversity. Still the question whether cover crops and their diversity increase resilience against drought, and how the combined effects of cover crops, their diversity and drought affect N₂O emissions, remain largely unknown. We study the combined effects of cover crop diversity and drought on cropland (oat) greenhouse gas emissions and belowground C and N processes in a field plot trial. The effect of drought on soil and crop C and N dynamics and greenhouse gas (CO₂, N₂O) emissions is studied with rainout shelters that remove 50% of incoming precipitation. The CO₂ and N₂O emissions are measured with the manual dark chamber method twice a week during the growing season and once a week during off-season, soil temperature and water content are measured continuously, and soil is sampled for mineral N and total C and N analysis seasonally.

The preliminary results show that reduced rainfall decreases CO₂ emissions but does not affect N₂O emissions significantly during the growing season. During off-season, reduced rainfall increases both CO₂, and particularly N₂O emissions irrespective of cover crop diversity treatments. During growing season there is a tendency of higher N₂O emissions from diverse cover crop treatments compared to oat only treatment, and during off-season, a higher cover crop diversity significantly increases N₂O emissions. Overall and in all treatments, off-season N₂O emissions dominate the annual N₂O balance. Our results highlight the need to include off-season measurements to the annual N₂O balance estimation, and when assessing the effects of cover crops and future climate change scenarios such as summer drought on annual N₂O emissions.