The effect of drought and rewetting on nitrogen cycling and nitrous oxide emissions in a controlled experiment with different cover crop species

Cover crops are recognized as a climate-smart agricultural practice that increases soil organic carbon content (SOC). As carbon (C) and nitrogen (N) cycles are coupled, an increase in SOC can impact the N cycle and nitrous oxide (N₂O) emissions. Another major driver affecting N cycling and N₂O emissions is soil moisture. With the increasing risk of summer droughts and wetter conditions during the off-season in Northern Europe, it is important to understand how drying-wetting and agricultural practices together affect N cycling and N₂O emissions.

To address this knowledge gap, we conducted a pot experiment with clay soil in controlled greenhouse conditions simulating summer drought with bare soil pots and oats sown either alone, with Italian ryegrass, or with alfalfa as plant treatments. The pots were initially watered to 70% degree of saturation to ensure that the plants start to grow, after which half the pots were let dry to 40% degree of saturation. The plants were grown for 36 days. At the end of the growth period, soil N₂O emissions were measured over three days. Following this, the pots were sampled destructively, and total N in plants, roots, and soil, as well as mineral N in soil, were analysed. Additionally, a follow-up pool-dilution incubation experiment using ¹⁵N-labelling with bare soil and soil previously covered with oats was conducted to study the effect of moisture content and rewetting on gross N transformation rates.

Contrary to our expectations, the results from the pot experiment showed that N₂O emissions in the plant treatments were higher in drought conditions than in moist conditions. This does not support our results from a cover crop field trial where reduced rainfall did not affect N₂O emissions during the growing season. However, during off-season reduced rainfall in the field led to higher N₂O emissions. Preliminary results from the incubation indicated lower N₂O emissions under drought conditions, with increased emissions upon rewetting and the highest emissions under moist conditions. The presence of plants decreased soil N₂O emissions in both experiments, but the plant species did not affect the emissions nor the total mineral N content in soil. As expected, in the pot experiment, total mineral N content in soil was higher in drought conditions than in moist soil as well as in bare soil compared with soil with growing plants. Results on the effects of drought and plants on gross N transformations during the incubation experiment with ¹⁵N labelling will be presented later.