Crop plant effects on denitrification – what have we learned in six years DASIM project?

Denitrification in agricultural crop production is one of the main sources of gaseous N₂O and N₂ losses to the environment. To successfully develop mitigation strategies, it is crucial to understand N₂O production pathways, but also to quantify all other gaseous N losses, especially N₂ emissions. Therefore, this project aimed (1) to identify the main drivers of denitrification during plant growth and in the post-harvest period, (2) to quantify denitrification derived N₂O and N₂ losses during the cropping season, and (3) to assess the interactions between plant litter quality, initial litter degradation, and formation of hotspots of N₂O and N₂ production. We conducted experiments on the laboratory, greenhouse, and field plot scale using the ¹⁵N gas flux method and HeO₂ atmosphere to directly measure N₂O and N₂ losses and to determine the fraction of denitrification derived N losses. We worked with soils, crops, temperature and moisture conditions that are typical for our central German humid-temperate climate.

Plant growth affected all controlling factors of denitrification, especially soil moisture, NO₃⁻ and C_org availability. Crop species differed in their growing patterns and N uptake throughout the growing season controlling both N and C availability in soil. Accordingly, N₂O and N₂ emission patterns differed between crop species. Overall, emissions were highest when plant N uptake was low, i.e., during early growth stages and ripening, and after harvest. On the field scale, soil moisture and temperature were major controls of N₂O+N₂ losses.

In a climate chamber study under controlled temperature conditions, N₂O and N₂ fluxes mainly derived from denitrification of labeled ¹⁵NO₃⁻ in anoxic microsites, while nitrification simultaneously occurred in more oxic parts of the soil, potentially contributing to formation of unlabeled N₂O. Increasing soil moisture with irrigation increased denitrification rates in anoxic hotspots, which corresponded with increasing N₂O and especially N₂ fluxes. At the same time, it restricted nitrification and thus decreased the share of nitrification-dependent processes contributing to N₂O formation.

Incorporation of plant litter increased CO₂, N₂O, and N₂ losses irrespective of litter quality, soil moisture or soil type/SOM content. We found that under O₂ limiting conditions (70 % WFPS), the fraction of easily degradable C controlled the magnitude of N₂O and N₂ losses after litter incorporation. Under moderate soil moisture (50-60 % WFPS), interactions between litter degradation and SOM turnover affected the time course and processes contributing to N₂O formation. Overall, our high-resolution gas flux measurements showed that N₂O+N₂ emissions from harvest residues can contribute significantly to the total N loss.