Low vs. upland - copper addition regulates denitrification in cropland soils from N2O emissions hotspots in Denmark

Agricultural land-use makes up around 38% of the total global land surface. Farming activities are a major source of greenhouse gas emissions (carbon dioxide, methane and nitrous oxide) worldwide. Nitrous oxide (N₂O) emissions from agricultural land are an important contributor to the overall greenhouse gas budget. The high spatial and temporal variation of N₂O emissions in agricultural lands makes it more challenging and important to quantify the actual emissions. Denmark’s glacial landscape is characterized by a high abundance of topographic depressions. These are typically flooded for 1-3 months per year, mainly during late winter and spring season. As these depressions are frequent in agricultural areas, their soils are exposed to an increased nitrate availability due to regular fertilization. The combination of high-water saturation and high nitrate availability results in distinct landscape denitrification and thus N₂O emission “hotspots”.

The reduction of N₂O to N₂ by N₂O reductase can be an important mechanism to mitigate N₂O emissions. The N₂O reductase is both Cu and pH sensitive. Studies have showed Cu-modified organic fertilizer have the potential to enhance the reduction from N₂O to N₂, and therefore decrease N₂O emissions from fields.

In this study, we aimed to elucidate if Cu addition alters the emission of N₂O from upland and depression soils in a different way. Therefore, we conducted an incubation experiment with both upland and depression soils, testing how different levels of Cu addition and two different water levels affect the emission of N₂O. In order to differentiate the N₂O production pathways (nitrification or denitrification), we applied ¹⁵N tracer in the form of ¹⁵NH₄¹⁵NO₃ or ¹⁴NH₄¹⁵NO₃. We also added ¹³C labelled maize residues to be able to trace the consumption of fresh substrate in the course of denitrification as affected by different Cu and water levels.

Interestingly, although the soils were homogenized and incubated at the same water availability, we demonstrated a clearly higher N₂O emission from the depression soils compared to the upland soils. In general, we were able to demonstrate that Cu addition clearly reduces the level of N₂O emissions from upland soils, which is clearly more pronounced at higher soil water levels.