Higher soil nitrous oxide production in landscape depressions linked to soil and hydrological legacy effects

Background and aims Eastern Denmark’s agricultural landscapes feature numerous topographic depressions that are frequently flooded during late winter and spring. These poorly drained, carbon- and nitrogen-rich depression soils receive eroded material from adjacent slopes. Fertilization and water saturation create N 2 O emission hotspots. However, the potential legacy effects of these topographic locations on microbial communities involved in N 2 O production and reduction remain unclear. One approach to mitigating high denitrification rates (as a source of N 2 O) is to alter microbial pathways by adding nonhazardous levels of copper. Methods We conducted an incubation study using upland and depression soils from the same site, incorporating varying Cu levels (0, 130, and 260 mM) and water levels (60% and 90% water holding capacity). Results Depression soils emitted eight times more N 2 O than upland soils at 90% WHC. Cu addition did not reduce cumulative N 2 O emissions but delayed or lowered the flux peak. Depression soils exhibited 3,000- and 4,000-fold higher 16S rRNA and nosZ clade I abundances, respectively, compared to upland soils. Cu addition significantly decreased 16S rRNA abundance, eliminated AOB amoA in upland soils, and slightly reduced the tested gene abundances in depression soils. The nosZ gene community structure differed significantly between the two soils. Conclusions Overall, our study suggests that erosional differentiation of soil properties, together with frequent waterlogging conditions, can result in distinct microbial communities, fostering legacy effects that lead to differences in N 2 O emissions between upland and depression soils. Adding Cu to these intensively managed soils is unlikely to be an effective strategy for mitigating N 2 O emission hotspots in arable fields.