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

Yujia Liu^1*Henri MP Siljanen²Björn Kemmann^3,4Per Ambus¹Dhiraj Paul²Krishnapriya Thiyagarasaiyar²Bo Elberling¹Florian Wichern⁵Kristian Thorup-Kristensen¹Carsten W Mueller^1,3Daniel M N Poultney^1,6,7

• ¹University of Copenhagen, Copenhagen, Denmark
• ²University of Eastern Finland, Kuopio, Northern Savonia, Finland
• ³Technical University of Berlin, Berlin, Brandenburg, Germany
• ⁴University of Alberta, Edmonton, Alberta, Canada
• ⁵Rhine-Waal University of Applied Sciences, Kleve, Germany
• ⁶UMR ASTRE - CIRAD, Montpellier, Languedoc-Roussillon, France
• ⁷Université de Montpellier, Montpellier, Languedoc-Roussillon, France

Eastern Denmark's agricultural landscapes feature numerous topographic depressions, 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 lead to N₂O emissions hotspots. The potential legacy effects of these topographic locations on microbial communities involved in N₂O production and reduction remain unclear.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).Depression soils emitted 8-times more N₂O than upland soils at 90% WHC. Cu addition did not reduce cumulative N₂O emissions, but delayed or lowered flux peak. Depression soils showed 3000-fold and 4000-fold higher 16S rRNA and nosZ clade I than upland soils respectively. Cu addition significantly decreased 16S rRNA abundance, eliminated AOB amoA in upland soils, and slightly lowered the tested gene abundance in depression soils. The nosZ gene community structure differed significantly between these two soils.Overall, our study suggests that erosional differentiation of soil properties together with frequent waterlogging conditions can result in distinct microbial communities, fostering legacy effects with differences in N₂O emissions between the upland and depression soils. Adding Cu to these intensively managed soils is unlikely an effective strategy to mitigate N₂O emission hotspots in arable fields.