CO2 and N2O Emissions after Digestate Application Responded Differently to Topsoil Dilution and Soil Erosion State

Abstract

The application of digestates supports soil fertility by restoring soil organic matter (SOM) and supplying nitrogen (N). However, their application can increase greenhouse gas (GHG) emissions from agriculture. Targeted digestate application to soils where erosion has mixed subsoil into topsoil may reduce emissions, as the lower SOM saturation in this diluted topsoil could enhance stabilization of organic inputs. This study investigates whether topsoil dilution through erosion can reduce carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) emissions following digestate application. We conducted an incubation experiment simulating erosion-induced topsoil dilution. Three different soils from Uckermark region, Germany—non-eroded (LL), moderately eroded (eLL), and strongly eroded (RZ)—were incubated for 26 days in an automated gas exchange system. Topsoil was diluted with 20% subsoil, and digestate applied as organic fertilizer. CO 2 and N 2 O emissions were measured; undiluted, unfertilized soils served as controls. Digestate increased CO 2 and significantly raised N 2 O emissions in all soils. Topsoil dilution reduced CO 2 emissions in LL and showed similar trends in eLL and RZ, though the effect weakened with erosion severity, likely due to existing C-undersaturation in these soils. N 2 O responses varied: emissions decreased in eLL (high clay and reactive mineral content), possibly due to enhanced N stabilization, but increased in RZ (calcareous, high-pH soil likely promoting nitrification) and slightly in LL, possibly due to lowered carbon-to-nitrogen (C: N) ratio. Topsoil dilution can mitigate digestate-induced CO 2 but may elevate N 2 O emissions depending on soil properties. Therefore, site-specific management is key to lowering GHGs in erosion-prone soils.

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