Effect of topsoil dilution and digestate application on CO2 and N2O emissions from erosion affected soils

Meliorative, partial deep tillage (PDT) has a high potential for a permanent increase in the carbon (C) and nitrogen (N) stocks. Through PDT, the lower-concentration, C and N-poor subsoil is tilled upwards towards the surface, the topsoil material is buried in the subsoil and simultaneously mixed, resulting in a C and N dilution of the topsoil. However, whether this might temporarily enhance the C and N accumulation until the former equilibrium of a diluted topsoil is reached is yet unclear. To clarify this, we conducted a model experiment simulating subsoil mixing into topsoil and analysed CO2 and N2O emissions, considering soil type, erosion state and digestate application.
We used a non-eroded Albic Luvisol (LL), eroded Albic Luvisol (eLL), and a strongly eroded Calcaric Regosol (RZ) soils from the CarboZALF experimental area in the Uckermark, Germany. Each soil's topsoil was mixed with 20% material from the respective subsoil and incubated for 26 days in an automated, gas exchange measuring system. Undiluted, unfertilised substrates served as controls, and were compared with those receiving organic fertiliser (Digestate). Digestate addition uniformly increased CO2 emissions across all soil types and notably elevated N2O emissions, particularly in eLL soil. A moderate increase in RZ and the smallest increase in LL soil were observed. Topsoil dilution reduced CO2 release in all variants, especially in fertilised variants, but only moderately reduced N2O emissions in eLL soil upon subsoil and digestate addition. Conversely, slight increases in N2O release were observed in the other two soils under these conditions.

Research domain: Soil Sciences

Research question: We pursued the following hypotheses: the addition of digestate always leads to a short-term increase in CO2 and N2O release; soil dilution leads to a significant reduction in CO2 and N2O release because the available C and N quantities are reduced in the diluted and undersaturated topsoil or a higher proportion of the C and N from the digestate is bound in the soil; and the effect of soil dilution and digestate is somewhat modified by the degree of erosion of the soil.