Small scale soil heterogeneity shows stable subsoil preferential flow paths of water and DOC over a 5 year period in a Dystric Cambisol

Preferential flow paths (PFPs) are intertwined soil regions that link top and subsoil and through which water and consequently nutrients flow across the soil profile. PFPs enable newly available carbon sources to reach deeper soil layers, enabling soil microorganisms to flourish in an otherwise substrate-poor subsoil. A reliable assessment of organic carbon (OC) translocation into the subsurface requires an understanding of the small scale variability of dissolved organic carbon (DOC) concentrations and fluxes into the subsoil.

Using segmented suction plates over a 5-year period, we measured DOC and water fluxes, and subsequently OC translocation, at three depths in three soil profiles down to 1.5 m in a sandy Dystric Cambisol in Lower Saxony (Germany). DOC fluxes and water fluxes were correlated and decreased with depth. Overall fluxes were dependent on seasonal fluctuations of precipitation, with the winter and spring months bearing the highest water fluxes. We found significant flux variability between suction plates and soil depths. Rank analysis showed stable regions of high and low water and DOC fluxes, suggesting stable subsoil PFPs over these five years. Furthermore, the significance of small scale spatial heterogeneity as estimated by intraclass correlation was higher than the seasonal variability in each hydrological year, strengthening the idea that PFPs in a soil profile persist over years. In addition, SUVA analysis showed a decrease in OM aromaticity with depth in all three profiles and it was moderately correlated with water fluxes, indicating selective retention of complex organic matter along the soil profile.

These findings highlight the potential for long-term stability of PFPs in subsoils and their significance for the development and maintenance of biogeochemical subsoil C hotspots, and that small scale soil heterogeneity plays a major role in controlling water and nutrient movements across the soil profile.