Event-scale dynamics of the total mobile inventory in soil – Highlights from a multi-year and -site lysimeter study

The matter exchange between surface and subsurface ecosystems cover a diverse size- and material spectrum of inorganic, organo-mineral and organic substances, comprising dissolved, colloidal, but also larger particulate matter, including microbiota. The conditions and field-scale factors controlling the long-term seasonal and episodic dynamics of this “total mobile inventory” (TMI, Lehmann et al., 2021) in undisturbed soil and its translocation through the subsurface of the Critical Zone are almost unknown. Therefore, we established long-term soil monitoring plots in the Hainich Critical Zone Exploratory; NW-Thuringia, central Germany). Soil seepage from 22 tension-controlled lysimeters in topsoil and subsoil, covering different land use (forest, pasture, cropland) in a topographic groundwater recharge area, was collected and analyzed by a broad variety of analytical methods on a regular (biweekly) and event-scale cycle. Atmospheric forcing was found to be the major factor triggering the translocation of the mobile inventory, mainly causing considerable seasonality in the solute signature (e.g., sulphate) and seepage pH. However, episodic high-flow (infiltration) events rather than seasonality caused mobilization of significant amounts of particulates, especially, after snow melts or rainstorms. Additionally, we explored the infiltration event-scale dynamics in detail, based on high-resolution monitoring of fluid flow and TMI fluxes during a snowmelt event (February 2021) and a simulated heavy rainstorm (artificial irrigation, August 2021). On the event-scale we observed pronounced fluctuating TMI (e.g nitrate, OC, particles) mainly forced by preferential flow. Consequently, in the light of climate change (i.e., extreme events and conditions) our study provides field-scale evidence for the importance of event-scale dynamics of mobile inventory in soil with implications for the understanding of subsurface ecosystem nutrition and functioning.