(Intra-event) dynamics of the total mobile inventory in soil - importance for carbon balances and coupling of subsurface ecosystems

A diverse and broad range of sizes and materials – the total mobile inventory (TMI) – is mobile in soil (Lehmann et al., 2021). Constituting matter exchange between surface and subsurface ecosystems, it is still almost unknown how dissolved, colloidal to particulate fractions (>0.45 µm) fluctuate in seepage of undisturbed soil in both the short (intra-event scale) and long terms (seasons and multiannual periods). In the topographic groundwater recharge area of the Hainich Critical Zone Exploratory (NW-Thuringia), we monitor TMI dynamics using tension-controlled lysimeters in topsoil and subsoil under forest, pasture, and cropland. Soil seepage and precipitation were sampled on a regular (biweekly) and event-scale cycle and analyzed by physico-/hydrochemical, spectroscopic, and molecular biological methods. Within >6.5 years, fluctuations in the TMI were mainly driven by atmospheric conditions (precipitation, temperature), showing pronounced seasonality in the signature of dissolved components (e.g., sulphate) and the seepage pH. In hydrological winter, the total export of bacteria from soil was 1.5-fold higher compared to summer. However, episodic, and strong infiltration events, for instance after snowmelts or heavy rain storms, resulted in increased mobilization of particles. Noteworthy, the export of particulate organic carbon (POC) during winter infiltration events accounted for ~80% of the total annual translocation. Taking >20% of the total mobile OC on average, mobile POC, thus, must be taken into account in carbon balances and for ecosystem interactions. Against the background of increasing climate variability and extreme conditions due to climate change, we furthermore investigated intra-event dynamics of the TMI by increased temporal resolution to improve the understanding of the factors influencing the translocation. During a snow melt event (February 2021) and a simulated heavy rain event (artificial irrigation, August 2021), strong intra-event fluctuations of the TMI (e.g., nitrate, DOM, microorganisms) were also caused by effects of preferential flow. Besides improving our understanding of the soil carbon balance, our long-term monitoring identifies possible controlling factors for the coupling and supply of subsurface ecosystems (aeration zone, groundwater) and for their chemical, biological and functional fluctuations.

Lehmann, K., Lehmann, R., & Totsche, K. U. (2021). Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon. Science of The Total Environment, 143774.