How Deep Can Surface Signals Be Traced in the Critical Zone? Merging Biodiversity with Biogeochemistry Research in a Central German Muschelkalk Landscape

Abstract

The Earth's Critical Zone (CZ) is a thin living layer connecting atmosphere and geosphere, including aquifers. Humans live in the CZ and benefit from the vital supporting services it provides. However, the CZ is increasingly impacted by human activities including land and resource use, pollution, and climate change. Recent interest in uniting the many disciplines studying this complex domain has initiated an international network of research infrastructure platforms that allow access to the CZ in a range of geologic settings. In this paper a new such infrastructure platform associated with the Collaborative Research Center AquaDiva is described, that uniquely seeks to combine CZ research with detailed investigation of the functional biodiversity of the subsurface. Overall, AquaDiva aims to test hypotheses about how water connects surface conditions set by land cover and land management to the biota and biogeochemical functions in the subsurface. With long-term and continuous observations, hypotheses about how seasonal variations and extreme events at the surface impact subsurface processes, community structure and function are tested. AquaDiva has established the Hainich Critical Zone Exploratory (CZE) in central Germany in an alkaline geological setting of German Triassic Muschelkalk formations. The Hainich CZE includes specialized monitoring wells to access the vadose zone and two main groundwater complexes in limestone and marlstone parent materials along a ∼6 km transect spanning forest, pasture, and agricultural land uses. Initial results demonstrate fundamental differences in the biota and biogeochemistry of the two aquifer complexes that trace back to the land uses in their respective recharge areas. They also show the importance of antecedent conditions on the impact of precipitation events on responses in terms of groundwater dynamics, chemistry and ecology. Thus, we find signals of surface land use and events can be detected in the subsurface CZ. Future research will expand to a second CZE in contrasting siliciclastic parent rock, to evaluate the relative importance of parent material lithology vs. surface conditions for the emergent characteristics of the subsurface CZ Küsel et al.Tracing Surface Signals into Subsurface and biodiversity. The Hainich CZE is open to researchers who bring new questions that the research platform can help answer.

Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages

“…Within the time period of monitoring there was no significant change from previous reported hydrochemical patterns (Table 1) (Herrmann et al, 2015;Küsel et al, 2016). Water chemistry reflects the limestone environment of the catchment.…”

“…The Hainich CZE comprises here a number of surface and belowground observational plots along a 5.4 km long hillslope transect in an intensively investigated area of about 29 km 2 (Kohlhepp et al, 2016;Küsel et al, 2016). A groundwater well transect consists of five locations (H1 (upslope) to H5 (toeslope)) that also span a land-use gradient from deciduous managed forest (H1), unmanaged woodland (H2), and grassland/pasture (H3) to cropland agriculture (H4 and H5) longitudinal to the assumed groundwater flow direction (Fig.…”

“…HTU comprises mainly an layered aquifer complex within the overlying Meissner formation (moM), comprising fracture aquifers with fine fissures and less pronounced karstification due to the finely alternating succession of limestones and low-permeable marlstone beds ( Fig. 1; Küsel et al, 2016;Kohlhepp et al, 2016). Vertical exchange is strongly inhibited by frequent and lowpermeable marlstone interbeds resulting in confined flow conditions and a layer-cake architecture (Kohlhepp et al, 2016).…”

Modeling macropore seepage fluxes from soil water content time series by inversion of a dual permeability model

“…It is deliberately kept simple to reduce the number of parameters and allow their estimation by inverse modeling using limited data. The model 30 is not meant to be an accurate physical model of water flow, but rather a simplified conceptual description and is intended to be used in field applications in the future. The model is set up as a DPM with a macropore and a soil matrix domain (Fig.…”

“…Therefore, we plan to apply the model to field data from a mixed beech forest plot in the AquaDiva critical zone observatory (Küsel et al, 2016). The field soil will be deeper and more heterogeneous than the substrate in the mesocosms.…”