Surface ecosystems are rapidly changing on a global scale and it is important to understand how this influences aquifers in the subsurface, as groundwater quality is a major concern for future generations. Dissolved organic matter (DOM) contains molecular and isotopic signals from surface-derived inputs as well as from the biotic and abiotic subsurface environment and is therefore ideal to study the connectivity between both environments. We evaluated a 3-year time series of DOM composition using ultrahigh resolution mass spectrometry and age using 14 C accelerator mass spectrometry along a hillslope well transect in the fractured bedrock of the Hainich Critical Zone Exploratory, Germany. We found a wide range of DOM 14 C depletion, from 14 C = −47.9 to 14 C = −782.4, within different zones of the shallow groundwater. The 14 C content of DOM mirrored the connectivity of the aquifers to the surface. The composition of DOM was highly interrelated with its 14 C age. The proportions of surface-derived DOM components decreased with DOM age, whereas microorganismderived DOM components increased. The intensity of surface-sourced DOM signals differed between the wells and likely reflected the hydrological complexity of fracturedrock environments. During recharge, DOM was more enriched in 14 C, contained more surface-derived molecular components and was more diverse. As a potential response to the varying DOM substrate, bacterial 16S rRNA gene analysis revealed community evolution and increased bacterial diversity during recharge. The influx of diverse, surface-derived DOM potentially fueled evolution within the autochthonous bacterial communities, as in contrast to DOM, the bacterial community did not retreat to the initial diversity and community composition during the recession period. Our results demonstrate on the one hand that combined analyses of the composition and age of groundwater DOM strongly contribute to the understanding of interconnections, community evolution and the functioning of subsurface ecosystems and on the other hand that changes in surface ecosystems have an imprint on subsurface ecosystems.

The origin, molecular composition and fate of dissolved organic matter (DOM) provides essential information that links surface and subsurface processes and explores the functioning of the Critical Zone. Therefore it is important to identify specific marker compounds that provide information on the temporal and spatial linkages in the Critical Zone. Here, we used ultra-high resolution mass spectrometry and accelerator mass spectrometry in order to identify markers for fast transport of surface-derived DOM through the Critical Zone. We assessed the molecular composition and radiocarbon age of solid phase extracted DOM (SPE-DOM) from forest top soils. The 14 C ages of SPE-DOM in our study were similar to that of bulk DOM. Calibrated ages ranged from 0 to 49 years and the within-site variability was larger than between sampling sites. Spearman rank correlation between the 14 C ages and the DOM composition identified 129 sum formulae that were significantly correlated to 14 C age. We found that molecular entities with younger 14 C ages had lower molecular weight, higher unsaturation and less oxygen and heteroatoms than those associated with older 14 C ages. A chemical library search suggested that phenylpropanoids and compounds, which are known to be lignin derived, are key molecular species for terrestrial DOM with young 14 C ages. Among them, lignin dimers emerged as prominent surface-derived compounds that can potentially be used as markers for fast transport of water and DOM into the subsurface and groundwater.

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Microbial communities play a key role for central biogeochemical cycles in the subsurface. Little is known about whether short-term seasonal drought and rewetting events influence the dominant microbes involved in C- and N-cycles. Here, we applied metaproteomics at different subsurface sites in winter, summer and autumn from surface litter layer, seepage water at increasing subsoil depths and remote located groundwater from two wells within the Hainich Critical Zone Exploratory, Germany. We observed changes in the dominance of microbial families at subsurface sampling sites with increasing distances, i.e., Microcoleaceae dominated in topsoil seepage, while Candidatus Brocadiaceae dominated at deeper and more distant groundwater wells. Nitrifying bacteria showed a shift in dominance from drought to rewetting events from summer by Nitrosomandaceae to autumn by Candidatus Brocadiaceae. We further observed that the reductive pentose phosphate pathway was a prominent CO2-fixation strategy, dominated by Woeseiaceae in wet early winter, which decreased under drought conditions and changed to a dominance of Sphingobacteriaceae under rewetting conditions. This study shows that increasing subsurface sites and rewetting event after drought alter the dominances of key subsurface microbes. This helps to predict the consequences of annual seasonal dynamics on the nutrient cycling microbes that contribute to ecosystem functioning.