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Contrasting environmental conditions in topsoil and subsoil determine both abundance and function of soil microbial communities, affecting carbon (C) dynamics throughout the entire soil profile. Although the response of soil microorganisms to single factors such as substrate availability or micro-climatic conditions has been frequently studied, fewer studies have focused on complex interactions between substrate availability and environmental conditions. To address this, we employed vertical soil translocations between topsoil and subsoil horizons of an acid and sandy Dystric Cambisol under European beech forest in Lower Saxony, Germany, to investigate the impact of changing habitat conditions on microbial decomposer communities. To follow microbial substrate utilization at different soil depths, we created hot spots of fresh organic matter (OM) by adding 13C-labelled root litter. Soil samples were taken every three months over an experimental period of twelve months (June 2014 to June 2015). Generally, microbial biomass was strongly controlled by C availability throughout the profile. The importance of root litter as a microbial C source increased from topsoil to subsoil, but changes in available C sources affected fungi and bacteria differently. Fungi preferentially used root litter-derived C throughout the entire soil profile, demonstrating that limited access to preferred substrates, rather than micro-climatic conditions, was the main driver of decreasing fungal abundance with soil depth. In contrast, bacteria intensified utilization of root-derived C only in the absence of alternative C sources in the subsoil and were more strongly affected by spatial separation from C sources. Low soil moisture in combination with the highly sandy subsoil environment limited bacterial access to their substrates and, consequently, bacterial growth. In conclusion, fungal C utilization relies mainly on the quantity of recent plant-derived substrates, whereas bacterial access to substrates is additionally controlled by environmental conditions. This study indicates that limited microbial access to their heterogeneously distributed substrates may be an important factor for C accumulation and stabilization in subsoils. • Micro-climatic conditions inhibit bacterial carbon utilization in subsoil. • Substrate limitation is the main driver of low fungal abundance in subsoil. • Altered carbon sources with soil depth affect the microbial community structure. [ABSTRACT FROM AUTHOR]

As the largest active terrestrial reservoir of organic carbon (OC), soils play an essential role inthe global carbon cycle. Their response to rising global temperatures is of uttermost interest,having the potential to move and store an extensive amount of carbon within the climatesystem. One of the critical aspects of soil OC is the turnover rate, which has been reported todecrease rapidly below the A horizon. While most studies have focused on processes thatpromote higher ages of organic matter including various stabilization mechanisms, recentobservations suggest that part of the soil OC might already have been deposited with thesediment parent material, i.e. geogenic carbon and thus has turnover rates of millennia orlonger. This could lead to underestimations of turnover rates and needs to be taken intoaccount in biogeochemical models. This study aims to characterize geogenic OC and quantifyits contribution to subsoils. Deep soil cores (10 m) were collected from three different parent materials incentral Germany: Pleistocene loess, Paleogene sand and red sandstone. These wereanalyzed using a set of geochemical methods, including bulk 14C, selected lipidbiomarkers, pyrolysis-GCMS and 13C-CPMAS spectroscopy in order to characterizeorganic matter at multiple depth intervals and identify contributions of geogenicorigin. In the Pleistocene loess, apparent bulk 14C ages increased with depth from 2,200 to30,700 years BP, reflecting the depositional history of this relatively young sediment. Thelipid analysis revealed multiple sections with high OC content and Carbon Preference Index(CPI) values (>13), suggesting the presence of paleosols. The oldest part yielded lower CPIvalues (<3.0), evidently phases where deposition was too rapid for pedologicalprocesses to take place. This was supported by 13C-CPMAS NMR spectroscopy,demonstrating a highly degraded (alkyl-C rich) but soil like chemical composition of theOM. In these sections the organic matter is scarce and characterized by aliphaticcompounds such as n-alkanes, possibly deposited along with the mineral material,and methyl ketones as well as monoaromatic compounds including benzene andtoluene. Apparent bulk 14C ages of the Paleogene sand varied between 6,750 and 12,000 years,significantly younger than the deposition of the sediment (23-34 Mio. years BP). The sampleswere absent of n-alkanes and only small traces of fatty acids (2.67-0.59 μg/g dry weight)were detected, dominated by even long chain homologues. Young ages were also displayedby the red sandstone (about 245 Mio. years old), ranging between 12,760 and 17,400 yearsBP. In summary, the hitherto investigations of the deep sediment cores of differentparent material show a significant variance in age and composition of the OC fordifferent parent material at various depths, owing to OC components deposited alongwith the sediment parent material mixed with more recently deposited biogeniccarbon. [ABSTRACT FROM AUTHOR]