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While habitat conditions influencing the abundance of microorganisms in topsoil are well known, these dynamics have been largely unexplored in deeper soil horizons. We investigated the effects of different substrate availabilities and environmental conditions on microbial community composition and carbon flow into specific groups of microorganisms in subsoils using a reciprocal soil transfer experiment within an acid and sandy Dystric Cambisol from a ∼100-year old European beech ( Fagus sylvatica L.) forest in Lower Saxony, Germany. Containers filled with subsoil from 10 to 20 cm (SUB20) and 110 to 120 cm (SUB120) soil depths and with additions of different amounts of 13 C labelled cellulose (1% and 5% of the respective organic carbon content of both soil layers) were exposed either in their home field environment or transferred reciprocally between SUB20 and SUB120 horizons for periods of one, four and twelve months. During the exposure of twelve months, 13 C accumulated up to 15 percent in total microbial biomass and up to 25 percent in fungal PLFAs. Similar microbial 13 C incorporation rates in SUB20 samples located at either 20 or 120 cm depth indicated comparable microclimatic conditions in both soil environments with no depth-dependent effects on the decomposer communities. While low nitrogen availability (when primary C-limitation was alleviated) and water content limited bacterial growth and activity at both depths, fungal abundance and activity were less affected due to their ability to efficiently exploit resources in surrounding soil by hyphal growth and higher drought resistance. Consequently, bacterial PLFAs (phospholipid fatty acids) incorporated less 13 C than fungi. The relatively high, from 1% to 5% cellulose addition linearly increased, 13 C incorporation rates in SUB120 samples at 120 cm depth clearly showed the potential of efficient carbon turnover in deeper soil layers. Spatial separation between subsoil microorganisms and their substrates may therefore be an important factor influencing carbon accumulation in subsoil. [ABSTRACT FROM AUTHOR]

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]

Forest subsoils may represent an important C sink in a warming world, but rhizodeposition as the key biogeochemical process determining the C sink strength of mature forests has not yet been quantified in subsoils. According to studies conducted in topsoil or laboratory experiments, soil C inputs by root exudation are increasing with increasing temperature and decreasing nutrient availability. We examined whether these relationships apply to forest subsoil by analyzing the response of root exudation to increasing soil depth up to 130 cm in a mature European beech ( Fagus sylvatica L.) forest. In two subsequent growing seasons differing in temperature and precipitation, we investigated in situ root exudation with a cuvette-based method and analyzed root morphology, microbial biomass, and soil nutrient availability. We proved that root exudation greatly decreases with soil depth as a consequence of a significant decrease in root-mass specific exudation activity to nearly a fifth of topsoil activity. The decrease in specific metabolic activity from 312 mg C g −1 yr −1 in the topsoil to 80 mg C g −1 yr −1 at 130 cm depth was amplified by an exponential decrease in root biomass per soil volume, leading to a relative decrease in root exudation per volume in the deep subsoil to 2% of topsoil root exudation (1 g C 10 cm −1 m −2 yr −1 at 130 cm depth). Specific root area decreased and mean fine root diameter and root tissue density increased with soil depth, indicating a shift in primary root functionality from fibrous roots in the topsoil to pioneer roots in the subsoil. The decrease in root exudation was accompanied by decreases in soil microbial biomass, extractable organic C (EOC), and N and P availability and increases in the aromatic C portion in SOM, but it did not relate to seasonal differences in climatic conditions. More specifically, it responded positively to an increase in EOC and ETN in the topsoil, but remained at its minimum rate in the SOC-poor subsoil, probably due to a lower organic N and higher mineral N content. The vertical pattern of beech root exudation is in accordance with a strategy to maximize whole-tree carbon-use efficiency, as it reduces C loss by exudation in soil spots where positive priming effects are unlikely, but enhances C exudation where microbes can mine less bioavailable SOM. The exudation patterns further suggest that increased C allocation to root systems as a likely tree response to elevated atmospheric [CO 2 ] may not lead to enhanced soil C input by root exudation to subsoils poor in SOM. [ABSTRACT FROM AUTHOR]

Incomplete knowledge on the environmental factors linking litter decomposition and the formation of soil organic matter (SOM) hampers the sustainable management of soil as a carbon (C) sink. Here, we explored the effect of soil texture on the fate of C from decomposing litter (Indiangrass; Sorghastrum nutans (L.) Nash) and the concurrent formation of SOM in mineral soils of different textures (sand- and clay-rich) and forest floor material. We quantified the amount of litter C respired, C remaining in the litter, and litter C retained in the soil/forest floor in a 186-day incubation employing stable isotope analyses (13C). We complemented our isotopic approach with the extraction of microbial biomarkers from the litter and soils/forest floor material and spectroscopic studies into the compositional changes of the incubated materials. We found that soil texture affected both the decomposition of litter and the retention of litter-derived C in the soil. The soil rich in clay provided conditions favorable for a more efficient microbial utilization of the litter material (high pH and high C use efficiency) as compared to the sand-rich soil and the forest floor. This resulted in lower amounts of litter C respired as CO 2 (25.0%, vs. 55.6 and 56.1% in clay vs. sand and forest floor material, respectively) and higher amounts of litter C retained in the clay-rich soil (12.6% vs. 3.5 and 5.3% in clay vs. sand and forest floor material, respectively). High contents of silt- and clay-sized mineral particles in the clay-rich soil likely resulted in the ability to stabilize litter C in aggregates and organo-mineral associations, perhaps as microbial residues. This ability was low in the sand-rich soil and virtually absent in the forest floor, where the recalcitrance of the litter and native SOM was probably more relevant, and a larger portion of litter C may have been retained in the soil as relatively untransformed plant compounds. We emphasize that litter decomposition, the formation of SOM, and soil texture are tightly linked, such that any differences in soil texture alter litter decomposition and SOM formation patterns for the same litter. • Role of clay/sand/forest floor in litter decomposition and SOM formation evaluated. • Clay-rich soil retained most litter C, while litter in this soil decomposed slowest. • Favorable conditions for efficient microbial use of litter C in clay-rich soil. • High amount of fine particles resulted in high ability to retain new litter C. • Soil texture differences affect litter decomposition & soil C sequestration rates. [ABSTRACT FROM AUTHOR]