. Soil formation is the result of a complex network of biological as well as chemical and physical processes. The role of soil microbes is of high interest, since they are responsible for most biological transformations and drive the development of stable and labile pools of carbon (C), nitrogen (N) and other nutrients, which facilitate the subsequent establishment of plant communities. Forefields of receding glaciers provide unique chronosequences of different soil development stages and are ideal ecosystems to study the interaction of bacteria, fungi and archaea with their abiotic environment. In this review we give insights into the role of microbes for soil development. The results presented are based on studies performed within the Collaborative Research Program DFG SFB/TRR 38 ( http://www.tu-cottbus.de/ecosystem ) and are supplemented by data from other studies. The review focusses on the microbiology of major steps of soil formation. Special attention is given to the development of nutrient cycles on the formation of biological soil crusts (BSCs) and on the establishment of plant–microbe interactions.

/pdf/the-role-of-microorganisms-at-different-stages-of-ecosystem-384dynyc6j.pdf

The role of microorganisms at different stages of ecosystem development for soil formation

Retreating ice fronts (as a result of a warming climate) expose large expanses of deglaciated forefield, which become colonized by microbes and plants. There has been increasing interest in characterizing the biogeochemical development of these ecosystems using a chronosequence approach. Prior to the establishment of plants, microbes use autochthonously produced and allochthonously delivered nutrients for growth. The microbial community composition is largely made up of heterotrophic microbes (both bacteria and fungi), autotrophic microbes and nitrogen-fixing diazotrophs. Microbial activity is thought to be responsible for the initial build-up of labile nutrient pools, facilitating the growth of higher order plant life in developed soils. However, it is unclear to what extent these ecosystems rely on external sources of nutrients such as ancient carbon pools and periodic nitrogen deposition. Furthermore, the seasonal variation of chronosequence dynamics and the effect of winter are largely unexplored. Modelling this ecosystem will provide a quantitative evaluation of the key processes and could guide the focus of future research. Year-round datasets combined with novel metagenomic techniques will help answer some of the pressing questions in this relatively new but rapidly expanding field, which is of growing interest in the context of future large-scale ice retreat.

/pdf/microbial-community-dynamics-in-the-forefield-of-glaciers-4d76igx7gq.pdf

Microbial community dynamics in the forefield of glaciers

The biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence (Damma glacier, Switzerland). This site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover. Calcium isotopic compositions, obtained by TIMS using a 43Ca–46Ca double spike, were measured in the main Ca pools. During this very early stage of weathering, the young soils which have formed (δ44/42Ca=+0.44‰)(δ44/42Ca=+0.44‰) were indistinguishable to the rocks from which they were derived (δ44/42Ca=+0.44‰)(δ44/42Ca=+0.44‰) and stream water (δ44/42Ca=+0.48‰)(δ44/42Ca=+0.48‰) was also within error of the average rock. This lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate Ca isotopes. The only Ca pool which was strongly fractionated from bulk rock was vegetation, which exhibited an enrichment of light Ca isotopes. Significant Ca isotope fractionation between bulk rock and the dissolved flux of Ca is likely to only occur where the Ca biogeochemical cycle is dominated by secondary processes such as biological cycling, adsorption and secondary mineral precipitation.

Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland

The litter layer is a major source of CO2, and it also influences soil-atmosphere exchange of N2O and CH4. So far, it is not clear how much of soil greenhouse gas (GHG) emission derives from the litter layer itself or is litter-induced. The present study investigates how the litter layer controls soil GHG fluxes and microbial decomposer communities in a temperate beech forest. We removed the litter layer in an Austrian beech forest and studied responses of soil CO2, CH4 and N2O fluxes and the microbial community via phospholipid fatty acids (PLFA). Soil GHG fluxes were determined with static chambers on 22 occasions from July 2012 to February 2013, and soil samples collected at 8 sampling events. Litter removal reduced CO2 emissions by 30 % and increased temperature sensitivity (Q10) of CO2 fluxes. Diffusion of CH4 into soil was facilitated by litter removal and CH4 uptake increased by 16 %. This effect was strongest in autumn and winter when soil moisture was high. Soils without litter turned from net N2O sources to slight N2O sinks because N2O emissions peaked after rain events in summer and autumn, which was not the case in litter-removal plots. Microbial composition was only transiently affected by litter removal but strongly influenced by seasonality. Litter layers must be considered in calculating forest GHG budgets, and their influence on temperature sensitivity of soil GHG fluxes taken into account for future climate scenarios.

/pdf/contribution-of-litter-layer-to-soil-greenhouse-gas-4scy59t5b2.pdf

Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added 14 C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the 14 C label in mineralized and leached DOM and quantified the spatial distribution of DO 14 C retained in soils using a new autoradiographic technique. Leaching of DO 14 C through the 10 cm-long soil columns amounted up to 28% of the added DO 14 C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO 14 C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of 14 C fluxes, measured 14 C activities in soils indicated that the major part of litter DO 14 C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO 14 C in the upper 3 cm of the soil columns. In the deeper soil, the 14 C label was concentrated along soil pores and textural discontinuities with similarly high 14 C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO 14 C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO 14 C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation, physico-chemical retention or preferential flow?

Soils are accumulating C during their initial development but little is known on the evolution of soil C fluxes and their components. In this study, we investigated soil CO 2 effluxes and leaching of dissolved organic carbon (DOC) during initial stages of soil formation along the granitic Damma glacier forefield in the Swiss Alps. We added isotopic labelled litter ( Leucanthemopsis alpina , δ 13 C = +88‰) to soil columns in three successional stages (10 yr, 70 yr, 120 yr) and traced the ‘new’ litter-derived C in soil CO 2 effluxes, DOC leaching, and in phospholipid fatty acids (PLFA), biomarkers for soil microbial communities. The results showed increasing total soil C fluxes with progressing soil development due to increasing C stocks in plants and soils. Throughout three summer months, 15–63% of the added litter C were lost via mineralisation and leaching. Along the initial soil chronosequence, litter-derived CO 2 C effluxes did not change systematically with soil age. The distribution of δ 13 C signatures in PLFAs was very similar at all site ages, suggesting that decomposer communities did almost not change with succession. Instead, moisture conditions of surface soils varying at the local scale seemed to primarily control litter mineralisation with the driest topsoils and lowest mineralisation at the oldest site. Only a small fraction of the added litter, less than 1% was leached as DOC at 10 cm depth. This indicates a strong removal of litter-derived DOC with passage through the mineral soils. The highest litter-derived DOC leaching occurred in the initial soils with the smallest contents of secondary Fe and Al oxides as potential sorbents for DOC. Litter addition had a negligible priming effect on soil respiration and DOC leaching in all three successional stages.

Mineralisation and leaching of C from 13C labelled plant litter along an initial soil chronosequence of a glacier forefield

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Mineralisation and leaching of C from 13C labelled plant litter along an initial soil chronosequence of a glacier forefield