A Microbial Link to Weathering of Postglacial Rocks and Sediments, Mount Viso Area, Western Alps, Demonstrated through Analysis of a Soil/Paleosol Bio/Chronosequence

Understanding the mechanism associated with rates of weathering and evolution of rocks→sediment→soil→paleosol in alpine environments raises questions related to the impact of microbial mediation versus various diverse abiotic chemical/physical processes, even including the overall effect of cosmic impact/airburst during the early stage of weathering in Late Glacial (LG) deposits. This study is of a chronosequence of soils/paleosols, with an age range that spans the post–Little Ice Age (post-LIA; <150 yr), the Little Ice Age (LIA; AD 1500–1850), the middle Neoglacial (∼3 ka)–Younger Dryas (YD; <12.8 ka), and the LG (<15 ka). The goal is to elicit trends in weathering, soil morphogenesis, and related eubacterial population changes over the past 13–15 k.yr. The older LG/YD paleosols in the sequence represent soil morphogenesis that started during the closing stage of Pleistocene glaciation. These are compared with undated soils of midto late Neoglacial age, the youngest of LIA and post-LIA age. All profiles formed in a uniform parentmaterial ofmetabasalt composition and in moraine, rockfall, protalus, and alluvial fan deposits. Elsewhere in Europe,North America, and Asia, the cosmic impact/airburst event at 12.8 ka often produced a distinctive, carbon-rich “black mat” layer that shows evidence of high-temperature melting. At this alpine site, older profiles of similar LG age contain scorched and melted surface sediments that are otherwise similar in composition to the youngest/thinnest profiles developing in the catchment today. Moreover, microbial analysis of the sediments offers new insight into the genesis of these sediments: the C and Cu (u = unweathered) horizons in LG profiles present at 12.8 ka (now Ah/Bw) show bacterial population structures that differ markedly from recent alluvial/protalus sample bacterial populations. We propose here that these differences are, in part, a direct consequence of the age/cosmic impact/weathering processes that have occurred in the chronosequence. Of the several questions that emerge from these sequences, perhaps the most important involve the interaction of biotic-mineral factors, which need to be understood if we are to generally fully appreciate the role played by microbes in rock weathering.

Eating from the same plate? Revisiting the role of labile carbon inputs in the soil food web

An increasing number of empirical studies are challenging the central fundamentals on which the classical soil food web model is built. This model assumes that bacteria consume labile substrates twice as fast as fungi, and that mycorrhizal fungi do not decompose organic matter. Here, we build on emerging evidence that points to significant consumption of labile C by fungi, and to the ability of ectomycorrhizal fungi to decompose organic matter, to show that labile C constitutes a major and presently underrated source of C for the soil food web. We use a simple model describing the dynamics of a recalcitrant and a labile C pool and their consumption by fungi and bacteria to show that fungal and bacterial populations can coexist in a stable state with large inputs into the labile C pool and a high fungal use of labile C. We propose a new conceptual model for the bottom trophic level of the soil food web, with organic C consisting of a continuous pool rather than two or three distinct pools, and saprotrophic fungi using substantial amounts of labile C. Incorporation of these concepts will increase our understanding of soil food web dynamics and functioning under changing conditions.