Browsing by Author "Schmidt, S"

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    Investigation of the mechanisms driving ancient soil carbon stability in deep B-horizons of a giant podzol chronosequence (Cooloola)
    (American Geophysical Union, 2018-12-13) Jones, AR; Sanderman, J; Dalal, RC; Jacobsen, GE; Grandy, AS; Schmidt, S; Gupta, VVSR; Oudyn, F
    Despite deep subsoil soil organic matter (SOM) being rich in nutrients and highly palatable for microbial decomposition, it does not completely decompose, as indicated by universally observed increasing SOM turnover time (radiocarbon age) with depth. It is not entirely clear whether this is a result of unfavourable environmental conditions, mineral protection of organic matter or carbon substrate limitation due to distance from plant inputs. In this study, we have assessed the biogeochemical mechanisms driving ancient SOM stability in deep B-horizons of a giant podzol chronosequence (Cooloola sand dune chronosequence, Queensland, Australia). The chronosequence is characterised by a sequence of spodic B-horizons of increasing depth (from 0.5 to 15 m) with overlying leached E horizons comprised of weathered, clean quartz grains likewise extending with dune age. In this way, the chronosequence provides a unique model system to track depth-related changes to SOM composition and stability in the mineral-rich B-horizon at subsoil horizon depths rarely characterised before. Categorising products from pyrolysis-GC-MS into their respective origins revealed a clear transition of SOM composition primarily from plant-derived SOM (lignins, polysaccharides and N-bearing compounds) in the shallow A-horizons (0.1 m) to microbial-derived SOM (proteins, aromatics) in the deepest B horizon (15 m; Figure 1). These observations were accompanied by increasing SOM radiocarbon age with depth, indicating that the stabilisation of SOM was attributed to depth-related processes. Complexation with aluminium in the B-horizon appears to be the main driver of carbon accumulation in these podzols (more so than iron), pointing to geochemical stabilisation of SOM. In this work, we discuss whether proteins and aromatics have accumulated at depth due to preferential aluminium-complexation or if these compounds are the products of long-term degradation processes by the deep soil microbial community.
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    Molecular complexity and diversity of persistent soil organic matter
    (Elseiver B. V., 2023-09) Jones, AR; Dalal, RC; Gupta, VVSR; Schmidt, S; Allen, DE; Jacobsen, GE; Bird, MI; Grandy, AS; Sanderman, J
    Managing and increasing organic matter in soil requires greater understanding of the mechanisms driving its persistence through resistance to microbial decomposition. Conflicting evidence exists for whether persistent soil organic matter (SOM) is molecularly complex and diverse. As such, this study used a novel application of graph networks with pyrolysis-gas chromatography-mass spectrometry to quantify the complexity and diversity of persistent SOM, defined as SOM that persists through time (soil radiocarbon age) and soil depth. We analyzed soils from the Cooloola giant podzol chronosequence across a large gradient of soil depths (0–15 m) and SOM radiocarbon ages (modern to 19,000 years BP). We found that the most persistent SOM on this gradient was highly aromatic and had the lowest molecular complexity and diversity. By contrast, fresh surface SOM had higher molecular complexity and diversity, with high contributions of plant-derived lignins and polysaccharides. These findings indicate that persisting SOM declines in molecular complexity and diversity over geological timescales and soil depths, with aromatic SOM compounds persisting longer with mineral association. © 2023 Elsevier Ltd