Organic carbon in subsoil generally has longer turnover times than that in surface soil, but little is known about how the stability of the specific organic compound classes changes with soil depth. The objective of this study was to analyze the composition and thermal stability of clay-associated organic matter (OM) at varying soil depths in the summit and footslope of a pasture hillslope using C X-ray absorption near edge structure (XANES) and pyrolysis-field ionization mass spectrometry (Py-FIMS). C XANES showed aromatic C was relatively enriched in the subsoil, relative to the surface soil. Py-FIMS demonstrated a relative enrichment of phenols/lignin monomers and alkylaromatics with increasing profile depth in the summit soil, and to a greater extent in the footslope soil, followed by a decreasing abundance of sterols. In surface soil, the thermostability of clay-associated OM increases in the order: carbohydrates and N compounds < phenols/lignin monomers < lignin dimers and alkylaromatics, suggesting the intrinsic chemical nature of OM as a major driver for OM persistent in surface soil. The thermal stability of clay-associated carbohydrates, N compounds, and phenols/lignin monomers increased with profile depth, likely due to stronger organic-organic/organic-mineral binding. In subsoil, the thermal stability of clay-associated carbohydrates and N compounds can be as high as that of alkylaromatic and lignin dimers, implying that persistent subsoil OM could be composed of organic compound classes, like carbohydrates, that were traditionally considered as biochemically labile compounds. In contrast, the thermally-stable compound classes, like lignin dimers and alkylaromatics, showed no changes in the thermal stability with soil depth. This study suggests that stability of the more labile OM compounds may be more strongly influenced by the change in environmental conditions, relative to the more stable forms.

Cadmium (Cd) has a high toxicity and resolving its speciation in soil is challenging but essential for estimating the environmental risk. In this study partial least-square (PLS) regression was tested for its capability to deconvolute Cd L(3)-edge X-ray absorption near-edge structure (XANES) spectra of multi-compound mixtures. For this, a library of Cd reference compound spectra and a spectrum of a soil sample were acquired. A good coefficient of determination (R(2)) of Cd compounds in mixtures was obtained for the PLS model using binary and ternary mixtures of various Cd reference compounds proving the validity of this approach. In order to describe complex systems like soil, multi-compound mixtures of a variety of Cd compounds must be included in the PLS model. The obtained PLS regression model was then applied to a highly Cd-contaminated soil revealing Cd(3)(PO(4))(2) (36.1%), Cd(NO(3))(2)·4H(2)O (24.5%), Cd(OH)(2) (21.7%), CdCO(3) (17.1%) and CdCl(2) (0.4%). These preliminary results proved that PLS regression is a promising approach for a direct determination of Cd speciation in the solid phase of a soil sample.

Soil organic matter (SOM) is unevenly distributed in arable fields in undulated landscapes, but the chemical composition resulting from their turnover, transport and deposition processes is insufficiently known. Therefore, we aimed at disclosing the molecular-chemical composition of SOM in four different catenae at shoulderslope, backslope and footslope positions in arable fields in the Baltic Sea catchment, Europe. The backslope positions always had the lowest organic C-contents (C_org) (1.6…11.8 g·kg⁻¹) and C-stocks (3.8…8.5 kg·m⁻²) compared to the shoulderslopes and footslopes (1.7…17.7 g·C_org·kg⁻¹, 5.4…15 kg·C_org·m⁻²). In the SOM-poor backslope positions, the organic matter was characterized by relatively high proportions of carbohydrates, phenols + lignin monomers, alkylaromatic compounds, N-compounds and amides, indicating intensive microbial decomposition. By contrast, the footslopes had the largest C_org-contents (9.3…16.5 g·kg⁻¹) and C-stocks (8.9…15 kg·m⁻²) in the catenae and particular enrichments in lipids, lignin dimers, sterols and free fatty acids. These relatively stabile SOM compound classes are interpreted as leftovers from erosive downslope transport and concurrent microbial decomposition, e.g., they are pronounced at backslope positions, followed by restricted microbial decomposition. This heterogeneous SOM distribution calls for an adapted soil management that reduces erosion and places amendments to field areas, such as the shoulderslope and backslope.

The dynamics of soil carbon in grassland are partly determined by soil organic matter (SOM) composition. However, it remains unclear which role grassland management plays in the interplay between SOM composition and carbon dynamics. Using pyrolysis‐field ionization mass spectrometry (Py‐FIMS), we studied the effect of meadow, mown pasture and pasture on the molecular SOM composition in German topsoils. In sandy soils of the Schorfheide‐Chorin region, SOM composition and stability were strongly affected by clay contents and concentrations of crystalline Fe‐oxides. Here, the grassland management type influenced lipid proportions, which accounted for a maximum of 11.1% of the total ion intensity (TII) under mown pasture. In the Hainich‐Dün region, SOM composition was mainly related to the SOM decomposition stage (abundance of potentially recalcitrant compounds) but not to minerals. Compound classes of carbohydrates (4.3% TII), phenols and lignin monomers (8.5% TII), N‐containing compounds (2.2% TII) and peptides (4.6% TII) were highest under meadow, while compound classes of lignin dimers (3.4% TII) and lipids (8.1% TII) were highest under pasture. In the Schwäbische Alb region, the proportion of free fatty acids (1.6 to 2.3% TII) was positively related to the C/N ratio ( r = 0.86); SOM stability was positively affected by poorly crystalline Fe‐oxide content ( r = 0.85). The results suggest that grassland management is affecting SOM composition and stability and thus influence SOM dynamics in grasslands. However, the proportion and composition (Fe‐oxide content) of the soil clay fraction overrode grassland management effects if soil clay/OC ratios were <10.

Over the decades our understanding of lichens has shifted to the fact that they are multiorganismic, symbiotic microecosystems, with their complex interactions coming to the fore due to recent advances in microbiomics. Here, we present a mutualistic-parasitic continuum dynamics scenario between an orange lichen and a lichenicolous fungus from the Atacama Desert leading to the decay of the lichen's photobiont and leaving behind a black lichen thallus. Based on isolation, sequencing, and ecophysiological approaches including metabolic screenings of the symbionts, we depict consequences upon infection with the lichenicolous fungus. This spans from a loss of the lichen's photosynthetic activity and an increased roughness of its surface to an inhibition of the parietin synthesis as a shared pathway between the photobiont and the mycobiont, including a shift of secondary metabolism products. This degree of relations has rarely been documented before, although lichenicolous fungi have been studied for over 200 years, adding an additional level to the view of interactions within lichens.