Impact of Climate on Soil Organic Matter Composition in Soils of Tropical Volcanic Regions Revealed by EGA-MS and Py-GC/MS

Soil organic matter (SOM) crucially influences the global carbon cycle, yet its molecular composition and determinants are understudied, especially for tropical volcanic regions. We investigated how SOM compounds change in response to climate, vegetation, soil horizon, and soil properties and the relationship between SOM composition and microbial decomposability in Tanzanian and Indonesian volcanic regions. We collected topsoil (0-15 cm) and subsoil (20-40 cm) horizons (n = 22; pH: 4.6-7.6; SOC: 10-152 g kg-1) with undisturbed vegetation and wide mean annual temperature and moisture ranges (14-26 °C; 800-3300 mm) across four elevational transects (340-2210 m asl.). Evolved gas analysis-mass spectrometry (EGA-MS) documented a simultaneous release of SOM compounds and clay mineral dehydroxylation. Subsequently applying double-shot pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) at 200 and 550 °C, we detailed the molecular composition of topsoil and subsoil SOM. A minor portion (2.7 ± 1.9%) of compounds desorbed at 200 °C, limiting its efficacy for investigating overall SOM characteristics. Pyrolyzed SOM closely aligns with the intermediate decomposable SOM pool, with most pyrolysates (550 °C) originating from this pool. Pyrolysates composition suggests tropical SOM is mainly microbial-derived and subsoil contains more degraded compounds. Higher litter inputs and attenuated SOM decomposition due to cooler temperatures and lower soil pH (<5.5) produce less-degraded SOM at higher elevations. Redundancy analyses revealed the crucial role of active Al/Fe (oxalate-extractable Al/Fe), abundant in low-temperature/high-moisture conditions, in stabilizing these less-degraded components. Our findings provide new insights into SOM molecular composition and its determinants, critical for understanding the carbon cycle in tropical ecosystems.

Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in Anoxic Soils

It is known that the association of soil organic matter (SOM) with iron minerals limits carbon mobilization and degradation in aerobic soils and sediments. However, the efficacy of iron mineral protection mechanisms under reducing soil conditions, where Fe(III)-bearing minerals may be used as terminal electron acceptors, is poorly understood. Here, we quantified the extent to which iron mineral protection inhibits mineralization of organic carbon in reduced soils by adding dissolved ¹³C-glucuronic acid, a ⁵⁷Fe-ferrihydrite-¹³C-glucuronic acid coprecipitate, or pure ⁵⁷Fe-ferrihydrite to anoxic soil slurries. In tracking the re-partitioning and transformation of ¹³C-glucuronic acid and native SOM, we find that coprecipitation suppresses mineralization of ¹³C-glucuronic acid by 56% after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks, owing to ongoing reductive dissolution of the coprecipitated ⁵⁷Fe-ferrihydrite. Addition of both dissolved and coprecipitated ¹³C-glucuronic acid resulted in increased native SOM mineralization, but the reduced bioavailability of the coprecipitated versus dissolved ¹³C-glucuronic acid decreased the priming effect by 35%. In contrast, the addition of pure ⁵⁷Fe-ferrihydrite resulted in negligible changes in native SOM mineralization. Our results show that iron mineral protection mechanisms are relevant for understanding the mobilization and degradation of SOM under reducing soil conditions.

Beyond bulk: Density fractions explain heterogeneity in global soil carbon abundance and persistence

Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of environmental factors on soil organic C partitioning among pools, abundance in each pool (mg C g^-1  soil), and persistence (as approximated by radiocarbon abundance) in relatively unprotected particulate and protected mineral-bound pools. We show that C within particulate and mineral-associated pools consistently differed from one another in degree of persistence and relationship to environmental factors. Soil depth was the best predictor of C abundance and persistence, though it accounted for more variance in persistence. Persistence of all C pools decreased with increasing mean annual temperature (MAT) throughout the soil profile, whereas persistence increased with increasing wetness index (MAP/PET) in subsurface soils (30-176 cm). The relationship of C abundance (mg C g^-1  soil) to climate varied among pools and with depth. Mineral-associated C in surface soils (<30 cm) increased more strongly with increasing wetness index than the free particulate C, but both pools showed attenuated responses to the wetness index at depth. Overall, these relationships suggest a strong influence of climate on soil C properties, and a potential loss of soil C from protected pools in areas with decreasing wetness. Relative persistence and abundance of C pools varied significantly among land cover types and soil parent material lithologies. This variability in each pool's relationship to environmental factors suggests that not all soil organic C is equally vulnerable to global change. Therefore, projections of future soil organic C based on patterns and responses of bulk soil organic C may be misleading.

Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon

Considerable portions of the total mobile inventory of soil seepage are the diverse colloidal and larger suspended materials that essentially contribute to pedogenesis, soil functioning, and nutritional supply of subsurface ecosystems. However, the size- and material-spectra of the total mobile inventory, and field-scale factors controlling its long-term seasonal and episodic dynamics in undisturbed soil, are scarcely investigated so far. In a 4.5-year field-scale study, we utilized automated tension-controlled lysimeters optimized for in situ-sampling of total mobile inventory. Covering different land uses in a low-mountain groundwater recharge area in central Germany, seepage of top- and subsoil was collected at least biweekly and analyzed by hydrochemical and spectromicroscopic techniques (SEM/EDX, nanoparticle tracking analysis). In undisturbed soil, diverse mineral-, mineral-organic, organic, and bioparticles (microbial cells, biotic detritus) up to 75 μm was mobile. Atmospheric forcing was the major factor that governed transport of the total mobile inventory, causing considerable seasonality in seepage pH and certain solutes (e.g. sulphate), as well as episodic fluctuation of particulates. Especially episodic high-flow events, like those following snow melts and lasting rainstorms, primarily contributed to the export of inorganic/organic matter beyond the subsoil-regolith boundary. Individual infiltration events during winter accounted for up to 80% of annual fluxes of particulate organic carbon. On average, a significant proportion of 21% of the mobile organic carbon belonged to the >0.45 μm fraction. The pedological setting and land use mostly impacted the solute signature but were of minor importance for the particle load. Our ongoing monitoring provides evidence of significant episodic nutrient fluxes and unveiled pronounced temporal patterns of field-scale pH fluctuations. We conclude that dynamics of the total mobile inventory, including particulates >0.45 μm must be considered in approaches that budget carbon and elemental fluxes, but also in concepts and models on nutrient cycles and subsurface ecosystem functioning.

Adsorption and Molecular Fractionation of Dissolved Organic Matter on Iron-Bearing Mineral Matrices of Varying Crystallinity

Iron (Fe)-bearing mineral phases contribute disproportionately to adsorption of soil organic matter (SOM) due to their elevated chemical reactivity and specific surface area (SSA). However, the spectrum of Fe solid-phase speciation present in oxidation-reduction-active soils challenges analysis of SOM-mineral interactions and may induce differential molecular fractionation of dissolved organic matter (DOM). This work used paired selective dissolution experiments and batch sorption of postextraction residues to (1) quantify the contributions of Fe-bearing minerals of varying crystallinity to DOM sorption, and (2) characterize molecular fractionation using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). A substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-MS analysis of extracted DOM revealed distinct chemical signatures across Fe-OM associations. Sorbed carbon (C) was highly correlated with Fe concentrations, suggesting that Fe-bearing phases are strong drivers of sorption in these soils. Molecular fractionation was observed across treatments, particularly those dominated by short-range-order (SRO) mineral phases, which preferentially adsorbed aromatic and lignin-like formulas, and higher-crystallinity phases, associated with aliphatic DOM. These findings suggest Fe speciation-mediated complexation acts as a physicochemical filter of DOM moving through the critical zone, an important observation as predicted changes in precipitation may dynamically alter Fe crystallinity and C stability.

Steep spatial gradients of volcanic and marine sulfur in Hawaiian rainfall and ecosystems

Sulfur, a nutrient required by terrestrial ecosystems, is likely to be regulated by atmospheric processes in well-drained, upland settings because of its low concentration in most bedrock and generally poor retention by inorganic reactions within soils. Environmental controls on sulfur sources in unpolluted ecosystems have seldom been investigated in detail, even though the possibility of sulfur limiting primary production is much greater where atmospheric deposition of anthropogenic sulfur is low. Here we measure sulfur isotopic compositions of soils, vegetation and bulk atmospheric deposition from the Hawaiian Islands for the purpose of tracing sources of ecosystem sulfur. Hawaiian lava has a mantle-derived sulfur isotopic composition (δ(34)S VCDT) of -0.8‰. Bulk deposition on the island of Maui had a δ(34)S VCDT that varied temporally, spanned a range from +8.2 to +19.7‰, and reflected isotopic mixing from three sources: sea-salt (+21.1‰), marine biogenic emissions (+15.6‰), and volcanic emissions from active vents on Kilauea Volcano (+0.8‰). A straightforward, weathering-driven transition in ecosystem sulfur sources could be interpreted in the shift from relatively low (0.0 to +2.7‰) to relatively high (+17.8 to +19.3‰) soil δ(34)S values along a 0.3 to 4100 ka soil age-gradient, and similar patterns in associated vegetation. However, sub-kilometer scale spatial variation in soil sulfur isotopic composition was found along soil transects assumed by age and mass balance to be dominated by atmospheric sulfur inputs. Soil sulfur isotopic compositions ranged from +8.1 to +20.3‰ and generally decreased with increasing elevation (0-2000 m), distance from the coast (0-12 km), and annual rainfall (180-5000 mm). Such trends reflect the spatial variation in marine versus volcanic inputs from atmospheric deposition. Broadly, these results illustrate how the sources and magnitude of atmospheric deposition can exert controls over ecosystem sulfur biogeochemistry across relatively small spatial scales.