n-Alkane content and compound-specific d13C values of soil samples, river samples, and marine surface samples

Southwestern Africa's coastal marine mudbelt, a prominent Holocene sediment package, provides a valuable archive for reconstructing terrestrial palaeoclimates on the adjacent continent. While the origin of terrestrial inorganic material has been intensively studied, the sources of terrigenous organic material deposited in the mudbelt are yet unclear. In this study, plant wax derived n-alkanes and their compound-specific d13C in soils, flood deposits and suspension loads from regional fluvial systems and marine sediments are analysed to characterize the origin of terrestrial organic material in the southwest African mudbelt. Soils from different biomes in the catchments of the Orange River and small west coast rivers show on average distinct n-alkane distributions and compound-specific d13C values reflecting biome-specific vegetation types, most notably the winter rainfall associated Fynbos Biome of the southwestern Cape. In the fluvial sediment samples from the Orange River, changes in the n-alkane distributions and compound-specific d13C compositions reveal an overprint by local vegetation along the river's course. The smaller west coast rivers show distinct signals, reflecting their small catchment areas and particular vegetation communities. Marine surface sediments spanning a transect from the northern mudbelt (29°S) to St. Helena Bay (33°S) reveal subtle, but spatially coherent, changes in n-alkane distributions and compound-specific d13C, indicating the influence of Orange River sediments in the northern mudbelt, the increasing importance of terrigenous input from the adjacent western coastal biomes in the central mudbelt, and contributions from the Fynbos Biome to the southern mudbelt. These findings indicate the different sources of terrestrial organic material deposited in the mudbelt, and highlight the potential the mudbelt has to preserve evidence of environmental change from the adjacent continent.

Soil properties and microbial indicators of samples from Lake Wellman, Darwin Mountains, Antarctica

Four pedons on each of four drift sheets in the Lake Wellman area of the Darwin Mountains were sampled for chemical and microbial analyses. The four drifts, Hatherton, Britannia, Danum, and Isca, ranged from early Holocene (10 ka) to mid-Quaternary (c. 900 ka). The soil properties of weathering stage, salt stage, and depths of staining, visible salts, ghosts, and coherence increase with drift age. The landforms contain primarily high-centred polygons with windblown snow in the troughs. The soils are dominantly complexes of Typic Haplorthels and Typic Haploturbels. The soils were dry and alkaline with low levels of organic carbon, nitrogen and phosphorus. Electrical conductivity was high accompanied by high levels of water soluble anions and cations (especially calcium and sulphate in older soils). Soil microbial biomass, measured as phospholipid fatty acids, and numbers of culturable heterotrophic microbes, were low, with highest levels detected in less developed soils from the Hatherton drift. The microbial community structure of the Hatherton soil also differed from that of the Britannia, Danum and Isca soils. Ordination revealed the soil microbial community structure was influenced by soil development and organic carbon.

Soil and sediment analyses of Lake La Thuile (Bauges, France)

Data used in the study of the evolution of soils of Lake La Thuile catchment, in relation with the long sediment sequence of the lake. Data of pH, Loss On Ignition, Oxygen and Hydrogen Index (Rock-eval analyses) and mineral geochemistry (Portative XRF, Al2O3/TiO2 and K2O/TiO2 ratios) are available for each soil horizons that have been studied in the catchment. For sediments, data of Oxygen and Hydrogen Index (Rock-eval analyses), mineral geochemistry (Portative XRF, Al2O3/TiO2 and K2O/TiO2 ratios), erosion, soil evolution modelization and the ages are available according to depth.

Redistribution of Soil Organic Matter by Permafrost Disturbance in the Canadian High Arctic

With increased warming in the Arctic, permafrost thaw may induce localized physical disturbance of slopes. These disturbances, referred to as active layer detachments (ALDs), redistribute soil across the landscape, potentially releasing previously unavailable carbon (C). In 2007–2008, widespread ALD activity was reported at the Cape Bounty Arctic Watershed Observatory in Nunavut, Canada. Our study investigated organic matter (OM) composition in soil profiles from ALD-impacted and undisturbed areas. Solid-state 13C nuclear magnetic resonance (NMR) and solvent-extractable biomarkers were used to characterize soil OM. Throughout the disturbed upslope profile, where surface soils and vegetation had been removed, NMR revealed low O-alkyl C content and biomarker analysis revealed low concentrations of solvent-extractable compounds suggesting enhanced erosion of labile-rich OM by the ALD. In the disturbed downslope region, vegetation remained intact but displaced material from upslope produced lateral compression ridges at the surface. High O-alkyl content in the surface horizon was consistent with enrichment of carbohydrates and peptides, but low concentrations of labile biomarkers (i.e., sugars) suggested the presence of relatively unaltered labile-rich OM. Decreased O-alkyl content and biomarker concentrations below the surface contrasted with the undisturbed profile and may indicate the loss of well-established pre-ALD surface drainage with compression ridge formation. However, pre-ALD profile composition remains unknown and the observed decreases may result from nominal pre-ALD OM inputs. These results are the first to establish OM composition in ALD-impacted soil profiles, suggesting reallocation of permafrost-derived soil C to areas where degradation or erosion may contribute to increased C losses from disturbed Arctic soils.

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.

Séquestration géologique du CO2 par carbonatation minérale dans les résidus miniers

La carbonatation minérale dans les résidus miniers est un moyen sûr et permanent de séquestrer le CO2 atmosphérique. C’est un processus naturel et passif qui ne nécessite aucun traitement particulier et donc avantageux d’un point de vue économique. Bien que la quantité de CO2 qu’il soit possible de séquestrer selon ce processus est faible à l’échelle globale, dans le cadre d’un marché du carbone, les entreprises minières pourraient obtenir des crédits et ainsi revaloriser leurs résidus. À l’heure actuelle, il y a peu d’informations pour quantifier le potentiel de séquestration du CO2 de façon naturelle et passive dans les piles de résidus miniers. Il est donc nécessaire d’étudier le phénomène pour comprendre comment évolue la réaction à travers le temps et estimer la quantité de CO2 qui peut être séquestrée naturellement dans les piles de résidus. Plusieurs travaux de recherche se sont intéressés aux résidus miniers de Thetford Mines (Québec, Canada), avec une approche principalement expérimentale en laboratoire. Ces travaux ont permis d’améliorer la compréhension du processus de carbonatation, mais ils nécessitent une validation à plus grande échelle sous des conditions atmosphériques réelles. L’objectif général de cette étude est de quantifier le processus de carbonatation minérale des résidus miniers sous des conditions naturelles, afin d’estimer la quantité de CO2 pouvant être piégée par ce processus. La méthodologie utilisée repose sur la construction de deux parcelles expérimentales de résidus miniers situées dans l’enceinte de la mine Black Lake (Thetford Mines). Les résidus miniers sont principalement constitués de grains et de fibres de chrysotile et lizardite mal triés, avec de petites quantités d’antigorite, de brucite et de magnétite. Des observations spatiales et temporelles ont été effectuées dans les parcelles concernant la composition et la pression des gaz, la température des résidus, la teneur en eau volumique, la composition minérale des résidus ainsi que la chimie de l’eau des précipitations et des lixiviats provenant des parcelles. Ces travaux ont permis d’observer un appauvrissement notable du CO2 dans les gaz des parcelles (< 50 ppm) ainsi que la précipitation d’hydromagnésite dans les résidus, ce qui suggère que la carbonatation minérale naturelle et passive est un processus potentiellement important dans les résidus miniers. Après 4 ans d’observations, le taux de séquestration du CO2 dans les parcelles expérimentales a été estimé entre 3,5 et 4 kg/m3/an. Ces observations ont permis de développer un modèle conceptuel de la carbonatation minérale naturelle et passive dans les parcelles expérimentales. Dans ce modèle conceptuel, le CO2 atmosphérique (~ 400 ppm) se dissout dans l’eau hygroscopique contenue dans les parcelles, où l’altération des silicates de magnésium forme des carbonates de magnésium. La saturation en eau dans les cellules est relativement stable dans le temps et varie entre 0,4 et 0,65, ce qui est plus élevé que les valeurs de saturation optimales proposées dans la littérature, réduisant ainsi le transport de CO2 dans la zone non saturée. Les concentrations de CO2 en phase gazeuse, ainsi que des mesures de la vitesse d’écoulement du gaz dans les cellules suggèrent que la réaction est plus active près de la surface et que la diffusion du CO2 est le mécanisme de transport dominant dans les résidus. Un modèle numérique a été utilisé pour simuler ces processus couplés et valider le modèle conceptuel avec les observations de terrain. Le modèle de transport réactif multiphase et multicomposant MIN3P a été utilisé pour réaliser des simulations en 1D qui comprennent l’infiltration d’eau à travers le milieu partiellement saturé, la diffusion du gaz, et le transport de masse réactif par advection et dispersion. Même si les écoulements et le contenu du lixivat simulés sont assez proches des observations de terrain, le taux de séquestration simulé est 22 fois plus faible que celui mesuré. Dans les simulations, les carbonates précipitent principalement dans la partie supérieure de la parcelle, près de la surface, alors qu’ils ont été observés dans toute la parcelle. Cette différence importante pourrait être expliquée par un apport insuffisant de CO2 dans la parcelle, qui serait le facteur limitant la carbonatation. En effet, l’advection des gaz n’a pas été considérée dans les simulations et seule la diffusion moléculaire a été simulée. En effet, la mobilité des gaz engendrée par les fluctuations de pression barométrique et l’infiltration de l’eau, ainsi que l’effet du vent doivent jouer un rôle conséquent pour alimenter les parcelles en CO2.

The impact of organic amendments on soil properties under mediterranean climatic conditions

Soil erosion and unsustainable land use produce adverse effects on SOC content. Soil management techniques and corrections can be applied for soil recovery, especially, with afforestaion purposes. This study presents the short term effects on the application of different treatments on soil properties for soil included in several sets of closed plots located in the experimental area of Pinarillo (Nerja, Spain). The analysed soil properties were: PH, EC, organic carbon, total nitrogen and total carbon. In order to verify possible differences, we applied the test of Mann-Whitney U in corroboration with the previous homogeneity test of variance.

Effects of Soil Organic Matter Properties and Microbial Community Composition on Enzyme Activities in Cryoturbated Arctic Soils

Enzyme-mediated decomposition of soil organic matter (SOM) is controlled, amongst other factors, by organic matter properties and by the microbial decomposer community present. Since microbial community composition and SOM properties are often interrelated and both change with soil depth, the drivers of enzymatic decomposition are hard to dissect. We investigated soils from three regions in the Siberian Arctic, where carbon rich topsoil material has been incorporated into the subsoil (cryoturbation). We took advantage of this subduction to test if SOM properties shape microbial community composition, and to identify controls of both on enzyme activities. We found that microbial community composition (estimated by phospholipid fatty acid analysis), was similar in cryoturbated material and in surrounding subsoil, although carbon and nitrogen contents were similar in cryoturbated material and topsoils. This suggests that the microbial community in cryoturbated material was not well adapted to SOM properties. We also measured three potential enzyme activities (cellobiohydrolase, leucine-amino-peptidase and phenoloxidase) and used structural equation models (SEMs) to identify direct and indirect drivers of the three enzyme activities. The models included microbial community composition, carbon and nitrogen contents, clay content, water content, and pH. Models for regular horizons, excluding cryoturbated material, showed that all enzyme activities were mainly controlled by carbon or nitrogen. Microbial community composition had no effect. In contrast, models for cryoturbated material showed that enzyme activities were also related to microbial community composition. The additional control of microbial community composition could have restrained enzyme activities and furthermore decomposition in general. The functional decoupling of SOM properties and microbial community composition might thus be one of the reasons for low decomposition rates and the persistence of 400 Gt carbon stored in cryoturbated material.

Rate of Belowground Carbon Allocation Differs with Successional Habit of Two Afromontane Trees

Background: Anthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO2 efflux is crucial for addressing the carbon footprint of creeping degradation. Methodology: We compared the carbon allocation pattern of the late successional gymnosperm Podocarpus falcatus (Thunb.) Mirb. and the early successional (gap filling) angiosperm Croton macrostachyus Hochst. es Del. in an Ethiopian Afromontane forest by whole tree (CO2)-C-13 pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO2. Further, we quantified the overall losses of assimilated C-13 with soil CO2 efflux. Principal Findings: C-13 in leaves of C. macrostachyus declined more rapidly with a larger size of a fast pool (64% vs. 50% of the assimilated carbon), having a shorter mean residence time (14 h vs. 55 h) as in leaves of P. falcatus. Phloem sap velocity was about 4 times higher for C. macrostachyus. Likewise, the label appeared earlier in the arbuscular mycorrhiza of C. macrostachyus and in the soil CO2 efflux as in case of P. falcatus (24 h vs. 72 h). Within one year soil CO2 efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one. Conclusions: Our results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e. g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.

Ecological stoichiometry controls the transformation and retention of plant-derived organic matter to humus in response to nitrogen fertilisation

Carbon (C) sequestration in soils is a means for increasing soil organic carbon (SOC) stocks and is a potential tool for climate change mitigation. One recommended management practice to increase SOC stocks is nitrogen (N) fertilisation, however examples of positive, negative or null SOC effects in response to N addition exist. We evaluated the relative importance of plant molecular structure, soil physical properties and soil ecological stoichiometry in explaining the retention of SOC with and without N addition. We tracked the transformation of 13C pulse-labelled buffel grass (Cenchrus ciliaris L.), wheat (Triticum aestivum L.) and lucerne (Medicago sativa L.) material to the <53 μm silt + clay soil organic C fraction, hereafter named “humus”, over 365-days of incubation in four contrasting agricultural soils, with and without urea-N addition. We hypothesised that: a) humus retention would be soil and litter dependent; b) humus retention would be litter independent once litter C:N ratios were standardised with urea-N addition; and c) humus retention would be improved by urea-N addition. Two and three-way factorial analysis of variance indicated that 13C humus was consistently soil and litter dependent, even when litter C:N ratios were standardised, and that the effect of urea-N addition on 13C humus was also soil and litter dependent. A boosted regression analysis of the effect of 44 plant and soil explanatory variables demonstrated that soil biological and chemical properties had the greatest relative influence on 13C humus. Regression tree analyses demonstrated that the greatest gains in 13C humus occurred in soils of relatively low total organic C, dissolved organic C and microbial biomass C (MBC), or with a combination of relatively high MBC and low C:N ratio. The greatest losses in 13C humus occurred in soils with a combination of relatively high MBC and low total N or increasing C:N ratio. We conclude that soil variables involved in soil ecological stoichiometry exert a greater relative influence on incorporating organic matter as humus compared to plant molecular structure and soil physical properties. Furthermore, we conclude that the effect of N fertilisation on humus retention is dependent upon soil ecological stoichiometry.

Conversion of sub-tropical native vegetation to introduced conifer forest: Impacts on below-ground and above-ground carbon pools

Land-use change can have a major influence on soil organic carbon (SOC) and above-ground C pools. We assessed a change from native vegetation to introduced Pinus species plantations on C pools using eight paired sites. At each site we determined the impacts on 0–50 cm below-ground (SOC, charcoal C, organic matter C, particulate organic C, humic organic C, resistant organic C) and above-ground (litter, coarse woody debris, standing trees and woody understorey plants) C pools. In an analysis across the different study sites there was no significant difference (P > 0.05) in SOC or above-ground tree C stocks between paired native vegetation and pine plantations, although significant differences did exist at specific sites. SOC (calculated based on an equivalent soil mass basis) was higher in the pine plantations at two sites, higher in the native vegetation at two sites and did not differ for the other four sites. The site to site variation in SOC across the landscape was far greater than the variation observed with a change from native vegetation to introduced Pinus plantation. Differences between sites were not explained by soil type, although tree basal area was positively correlated with 0–50 cm SOC. In fact, in the native vegetation there was a significant linear relationship between above-ground biomass and SOC that explained 88.8% of the variation in the data. Fine litter C (0–25 mm diameter) tended to be higher in the pine forest than in the adjacent native vegetation and was significantly higher in the pine forest at five of the eight paired sites. Total litter C (0–100 mm diameter) increased significantly with plantation age (R2 = 0.64). Carbon stored in understorey woody plants (2.5–10 cm DBH) was higher in the native vegetation than in the adjacent pine forest. Total site C varied greatly across the study area from 58.8 Mg ha−1 at a native heathland site to 497.8 Mg ha−1 at a native eucalypt forest site. Our findings suggest that the effects of change from native vegetation to introduced Pinus sp. forest are highly site-specific and may be positive, negative, or have no influence on various C pools, depending on local site characteristics (e.g. plantation age and type of native vegetation).

Influence of mucuna fallow crop on selected soil properties, weed suppression and taro yields in Taveuni, Fiji

Two field experiments were carried out in Taveuni, Fiji to study the effects of mucuna (Mucuna pruriens) and grass fallow systems at 6 and 12 month durations on changes in soil properties (Experiment 1) and taro yields (Experiment 2). Biomass accumulation of mucuna fallow crop was significantly higher (P<0.05) than grass fallow crop at both 6 and 12 month durations. The longer fallow duration resulted in higher (P<0.05) total soil organic carbon, total soil nitrogen and earthworm numbers regardless of fallow type. Weed suppression in taro grown under mucuna was significantly greater (P<0.05) than under natural grass fallow. Taro grown under mucuna fallow significantly outyielded taro grown under grass fallow (11.8 vs. 8.8 t ha-1). Also, the gross margin of taro grown under mucuna fallow was 52% higher than that of taro grown under grass fallow. © ISHS.

Bioeconomic modelling of woody regrowth carbon offset options in productive grazing systems

Agricultural land has been identified as a potential source of greenhouse gas emissions offsets through biosequestration in vegetation and soil. In the extensive grazing land of Australia, landholders may participate in the Australian Government’s Emissions Reduction Fund and create offsets by reducing woody vegetation clearing and allowing native woody plant regrowth to grow. This study used bioeconomic modelling to evaluate the trade-offs between an existing central Queensland grazing operation, which has been using repeated tree clearing to maintain pasture growth, and an alternative carbon and grazing enterprise in which tree clearing is reduced and the additional carbon sequestered in trees is sold. The results showed that ceasing clearing in favour of producing offsets produces a higher net present value over 20 years than the existing cattle enterprise at carbon prices, which are close to current (2015) market levels (~$13 t–1 CO2-e). However, by modifying key variables, relative profitability did change. Sensitivity analysis evaluated key variables, which determine the relative profitability of carbon and cattle. In order of importance these were: the carbon price, the gross margin of cattle production, the severity of the tree–grass relationship, the area of regrowth retained, the age of regrowth at the start of the project, and to a lesser extent the cost of carbon project administration, compliance and monitoring. Based on the analysis, retaining regrowth to generate carbon income may be worthwhile for cattle producers in Australia, but careful consideration needs to be given to the opportunity cost of reduced cattle income.