Cornelia Rumpel

Stability of organic carbon in deep soil layers controlled by fresh carbon supply

The world’s soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.

Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation

Understanding the origin of the carbon (C) stabilised in soils is crucial in order to device management practices that will foster Caccumulation in soils. The relative contributions to soilC pools of roots vs. shoots is one aspect that has been mostly overlooked, although it appears a key factor that drives the fate of plant tissueC either as mineralized CO2 or as stabilized soil organic matter (SOM). Available studies on the subject consistently indicate that rootC has a longer residence time in soil than shootC. From the few studies with complete datasets, we estimated that the mean residence time in soils of root-derived C is 2.4times that of shoot-derived C. Our analyses indicate that this value is biased neither by an underestimation of root contributions, as exudation was considered in the analysis, nor by a priming effect of shoot litter on SOM. Here, we discuss the main SOM stabilisation mechanisms with respect to their ability to specifically protect root-derived SOM. Comparing in situ and incubation experiments suggests that the higher chemical recalcitrance of root tissues as compared to that of shoots is responsible for only a small portion, i.e. about one fourth, of the difference in mean residence time in soils of root-derived vs. shoot-derivedC. This suggests that SOM protection mechanisms other than chemical recalcitrance are also enhanced by root activities: (1)physico-chemical protection, especially in deeper horizons, (2)micrometer-scale physical protection through myccorhiza and root-hair activities, and (3)chemical interactions with metal ions. The impact of environmental conditions within deeper soil layers on rootC stabilisation appear difficult to assess, but is likely, if anything, to further increase the ratio between the mean residence time of root vs. shootC in soils. Future advances are expected from isotopic studies conducted at the molecular level, which will help unravel the fate of individual shoot and root compounds, such as cutins and suberins, throughout soil profiles.

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.

Vertical distribution, age, and chemical composition of organic carbon in two forest soils of different pedogenesis

Recent carbon inventories have shown that significant amounts of soil organic matter (SOM), even though at low concentrations, can be stored in the subsoil (B and C horizons). Its quantity, turnover and chemical composition are largely unknown. The objective of the present study was to analyse the organic matter in the mineral horizons of two forest soils with different pedogenetic history and to assess the quantity, turnover and chemical composition of SOM stored in the subsoil compared to the topsoil (A horizon and litter layers). Samples were taken from a Dystric Cambisol under beech and a Haplic Podzol under spruce forest down to a depth of 140 and 80 cm, respectively. They were analysed for elemental composition, carbon storage and chemical structure of SOM by 13C CPMAS NMR spectroscopy, radiocarbon age by accelerator mass spectrometry and plant derived phenols as well as hydroxyalkanoic acids by CuO oxidation. Special attention was drawn to the contribution of phenols and hydroxyalkanoic acids, because they are major contributors of root litter. Up to 75% of the organic carbon present in the mineral soil of the two profiles was found below the A horizon. Radiocarbon measurements showed that the organic carbon in the subsoil had an apparent age of several thousand years. The structural analyses indicated a similar chemical composition of the organic matter present in the litter layers of both soils. In the mineral soils, the chemical composition of the SOM differed according to the pedogenetic processes operating at the two sites. A high contribution of alkyl carbon was recorded in the B horizons of the Dystric Cambisol which may be partly explained by the contribution of hydroxyalkanoic acids which are preserved preferentially in this soil compared to phenols. In the Haplic Podzol, spectra of the B horizons indicate a higher contribution of O-alkyl and carboxylic carbon due to carbon leaching during podzolisation. In the C horizons of both soils, most of the organic carbon was mobilised after demineralisation by treatment with 10% hydrofluouric acid (HF) and may therefore be adsorbed to the soil minerals. Our data indicated that there is, apart from root litter, a strong influence of soil-forming processes on the composition of organic carbon in subsoils.

Stabilization of organic matter by soil minerals — investigations of density and particle‐size fractions from two acid forest soils

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle-size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF-soluble, or located in the density fraction d >1.6 g cm—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm—3 were closely correlated to the contents of oxalate and dithionite-citrate-bicarbonate-extractable Fe (r2 = 0.93 and 0.88, P <0.001). SEM-EDX analyses suggested associations of OC with aluminum whereas silicon-enriched regions were poor in OC. The specific surface area and the microporosity of the soil mineral matrix after destruction of organic matter were less closely correlated to OC than the extractable iron fractions. This is possibly due to variable surface loadings, depending on different OC inputs with depth. Our results imply that subsoils are important for the storage of OC in terrestrial ecosystems because of intimate association of organic matter with secondary hydrous aluminum and iron phases leading to stabilization against biological degradation. Stabilisierung organischer Substanz im Unterboden Wir gingen der Frage nach, inwieweit der Unterboden zur Speicherung organischen Kohlenstoffs in terrestrischen Okosystemen beitragt und welche Rolle dabei der Bindung an die Mineralphase zukommt. Untersucht wurden zwei Profile von Waldboden (Braunerde-Podsol, schwach pseudovergleyte Braunerde). Wir bestimmten die Verteilung organischen Kohlenstoffs im Profil sowie in Dichte- und Korngrosenfraktionen. Auserdem wurde die Extrahierbarkeit von Kohlenstoff nach Zerstorung der Minerale mit Flusssaure (HF) ermittelt. Die Ergebnisse setzten wir mit chemischen Indikatoren der Mineralogie sowie der spezifischen Oberflache in Beziehung. Rasterelektronenmikroskopische Aufnahmen in Kombination mit energiedispersiver Rontgenspektroskopie (REM-EDX) wurden zur Visualisierung der organischen Substanzen auf Mineraloberflachen und ihrer Assoziation mit Elementen der Mineralphasen verwendet. Auf die Unterboden entfiel ein groser Teil (40—50%) des in den untersuchten Profilen einschlieslich der organischen Auflagen gespeicherten organischen Kohlenstoffs. Mit zunehmender Bodentiefe nahmen (1) die 14C-Alter zu und waren (2) zunehmende Anteile des Kohlenstoffs HF-loslich bzw. in der Dichtefraktion d >1.6 g cm—3 und in den Tonfraktionen angereichert. Die C-Anteile in der Dichtefraktion d >1.6 g cm—3 waren eng mit den Gehalten an oxalat- und dithionitloslichem Eisen korreliert (r2 = 0.93 bzw. 0.88, P <0.001). Die REM-EDX-Untersuchungen zeigten, dass die Akkumulation von Kohlenstoff haufig auch mit Aluminiumanreicherungen einhergehen. Zonen mit Siliciumanreicherungen waren dagegen immer an Kohlenstoff abgereichert. Die spezifische Oberflache bzw. die Mikroporositat der Mineralmatrix nach Zerstorung der organischen Substanz waren weniger eng mit organischem Kohlenstoff korreliert als dies fur extrahierbares Eisen der Fall war. Dies hangt moglicherweise mit der mit der Bodentiefe variierenden Kohlenstoffbelegung der Oberflachen infolge unterschiedlicher Eintrage an organischer Substanz zusammen. Unsere Ergebnisse weisen Unterboden als wichtige Kohlenstoffspeicher terrestrischer Okosysteme aus. Ursache ist die enge Bindung der organischen Substanz an sekundare Aluminium- und Eisen-Hydroxidphasen, die zu einer Stabilisierung gegenuber biologischem Abbau fuhrt.

Global change pressures on soils from land use and management

Soils are subject to varying degrees of direct or indirect human disturbance, constituting a major global change driver. Factoring out natural from direct and indirect human influence is not always straightforward, but some human activities have clear impacts. These include land-use change, land management and land degradation (erosion, compaction, sealing and salinization). The intensity of land use also exerts a great impact on soils, and soils are also subject to indirect impacts arising from human activity, such as acid deposition (sulphur and nitrogen) and heavy metal pollution. In this critical review, we report the state-of-the-art understanding of these global change pressures on soils, identify knowledge gaps and research challenges and highlight actions and policies to minimize adverse environmental impacts arising from these global change drivers. Soils are central to considerations of what constitutes sustainable intensification. Therefore, ensuring that vulnerable and high environmental value soils are considered when protecting important habitats and ecosystems, will help to reduce the pressure on land from global change drivers. To ensure that soils are protected as part of wider environmental efforts, a global soil resilience programme should be considered, to monitor, recover or sustain soil fertility and function, and to enhance the ecosystem services provided by soils. Soils cannot, and should not, be considered in isolation of the ecosystems that they underpin and vice versa. The role of soils in supporting ecosystems and natural capital needs greater recognition. The lasting legacy of the International Year of Soils in 2015 should be to put soils at the centre of policy supporting environmental protection and sustainable development.

Types and chemical composition of organic matter in reforested lignite-rich mine soils

Abstract In the post-mining landscapes of Lusatia, forest soils develop from extremely acid, lignite-rich open cast mine spoils. The sites have been ameliorated with ash from lignite-fired power stations prior to afforestation. During stand development, incorporation of plant-derived organic matter leads to an intimate mixture with the substrate-derived lignite in the first centimetres of the soil (Ai horizon). The objective of the study was to characterise and to compare the composition of organic matter of mine soils under forest which contain substantial amounts of lignite. Therefore, the forest floor and the mineral soil (Ai and Cv horizon) under a 20-year-old pine and a 36-year-old red oak site were analysed for elemental composition, magnetic susceptibility, chemical structure by cross-polarisation magic angle spinning (CPMAS) 13 C and 15 N nuclear magnetic resonance (NMR) spectroscopy and lignite content by radiocarbon dating. The 13 C CPMAS NMR spectra of the forest floor and Ai horizon reveal signals at 56, 72, 105, 119, 130 and 150 ppm, indicating the presence of carbohydrates and lignin originating from plant material. Additionally, structures characteristic for lignite material (aromatic and aliphatic carbon) could be observed in the Oh and Ai horizons. Using radiocarbon dating, 25 to 83% of the total carbon in these horizons can be assigned to lignite. Lignite carbon may also indicate carbonaceous particles derived from amelioration ash as well as from lignite-derived airborne contamination, which are possible carbon sources of the forest floor and the surface soil. 13 C NMR and radiocarbon dating show that the subsoil (Cv horizon) is dominated by carbon derived from lignite. From these results it is concluded that mine soils, rich in lignite contain up to four organic matter types, namely lignite inherent to the parent substrate, organic matter derived from decomposition of plant residues, carbonaceous particles in amelioration ash and carbonaceous particles from airborne lignite-derived contamination. 15 N NMR spectroscopy revealed that most of the nitrogen of these soils is of recent biogenic origin.

Impact of compost, vermicompost and biochar on soil fertility, maize yield and soil erosion in Northern Vietnam: A three year mesocosm experiment

Compost, vermicompost and biochar amendments are thought to improve soil quality and plant yield. However, little is known about their long-term impact on crop yield and the environment in tropical agro-ecosystems. In this study we investigated the effect of organic amendments (buffalo manure, compost and vermicompost) and biochar (applied alone or with vermicompost) on plant yield, soil fertility, soil erosion and water dynamics in a degraded Acrisol in Vietnam. Maize growth and yield, as well as weed growth, were examined for three years in terrestrial mesocosms under natural rainfall. Maize yield and growth showed high inter-annual variability depending on the organic amendment. Vermicompost improved maize growth and yield but its effect was rather small and was only significant when water availability was limited (year 2). This suggests that vermicompost could be a promising substrate for improving the resistance of agrosystems to water stress. When the vermicompost-biochar mixture was applied, further growth and yield improvements were recorded in some cases. When applied alone, biochar had a positive influence on maize yield and growth, thus confirming its interest for improving long-term soil productivity. All organic amendments reduced water runoff, soil detachment and NH₄(+) and NO₃(-) transfer to water. These effects were more significant with vermicompost than with buffalo manure and compost, highlighting that the beneficial influence of vermicompost is not limited to its influence on plant yield. In addition, this study showed for the first time that the combination of vermicompost and biochar may not only improve plant productivity but also reduce the negative impact of agriculture on water quality.

Effect of physical weathering on the carbon sequestration potential of biochars and hydrochars in soil.

Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250 °C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200 °C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half‐life (t1/2) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.