Soil Organic Matter Degradation Research Articles

Carbohydrates protect protein against abiotic fragmentation by soil minerals

The degradation and turnover of soil organic matter is an important part of global carbon cycling and of particular importance with respect to attempts to predict the response of ecosystems to global climate change. Thus, it is important to mechanistically understand the processes by which organic matter can be degraded in the soil environment, including contact with reactive or catalytic mineral surfaces. We have characterized the outcome of the interaction of two minerals, birnessite and kaolinite, with two disaccharides, cellobiose and trehalose. These results show that birnessite reacts with and degrades the carbohydrates, while kaolinite does not. The reaction of disaccharides with birnessite produces Mn(II), indicating that degradation of the disaccharides is the result of their oxidation by birnessite. Furthermore, we find that both sugars can inhibit the degradation of a model protein by birnessite, demonstrating that the presence of one organic constituent can impact abiotic degradation of another. Therefore, both the reactivity of the mineral matrix and the presence of certain organic constituents influence the outcomes of abiotic degradation. These results suggest the possibility that microorganisms may be able to control the functionality of exoenzymes through the concomitant excretion of protective organic substances, such as those found in biofilms.

Reviewing our options: Managing water‐limited soils for conservation and restoration

AbstractThe Food and Agriculture Organization considers around a quarter of global land to be degraded. Of particular concern are threats to soils in water‐limited regions, which are critical to food and economic security in countries across the globe but are under increasing pressure due to human use and climatic forcing. These soils have been used to feed and provide resources and services to human societies for millennia, with earliest land‐uses dating back to prehistoric times. With the adoption of modern, frequently unsuitable agricultural practices combined with the population pressures and shifting consumption patterns, soils in water‐limited regions have come under threat, resulting in degradation and in worst‐case scenarios, desertification. Here, we review the current state of soils in water‐limited environments and provide a guide to management for conservation and restoration of these fragile soils. Options to manage specific threats to soil functionality, namely, erosion, soil salinity, loss of functionality due to landscape homogenization, degradation of soil organic matter, and climate vulnerability are presented for specific land‐uses using a whole‐system approach management framework.

Acid pretreatment increased lipid biomarker extractability: a case study to reveal soil organic matter input from rubber trees after long‐term cultivation

The lipids extractable by organic solvents are important components of soil organic matter (SOM) and were used to trace the sources and degradation of SOM. Previous studies have suggested soil mineral protection of lipids, which might decrease the efficiency of extraction of some lipid compounds by organic solvents. Therefore, in this study we applied a mild acid treatment to remove most of the reactive mineral particles without altering SOM chemical structures. The SOM information provided by lipid biomarkers was different before and after acid treatment in this research. Because of the various chemical contents of lipids, the presence of reactive minerals might alter lipid biomarker signals considerably and could lead to the wrong conclusions. Based on lipid biomarker information obtained through acid‐aided extraction, we identified that the source for alkanoic acids was different from that for alkanols and alkanes. Alkanoic acids were derived from both original vegetation (bamboos) and new inputs from fresh rubber tree tissues, whereas alkanols and alkanes were mainly from the original vegetation. Various biomarker indices indicated an extended degradation of lipids after intensive rubber tree cultivation. The leaching of short‐chain aliphatic lipids and the new input of alkanoic acids in the surface soil were also considered to avoid the incorrect orientation of biomarker information. This study suggested that although the new input from rubber trees was evident in cultivated soil, cultivation activities might enhance the degradation of SOM and accelerate its turnover. The cycling of SOM thus needs to be investigated carefully to protect the ecosystems affected by intense human activity.Highlights The extractability of lipids is enhanced after removal of reactive minerals. Various biomarkers indicate enhanced lipid degradation after cultivation. New input of SOM from rubber trees is suggested by alkanoic acid biomarkers. Cultivation activities accelerate SOM degradation and turnover.

Factors influencing soil enzyme activity in China’s forest ecosystems

Enzyme activity (EA) mediates soil organic matter (SOM) degradation, transformation, and mineralization, thereby maintaining the biogeochemical cycles and energy flow of ecosystems. To determine the main factors explaining EA variations in China’s forest ecosystems, we created a database of soil EAs and relevant variables using data from the literature and analysed relationships between EAs and both climatic and edaphic variables. Catalase, phenol oxidase, acid (alkaline) phosphatase, and protease activities differed significantly among different types of forests. Catalase and urease activities were generally higher in primosols, cambisols, and argosols than in ferrosols. EA largely decreased with soil depth and increased with SOM. Phenol oxidase and urease activities were negatively correlated with mean annual temperature (MAT); in contrast, catalase, invertase, and protease activities first decreased ( 17.5 °C) with increasing MAT. Although protease activity was slightly positively correlated with mean annual precipitation (MAP), catalase, phenol oxidase, and urease activities were all negatively related to MAP. Catalase, invertase, acid (alkaline) phosphatase, urease, and protease activities first increased (< 2000 m.a.s.l.) and then decreased (2000–4100 m.a.s.l.) with increasing elevation. Principal component analysis revealed most EAs to be correlated with climate conditions and soil pH. These findings suggest that climatic and edaphic variables directly and indirectly correlate with forest type and greatly impact soil EA.

Soil bacterial community mediates the effect of plant material on methanogenic decomposition of soil organic matter

Input of plant material may strongly change decomposition rates of soil organic matter (SOM), i.e. causing priming effect (PE), but the underlying mechanisms are largely unknown. We found that rice straw addition in anoxic Fuyang (F) rice field soil stimulated CH4 production from SOM at the expense of CO2, whereas in Uruguay (U) soil it suppressed SOM degradation to CO2 plus CH4 (negative PE). Reciprocal inoculation experiments with non-sterile and sterile soils showed that the soils always displayed the effect of rice straw characteristic for the live microbial community rather than for the soil physicochemical properties. Pyrosequencing of 16S rRNA genes showed that bacterial communities in these soil samples were separated into two clusters (F and U). Symbiobacterium was abundant or dominant in microbiota from U soil, but negligible in those from F soil. Network analysis indicated that the bacterial populations involved in SOM decomposition were different between soils of F and U clusters; moreover, they were more tightly connected to methanogens in U than in F clusters. Ultimately, our results suggested that the PE of rice straw is mediated by the composition and activity of soil microbial community.

Agricultural land use determines functional genetic diversity of soil microbial communities

Microbial communities play a major role in the degradation of soil organic matter (SOM) in soils. Despite its significance, the functional diversity of the highly diverse microbial communities is poorly understood. To address this, we applied a recently developed technique, captured metagenomics, to determine the effects of land-use on the functional genetic diversity of genes involved in the carbon degradation of SOM in five pairs of agricultural soils with either winter wheat or grass as management. In addition, 16S rRNA based amplicon sequencing was used to study the taxonomic composition in the same soils. The functional genes resulting from the captured metagenomes had a higher abundance and diversity of sequences coding for enzymes degrading SOM in the grasslands compared to the wheat soils. Though the taxonomic diversity did not correlate with the land use. Amounts of C and N (organic matter content) in the soils affected both functional and taxonomic diversity of the microbial communities, where N was highly correlated to their functions and C was highly correlated to their taxonomy. Captured metagenomic analyses of the functional genes may provide a measure of the potential SOM degradation capacity by soil microbial communities at a high resolution. This can be used for assessments of how agricultural management affects the functioning of soil communities.

Use of Piptatherum miliaceum for the phytomanagement of biochar amended Technosols derived from pyritic tailings to enhance soil aggregation and reduce metal(loid) mobility

Tailings and overburdens pose high environmental and human health risks. The development of Technosols from anthropogenic materials and growth of native metal-tolerant plant species could be a sustainable strategy to minimize such risks. In this study, a 10-month pot experiment was carried out with Technosols developed from acidic tailings amended with marble waste (CaCO3) and two rates of biochar (10gCkg−1 and 20gCkg−1), with and without Piptatherum miliaceum. The main aims were to assess if biochar addition influenced plant metal uptake, increased soil aggregation, decreased metal mobility and enhanced plant growth. In addition, we assessed if the presence of P. miliaceum improved soil structure, stimulated microbial communities, and increased metal immobilization. Biochar and CaCO3 additions were the main factors responsible for acid neutralization. However, the growth of P. miliaceum also contributed to increase soil pH, likely due to increases in electrical conductivity. High organic matter recalcitrance with low content of labile and soluble organic compounds was observed in the developed Technosols. The presence of P. miliaceum positively stimulated microbial biomass and enzyme activities, and contributed to significantly increase the ratio recalcitrant C-to-recalcitrant N. This suggests an effect of rhizosphere in soil organic matter stabilization and degradation. Thus, microorganisms degrade N functional groups in biochar more easily than C functional groups. Greenhouse gas emissions were not increased by addition of biochar. Methane emissions decreased, with no effect of plant growth. The use of biochar and marble waste reduced the mobility of metal(loid)s due to increase in pH, and the presence of plant increased the fraction of Cd retained in Fe/Mn oxides. Plants showed low metal(loid) uptake in shoots, with no effect of biochar rate. Technosols showed higher proportion of macro and microaggregates than original tailings. The presence of plant significantly contributed to increase the proportion of microaggregates. The highest rate of biochar had a negative effect on P. miliaceum biomass which resulted in limited growth compared with the lower biochar rate.

Long-term litter manipulation alters soil organic matter turnover in a temperate deciduous forest

Understanding soil organic matter (OM) biogeochemistry at the molecular-level is essential for assessing potential impacts from management practices and climate change on shifts in soil carbon storage. Biomarker analyses and nuclear magnetic resonance (NMR) spectroscopy were used in an ongoing detrital input and removal treatment experiment in a temperate deciduous forest in Pennsylvania, USA, to examine how above- and below-ground plant inputs control soil OM quantity and quality at the molecular-level. From plant material to surface soils, the free acyclic lipids and cutin, suberin, and lignin biomarkers were preferentially retained over free sugars and free cyclic lipids. After 20years of above-ground litter addition (Double Litter) or exclusion (No Litter) treatments, soil OM composition was relatively more degraded, as revealed by solid-state 13C NMR spectroscopy. Under Doubled Litter inputs, soil carbon and phospholipid fatty acid (PLFA) concentrations were unchanged, suggesting that the current OM degradation status is a reflection of microbial-mediated degradation that occurred prior to the 20-year sampling campaign. Soil OM degradation was higher in the No Litter treatments, likely due to the decline in fresh, above-ground litter inputs over time. Furthermore, root and root and litter exclusion treatments (No Roots and No Inputs, respectively) both significantly reduced free sugars and PLFAs and increased preservation of suberin-derived compounds. PLFA stress ratios and the low N-acetyl resonances from diffusion edited 1H NMR also indicate substrate limitations and reduced microbial biomass with these treatments. Overall, we highlight that storage of soil carbon and its biochemical composition do not linearly increase with plant inputs because the microbial processing of soil OM is also likely altered in the studied forest.

The effect of roots and easily available carbon on the decomposition of soil organic matter fractions in boreal forest soil

The priming effect induced by carbon (C) that is easily available to microbes has been shown to increase the mineralization of soil organic matter (SOM) that is resistant to decomposition, but the combined effects of easily available carbon and the living root system have rarely been studied. The aim of this research was to study the decomposition of SOM fractions of different solubility in water and their 13C, 14C and 15N abundance with and without the presence of a living root system and easily available carbohydrate in the form of glucose. The SOM collected from the organic horizon of a boreal forest soil in Hyytiälä, southern Finland (61°51′N, 24°17′E), and exposed to laboratory incubations with and without the presence of Pinus sylvestris L. seedlings and glucose, was separated into three chemical fractions with accelerated solvent (ASE) and pressurized hot water extractions (PHWE). Changes in the natural abundance of 13C, 14C and 15N, spectral properties assessed by Fourier transform infrared spectroscopy (FTIR) and the C and N pools of SOM fractions were studied after incubation for 6 months. The extractions separated SOM into fractions with distinctive isotopic composition. The most easily soluble SOM fraction showed the largest abundance of 15N and 14C, and the living root system induced changes in the abundance of 15N and FTIR spectra. Our research suggests that plant roots may induce SOM degradation and N uptake from soluble SOM fractions, but 13C, 14C, 15N or FTIR alone cannot be used to describe the recalcitrance of SOM and its accessibility to microorganisms. It is better to use several methods in parallel to study the decomposability of SOM.HighlightsWe studied the effects of tree roots and addition of glucose on the solubility of soil organic matter (SOM). SOM can be separated into pools with distinctive isotopic concentrations with pressurized hot water. Roots may induce organic matter degradation and N uptake from soluble SOM fractions. Larger 14C abundance in soluble SOM indicated it contained older C than the recently assimilated C.

Abiotic and seasonal control of soil-produced CO2 efflux in karstic ecosystems located in Oceanic and Mediterranean climates

This study characterizes the processes involved in seasonal CO2 exchange between soils and shallow underground systems and explores the contribution of the different biotic and abiotic sources as a function of changing weather conditions. We spatially and temporally investigated five karstic caves across the Iberian Peninsula, which presented different microclimatic, geologic and geomorphologic features. The locations present Mediterranean and Oceanic climates. Spot air sampling of CO2 (g) and δ13CO2 in the caves, soils and outside atmospheric air was periodically conducted. The isotopic ratio of the source contribution enhancing the CO2 concentration was calculated using the Keeling model. We compared the isotopic ratio of the source in the soil (δ13Cs–soil) with that in the soil-underground system (δ13Cs–system). Although the studied field sites have different features, we found common seasonal trends in their values, which suggests a climatic control over the soil air CO2 and the δ13CO2 of the sources of CO2 in the soil (δ13Cs–soil) and the system (δ13Cs–system). The roots respiration and soil organic matter degradation are the main source of CO2 in underground environments, and the inlet of the gas is mainly driven by diffusion and advection. Drier and warmer conditions enhance soil-exterior CO2 interchange, reducing the CO2 concentration and increasing the δ13CO2 of the soil air. Moreover, the isotopic ratio of the source of CO2 in both the soil and the system tends to heavier values throughout the dry and warm season.We conclude that seasonal variations of soil CO2 concentration and its 13C/12C isotopic ratio are mainly regulated by thermo-hygrometric conditions. In cold and wet seasons, the increase of soil moisture reduces soil diffusivity and allows the storage of CO2 in the subsoil. During dry and warm seasons, the evaporation of soil water favours diffusive and advective transport of soil-derived CO2 to the atmosphere. The soil CO2 diffusion is enough important during this season to modify the isotopic ratio of soil produced CO2 (3–6‰ heavier). Drought induces release of CO2 with an isotopic ratio heavier than produced by organic sources. Consequently, climatic conditions drive abiotic processes that turn regulate a seasonal storage of soil-produced CO2 within soil and underground systems. The results here obtained imply that abiotic emissions of soil-produced CO2 must be an inherent consequence of droughts, which intensification has been forecasted at global scale in the next 100 years.

Increasing molecular structural complexity and decreasing nitrogen availability depress the mineralization of organic matter in subtropical forest soils

The degradation of soil organic matter (SOM) is determined by its compositional and structural properties, but the properties of the remaining SOM can be modified during the mineralization process. In this study, 12 soils (Ultisols, Alfisols and Inceptisols) from subtropical forests were incubated under laboratory conditions (25 °C, aeration) for 90 days. The mechanisms underlying the changes of these properties were investigated by analyzing pre- and post-incubation soils using UV absorption, fluorescence spectroscopy and acid hydrolysis methods. The turnover rate of SOM decreased with increasing soil depth. The amount of CO2 released ranged from 90 to 620 mg CO2-C kg−1, representing c. 0.9–3.1% of soil organic C (SOC). The proportion of nonhydrolyzable (recalcitrant) N in total N (RN) increased with soil depth, whereas it was not apparent in relation to the proportion of nonhydrolyzable (recalcitrant) C in total C (RC). Furthermore, a negative correlation was observed between SOM mineralization and RN (P < 0.05) rather than RC, implying that the availability of N was a limiting factor in SOM decomposition when compared with C. The decrease in E2-to-E4 ratio and fluorescence intensity (FI) of water-extractable organic C (WEOC) with increasing soil depth suggested that WEOC in the deeper horizons was probably rich in electron-withdrawing groups (COOH) but poor in electron-donating groups (OH, NH2), and it was composed of components with larger molecular size, higher humification degree and structural complexity. These could be responsible for the lower SOM mineralization rates in deeper horizons. After 90 days of incubation, a blue shift of wavelengths in the synchrotron-scan spectra, and an obvious increase of FI values, together with the increase in E2-to-E4 ratio of WEOC implied a reduction in molecular size and the degree of aromatic condensation and humification, resulting in a shift to less complicated structures of WEOC.

Tannins can slow-down but also speed-up soil enzymatic activity in boreal forest

The boreal forest ecosystem stores substantial amounts of soil organic matter (SOM), which may act either as a source or as a sink for atmospheric CO2 under climate change and that is why enzymatic SOM degradation is gaining increasing attention. The boreal forest ecosystem is rich in plant secondary compounds and in particular tannins which are seen as enzyme inhibitors. We studied changes in enzymatic activity after the addition of tannins. Our experimental design combined direct studies of the tannin effects on enzymes with laboratory soil mesocosm experiments. Our results showed that the addition of tannins directly led to both decreases and increases in the catalytic activity of enzymes, however, some differences between enzymes were observed. Overall, low concentrations of tannins increased the coiled structures of the enzymes boosting their catalytic activity. High concentrations of tannins acted in the opposite way, thereby diminishing the catalytic activity. We observed that tannins caused a similar change in enzymatic activity in soil mesocosm experiments. Tannin-enzyme synergy needs more study as these interactions can potentially play an important role in SOM decomposition of future climate, especially in the tannin-rich ecosystems.

The contribution of ericoid plants to soil nitrogen chemistry and organic matter decomposition in boreal forest soil

Nitrogen (N) availability restricts plant carbon assimilation from the atmosphere in N-limited boreal ecosystems. Boreal forest floor is covered with ericoid plants and in soil, ectomycorrhizal and ericoid mycorrhizal plant roots and fungal hyphae are intermixed. How different mycorrhizal plants affect N mobilization from soil organic matter (SOM) has yet to be elucidated. Here we compared the effects of ericoid plants (Calluna vulgaris, Vaccinium myrtillus, Vaccinium vitis-idaea), forming ericoid mycorrhiza (ERM), on soil N chemistry, plant N uptake and SOM decomposition and compared them with ectomycorrhizal (ECM) Pinus sylvestris in a microcosm experiment. Pine and ericoid plants affected soil N similarly, accessing both degradable and recalcitrant N pools. Both ericoid plants and pine took up N from organic source. Ericoid plant roots contained more phenolic compounds and condensed tannins than pine roots which together with lower pH under ericoids suggests substantial changes in soil chemistry and biology. This study underlines the plant stimulation of SOM degradation with special emphasis on ericoid plants. We point out that both ericoid and ectomycorrhizal plants and their interactions should be of interest when N uptake and SOM decomposition in boreal forests are studied.

Invasion of moso bamboo into a Japanese cedar plantation affects the chemical composition and humification of soil organic matter.

Bamboo, which has dense culms and root rhizome systems, can alter soil properties when it invades adjacent forests. Therefore, this study investigated whether bamboo invasions can cause changes in soil organic matter (SOM) composition and soil humification. We combined solid-state 13C NMR spectroscopy and chemical analysis to examine the SOM in a Japanese cedar (Cryptomeria japonica) and adjacent bamboo (Phyllostachys edulis) plantation. Bamboo reduced soil organic C (SOC) content, compared to the cedar plantation. The value of ∆logK (ratio of absorbance of humic acids at 400 and 600 nm) was cedar > transition zone > bamboo soils. Our results indicated that bamboo increased SOM humification, which could be due to the fast decomposition of bamboo litter with the high labile C. Furthermore, intensive management in the bamboo plantation could enhance the humification as well. Overall, litter type can control an ecosystem’s SOC nature, as reflected by the finding that higher labile C in bamboo litter contributed the higher ratios of labile C to SOC and lower ratios of recalcitrant C to SOC in bamboo soils compared with cedar soils. The invasion of bamboo into the Japanese cedar plantation accelerated the degradation of SOM.

Costimulation of soil glycosidase activity and soil respiration by nitrogen addition.

Unprecedented levels of nitrogen (N) have been deposited in ecosystems over the past century, which is expected to have cascading effects on microbially mediated soil respiration (SR). Extracellular enzymes play critical roles on the degradation of soil organic matter, and measurements of their activities are potentially useful indicators of SR. The links between soil extracellular enzymatic activities (EEAs) and SR under N addition, however, have not been established. We therefore conducted a meta-analysis from 62 publications to synthesize the responses of soil EEAs and SR to elevated N. Nitrogen addition significantly increased glycosidase activity (GA) by 13.0%, α-1,4-glucosidase (AG) by 19.6%, β-1,4-glucosidase (BG) by 11.1%, β-1,4-xylosidase (BX) by 21.9% and β-D-cellobiosidase (CBH) by 12.6%. Increases in GA were more evident for long duration, high rate, organic and mixed N addition (combination of organic and inorganic N addition), as well as for studies from farmland. The response ratios (RRs) of GA were positively correlated with the SR-RRs, even when evaluated individually for AG, BG, BX and CBH. This positive correlation between GA-RR and SR-RR was maintained for most types of vegetation and soil as well as for different methods of N addition. Our results provide the first evidence that GA is linked to SR under N addition over a range of ecosystems and highlight the need for further studies on the response of other soil EEAs to various global change factors and their implications for ecosystem functions.

Quantifying Charcoal Degradation and Negative Priming of Soil Organic Matter with a 14C-Dead Tracer

AbstractConverting biomass to charcoal produces physical and chemical changes greatly increasing environmental recalcitrance, leading to great interest in the potential of this carbon form as a long-term sequestration strategy for climate change mitigation. Uncertainty remains, however, over the timescale of charcoal’s environmental stability, with estimates varying from decadal to millennial scales. Uncertainty also remains over charcoal’s effect on other aspects of carbon biogeochemical cycling and allied nutrient cycles such as nitrogen. Radiocarbon is a powerful tool to investigate charcoal mineralization due to its sensitivity; here we report the results of a study using 14C-dead charcoal (pMC=0.137±0.002) in organic-rich soil (pMC=99.76±0.46), assessing charcoal degradation over 55 days of incubation. Using this method, we discriminated between decomposition of indigenous soil organic matter (SOM) and charcoal by microorganisms. SOM was the major source of carbon respired from the soil, but there was also a contribution from charcoal carbon mineralization. This contribution was 2.1 and 1.1% on days 27 and 55, respectively. We also observed a negative priming effect due to charcoal additions to soil, where SOM mineralization was repressed by up to 14.1%, presumably arising from physico-chemical interactions between soil and charcoal.

Mycorrhizal contribution to soil respiration in an apple orchard

Soil respiration (Rsoil) partitioning into its components is pivotal to understand C cycling in ecosystems. Notwithstanding the importance of mycorrhizal fungi in the belowground C cycle, their respiration (Rmyc) is poorly studied in agricultural ecosystems. In this study, we applied a modified trenching method and the 13C natural abundance technique to distinguish Rmyc from root respiration (Rroot) and from the respiration due to soil organic matter (SOM) degradation (Rsom) in an apple orchard (Malus domestica Borkh.) located in Northern Italy. Membranes with different mesh size were used to physically exclude roots or mycorrhizae from soil cores with a different δ13C than the orchard average value. The CO2 efflux from soil cores was determined with an infrared gas analyser, and isotopic measurements were performed on the soil-emitted CO2. The different Rsoil components were determined both by difference and by the isotopic mass balance. Mycorrhizal contribution to soil respiration (11±6%) was of similar magnitude to that of roots (12±4%) while Rsom accounted for 73±3% of Rsoil. In presence of apple roots, respiration of SOM and mycorrhizae (Rsom+myc) significantly increased in late summer and autumn, likely because of a priming effect of roots on SOM degradation or to a stimulation of mycorrhizal respiration. Our results suggest that respiration of mycorrhizal fungi can significantly contribute to Rsoil, and need to be considered to correctly partition soil respiration. Furthermore, as roots increased Rsom+myc, the root exclusion method for soil respiration partitioning may overestimate Rroot.

Suppression of soil organic matter decomposition by gasoline and diesel as assessed by 13C natural abundance

Petroleum products are common contaminants in soils due to human activities. They are toxic for microorganisms and threat their functions, including decomposition of soil organic matter (SOM). The direct estimation of altered SOM decomposition – based on the CO2 efflux – is impossible after petroleum contamination because petroleum decomposition also contributes to these CO2 fluxes. We used the natural differences in the isotopic signature (δ13C) of SOM and of petroleum products to partition the total CO2 for both sources and to analyse the suppression of SOM decomposition. The dynamics of 13C fractionation during the mineralization of gasoline and diesel was measured during 42 days. The 13C fractionation varied between −8.8‰ and +3.6‰ within the first 10 days, and stabilized thereafter at about −5.3‰ for gasoline and +3.2‰ for diesel. These 13C fractionations and δ13C values of CO2 emitted from the soil were used to partition the total CO2. Contamination with gasoline reduced the CO2 efflux from SOM by a factor of 25 (from 151 to 6.1 mg C-CO2 kg−1 soil during 42 days). The negative effect of diesel was much lower: the CO2 efflux from SOM was decreased by less than a factor of 2. The strong effect of gasoline versus diesel reflects the lower absorption of gasoline to mineral particles and the development of a thin film on water surfaces, leading to toxicity for microorganisms. We conclude that the soil contamination by gasoline and diesel strongly decreased microbial functions and so, the degradation of native SOM.

Stand scale variability of topsoil organic matter composition in a high-elevation Norway spruce forest ecosystem

Our knowledge about the effect of single-tree influence areas on the physicochemical properties of the underlying mineral soil in forest ecosystems is still limited. This restricts our ability to adequately estimate future changes in soil functioning due to forest management practices. We studied the stand scale spatial variation of different soil organic matter species investigated by 13C NMR spectroscopy, lignin phenol and neutral sugar analysis under an unmanaged mountainous high-elevation Norway spruce (Picea abies L.) forest in central Europe. Multivariate geostatistical approaches were applied to relate the spatial patterns of the different soil organic matter species to topographic parameters, bulk density, oxalate- and dithionite-extractable iron, pH, and the impact of tree distribution. Soil samples were taken from the mineral top soil. Generally, the stand scale distribution patterns of different soil organic matter compounds could be divided into two groups: Those compounds, which were significantly spatially correlated with topography/altitude and those with small scale spatial pattern (range≤10m) that was closely related to tree distribution. The concentration of plant-derived soil organic matter components, such as lignin, at a given sampling point was significantly spatially related to the distance of the nearest tree (p≤0.05). In contrast, the spatial distribution of mainly microbial-derived compounds (e.g. galactose and mannose) could be attributed to the dominating impact of small-scale topography and the contribution of poorly crystalline iron oxides that were significantly larger in the central depression of the study site compared to crest and slope positions.Our results demonstrate that topographic parameters dominate the distribution of overall topsoil organic carbon (OC) stocks at temperate high-elevation forest ecosystems, particularly in sloped terrain. However, trees superimpose topography-controlled OC biogeochemistry beneath their crown by releasing litter and changing soil conditions in comparison to open areas. This may lead to distinct zones with different mechanisms of soil organic matter degradation and also stabilization in forest stands.

Biochar amendment altered the molecular-level composition of native soil organic matter in a temperate forest soil

Environmental contextBiochar amendment in soil can sequester carbon but may also stimulate microbial activity, potentially enhancing soil organic matter degradation. We incubated biochar in a temperate forest soil and characterised the soil organic matter composition using molecular-level biomarker and nuclear magnetic resonance techniques. Biochar amendment altered the native soil organic matter composition and decreased the concentration of easily degradable soil organic matter components. AbstractBiochar amendment in soil can sequester carbon and improve soil water and nutrient retention, fertility and plant productivity. However, biochar may stimulate microbial activity, leading to priming or accelerated soil organic matter (OM) degradation, which could alter the native soil OM molecular composition. To investigate this, we amended sugar maple wood biochar (pyrolysed at 500°C) at four concentrations (0, 5, 10 and 20 metric tons per hectare) in a temperate forest soil for 32 weeks. Solvent extraction and CuO oxidation were used to characterise free compounds and lignin-derived phenols respectively at 8 week intervals, while base hydrolysis was used to examine plant wax, cutin and suberin components at the end of the incubation. Stimulated soil microbial activity following an adaptation period (16 weeks) resulted in increased inputs of microbial- and plant-derived soil OM components including solvent-extractable short-chain n-alkanols and n-alkanoic acids, long-chain n-alkanes and n-alkanols, and sugars. Degradation parameters for base-hydrolysable cutin- and suberin-derived compounds did not show any significant degradation of these plant biopolymers. Analysis of lignin-derived phenols revealed lower concentrations of extractable phenols and progressive oxidation of syringyl and vanillyl phenols at higher biochar application rates over time. Solution-state 1H nuclear magnetic resonance analysis of base-extractable soil OM after 32 weeks showed a decrease in the proportion of labile OM components such as carbohydrates and peptides and a relative increase in more recalcitrant polymethylene OM constituents in the amended soils. The biochar-mediated shifts in soil OM composition and reduction in labile carbon may reduce soil fertility in biochar-amended systems with long-term amendment.