Organic matter (OM) is an important soil component controlling many chemical and physical properties ranging from nutrient and pollutant sorption to aggregate stability and water holding capacity. Advanced techniques and sophisticated methods have been developed to characterise and separate qualitatively different fractions of soil organic matter SOM(SOM). Despite its importance and the technical progress, two main deficiencies still constrain the comprehensive analysis of OM quantity and quality in soils: 1) Most of the techniques (e.g. solid state 13C NMR spectroscopy or fourier-transformed infrared spectroscopy) are destructive so that the sample is lost after the measurement; and 2) SOM quantity and quality generally show a heterogeneous spatial distribution across many scales which is not considered in an adequate way in most studies since the analytical techniques require substantial amounts of soil material impeding the sampling of small structures. Therefore, a fast imaging technique that non-destructively measures SOM quantity and quality in a non-destructive way with a high spatial resolution on the pedon scale is requiredneeded.
We sampled two different sites in a semiarid steppe in Inner Mongolia, China - one continuously grazed site and the other ungrazed since 1979, with both were classified as calcic Chernozems. We expect lLongterm grazing is expected to decrease carbon contents and the most pronounced particulate organic matter fractions (POM) fractions). Stainless steel boxes (100×100×300 mm³) were used to sample undisturbed soil profiles. Two hyperspectral cameras recorded the visible, near and short wave infrared reflectance (400 to 2500 nm in 416 bands) of the profiles with a ground sampling distance of 63×63 µm² per pixel for VisNIR and 256×256 µm² per pixel for SWIR. This procedure was repeated over three vertical slicescuts at a lateral distance of 25 mm through the soil boxes. After each image recording the profile was divided into ten equal squares (each 50 × 50 mm2) and mixed samples were extracted from each square to a depth of 5 mm. Density fractionation is a standard lab approach to separate SOM fractions with different degrees of decomposition and chemical compositions. We used three image processing approaches to spectroscopically identify and quantify different SOM fractions and validate these results with density fractionation, which is a standard lab approach to separate SOM fractions with different degrees of decomposition and chemical compositions.the standard fractionation approach. We show the potential of laboratory imaging Vis-NIR-SWIR spectroscopy for the qualitative and quantitative mapping of SOM fractions on the pedon scale.
Though northern peatlands cover only 3 % of the land surface they count as one of the largest terrestrial organic C pools. This huge C pool is threatened by increasing temperatures, related microbial degradation and indirect effects of climate change leading to vascular plant dominance over sphagnum mosses and a shift from graminoids to shrubs. Effects of these changes in vegetation on peat degradation are unknown. Dissolved organic matter (DOM) as an important component of the C cycle in peatlands might be used as a sensitive indicator of enhanced peat degradation. Furthermore, peatlands are the major source of DOM in many surface waters and understanding the mechanisms of peat degradation will help to elucidate the reasons for the ongoing trends of increasing concentrations of dissolved organic carbon (DOC) in surface waters. In this study we aimed to determine effects of temperature and plant functional types (PFT: graminoids, shrubs) on amounts and composition of DOM allowing conclusions about ongoing changes in peat degradation. We selected two ombrotrophic peatlands in the Italian Alps, reflecting a temperature gradient where we manipulated the vascular plant cover by selective clipping. On the established plant functional type plots we collected DOM directly after plant removal and during the following seasons over a period of one year. Besides DOC concentrations we determined DOM composition by C-13 of DOC and UV and fluorescence spectroscopy. The short term response (2-24h) of DOM to the plant clipping enabled us to estimate the C input of vascular plants via roots. The medium to long term data showed a clear relation of DOM to the temperature gradient and the PFT. All in all our results indicated a substantial contribution of the roots from vascular plants to DOM in peatlands. The release of DOM from peat clearly increased with temperature and vascular plant biomass. The difference between graminoids and shrubs seems to be marginal. We conclude that higher temperatures and greater vascular plant biomass result in increasing peat degradation as one likely reason for increasing DOC concentrations in many surface waters across Europe and North America.
Abstract. Peatlands, storing significant amounts of carbon, are extremely vulnerable to climate change. The effects of climate change are projected to lead to a vegetation shift from Sphagnum mosses to sedges and shrubs. Impacts on the present moss-dominated peat remain largely unknown. In this study, we used a multiproxy approach to investigate the influence of contrasting vascular plant types (sedges, shrubs) on peat chemistry and decomposition. Peat cores of 20 cm depth and plant material (Sphagnum spp., Calluna vulgaris and Eriophorum vaginatum) from two ombrotrophic peatlands in the Italian Alps with a mean annual temperature difference of 1.4 ∘C were analyzed. Peat cores were taken under adjacent shrub and sedge plants growing at the same height above the water table. We used carbon, nitrogen and their stable isotopes to assess general patterns in the degree of decomposition across sampling locations and depths. In addition, analytical pyrolysis was applied to disentangle effects of vascular plants (sedge, shrub) on chemical properties and decomposition of the moss-dominated peat. Pyrolysis data confirmed that Sphagnum moss dominated the present peat irrespective of depth. Nevertheless, vascular plants contributed to peat properties as revealed by, e.g., pyrolysis products of lignin. The degree of peat decomposition increased with depth as shown by, e.g., decreasing amounts of the pyrolysis product of sphagnum acid and increasing δ13C with depth. Multiple parameters also revealed a higher degree of decomposition of Sphagnum-dominated peat collected under sedges than under shrubs, particularly at the high temperature site. Surprisingly, temperature effects on peat decomposition were less pronounced than those of sedges. Our results imply that vascular plants affect the decomposition of the existing peat formed by Sphagnum, particularly at elevated temperature. These results suggest that changes in plant functional types may have a stronger impact on the soil carbon feedback in a warmer world than hitherto assumed.
Abstract. Peatlands, storing significant amounts of carbon are extremely vulnerable to climate change. The effects of climate change are projected to lead to a vegetation shift from Sphagnum mosses to sedges and shrubs. Impacts on the present moss-dominated peat remain largely unknown. In this study, we used a multi proxy approach to investigate the influence of contrasting vascular plant types (sedges, shrubs) on peat chemistry and decomposition. Peat cores of 20 cm depth and plant material (Sphagnum spp., Calluna vulgaris, Eriophorum vaginatum) from two ombrotrophic peatlands in the Italian Alps with a mean annual temperature difference of 1.4 °C were analysed. Peat cores were taken under adjacent shrub and sedge plants growing at the same height above the water table. We used carbon, nitrogen and their stable isotopes to assess general patterns in the degree of decomposition across sampling locations and depths. In addition, analytical pyrolysis was applied to disentangle effects of vascular plants (sedge, shrub) on chemical properties and decomposition of the moss-dominated peat. Pyrolysis data confirmed that Sphagnum moss dominated the present peat irrespectively of depth. Nevertheless, vascular plants contributed to peat properties as revealed by e.g. pyrolysis products of lignin. The degree of peat decomposition increased with depth as shown by e.g. decreasing amounts of the pyrolysis product of sphagnum acid and increasing δ13C with depth. Multiple parameters also revealed a higher degree of decomposition of Sphagnum-dominated peat collected under sedges than under shrubs, particularly at the high temperature site. Surprisingly, temperature effects on peat decomposition were less pronounced than those of sedges. Our results imply that vascular plants affect the decomposition of the existing peat formed by Sphagnum, particularly at elevated temperature. These results suggest that changes in plant functional types may have a stronger impact on the soil carbon feedback in a warmer world than hitherto assumed.
Abstract Background Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above‐ to belowground allocation is affected by temperature and plant functional types. Methods We traced the C allocation pathways of two dominant plant functional types (i.e., sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge‐shrub vegetation and in situ 13 C pulse‐labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying 13 C in plant leaves and soil respiration and by measuring δ 13 C in dissolved organic C. A depth‐resolved quantification of 13 C in the peat soil gave additional insight into belowground C allocation patterns. Results Temperature did not affect the rate at which 13 C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO 2 faster into their shoot biomass (faster depletion in 13 C in shoots) and allocated more of the assimilated 13 C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower 13 C in soil respiration measured under sedges. Conclusions Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above‐ to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers.
Organic matter is an important constituent of soils that controls many soil functions and is of vital importance for ecosystem services like climate regulation and food security. Soil organic matter (SOM consists of a wide spectrum of different organic substances that are highly heterogeneous in terms of chemical composition, stability against microbial decomposition and turnover time. SOM is heterogeneously distributed in the soil profile impeding its fast assessment. A technique to accurately measure SOM quality and quantity with a high spatial resolution in the soil profile is presently lacking. Imaging visible light and near infrared spectroscopy (imVisIR) is a promising technique for the fast and spatially resolved assessment of SOM quality and quantity. In this study, we evaluate the potential of imVisIR to quantitatively map the labile particulate organic matter fraction in undisturbed cores from mineral soils.
Peatlands, storing significant amounts of carbon (C), are extremely vulnerable to climate change. Indirect effects of climate change are projected to lead to a growing dominance of vascular plants in moss dominated peatlands with unknown effects on peat decomposition. In this study we investigated the influence of different plant functional types (moss, graminoid, shrub) on peat composition and decomposition. Peat cores (20 cm depth) and plant material (Sphagnum sp., Calluna vulgaris, Eriophorum vaginatum) of two ombrotrophic moss dominated peatlands on a temperature gradient in the Italian Alps were analyzed. Peat cores were taken under shrub and graminoid coverage at the low temperature site (Low-T-Site) and the high temperature site (High-T-Site). We used carbon to nitrogen ratios, C-13 and N-15 and pyrolysis gas chromatography/mass spectrometry (py-GC/MS) to assess the influence of vascular plants on peat composition and degradation. In these moss dominated peatlands, methoxyphenols from lignin indicated highest contribution of vascular plant material at 2-5 cm under shrub coverage and 5-12 cm depth under graminoid coverage. Increasing C-13 ratios with depth could be related to increasing peat decomposition. This increase was higher for peat cores under graminoid coverage than under shrub coverage. Furthermore, the enrichment in C-13 with depth was higher at the High-T-Site than at the Low-T-Site. More detailed effects of plant functional type on peat degradation were established using species specific pyrolysis products as e.g. methoxyphenols from lignin (marker compounds for vascular plants) and 4-isopropenylphenol reflecting degradation of the sphagnum peat matrix. Comparing depth records of these molecular parameters indicated higher peat degradation in the presence of graminoids compared to shrubs and at the High-T-Site compared to the Low-T-Site confirming conclusions from C-13 data. Consequently, plant functional types are very likely to influence peat composition and degradation especially at elevated temperatures, while the projected vegetation shifts from graminoids to shrubs should counteract increasing peat degradation with increasing temperature. Therefore, vegetation shifts in response to climate change may play a crucial role in determining peat composition and degradation.
Dead wood has important functions in forest ecosystems. It is a biodiversity hot spot, serves as a storage of water and stores 8% (73 +- 6 Pg) of the world’s forest carbon (Pan et al., 2011). The fate of this carbon (C) is still highly debated particularly concerning its influence on soil organic matter (SOM) and its contribution to the forest soil’s C sink. The aim of this research is to investigate how downed beech dead wood affects the stable soil C pool of temperate beech forests, and how this depends on soil moisture. The research was conducted in a near natural beech forest near Leipzig, Germany (Dübener Heide) and is part of the BENEATH-Project. We sampled three sites representing a soil moisture gradient, i.e. dry, intermediate (i.e. moist) and wet conditions. Undisturbed soil cores were taken from these sites in three depth (0-10 cm, 10-20 cm and 20-30 cm) beneath dead wood at an advanced stage of decay. Reference soils were sampled at a distance of about 2 m. Soil moisture and soil temperature are constantly monitored. We applied a physical fractionation scheme to identify the effects of dead wood on differently stable SOM fractions. The samples were separated in the free light fraction (F-LF), the occluded light fraction (O-LF) and the heavy fraction (HF) via density fractionation using sodium polytungstate solution (ρ =1,6 g cm-³). For each fraction, the organic C and N contents were determined. Our results indicate a positive influence of dead wood on SOC stocks in the dry and wet regions of our soil moisture gradient. In the intermediate region of the soil moisture gradient, dead wood has no or even a negative effect on SOC stocks. Changes in the SOC content under dead wood compared to the reference soil occurred manly in the F-LF and HF fraction at 0 cm and 10 cm depth. The observed pattern of dead woods effect on SOC along the moisture gradient is suggested to be a result of the relationship between soil moisture and microbial activity. According to the literature, we assume that the microbial activity should be highest in the intermediate moist soil and to some extent inhibited under either wet or dry conditions. In this case, it is not the input but the rate of decomposition that changes with soil moisture, resulting in a different net increase in SOC. To test our hypothesis, we attempt to estimate the theoretical time of effective microbial decomposition per year based on soil moisture and soil temperature data for our three sites. Correlation analysis will be used to test this indicator of microbial activity for the effect of dead wood on SOC. Our results should sharpen the picture of the dead wood’s role for long-term C stabilization in forest soils and how this process is affected by differences in the soil moisture status. They will give implications for climate mitigating forest management.