Peat and other organic soils under agricultural use in germany: Properties and challenges for classification

Under natural conditions, peatlands store large amounts of soil organic carbon (SOC). However, they are under threat due to drainage which leads to mineralisation of soil organic matter to carbon dioxide (CO2). This situation is especially severe in Germany, where more than 70 % of peat and other organic soils are used for agriculture. This study assessed the properties of these soils within the framework of the first German Agricultural Soil Inventory. In a nationwide 8 × 8 km grid, soils from a total of 3104 sites were sampled to depths of up to one metre or down to the peat base. Of these sites, 146 were on peat and other organic soils; and 31 % of the 146 sites were being affected not only by drainage but also by changes in horizonation (e.g. mineral covers, deep ploughing). The classification of heavily disturbed sites is limited within the German Manual of Soil Mapping, which has led to the development of an adapted classification scheme for peat and other organic soils under agricultural use in Germany. The respective peat classes showed distinct patterns of SOC and total nitrogen (Nt) contents and stocks, bulk density (BD) and C:N ratios. Overall, a SOC stock of 529 ± 201 t ha-1 and a Nt stock of 29.3 ± 13.9 t ha-1 were found within a depth of 0–100 cm. However, in deeper profiles, 48 % of the total SOC was stored below 100 cm depth down to the peat base. High SOC stocks were also found in peat-derived, mineral-covered and deep-ploughed organic soils, which might be classified as mineral soils depending on the classification system used but are still prone to mineralisation and need to be considered in terms of emissions reporting and mitigation. Logarithmic and quadratic pedotransfer functions were developed to estimate BD and SOC density, respectively, from SOC contents. This is necessary for the calculation of SOC stocks when analyses of BD are absent. The quadratic relationship between SOC content and SOC density clearly showed that heavily degraded organic soils store as much SOC in a defined volume as more natural ones, and that any estimates of differences in potential CO2 emissions should not be based on SOC content, but on SOC density instead

Reviewing and analyzing shrinkage of peat and other organic soils in relation to selected soil properties

Peat and other organic soils (e.g., organo-mineral soils) show distinctive volume changes through desiccation and wetting. Important processes behind volume changes are shrinkage and swelling. There is a long history of studies on shrinkage which were conducted under different schemes for soil descriptions, nomenclatures and parameters, measurement approaches, and terminologies. To date, these studies have not been harmonized in order to compare or predict shrinkage from different soil properties, for example, bulk density or substrate composition. This, however, is necessary to prevent biases in the determination of volume-based soil properties or for the interpretation of elevation measurements in peatlands, in order to predict carbon dioxide emissions or uptake caused by microbial decomposition or peat formation. This study gives a comprehensive overview of shrinkage studies carried out in the last 100 years. Terminology and approaches are systematically classified. In part I, the concepts for shrinkage characteristics, measurement methods, and model approaches are summarized. Part II is a meta-analysis of shrinkage studies on peat and other organic soils amended by own measurement data obtained by a three-dimensional structured light scanner. The results show that maximum shrinkage has a wide range from 11% to 93% and is strongly affected by common soil properties (botanical composition, degree of decomposition, soil organic carbon, and bulk density). Showing a stronger correlation, bulk density was a better predictor than soil organic carbon, but maximum shrinkage showed a large spread over all types of peat and other organic soils and ranges of bulk density and soil organic carbon

Improving the determination of soil hydraulic properties of peat soils at different scales

This thesis improves the characterization of unsaturated hydraulic properties for different types of peat and other organic soils at the laboratory and the field scale. First, suggestions for a general improvement of the unsaturated hydrological modeling in peatlands are made. Second, a novel one-dimensional expression for calculating specific yield for shallow groundwater systems with microrelief has been derived which was the basis for the development of a novel in situ method for determining soil water retention characteristics in shallow groundwater systems

Greenhouse Gas Balance of Sphagnum Farming on Highly Decomposed Peat at Former Peat Extraction Sites

For two years, we quantified the exchange of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) at two different large-scale Sphagnum farming sites. At both, peat extraction left a shallow layer of highly decomposed peat and low hydraulic conductivities. One site was characterized by preceding multi-annual inundation and irrigated by ditches, while the other one was inoculated directly after peat extraction and irrigated by ditches and drip irrigation. Further, GHG emissions from an irrigation polder and the effect of harvesting Sphagnum donor material at a near-natural reference site were determined. GHG mitigation potentials lag behind the results of less decomposed sites, although our results were also affected by the extraordinary hot and dry summer 2018. CO2 exchanges ranged between -0.6 and 2.2 t CO2-C ha−1 y−1 and were mainly influenced by low water table depths. CH4 emissions were low with the exception of plots with higher Eriophorum covers, while fluctuating water tables and poorly developing plant covers led to considerable N2O emissions at the ditch irrigation site. The removal of the upper vegetation at the near-natural site resulted in increased CH4 emissions and, on average, lowered CO2 emissions. Overall, best plant growth and lowest GHG emissions were measured at the previously inundated site. At the other site, drip irrigation provided more favourable conditions than ditch irrigation. The size of the area needed for water management (ditches, polders) strongly affected the areal GHG balances. We conclude that Sphagnum farming on highly decomposed peat is possible but requires elaborate water management. © 2021, The Author(s)

Substrate quality of drained organic soils—Implications for carbon dioxide fluxes

Background: Peatlands only cover a minor fraction of the global terrestrial surface, but due to drainage, they are major contributors to carbon dioxide (CO2) emissions from soils. Previous studies have shown that hydrological conditions, nutrient availability and anthropogenic disturbance play an important role in the mineralisation of organic matter. Furthermore, microbial turnover depends on peat quality, which is determined by its botanical origin and degree of transformation under natural conditions. Aims: The objective of this study was to shed light on the interdependence between mineralisation rates, secondary transformation of peat and chemical composition by examining the differences between bog and fen peat and between strongly degraded topsoil and well-preserved subsoil. Methods: Bog and fen peat from ten different peatlands under grassland use in Germany were analysed for their chemical composition using standard 13C nuclear magnetic resonance (NMR) spectroscopy and wet chemical extractions for fibre analysis. The radiocarbon age was determined as well. The results were combined with CO2 fluxes from a previous incubation study. Results: Topsoils had higher shares of proteins and lipids, and lower shares of carbohydrates and aromatics than subsoils. Bog peat subsoils were characterised by higher shares of carbohydrates and lower shares of aromatics than fen peat subsoils. Topsoils were more similar to each other in their chemical composition than the subsoils. Considering all samples, aromatics and phenolics were negatively correlated with CO2 fluxes. Measured CO2 fluxes from topsoils were significantly higher than from subsoils. However, no influences of chemical composition on CO2 fluxes were detected when examining topsoils and subsoils separately. Even though aromatics and phenolics showed positive relationships with radiocarbon age, differences in age alone were unable to explain the higher amounts of these compounds in the subsoil. Conclusions: The results imply that chemical composition of topsoil peat is not the reason for higher mineralisation rates compared to subsoil peat, but rather a consequence of decomposition and transformation. Thus, peat mineralisation of drained organic soils under agriculture might not slow down over time due to gradually decreasing peat quality but could increase further

Analysis of peat soil organic carbon, total nitrogen, soil water content and basal respiration: Is there a ‘best’ drying temperature?

Soil needs to be dried in order to determine water content, soil organic carbon content (SOC) and total nitrogen content (N). Water content is commonly measured using standard methods that involve drying temperatures of 105–110 °C. Recommended drying temperatures differ for the determination of SOC and N. However, at moderate drying temperatures, microbial activity might lead to organic matter mineralisation and nitrification, and thus to an underestimation of SOC and N. Furthermore, low drying temperatures might not dewater soils sufficiently to correctly determine water content or bulk density. Chemical processes such as thermal decomposition and volatilisation might occur at higher temperatures. This raises the question of whether the same sample can be used to determine water content, SOC and N. Further, the effect of drying, especially at different temperatures, on basal respiration of peat soils determined by incubation experiments is so far unknown. Effects of drying temperature might be especially severe for peat soils, which have high SOC and water contents. This study systematically evaluated the effect of different drying temperatures (20, 40, 60, 80 and 105 °C) on the determination of mass loss (proxy for water content), SOC and N over a wide range of 15 different peat soils comprising amorphous, Sphagnum and sedge peat substrate. The investigated peat soils had SOC contents ranging from approximately 16.8–52.5% with different degrees of decomposition. They were thus separated into two ‘peat groups’ (amorphous and weakly decomposed). In a subsequent investigation, an incubation experiment was carried out on a subset of five peat soils to investigate the pre-treatment effect of different drying temperatures on basal respiration. The results showed that amorphous samples should be dried at 105 °C to determine water content. The weakly decomposed peat soils in the study had reliable water contents for drying temperatures above 60 °C. For temperatures below 80 °C, the determined SOC and N were biased by residual water. This could be corrected for weakly decomposed samples, but for amorphous samples only for drying temperatures ≥60 °C. Thus, mineralisation of soil organic matter is likely to take place at lower drying temperatures which are not recommendable especially for amorphous peat prone to high mineralisation rates. This is supported by the results of the incubation experiment: The effect of peat type (amorphous topsoil vs. weakly decomposed subsoil) was greater than the effect of different drying temperatures, which nonetheless affected respiration rates. The differences between all five soils were consistent, irrespective of the drying temperature. Thus, incubation experiments might be possible using peat dried at moderate temperatures. © 2021 The Author

Effects of water management and grassland renewal on the greenhouse gas emissions from intensively used grassland on bog peat

Artificial drainage is prerequisite for conventional agricultural use of peatlands, but causes high emissions of greenhouse gases (GHG), mainly carbon dioxide (CO2). Furthermore, grassland renewal is regularly practiced to maintain high fodder quality, but might cause high emissions of nitrous oxide (N2O). Raising water levels is necessary to reduce CO2 emissions. Water management by subsurface irrigation (SI) and ditch blocking (DB) is thus discussed as potential compromise between maintaining intensive grassland use and reducing GHG emissions. Here, we present results of a four year study on the effects of SI and DB in combination with grassland renewal on GHG emissions from an intensively used grassland on bog peat in North-Western Germany. The water management itself was successful and lead to average mean annual water levels of -0.33 m at the parcels with SI. This was 0.38 m higher than at the control parcels. Ditch blocking also raised the mean water levels to -0.33 m, but the parcel was dryer in summer and wetter in spring than those with SI. Despite clear effects on water levels, CO2 and total GHG emissions were much (38 % and 31 %) higher from SI parcels than from the control parcels. CO2 and GHG emissions of the DB parcel were similar to those of the control. Shallow ploughing increased N2O emissions for around 1.5 years, but there was no clear effect of direct sowing. Methane emission from all parcels were low. The surprising results regarding CO2 might be explained by an interaction of increased soil moisture in the topsoil and improved nutrient retention during periods of high soil temperatures facilitated by SI and, concurrently, by limitations of microbial activity due to dry conditions at the control parcels. Thus, results of this study do not support subsurface irrigation as a GHG mitigation measure at intensively used bog peatlands

High greenhouse gas emissions after grassland renewal on bog peat soil

Drained agriculturally used peatlands are hotspots for greenhouse gas (GHG) emissions, especially carbon dioxide (CO2) and nitrous oxide (N2O). To reduce GHG emissions and simultaneously maintain intensive grassland use, raising water levels by subsurface irrigation (SI) is controversially discussed. Both, intensive grassland use and installations of SI may require grassland renewal. We investigated an experimental intervention site (INT) (SI target water levels: -0.30 m) and a deeply drained reference site (REF), both intensive grassland on deep bog peat. After installation of the SI system, a mechanical grassland renewal took place at INT. At both sites, CO2 (eddy covariance), N2O and methane (manual closed chamber technique) were measured. Additionally, soil water was analyzed for nitrogen species. Here, we report on the initial year of GHG measurements including grassland renewal and rising water levels. Overall, GHG emissions were strongly influenced by grassland renewal at INT. Despite progressively rising water levels, soil moisture in the upper centimeters was low and thus grass growth was slow, resulting in an almost complete loss of harvest. This resulted in a net ecosystem carbon balance (NECB) of 4.64 ± 1.03 t C ha−1 containing only 0.57 ± 0.09 t C ha−1 harvest at INT, while NECB at REF was 6.08 ± 1.74 t C ha−1 including harvest from five grass cuts. Methane fluxes were negligible at both sites. Nitrous oxide emissions dominated the GHG balance at INT. With 144.5 ± 45.5 kg N2O–N ha–1 a–1, they were much higher than at REF (3.9 ± 3.1 kg N2O–N ha–1 a–1) and any other values published so far. Peak fluxes occurred when nitrate concentrations in soil water were extremely high, soil moisture was increased, and vegetation development was struggling. This study highlights the risk of grassland renewals on peat soils regarding yield losses as well as high GHG emissions

How do sand addition, soil moisture and nutrient status influence greenhouse gas fluxes from drained organic soils?

Drainage turns peatlands from natural carbon sinks into hotspots of greenhouse gas (GHG)emissions from soils due to alterations in hydrological and biogeochemical processes. As a consequence of drainage-induced mineralisation and anthropogenic sand addition, large areas of former peatlands under agricultural use have soil organic carbon (SOC)contents at the boundary between mineral and organic soils. Previous research has shown that the variability of GHG emissions increases with anthropogenic disturbance. However, how and whether sand addition affects GHG emissions remains a controversial issue. The aim of this long-term incubation experiment was to assess the influence of hydrological and biogeochemical soil properties on emissions of carbon dioxide (CO 2 ), nitrous oxide (N 2 O)and methane (CH 4 ). Strongly degraded peat with sand addition (peat-sand mixtures)and without sand addition (earthified peat)was systematically compared under different moisture conditions for fen and bog peat. Soil columns originating from both the topsoil and the subsoil of ten different peatlands under grassland use were investigated. Over a period of six months the almost saturated soil columns were drained stepwise via suction to −300 hPa. The CO 2 fluxes were lowest at water-saturated and dry soil moisture conditions, resulting in a parabolic dependence of CO 2 fluxes on the water-filled pore space (WFPS)peaking at 56–92% WFPS. The highest N 2 O fluxes were found at between 73 and 95% WFPS. Maximum CO 2 fluxes were highest from topsoils, ranging from 21 to 77 mg C m −2 h −1 , while the maximum CO 2 fluxes from subsoils ranged from 3 to 14 mg C m −2 h −1 . No systematic influence of peat type or sand addition on GHG emissions was found in topsoils, but CO 2 fluxes from subsoils below peat-sand mixtures were higher than from subsoils below earthified peat. Maximum N 2 O fluxes were highly variable between sites and ranged from 18.5 to 234.9 and from 0.2 to 22.9 μg N m −2 h −1 for topsoils and subsoils, respectively. CH 4 fluxes were negligible even under water-saturated conditions. The highest GHG emissions occurred at a WFPS that relates – under equilibrium conditions – to a water table of 20–60 cm below the surface in the field. High maximum CO 2 and N 2 O fluxes were linked to high densities of plant-available phosphorus and potassium. The results of this study highlight that nutrient status plays a more important role in GHG emissions than peat type or sand addition, and do not support the idea of peat-sand mixtures as a mitigation option for GHG emissions

Improving the determination of soil hydraulic properties of peat soils at different scales

This thesis improves the characterization of unsaturated hydraulic properties for different types of peat and other organic soils at the laboratory and the field scale. First, suggestions for a general improvement of the unsaturated hydrological modeling in peatlands are made. Second, a novel one-dimensional expression for calculating specific yield for shallow groundwater systems with microrelief has been derived which was the basis for the development of a novel in situ method for determining soil water retention characteristics in shallow groundwater systems