Soil properties of drained and rewetted fen soils

The intensive agricultural use and consequently the drainage of fen soils have caused modifications in structure and nutrient dynamics. Pedogenetic processes result in the formation of typical soil horizons with distinctive soil properties. These are the basis for soil classification. In the present review, results are compiled. Modifications of abiotic and biotic parameters of fen soils due to drainage and rewetting are presented. Recommendations on the further use of fen soils are submitted.     

Methane fluxes and nitrogen dynamics from a drained fenland peat

 Rates of methane uptake were measured in incubation studies with intact cores from adjacent fenland peats that have been under arable management and woodland management for at least the past 30 years. On two separate occasions the woodland peat showed greater rates of uptake than the arable peat. These rates ranged from 23.1 to 223.3 μg CH₄ m^–2 day^–1 for the woodland peat and from 29.6 to 157.6 μg CH₄ m^–2 day^–1 for the arable peat. When the peats were artificially flooded there was a decrease in the rate of methane oxidation, but neither site showed any net efflux of methane. ¹⁵N isotopic dilution was used to characterise nitrogen cycling within the two peats. Both showed similar rates of gross nitrogen mineralisation (3.58 mg N kg^–1 day^–1, arable peat; 3.54 N kg^–1 day^–1, woodland peat) and ammonium consumption (4.19 arable peat and 4.70 mg N kg^–1 day^–1 woodland peat). There were significant differences in their inorganic ammonium and nitrate pool sizes, and the rate of gross nitrification was significantly higher in the woodland peat (4.90 mg N kg^–1 day^–1) compared to the arable peat (1.90 mg N kg^–1 day^–1). These results are discussed in the light of high atmospheric nitrogen deposition. Received: 1 December 1997     

Mineral nitrogen dynamics in poorly drained blanket peat

Summary Mineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m²) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NH _in4 ^sup+ , accumulated in winter, reaching 42 kg N ha^–1 in the surface 10 cm in April before the application of 112.5 kg N ha^–1 as NH₄NO₃ or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha^–1 day^–1, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha^–1 day^–1. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH₄NO₃ in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×10⁹ litre^–1) and August for anaerorbes (7.1×10⁹ litre^–1). N fertilizer increased these counts significantly (P<0.05) to 56×10⁹ aerobes and 13×10⁹ anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha^–1 were recorded.     

Carbon budget of drained peat bogs in Ukrainian Polesie

Methodological aspects of studying the organic carbon budget of drained peat bogs used in agriculture are discussed. Difficulties in the assessment of carbon budget on the basis of measured carbon inflows and outflows are shown. The “soil pool” model of the carbon budget is suggested. It is based on the results of long-term experimental studies of a drained peatland in Ukrainian Polesie. An algorithm for calculating the carbon budget in the aerated part of the peatland—the peat soil proper—is developed with due account for a gradual involvement of the deep peat layers into the zone of soil processes. Data on the loss of dry peat mass and organic carbon per in the course of peat mineralization and surface subsidence are given with due account for the nature of the peatland and the duration of its agricultural use.     

Solute recycling by crops and leaching in a drained arable soil

Preferential flow, as it bypasses the soil matrix, can greatly enhance the leaching of chemicals. When a soil is drained there is the risk that such short‐circuiting results in more or less direct passage of polluting chemicals from the soil to the groundwater. If the groundwater table is shallow the chemicals could be transferred back into the surface soil by hydraulic lift through roots and subsequent release by exudation or from decaying plant residues and again become exposed to leaching by preferential flow, thus strongly enhancing the chance of export via the drains. We investigated the leaching of bromide in a tile‐drained arable field over 2 years of crop rotation. The site was a former wetland, artificially drained a century ago for agriculture. Bromide was applied over 1.6 ha at a dosage of 10 g Br per m² in August 1995 after the harvest of wheat. During the 2 years 18% of the applied bromide was exported via the drainage system, most of it in preferential flow events and more than half of it in a single winter storm 5 months after the application. Within 7 months 56% of the applied tracer was leached out of the main root zone into the groundwater. Subsequently the tracer re‐emerged in water taken up by sugar beet in the following season. The beet accumulated 50% of the initially applied bromide in their leaves and released it again after harvest when the leaves were left as green manure on the field. Our results show that this recycling of solutes to the topsoil can have an important influence on their leaching as the solutes are thus again exposed to preferential transport into drains in the course of preferential flow events.     

Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N₂O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm ¹⁵N tracer experiments were performed to evaluate the effect of common wetland plants (Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the ¹⁵N balance method. In two experiments (wet and reflooded soil conditions) the N₂O and N₂ emissions were also directly determined.Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the ¹⁵N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77-95% of the ¹⁵N-nitrate added) and the transformation into organic N compounds was very low (1.8% of ¹⁵N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20-25% of the ¹⁵N nitrate added due to plant uptake. The N₂ emissions exceeded the N₂O emissions by a factor of 10-20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N₂O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N₂O and N₂ formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N₂O emissions and no emission via the shoots was observed.     

Composition of carbon fractions and potential denitrification in drained peat soils

The organic matter of five low-moor peat soils and one eutrophic raised-bog peat soil was chemically characterized by C fractionation and ion-exchange chromatography of amino acids and carbohydrates. C fractions were related to potential denitrification, D_pot, as a measure of microbial activity and C availability, determined by the acetylene inhibition technique. Chemical and physical properties vary distinctively between different kinds of peat, and show increasing C/N ratio and decreasing bulk density and ash content within the profile. Generally, the carbon composition reflects the geobotanical origin of the peat. In most samples more than 65% of organic C consists of non-hydrolysable C. Readily hydrolysable neutral sugar C represents up to about 12% of organic C, usually decreasing with depth. The recalcitrant fraction of neutral sugar C is much smaller (1 to 4.2% of organic C) and does not vary with depth. The content of readily hydrolysable glucose exhibits a strong profile differentiation that decreases with depth, whereas the higher contents of recalcitrant glucose carbon (12/0.5 M H₂SO₄) in the lower peat horizons reflect their cellulose character. Regression analysis between D_pot and single C components explains up to 51.5% of the variability. Combining fractions which point to C availability (readily hydrolysable glucose) and microbial metabolism (amino acids), it is possible to estimate D_pot with a certainty of more than 80%.     

Microbial activity and community structure in two drained fen soils in the Ljubljana Marsh

Fen peatlands are specific wetland ecosystems containing high soil organic carbon (SOC). There is a general lack of knowledge about the microbial communities that abound in these systems. We examined the microbial activity and community structure in two fen soils differing in SOC content sampled from the Ljubljana Marsh under different seasonal conditions. Substrate-induced respiration and dehydrogenase activity were used as indicators of total microbial activity. Both methods indicated higher microbial activities in the fen soil with the higher SOC content on all dates of sampling. To determine whether the differences in microbial activity were associated with differences in the microbial community structures, terminal restriction fragment length polymorphism (T-RFLP) of bacterial 16S rRNA genes was performed. Comparison of the T-RFLP profiles revealed very similar community structures in both fens and in the two seasonal extremes investigated. This suggested a stable community structure in the two fens, which is not affected by the SOC content or seasonal variation. In addition, a bacterial 16S ribosomal RNA gene based clone library was prepared from the fen soil with the higher SOC content. Out of 114 clones analysed, approximately 53% belonged to the Proteobacteria, 23% to the Acidobacteria, 21% to a variety of other taxa, and less than 3% were affiliated with the Firmicutes.     

Two- and three-domain bacterial laccase-like genes are present in drained peat soils

Laccases of fungal origin have been intensively studied due to their importance in various biotechnological applications. There is a constant demand for new laccases with improved properties such as stability at higher temperatures or at an alkaline pH. Growing molecular evidence suggests that laccases may also be widespread in bacteria. While only a handful of bacterial laccases have been purified and characterized, several novel traits have already been discovered (e.g. pH-stability and 2-domain organization of the enzyme as opposed to the usual 3-domain structure of fungal laccases). The aim of this study was to examine the diversity of bacterial laccase-like genes in two types of high-organic peat soil using a cloning and sequencing approach. Gene libraries prepared of small fragments (150 base pairs) revealed an amazing diversity of bacterial laccases. The fragments clustered in 11 major lineages, and one third of the 241 sequences resembled laccase-like genes of Acidobacteria. Additionally, a new primer was used to retrieve several larger fragments of the putative bacterial laccase genes that spanned all four copper-binding sites. Both “conventional” 3-domain laccases and the recently described 2-domain small laccases have been obtained using this approach, demonstrating the potential of the primer. The present study thus contributes to the understanding of the diversity of bacterial laccases and provides a new tool for finding laccase-like sequences in bacterial strains and soil samples.     

Prediction of the thawing depth for artificially drained and cultivated peat soils

The analysis of experimental materials obtained in various regions of Russia and Belarus suggests that the thawing depth of plowed peat soils depends on the sum of mean daily air temperatures after the snow melting. The effect of other factors, including the latitudinal position of particular places, the water-physical properties of soils, and their geomorphic position on slopes of different aspects, is concealed by the major role of the heat flux from the atmosphere. The suggested exponential equation to describe this dependence can be applied for prediction of the thawing depth of arable peat soils in the European part of Russia and in West Siberia.     

Actinomycetal complexes in drained peat soils of the taiga zone upon pyrogenic succession

The number and diversity of actinomycetes in peat soils vary in dependence on the stage of pyrogenic succession. In the cultivated peat soil, the number of actinomycetes after fires decreases by three-four times, mainly at the expense of acidophilic and neutrophilic groups. An increase in the number of mycelial prokaryotes (at the expense of alkaliphilic forms) is seen on the fifth year of functioning of the pyrogenic peat soil. The species diversity of streptomycetes in peat soils also decreases after fires. An increase in the range of streptomycetal species at the expense of neutrophilic and alkaliphilic forms takes place on the fifth year of the pyrogenic succession. Parameters of the actinomycetal complex—the population density, species composition, and ecological features—are the criteria whose changes allow us to judge the state of peat soils in the course of their pyrogenic succession.     

The effect of drainage on nutrient release from fen peat and its implications for water quality —a laboratory simulation

The effect of peat moisture status on N, S, Ca, and Mg release to drainage waters was examined using a constant temperature laboratory incubation. Peat samples originating from drained and undrained sites in West Sedgemoor, Somerset Levels, SW England were compared. Three treatments: long term waterlogging, aeration, and fluctuating aeration and waterlogging were imposed on all peat samples. These treatments resulted in different rates and total amounts of N, S, Ca, and Mg release, with waterlogging resulting in highest solute release. The total amounts and rates of release of S, Ca, and Mg from peat that was undrained prior to incubation always exceeded that from drained site peat samples regardless of peat moisture status. Although the degree of waterlogging or aeration affected the rate and total amount of watersoluble N released during incubation, there was no difference between peat that was drained, and peat that was undrained, prior to incubation. Drainage of currently undrained and waterlogged peat in West Sedgemoor will result in the transfer of high concentrations of S, Ca, and Mg from the peat to the drainage ditch.     

Denitrification in drained and rewetted minerotrophic peat soils in Northern Germany (Pohnsdorfer Stauung)

This study was conducted to assess the nitrogen removal potential of a minerotrophic peatland in Northern Germany, where hydrological conditions were partly restored in the beginning of the 1990s. Actual denitrification and the effect of nitrate (NO₃^—) and glucose additions on denitrification rates were determined in two flooded and one drained histosols in spring and summer 1998. In the flooded soils, denitrification was insignificant, but the drained field emitted significant rates. Additions of NO₃^— stimulated denitrification at all sites in spring and summer, whereas glucose additions had no effect. Low NO₃^— concentration in floodwater was obviously limiting denitrification in the flooded soils. In the drained soil, a coupled nitrification/denitrification might explain the low but significant denitrification rates. No spontaneous production of nitrous oxide occurred in the flooded soils, whereas at the drained site an increase in spontaneous nitrous oxide concentration was measured during incubation in the summer samples. The suggested introduction of NO₃^— rich water from a stream flowing through the area would apparently induce denitrification in the flooded fields.     

Effects of experimental drying intensity and duration on respiration and methane production recovery in fen peat incubations

Drying and rewetting to a variable extent influence the C gas exchange between peat soils and the atmosphere. We incubated a decomposed and compacted fen peat and investigated in two experiments 1) the vertical distribution of CO₂ and CH₄ production rates and their response to drying and 2) the effects of temperature, drying intensity and duration on CO₂ production rates and on CH₄ production recovery after rewetting. Surface peat down to 5 cm contributed up to 67% (CO₂) and above 80% (CH₄) of the depth-aggregated (50 cm) production. As CO₂ production sharply decreased with depth water table fluctuations in deeper peat layers are thus not expected to cause a substantial increase in soil respiration in this site. Compared to anaerobic water saturated conditions drying increased peat CO₂ production by a factor between 1.4 and 2.1. Regarding the effects of the studied factors, warmer conditions increased and prolonged drying duration decreased CO₂ production whereas the soil moisture level had little influence. No significant interactions among factors were found. Short dry events under warmer conditions are likely to result in greatest peaks of CO₂ production rates. Upon rewetting, CH₄ production was monitored over time and the recovery was standardized to pre-drying levels to compare the treatment effects. Methane production increased non-linearly over time and all factors (temperature, drying intensity and duration) influenced the pattern of post-drying CH₄ production. Peat undergoing more intense and longer drying events required a longer lag time before substantial CH₄ production occurred and warmer conditions appeared to speed up the process.     

Optimizing fen peatland water-table depth for romaine lettuce growth to reduce peat wastage under future climate warming

Forty percentage of UK peatlands have been drained for agricultural use, which has caused serious peat wastage and associated greenhouse gas emissions (carbon dioxide (CO₂) and methane (CH₄)). In this study, we evaluated potential trade-offs between water-table management practices for minimizing peat wastage and greenhouse gas emissions, while seeking to sustain romaine lettuce production: one of the most economically relevant crop in the East Anglian Fenlands. In a controlled environment experiment, we measured lettuce yield, CO_2, CH₄ fluxes and dissolved organic carbon (DOC) released from an agricultural fen soil at two temperatures (ambient and +2°C) and three water-table levels (−30 cm, −40 cm and −50 cm below the surface). We showed that increasing the water table from the currently used field level of −50 cm to −40 cm and −30 cm reduced CO₂ emissions, did not affect CH₄ fluxes, but significantly reduced yield and increased DOC leaching. Warming of 2°C increased both lettuce yield (fresh leaf biomass) and peat decomposition through the loss of carbon as CO₂ and DOC. However, there was no difference in the dry leaf biomass between the intermediate (−40 cm) and the low (−50 cm) water table, suggesting that romaine lettuce grown at this higher water level should have similar energetic value as the crop cultivated at −50 cm, representing a possible compromise to decrease peat oxidation and maintain agricultural production.     

Differences in the activity and bacterial community structure of drained grassland and forest peat soils

The microbial activity and bacterial community structure were investigated in two types of peat soil in a temperate marsh. The first, a drained grassland fen soil, has a neutral pH with partially degraded peat in the upper oxic soil horizons (16% soil organic carbon). The second, a bog soil, was sampled in a swampy forest and has a very high soil organic carbon content (45%), a low pH (4.5), and has occasional anoxic conditions in the upper soil horizons due to the high water table level. The microbial activity in the two soils was measured as the basal and substrate-induced respiration (SIR). Unexpectedly, the SIR (μl CO₂ g⁻¹ dry soil) was higher in the bog than in the fen soil, but lower when CO₂ production was expressed per volume of soil. This may be explained by the notable difference in the bulk densities of the two soils. The bacterial communities were assessed by terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA genes and indicated differences between the two soils. The differences were determined by the soil characteristics rather than the season in which the soil was sampled. The 16S rRNA gene libraries, constructed from the two soils, revealed high proportions of sequences assigned to the Acidobacteria phylum. Each library contained a distinct set of phylogenetic subgroups of this important group of bacteria.     

Estimating leaching losses from sub-surface drained soils

Abstract. Leaching losses of solutes can be calculated if two variables, the amount of water passing through the soil and the concentration of solute in that water (a flux concentration), are known. Two simple approaches, soil extraction and suction cup sampling, were used to estimate the concentration of solutes in the water moving through a silt loam soil. The results were compared with actual concentrations measured in the drainage water from a sub-surface (mole-pipe) drained soil.
Seasonal leaching losses were calculated as the sum of the products of estimated monthly drainage and the estimated average monthly solute concentration in the soil solution. These results were compared with the leaching losses measured in drainage water from the mole-pipe system. For non-reactive solutes such as bromide (an applied solute) and chloride (a resident solute), the suction cup data provided better estimates of the leaching losses than did the soil extraction data. The leaching losses calculated using volume-averaged soil solution concentrations (obtained by soil extraction) overestimated the loss for the resident solute, but under-estimated the loss for the surface-applied solute. On the other hand, the data for non-reactive solutes suggest that measurements on suction cup samples may be representative of the flux concentration of a solute during leaching. For nitrate, a biologically reactive solute, there was no clear pattern in the differences between the estimated and measured leaching losses. The flux-averaged concentration in the drainage water was about midway between those measured in the suction cup samples and in the soil solution.     

Phosphorus uptake by rice from soil that is flooded, drained or flooded then drained

To investigate the mechanisms by which rice plants growing in alternately flooded and drained soils absorb soil phosphate, we grew rice in moist, flooded and flooded then moist soils, and compared the measured uptake of phosphorus (P) with that calculated using a mathematical model of uptake allowing for solubilization by various means. The theory and equations for the model are given, together with a method for solving diffusion equations near roots in a root system of increasing density. The diffusion coefficients and buffer powers of P in the soil under the different water regimes are measured by following diffusion of P to a resin sink, and the parameters describing solubilization are estimated from previously published results. In all the water regimes studied, the plants relied upon solubilization for most of their P. The roots were not mycorrhizal, as they will often not be in intermittently flooded soils. In the flooded soil, uptake was three times that in the moist soil, and was consistent with solubilization by acidification caused by roots as a result of oxidation of iron and imbalance between the intake of cations and anions. In the moist soil, the uptake was consistent with solubilization by excretion of organic anions from the roots. In the flooded then moist soil, uptake declined sharply as the soil dried because P became immobilized in the soil. However, the final uptake was similar to that in the continuously moist soil, indicating that some of the immobilized P was re‐solubilized by roots, possibly by excretion of organic anions.     

Peatland subsidence and carbon loss from drained temperate fens

Peat‐forming organic soils lose large amounts of carbon and soil volume when drained. Although surface subsidence is often taken as a proxy for the associated carbon loss, other mechanisms also cause a change in volume. To infer the reliability of subsidence for estimating carbon loss, we compared long‐term subsidence rates of an 80 ha area of temperate fen that was drained 140 yr ago against estimates of subsidence based on soil bulk densities measured at four sites. Both methods correlate significantly, yield similar subsidence rates of 0.8–1.6 cm/yr and underpin the value of using profile information for inferring volumetric loss. Peat oxidation accounts for 28–64% of the loss in volume, which is equivalent to annual carbon loss rates of 2.5–5.5 t C/ha. Whereas the profile‐based method is also suitable for estimating carbon loss, the wide range of oxidative contribution to the overall subsidence indicates that subsidence alone cannot provide an unbiased estimate of carbon emission factors from drained fens.