Winter CO2 CH4 and N2O fluxes on some natural and drained boreal peatlands

CO₂ and CH₄ fluxes during the winter were measured at natural and drained bog and fen sites in eastern Finland using both the closed chamber method and calculations of gas diffusion along a concentration gradient through the snowpack. The snow diffusion results were compared with those obtained by chamber, but the winter flux estimates were derived from chamber data only. CH₄ emissions from a poor bog were lower than those from an oligotrophic fen, while both CO₂ and CH₄ fluxes were higher in theCarex rostrata- occupied marginal (lagg) area of the fen than in the slightly less fertile centre. Average estimated winter CO₂-C losses from virgin and drained forested peatlands were 41 and 68 g CO₂-C m^–2, respectively, accounting for 23 and 21% of the annual total CO₂ release from the peat. The mean release of CH₄-C was 1.0 g in natural bogs and 3.4 g m^–2 in fens, giving rise to winter emissions averaging to 22% of the annual emission from the bogs and 10% of that from the fens. These wintertime carbon gas losses in Finnish natural peatlands were even greater than reported average long-term annual C accumulation values (less than 25g C m^–2). The narrow range of 10–30% of the proportion of winter CO₂ and CH₄ emissions from annual emissions found in Finnish peatlands suggest that a wider generalization in the boreal zone is possible. Drained forested bogs emitted 0.3 g CH₄-C m^–2 on the average, while the effectively drained fens consumed an average of 0.01 g CH₄-C m^–2. Reason for the low CH₄. efflux or net oxidation in drained peatlands probably lies in low substrate supply and thus low CH₄ production in the anoxic deep peat layers. N₂O release from a fertilized grassland site in November–May was 0.7 g N₂O m^–2, accounting for 38% of the total annual emission, while a forested bog released none and two efficiently drained forested fens 0.09 (28% of annual release) and 0.04 g N₂O m^–2 (27%) during the winter, respectively.     

Accelerated vegetation succession but no hydrological change in a boreal fen during 20 years of recent climate change

Northern mires (fens and bogs) have significant climate feedbacks and contribute to biodiversity, providing habitats to specialized biota. Many studies have found drying and degradation of bogs in response to climate change, while northern fens have received less attention. Rich fens are particularly important to biodiversity, but subject to global climate change, fen ecosystems may change via direct response of vegetation or indirectly by hydrological changes. With repeated sampling over the past 20 years, we aim to reveal trends in hydrology and vegetation in a pristine boreal fen with gradient from rich to poor fen and bog vegetation. We resampled 203 semi‐permanent plots and compared water‐table depth (WTD), pH, concentrations of mineral elements, and dissolved organic carbon (DOC), plant species occurrences, community structure, and vegetation types between 1998 and 2018. In the study area, the annual mean temperature rose by 1.0°C and precipitation by 46 mm, in 20‐year periods prior to sampling occasions. We found that wet fen vegetation decreased, while bog and poor fen vegetation increased significantly. This reflected a trend of increasing abundance of common, generalist hummock species at the expense of fen specialist species. Changes were the most pronounced in high pH plots, where Sphagnum mosses had significantly increased in plot frequency, cover, and species richness. Changes of water chemistry were mainly insignificant in concentration levels and spatial patterns. Although indications toward drier conditions were found in vegetation, WTD had not consistently increased, instead, our results revealed complex dynamics of WTD as depending on vegetation changes. Overall, we found significant trend in vegetation, conforming to common succession pattern from rich to poor fen and bog vegetation. Our results suggest that responses intrinsic to vegetation, such as increased productivity or altered species interactions, may be more significant than indirect effects via local hydrology to the ecosystem response to climate warming.     

Long-term effects of drainage and hay-removal on nutrient dynamics and limitation in the Biebrza mires,Poland

To provide a reference for wetlands elsewhere we analysed soil nutrients and the vegetation of floodplains and fens in the relatively undisturbed Biebrza-valley, Poland. Additionally, by studying sites along a water-table gradient, and by comparing pairs of mown and unmown sites, we aimed with exploring long-term effects of drainage and annual hay-removal on nutrient availabilities and vegetation response. In undrained fens and floodplains, N mineralization went slowly (0–30 kg N ha⁻¹ year⁻¹) but it increased strongly with decreasing water table (up to 120 kg N ha⁻¹ year⁻¹). Soil N, P and K pools were small in the undisturbed mires. Drainage had caused a shift from fen to meadow species and the disappearance of bryophytes. Biomass of vascular plants increased with increasing N mineralization and soil P. Annual hay-removal tended to have reduced N mineralization and soil K pools, but it had increased soil P. Moreover, N concentrations in vascular plants were not affected, but P and K concentrations and therefore N:P and N:K ratios tended to be changed. Annual hay-removal had induced a shift from P to K limitation in the severely drained fen, and from P to N limitation in the floodplain. The low nutrient availabilities and productivity of the undisturbed Biebrza mires illustrate the vulnerability of such mires to eutrophication in Poland and elsewhere. In nutrient-enriched areas, hay removal may prevent productivity increase of the vegetation, but also may severely alter N:P:K stoichiometry, induce K-limitation at drained sites, and alter vegetation structure and composition.     

Phosphorus Retention at the Redox Interface of Peatlands Adjacent to Surface Waters in Northeast Germany

It is demanded currently in public discussions to rewet peatlands and re-establish their function as nutrient sinks. But due to high phosphorus (P) concentrations in the pore water of rewetted peatlands (40–420 μM) it is hypothesized that they can act as a surplus P source for adjacent surface waters and consequently support the eutrophication of such waters. Our detailed investigations of processes at the redox interface in four fens with different geochemical character show the dependence of P retention from the chemistry of the pore water. The precipitation of Fe(III) oxyhydroxide led to high retention of phosphorus and other substances such as DOC and sulphate in the eutrophic fens. When molar Fe/P ratios were larger than about 3 the initially high P concentrations in the anaerobic pore water (20–210 μM) decreased to concentrations below 1 μM under aerobic conditions. Thus, after rewetting high pore water concentrations of P do not automatically result in an increased P load to adjacent surface waters compared to pre-rewetting conditions. An enhanced P export to adjacent surface waters from eutrophic fens can be expected when the Fe/P ratio is smaller than 3 in the anaerobic pore water. In our investigations of natural, oligotrophic to mesotrophic fens the precipitation of Fe(III) oxyhydroxide was inhibited by the formation of stable dissolved Fe ∼ humic complexes. P retention in these fens was only related to the DOC concentrations at the redox interface, so that lower DOC concentrations concurred with higher P retention. The P equilibrium concentrations in an aerobic environment can be higher than that of eutrophic fens with Fe/P ratios larger than about 3 in the anaerobic pore water.     

Nutrient concentrations in mire vegetation as a measure of nutrient limitation in mire ecosystems

Abstract. The above-ground standing crop and nutrient concentrations in plant material were examined in 45 stands of mire vegetation in the Biebrza peatland, Poland. The stands included flood-plains, rich fens, transitional fens and bogs. The pattern in nutrient concentrations in the above-ground plant material resembled the pattern in nutrient concentrations in peatwater and peat which had been investigated in an earlier study. Concentrations of N were quite uniform along the gradient. P-concentrations were highest in the transitional fen. Critical nutrient concentrations were defined on the basis of a review of nutrient concentrations in plant material from peatlands in which a fertilization experiment had been carried out. Defined critical values for phanerogams were: 13-14 and 0.7 mg/g dry wt for N and P respectively. Concentrations lower than these values indicate deficiency. P/N ratios ≥ 0.07 indicate N-deficiency and P/N ratios ≤ 0.04 — 0.05 indicate P-deficiency. According to these values the Biebrza fens and bogs appear to be primarily deficient in N. The growth of the flood-plain vegetation does not appear to be restricted by nutrients.     

Hydrology,water chemistry and nutrient accumulation in the Biebrza fens and floodplains (Poland)

The composition, structure and above-ground biomass production of floodplain- and fen-vegetation of the Biebrza valley (N.E. Poland) are strongly correlated with water flow characteristics and water chemistry. Groundwater flow and flooding are the major conditioning factors for the vegetation in the valley.The highly productive vegetation is restricted to the dynamic floodplain where it receives nutrient-rich river water during spring floods. The non-flooded parts of the valley contain rich fen and transitional fen vegetation that have a lower biomass production. The rich fen is fed by calcareous and phosphate-poor groundwater coming from the moraines. In the transitional fen, where rainwater infiltrates, phosphate availability is large.Annual nutrient accumulation in the above-ground biomass of the floodplains is estimated to be about 8–9 § 10³ kg/km² for N and K and 1 § 10³ kg/km² for P. For the less-productive fens these figures are 60 to 70% lower. The total annual nutrient accumulation by vegetation of both floodplains and fens for the entire Biebrza valley is estimated to be about 5600 × 10³ kg N, 560 × 103 kg P and 4500 × 10³ kg K. This is high compared to the loading rates in the river near to where the Biebrza River discharges into the Narew River (N-, P- and K-loading rates are c. 900, 200 and 3000 × 103 kg/y, respectively). This implies that floodplain and fen vegetation are important sinks for nutrients, especially for N and P.This paper was presented at the INTECOL IV International Wetlands Conference in Columbus, Ohio, 1992, as part of a session organized by Prof. S. E. Jørgensen and sponsored by the International Lake Environment Committee.Corresponding Editor: J. Kvt     

Effect of peat re-wetting on carbon and nutrient fluxes, greenhouse gas production and diversity of methanogenic archaeal community

Many peatlands were affected by drainage in the past, and restoration of their water regime aims to bring back their original functions. The purpose of our study was to simulate re-wetting of soils of different types of drained peatlands (bogs and minerotrophic mires, located in the Sumava Mountains, Czech Republic) under laboratory conditions (incubation for 15 weeks) and to assess possible risks of peatland water regime restoration - especially nutrient leaching and the potentials for CO₂ and CH₄ production. After re-wetting of soils sampled from drained peatlands (simulated by anaerobic incubation) (i) phosphorus concentration (SRP) did not change in any soil, (ii) concentration of ammonium and dissolved organic nitrogen (DON) increased, but only in a drained fen, (iii) DOC increased significantly in the drained fen and degraded drained bog, (iv) CO₂ production decreased, (v) CH₄ production and the number of methanogens increased in all soils, and (vi) archaeal methanogenic community composition was also affected by re-wetting; it differed significantly between drained and pristine fens, whereas it was more similar between drained and pristine bogs. Overall, the soils from fens reacted more dynamically to re-wetting than the bogs, and therefore, some nutrients (especially nitrogen) and DOC leaching may be expected from drained fens after their water regime restoration. However, if compared to their state before restoration, ammonium and phosphorus leaching should not increase and leaching of nitrates and DON should even decrease after restoration, especially during the vegetation season. Further, CO₂ production in soils of fens and bogs should decrease after their water regime restoration, whereas CH₄ production in soils should increase. However, we cannot derive any clear conclusions about CH₄ emissions from the ecosystems based on this study, as they depend strongly on environmental factors and on the actual activity of methanotrophs in situ.     

Patterns of Fungal and Bacterial Carbon Mineralization Across Northern European Peatlands

The fungal and bacterial activity was determined in 20 northern European peatlands ranging from ombrotrophic bogs to eutrophic fens with key differences in degree of humification, pH, dry bulk density, carbon (C) content and vegetation communities using the selective inhibition (SI) technique. These peatlands were partly disturbed and the respective water tables lowered below the surface layer. Basal respiration ranged from 24 to 128 µg CO₂-C g^?1 dry peat d^?1. Bacterial contributions to CO₂ production were high in most peatlands and showed the following pattern: eutrophic >> transitional ≥ mesotrophic >> ombrotrophic peatland types. The fungal-to-bacterial (F:B) ratios varied substantially within peatland type, and this was mainly attributed to differences in peat botanical compositions and chemistry. The computed mean Inhibitor Additivity Ratio (IAR) was quite close to 1 to suggest that the SI techniques can be used to partition eukaryotic and prokaryotic activity in wide range of peatlands. Overall, basal respiration, microbial biomass-C, fungal and bacterial activities varied across the studied peatland types, and such differences could have consequences for C- and nutrient-cycling as well as how bogs and fens will respond to environmental changes.     

Methane emissions from fen,bog and swamp peatlands in Quebec

A static chamber technique was used weekly from spring thaw to winter freezing to measure methane emissions from 10 sites representing subarctic fens and temperate swamps and bogs. Rates of < 200 mg CH₄ m^–2 d^–1 were recorded in subarctic fens: within-site emissions were primarily controlled by the evolution of the peat thermal regime, though significant releases during spring thaw were recorded at some sites. Between subarctic fens, topography and water table elevation were important controls on methane emissions, with the general sequence: pool = horizontal fen> string. Emission rates from the 2 swamp sites were lower (< 20 mg CH₄ m^–2 d^–1 ), except during the spring thaw and when the sites were saturated. The low water table ( < 80 cm depth) in abnormally dry years reduced emission rates; rates were also low from a swamp site which had been drained and cleared of vegetation for horticulture. Methane emission rates were also low (< 5 mg CH₄ m^–2 d^–1) from 2 ombrotrophic bog sites. Laboratory measurements of rates of methane production under anaerobic conditions and methane consumption under aerobic conditions revealed that production rates were generally highest in the surface layers (0 to 2.5 cm depth); production was high in the fens and very low in the bogs. The swamp samples were able to produce methane under anaerobic conditions, but were also able to consume methane under aerobic conditions. Annual methane emission rates are estimated to be 1 to 10 g CH₄ m^–2 from the fens, 1 to 4 g CH4 m^–2 from the swamps and <0.2 g CH₄ m^–2 from the bogs and drained swamp.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

Summary Release of inorganic N and P in the organic soils of three small quaking fens in The Netherlands was studied by means of in situ incubation of the peat soil in plastic bottles. One of the fens had higher biomass production and lower species richness than the other two. The former fen is located in an area with downward groundwater percolation, whereas the latter fens are situated in an area of upward seepage of groundwater rich in calcium and bicarbonate.Mineralization of N proved to be slow in the low-productive fens, and 6 times faster in the highly productive fen. In the latter fen the amount of N mineralized during a certain period exceeded by far the amount accumulated in that period in the above-ground parts of the vegetation.The release of inorganic P was extremely slow in the two low-productive fens and rapid in the highly productive fen. The release rates were not related to the total P content of the soils. The slow P release in the low-productive fens may be due to the continuous inflow of groundwater rich in Ca, Al and Fe compounds.It can be concluded that the soil is a potentially large source of inorganic N and P in quaking fens. The importance of the soil versus other N and P sources is greater in nutrient-rich fens.     

Epiphytic Rotifer Abundance and Diversity in Moss Patches in Bogs and Fens in the Polesie National Park (Eastern Poland)

Moss‐inhabiting rotifers were studied in 2 raised bogs, 2 poor fens (i.e. intermediate between raised bogs and typical fens), 1 typical fen, and 1 base‐rich fen, located in the Polesie National Park (eastern Poland). Moss samples were collected from April till November 2007 in monthly intervals. In total, 42 taxa of Monogononta and 26 of Bdelloidea were found. Analysis of variance (ANOVA) detected highly significant differences between the habitats in conductivity (F = 37.19, P < 0.001), total organic content (F = 9.34, P < 0.001), total P (F = 51.71, P < 0.001), and nitrate N (F = 48.98, P < 0.001). The mean density of rotifers was the highest in a bog dominated by Sphagnum angustifolium Jensen (Durne Bagno), and the lowest in the base‐rich fen (Bubnów). In all the studied habitats, the Bdelloidea were the most abundant, accounting for 56 to 85% of the total rotifer density. Canonical correspondence analysis (CCA), made for selected physicochemical parameters and taxa, confirms strong relationships between abiotic conditions and selected rotifer taxa: Habrotrocha angusticolis, Dicranophorus capucinus, Keratella serrulata, and Lepadella elliptica. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nutrient limitation in species-rich lowland fens

Abstract. Nitrogen, phosphorus and potassium were supplied to some Belgian fens of varying nutrient status and productivity. Plant growth in the lowest productive fen with a species-rich Caricion davallianae vegetation was strongly P-limited. N was ineffective when applied alone, but increased the effect of P-addition when applied together. Summer biomass and plant nutrient concentrations were monitored for four years, and showed partial recovery of nutrient limitation. In a more productive fen dominated by Carex lasiocarpa and in a fen meadow, nutrient limitation was less strong. N limited growth in the productive fen, and N and K were co-limiting in the fen meadow. The P-concentration in the productive fen vegetation showed a marked increase after P-fertilization, but it did not result in higher standing crop. The significance of P-limitation for the conservation of species rich low productive fens is discussed. P-limitation may be an essential feature in the conservation of low productive rich fens: because it is less mobile in the landscape than N and/or because it is an intrinsic property of this vegetation type. Plant nutrient concentrations and N:P-ratios may be used as an indication for the presence and type of nutrient limitation in the vegetation. We found N:P-ratios of 23 to 31 for a P-limited site and 8 to 15 in N-limited sites. This was in agreement with critical values from the literature: N:P > ca. 20 for P-limitation and N:P < 14 for N-limitation. Thus, this technique appears valid in the vegetation types that were studied here.     

Classification of peatland vegetation in Atlantic Canada

Abstract. The peatlands of Atlantic Canada are classified in four plant alliances, consisting of 10 plant associations. The four plant alliances comprise the dry bog communities (Kalmio-Cladonion Wells 1981), wet bog communities (Scirpo-Sphagnion Wells 1981), hummock and ridge communities of slope and string fens (Betulo-Sphagnion Wells all. nov.), and poor, intermediate and rich fen plant communities (Scirpo-Myricion Wells 1981). Distribution maps are presented for relevés in each of the 10 associations. Based on species distributions, floristic regions are defined for peatlands in Atlantic Canada. Nutritional characteristics are also described for each plant association. pH and total soil concentrations of calcium, nitrogen and iron proved reliable in separating bog from fen. A boundary between ombrotrophic peatlands and minerotrophic peatlands is suggested, based on a soil pH of 4.0, total soil concentrations of 3.0 mg g^?1 Ca, 4.0 mg g^?1 Fe, 13.0 mg g^?1 N and a Ca/Mg ratio of 2.5. Comparisons between the syntaxa for peatlands in Atlantic Canada and those in Europe are discussed for higher taxa. The possibility of establishing a new order (Chamaedaphno-Scirpetalia Wells ord. nov.) for peatlands in Atlantic Canada is also discussed.     

Short-Term Summer Inundation as a Measure to Counteract Acidification in Rich Fens

In regions with intensive agriculture, water level fluctuation in wetlands has generally become constricted within narrow limits. Water authorities are, however, considering the re-establishment of fluctuating water levels as a management tool in biodiverse, base-rich fens (‘rich fens’). This includes temporary inundation with surface water from ditches, which may play an important role in counteracting acidification in order to conserve and restore biodiversity. Inundation may result in an increased acid neutralizing capacity (ANC) for two reasons: infiltration of base-rich inundation water into peat soils, and microbial alkalinity generation under anaerobic conditions. The main objectives of this study were to test whether short-term (2 weeks) summer inundation is more effective than short-term winter inundation to restore the ANC in the upper 10 cm of non-floating peat soils, and to explain potential differences. Large-scale field experiments were conducted for five years in base-rich fens and Sphagnum-dominated poor fens. Winter inundation did not result in increased porewater ANC, because infiltration was inhibited in the waterlogged peat and evapotranspiration rates were relatively low. Also, low temperatures limit microbial alkalinity generation. In summer, however, when temperature and evapotranspiration rates are higher, inundation resulted in increased porewater Ca and HCO₃ ^- concentrations, but only in areas with characteristic rich fen bryophytes. This increase was not only due to stronger infiltration into the soil, but also to higher microbial alkalinity generation under anaerobic conditions. In contrast, porewater ANC did not increase in Sphagnum-plots as a result of the ability of Sphagnum spp. to acidify their environment. In both rich and poor fens, flooding-induced P-mobilization remained sufficiently low to safeguard P-limited vegetation. NO₃ ^- and NH₄ ⁺ dynamics showed no considerable changes either. In conclusion, short-term summer inundation with base-rich and nutrient-poor surface water is considered beneficial in the management of non-floating rich fens, and much more effective than winter inundation.     

Peatland distibution along a north-south transect in the Mackenzie River Basin in relation to climatic and environmental gradients

Climate is a major factor affecting the development and form of peatlands, as well as the distribution of individual bryophyte species. This paper examines the climatic and ecological gradients affecting the distribution of peatland types along a north-south gradient in the Mackenzie River Basin. Based on a TWINSPAN analysis of bryophyte abundance from 82 peatlands in the Mackenzie River Basin, seven peatland types, two with southerly geographical distributions are recognized. In the Mackenzie River Basin, such local factors as surface water chemistry, pH, and solute concentration as well as height above the water table play a significant role in determinining bryophyte species distributions. Climate is secondary. Amongst the climatic variables, precipitation, length of the growing season, and annual temperature are the most signifcant. The seven peatland groups are: widespread poor fens; peat plateaus with thermokarst pools, low-Boreal bogs; bogs and peat plateus without thermokarst pools; low-Boreal dry poor fens; wet moderate-rich fens; and wet extreme-rich fens.     

The effect of harvesting intensity on the fate of applied 15N-ammonium to the organic horizons of a coniferous forest in N. Wales

¹⁵N-ammonium sulphate equivalent to 0.5 kg N/ha was added as a tracer to lysimeters containing the organic horizons of an acid forest soil. The effect of logging debris (brash), vegetation and second rotationPicea sitchensis seedlings on the amount of the¹⁵N found in various soil, vegetation and leachate pools was followed over a period of 60 days. Transformation of¹⁵N-ammonium to nitrate occurred within 24 hours. Although total nitrate leachate losses were high, tracer-derived nitrate represented only 0.4%–4.2% of the applied¹⁵N-ammonium. The atom % excess of the KCI-extractable organic-N pool was initially lower than for the inorganic species but due to the large pool size, consistently represented 3–6% of the applied¹⁵N-ammonium. The similarity of the atom % excess of the ammonium and nitrate pools indicated an autotrophic nitrification pathway.A significant proportion of the¹⁵N-ammonium passed through the microbial biomass which contained between 16 and 48% of the¹⁵N-ammonium 2 days after addition of the¹⁵N-ammonium. This nitrogen was in a readily available form or short-term pool for the first two weeks (with no change in the overall biomass pool), after which the nitrogen appeared to become transformed into more stable compounds representing a long-term pool. Total recovery of the¹⁵N was between 68% and 99% for the different treatments. The presence of brash reduced microbial immobilisation of the¹⁵N-ammonium and total retention in the organic matter. This is suggested to be a consequence of greater nitrification and denitrificatiion rate in organic horizons beneath a brash covering due to different microclimatic conditions.     

Peat and water chemistry at Big Run Bog,a peatland in the Appalachian mountains of West Virginia,USA

At Big Run Bog, aSphagnum-dominated peatland in the unglaciated Appalachian Plateau of West Virginia, significant spatial variation in the physical and chemical properties of the peat and in surface and subsurface (30 cm deep) water chemistry was characterized. The top 40 cm of organic peat at Big Run Bog had average values for bulk density of 0.09 g · cm^–3, organic matter concentration of 77%, and volumetric water content of 88%. Changes in physical and chemical properties within the peat column as a function of depth contributed to different patterns of seasonal variation in the chemistry of surface and subsurface waters. Seasonal variation in water chemistry was related to temporal changes in plant uptake, organic matter decomposition and element mineralization, and to varying redox conditions associated with fluctuating water table levels. On the average, total Ca, Mg, and N concentrations in Big Run Bog peat were 33, 15, and 1050 mol · g^–1, respectively; exchangeable Ca and Mg concentrations were 45 and 14 eq · g^–1 , respectively. Surface water pH averaged 4.0 and Ca⁺⁺ concentrations were less than 50 eq · L^–1 . These chemical variables have all been used to distinguish bogs from fens. Physiographically, Big Run Bog is a minerotrophic fen because it receives inputs of water from the surrounding forested upland areas of its watershed. However, chemically, Big Run Bog is more similar to true ombrotrophic bogs than to minerotrophic fens.     

Pulses of soil phosphorus availability in a Mexican tropical dry forest: effects of seasonality and level of wetting

Intact cores from the upper soil profile and surface litter were collected at the peak of the dry season and during the rainy period in the tropical deciduous forest of the Chamela region, Jalisco, México, to (1) analyze upper soil phosphorus (P) movement and retention, (2) compare soil P dynamic pools (soluble, bicarbonate, and microbial) in dry and rainy seasons, and (3) determine the response of these P pools to wetting. Unperturbed litter-soil cores were treated in the laboratory with either 10 mm or 30 mm of simulated rain with carrier-free ³²P and compared to a control (no water addition) to determine the fate and retention of added P. ³¹P concentrations and pools in most litter and soil fractions were higher in the dry than in the rainy season. Soluble P was 0.306 g/m² and microbial P was 0.923 g/m² in the dry season (litter plus soil) versus 0.041 (soluble) and 0.526 (microbial) g P/m² in the rainy season. After water addition, rainy-season cores retained 99.9 and 94% of ³²P in the 10- and 30-mm treatments, respectively. Dry-season samples retained 98.9 and 80% of inputs in the same treatments. Retention after wetting occurred mostly in soil (bicarbonate and microbial fractions). Simulated rainfall on rainy-season soils increased P immobilization. On the other hand, simulated rainfall on dry-season soils released P through mineralization. The P release represents between 46 and 99% of the annual litterfall return. Our results suggest that both soluble and microbial P constitute important sources for initiation of plant growth at the onset of the rainy season in tropical dry forest. Received: 23 September 1997 / Accepted: 2 February 1998     

Permafrost conditions in peatlands regulate magnitude,timing, and chemical composition of catchment dissolved organic carbon export

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m^?2) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m^?2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.