Microbial activity and functional composition among northern peatland ecosystems

The extent to which complex interrelationships between plants and microorganisms influence organic matter dynamics is critical to our understanding of global C cycles in changing environments. We examined the hypothesis that patterns of soil microbial activity and functional composition differ among vegetation types in northern peatland ecosystems. Microbial characteristics were compared among peatlands differing in plant growth form (tree, shrub/moss, sedge) in two regions (New York State and West Virginia). Microbial activity (basal respiration) was greater in surface (0-15 cm) than subsurface (15-30 cm) peat and from sites dominated by shrubs and Sphagnum moss (3.9±0.65 μg C g⁻¹ h⁻¹) compared to forested (1.8±0.20 μg C g⁻¹ h⁻¹) or sedge-dominated sites (1.9±0.38 μg C g⁻¹ h⁻¹). Microbial activity was not related to decomposability of peat organic matter among vegetation types, and activity was unexpectedly higher in sites with lower peat pH and higher water table level. Substrate-induced respiration (SIR) did not show a clear pattern among vegetation types, but was greater in surface than subsurface peat. Microbial responsiveness to added glucose was very low. The ratio of basal respiration to SIR varied between 0.39 and 0.72 and, like activity, was highest in shrub/Sphagnum sites. Microbial substrate utilization patterns (assayed with BIOLOG^® GN plates) also differed between shrub/Sphagnum sites and forest or sedge sites, suggesting that C fluxes were mediated by different assemblages of microorganisms in shrub/Sphagnum peatlands. Principal component (PC) scores indicated more utilization of N-containing compounds and carboxylic acids, and less utilization of carbohydrates by microbial communities in shrub/Sphagnum sites. PC scores were much more variable both within and among vegetation types for sites in West Virginia than in New York State, and a greater diversity of C sources were utilized in WV (57±3) than NYS (47±2) peat. Our results suggest a link between microbial respiratory activity and microbial functional composition as they vary among these peatland vegetation types.     

Isotope (C and C) analysis of deep peat CO2 using a passive sampling technique

We developed and tested a new method to collect CO₂ from the surface to deep layers of a peatland for radiocarbon analysis. The method comprises two components: i) a probe equipped with a hydrophobic filter that allows entry of peat gases by diffusion, whilst simultaneously excluding water, and, ii) a cartridge containing zeolite molecular sieve that traps CO₂ passively. We field tested the method by sampling at depths of between 0.25 and 4 m at duplicate sites within a temperate raised peat bog. CO₂ was trapped at a depth-dependent rate of between ∼0.2 and 0.8 ml d⁻¹, enabling sufficient CO₂ for routine ¹⁴C analysis to be collected when left in place for several weeks. The age of peatland CO₂ increased with depth from modern to ∼170 BP for samples collected from 0.25 m, to ∼4000 BP at 4 m. The CO₂ was younger, but followed a similar trend to the age profile of bulk peat previously reported for the site (Langdon and Barber, 2005). δ¹³C values of recovered CO₂ increased with depth. CO₂ collected from the deepest sampling probes was considerably ¹³C-enriched (up to ∼+9‰) and agreed well with results reported for other peatlands where this phenomenon has been attributed to fermentation processes. CO₂ collected from plant-free static chambers at the surface of the mire was slightly ¹⁴C-enriched compared to the contemporary atmosphere, suggesting that surface CO₂ emissions were predominantly derived from carbon fixed during the post-bomb era. However, consistent trends of enriched ¹³C and depleted ¹⁴C in chamber CO₂ between autumn and winter samples were most likely explained by an increased contribution of deep peat CO₂ to the surface efflux in winter. The passive sampling technique is readily portable, easy to install and operate, causes minimal site disturbance, and can be reliably used to collect peatland CO₂ from a wide range of depths.     

Short-term effects of single or combined application of mineral N fertilizer and cattle slurry on the fluxes of radiatively active trace gases from grassland soil

After implementation of legislative measures for the reduction of environmental hazards from nitrate leaching and ammonia volatilisation when using organic manures and fertilizers in Europe, much attention is now paid to the specific effects of these fertilizers on the dynamics of global warming-relevant trace gases in soil. Particularly nitrogen fertilizers and slurry from animal husbandry are known to play a key role for the CH₄ and N₂O fluxes from soils. Here we report on a short-term evaluation of trace gas fluxes in grassland as affected by single or combined application of mineral fertilizer and organic manure in early spring. Methane fluxes were characterised by a short methane emission event immediately after application of cattle slurry. Within the same day methane fluxes returned to negative, and on average over the 4-day period after slurry application, only a small but insignificant trend to reduced methane oxidation was found. Nitrous oxide emissions showed a pronounced effect of combined slurry and mineral fertilizer application. In particular fresh cattle slurry combined with calcium ammonium nitrate (CAN) mineral fertilizer induced an increase in mean N₂O flux during the first 4 days after application from 10 to 300 μg N₂O-N m⁻² h⁻¹. ¹⁵N analysis of emitted N₂O from ¹⁵N-labelled fertilizer or manure indicated that easily decomposable slurry C compounds induced a pronounced promotion of N₂O-N emission derived from mineral CAN fertilizer. Fluxes after application of either mineral fertilizer or slurry alone showed an increase of less than 5-fold. The NO_x sink strength of the soil was in the range of −6 to −10 μg NO_x-N m⁻² h⁻¹ and after fertilization it showed a tendency to be reduced by no more than 2 μg NO_x-N m⁻² h⁻¹, which was a result of both, increased NO emission and slightly increased NO₂ deposition. Associated determination of the N₂O:N₂ emission ratio revealed that after mineral N application (CAN) a large proportion (c. 50%) was emitted as N₂O, while after application of slurry with easily decomposable C and predominantly -N serving as N-source, the N₂O:N₂ emission ratio was 1:14, i.e. was changed in favour of N₂. Our work provides evidence that particularly the combination of slurry and nitrate-containing N fertilizers gives rise to considerable N₂O emissions from mineral fertilizer N pool.     

Changes in microbial community structure and function following Sphagnum peatland restoration

This study examines the recovery of the microbial compartment following active restoration of a North American ombrotrophic peatland extracted for horticultural peat-based substrates and restored by the Sphagnum moss transfer method. We used phospholipid fatty acids (PLFAs) to portrait the microbial community structure and Community Level Physiological Profiles (CLPP) to describe the functional diversity of the microbial communities. Our results indicate that the PLFA profiles were different between the beginning and the end of the growing season, but that it was impossible to distinguish five different vegetation classes found along the disturbance-recovery gradient on the basis of the microbial community structure. The pH, the cover of mosses, Ledum groenlandicum and Eriophorum vaginatum var. spissum were the best environmental predictors for the PLFA composition. The newly formed peat found in aerobic conditions beneath restored Sphagnum carpets had the highest decomposition capacity, whereas the lowest rates were found in the surface samples of non-restored conditions or in the deepest horizons of the natural samples. A large proportion of the variation in the physiological profiles was explained with variables related to the vegetation cover, the physicochemical environment and the microbial structure of the community, which is very promising for future monitoring studies. Overall, this study demonstrates that the recovery of particular plant groups, namely mosses and shrubs in restored peatlands might be the driver of changes occurring in the structure of the microbial communities in restored peatlands.     

The effects of drying and re-wetting and increased temperature on sulphate release from upland and wetland material

In central Ontario, elevated SO₄ concentrations and export have been measured in both upland and wetland-draining catchments following summer droughts, although the source of excess SO₄ is unclear. The objective of this study was to determine the effects of drying and re-wetting and temperature, respectively, on the release of SO₄ from the primary S pools in wetlands (Sphagnum and peat) and uplands (forest floor and mineral soil), using material collected from the PC1 catchment in Haliburton County, and from catchment S50 in the Turkey Lakes Watershed. Peat exhibited the most marked response to drying of the four materials considered, and within 24 h of re-wetting dried peat from both catchments released 3-4 times more SO₄ (50-67 mg kg⁻¹ S-SO₄) than continuously moist peat (16 mg kg⁻¹ S-SO₄), although temperature had only a marginal effect on SO₄ concentrations. There was no immediate response of Sphagnum to either drying or temperature, although S-SO₄ concentrations in Sphagnum tended to increase over the 30-day (d) incubation. There was a small but immediate increase in S-SO₄ concentrations in forest floor material (LFH) from both catchments within the first 24 h of incubation, which was greatest in treatments that were dried and/or incubated at a higher temperature. In contrast, neither temperature nor drying appeared to affect SO₄ release from mineral soil collected from either site. Results of laboratory incubations suggest that increases in SO₄ concentration that have been reported in wetland-draining streams immediately following summer dry periods may be quantitatively explained by drying and re-wetting of peat rather than increased mineralization in Sphagnum. Similarly, the higher SO₄ concentrations that have been measured in upland streams following summer droughts may in part be due to enhanced SO₄ release from the forest floor following drying and re-wetting. In contrast, while the mineral soil constitutes a large pool of total S, it does not appear to be responsive to changes in moisture or temperature in the short-term (<30 d) and therefore likely does not contribute to reported climate-related temporal variations in stream SO₄.     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Use of C abundance to study short-term pig slurry decomposition in the field

Applying pig slurry (PS) on agricultural soils is a common practice. However, its impact on soil organic C dynamics is not clear. This experiment investigated the use of natural ¹³C abundance to study the short-term C mineralization of anaerobically stored PS under field conditions. Measurements of δ¹³C-CO₂ were made on soil air samples obtained from a bare sandy loam during 22 d following incorporation of either PS alone, PS+barley straw, or barley straw alone; an unamended treatment was used as a control. Slurry C was enriched in ¹³C (−20.0‰) because of the high corn (Zea mays L.) content of the animal diet. This value contrasted with δ¹³C of −28.4‰ for the soil organic matter and of −29.0‰ for the barley straw. A peak of high δ¹³CO₂ values (average of −9.2‰) was observed on the day of PS application and was attributed to the dissociation of PS carbonates when mixed with the relatively acidic soil. After this initial burst, 36% of the evolved CO₂ originated from the decomposing PS. After 22 d of incubation, approx. 20% of the PS-C had been lost as CO₂. This short-term field study did not show any priming effect of PS on the mineralization of straw or native soil C. Due to its heterogeneity, the use of the isotopic composition of the evolved CO₂ for estimating PS decomposition requires precaution either through the use of a specific experimental design involving comparable C3 and C4 treatments, or calculations to account for the presence of ¹³C-enriched inorganic C in the PS.     

Fe,Al, Mn,and S chemistry of Sphagnum peat in four peatlands with different metal and sulfur input

Comparisons among 4 peatland sites representing a gradient of increasing Fe, Al, Mn, and S loading revealed significant accumulation of total Fe, Al, and S, but not Mn, in surface (0 to 20 cm deep) peat along the gradient. Iron and Al accumulation were contributed mainly by organically bound fractions, with oxides contributing to a lesser extent. Although SO₄ ^2? and Fe sulfides showed significant increases in concentration along the gradient, most of the accumulation of total S was contributed by organic, rather than inorganic S. Laboratory studies of Fe²⁺ adsorption by peat indicated that increasing the pH of added Fe²⁺ solutions (pH values of 3, 4, 5, and 6) did not significantly affect Langmuir equation estimates of either maximum Fe²⁺ adsorption capacity or the affinity of peat for Fe²⁺. Regardless of the pH of the added Fe²⁺ solutions, final solution pH values were relatively uniform, averaging about 3.4, reflecting a considerable bufferring capacity of Sphagnum peat. Factors affecting the accumulation of metals and S in peat remain topics for further investigation.     

Fluxes of nitrous oxide and nitric oxide from experimental excreta patches in boreal agricultural soil

Nitric oxide (NO) and nitrous oxide (N₂O) emissions were measured from experimental dung and urine patches placed on boreal pasture soil during two growing seasons and one autumn period until soil freezing. N₂O emissions in situ were studied by a static chamber method. NO was measured with a dynamic chamber method using a NO analyser in situ. Mean emissions from the control plots were 47.6±4.5 μg N₂ON m⁻² h⁻¹ and 12.6±1.6 μg NON m⁻² h⁻¹. N₂O and NO emissions from urine plots (132±21.2 μg N₂ON m⁻² h⁻¹ and 51.9±7.6 μg NON m⁻² h⁻¹) were higher than those from dung plots (110.0±20.1 μg N₂ON m⁻² h⁻¹ and 14.7±2.1 μg NON m⁻² h⁻¹). There was a large temporal variation in N₂O and NO emissions. Maximum N₂O emissions were measured a few weeks after dung or urine application, whereas the maximum NO emissions were detected the following year. NO was responsible on average 14% (autumn) and 34% (summer) of total (NO+N₂O)N emissions from the pasture soil. NO emissions increased with increasing soil temperature and with decreasing soil moisture. N₂O emissions increased with increasing soil moisture, but did not correlate with soil temperature. Therefore we propose that N₂O and NO were produced mainly during different microbial processes, i.e., nitrification and denitrification, respectively. The results show that the overall conditions and mechanism especially for emissions of NO are still poorly understood but that there are differences in the mechanisms regulating N₂O and NO production.     

Methane fluxes from three ecosystems in tropical peatland of Sarawak, Malaysia

Methane fluxes were measured monthly over a year from tropical peatland of Sarawak, Malaysia using a closed-chamber technique. The CH₄ fluxes in forest ecosystem ranged from −4.53 to 8.40 μg C m⁻² h⁻¹, in the oil palm ecosystem from −32.78 to 4.17 μg C m⁻² h⁻¹ and in the sago ecosystem from −7.44 to 102.06 μg C m⁻² h⁻¹. A regression tree approach showed that CH₄ fluxes in each ecosystem were related to different underlying environmental factors. They were relative humidity for forest and water table for both sago and oil palm ecosystems. On an annual basis, both forest and sago were CH₄ source with an emission of 18.34 mg C m⁻² yr⁻¹ for forest and 180 mg C m⁻² yr⁻¹ for sago. Only oil palm ecosystem was a CH₄ sink with an uptake rate of −15.14 mg C m⁻² yr⁻¹. These results suggest that different dominant underlying environmental factors among the studied ecosystems affected the exchange of CH₄ between tropical peatland and the atmosphere.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Dung application increases CH<Subscript>4</Subscript> production potential and alters the composition and abundance of methanogen community in restored peatland soils from Europe

Peatland restoration via rewetting aims to recover biological communities and biogeochemical processes typical to pristine peatlands. While rewetting promotes recovery of C accumulation favorable for climate mitigation, it also promotes methane (CH₄) emissions. The potential for exceptionally high emissions after rewetting has been measured for Central European peatland sites previously grazed by cattle. We addressed the hypothesis that these exceptionally high CH₄ emissions result from the previous land use. We analyzed the effects of cattle dung application to peat soils in a short- (2 weeks), a medium- (1 year) and a long-term (grazing) approach. We measured the CH₄ production potentials, determined the numbers of methanogens by mcrA qPCR, and analyzed the methanogen community by mcrA T-RFLP-cloning-sequencing. Dung application significantly increased the CH₄ production potential in the short- and the medium-term approach and non-significantly at the cattle-grazed site. The number of methanogens correlated with the CH₄ production in the short- and the long-term approach. At all three time horizons, we found a shift in methanogen community due to dung application and a transfer of rumen methanogen sequences (Methanobrevibacter spp.) to the peatland soil that seemed related to increased CH₄ production potential. Our findings indicate that cattle grazing of drained peatlands changes their methanogenic microbial community, may introduce rumen-associated methanogens and leads to increased CH₄ production. Consequently, rewetting of previously cattle-grazed peatlands has the potential to lead to increased CH₄ emissions. Careful consideration of land use history is crucial for successful climate mitigation with peatland rewetting.     

Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting

Soil compaction and soil moisture are important factors influencing denitrification and N₂O emission from fertilized soils. We analyzed the combined effects of these factors on the emission of N₂O, N₂ and CO₂ from undisturbed soil cores fertilized with (150 kg N ha⁻¹) in a laboratory experiment. The soil cores were collected from differently compacted areas in a potato field, i.e. the ridges (ρ_D=1.03 g cm⁻³), the interrow area (ρ_D=1.24 g cm⁻³), and the tractor compacted interrow area (ρ_D=1.64 g cm⁻³), and adjusted to constant soil moisture levels between 40 and 98% water-filled pore space (WFPS).High N₂O emissions were a result of denitrification and occurred at a WFPS≥70% in all compaction treatments. N₂ production occurred only at the highest soil moisture level (≥90% WFPS) but it was considerably smaller than the N₂O-N emission in most cases. There was no soil moisture effect on CO₂ emission from the differently compacted soils with the exception of the highest soil moisture level (98% WFPS) of the tractor-compacted soil in which soil respiration was significantly reduced. The maximum N₂O emission rates from all treatments occurred after rewetting of dry soil. This rewetting effect increased with the amount of water added. The results show the importance of increased carbon availability and associated respiratory O₂ consumption induced by soil drying and rewetting for the emissions of N₂O.     

Use of 13C and 15N natural abundance techniques to characterize carbon and nitrogen dynamics in composting and in compost-amended soils

Isotope fractionation during composting may produce organic materials with a more homogenous δ¹³C and δ¹⁵N signature allowing study of their fate in soil. To verify this, C, N, δ¹³C and δ¹⁵N content were monitored during nine months covered (thermophilic; >40 °C) composting of corn silage (CSC). The C concentration reduced from 10.34 to 1.73 g C (g ash)⁻¹, or 83.3%, during composting. Nitrogen losses comprised 28.4% of initial N content. Compost δ¹³C values became slightly depleted and increasingly uniform (from −12.8±0.6‰ to −14.1±0.0‰) with composting. Compost δ¹⁵N values (0.3±1.3 to 8.2±0.4‰) increased with a similar reduced isotope variability.The fate of C and N of diverse composts in soil was subsequently examined. C, N, δ¹³C, δ¹⁵N content of whole soil (0-5 cm), light (<1.7 g cm⁻³) and heavy (>1.7 g cm⁻³) fraction, and (250-2000 μm; 53-250 μm and <53 μm) size separates, were characterized. Measurements took place one and two years following surface application of CSC, dairy manure compost (DMC), sewage sludge compost (SSLC), and liquid dairy manure (DM) to a temperate (C₃) grassland soil. The δ¹³C values and total C applied (Mg C ha⁻¹) were DM (−27.3‰; 2.9); DMC (−26.6‰; 10.0); SSLC (−25.9‰; 10.9) and CSC (−14.0‰; 4.6 and 9.2). The δ¹³C of un-amended soil exhibited low spatial (−28.0‰±0.2; n=96) and temporal (±0.1‰) variability. All C₄ (CSC) and C₃ (DMC; SSLC) composts, except C₃ manure (DM), significantly modified bulk soil δ¹³C and δ¹⁵N. Estimates of retention of compost C in soil by carbon balance were less sensitive than those calculated by C isotope techniques. One and two years after application, 95 and 89% (CSC), 75 and 63% (SSLC) and 88 and 42% (DMC) of applied compost C remained in the soil, with the majority (80-90%) found in particulate (>53 μm) and light fractions. However, C₄ compost (CSC) was readily detectable (12% of compost C remaining) in mineral (<53 μm) fractions. The δ¹⁵N-enriched N of compost supported interpretation of δ¹³C data. We can conclude that composts are highly recalcitrant with prolonged C storage in non-mineral soil fractions. The sensitivity of the natural abundance tracer technique to characterize their fate in soil improves during composting, as a more homogeneous C isotope signature develops, in addition to the relatively large amounts of stable C applied in composts.     

Species-specific effects of plants colonising cutover peatlands on patterns of carbon source utilisation by soil microorganisms

Root exudates and litter are the main sources of inputs of labile carbon into the microbial pool in successional ecosystems. Here we studied whether typical pioneer species (Eriophorum vaginatum, Eriophorum angustifolium and Calluna vulgaris) alter the functional response of the microbial community of a previously cutover peatland. Peat was sampled at three depths (0–5, 20–25 and 40–45 cm) from beneath these species and from bare soil areas. MicroResp analysis using ecologically relevant, radiolabelled, carbon sources showed significant separation in community level physiological profiles (CLPP) of soil microorganisms according to peat depth. This effect was also reflected in microbial biomass carbon, which also decreased with increasing depth. Furthermore, distinct differences in CLPP were observed between the three plant species and the bare soil in the absence of an effect on microbial biomass carbon or total soil carbon. The plant species effects were driven by differential utilisation of xylose, glutamic acid, lysine and phenylethylamine. The data suggest that ‘new’ carbon inputs from plants colonising abandoned cutover peatland may support communities of microorganisms that have functionally distinct roles in carbon turnover.     

CO2 and inorganic N supply modify competition for N between co-occurring grass plants, tree seedlings and soil microorganisms

Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO₂) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol⁻¹) or high (645 μmol mol⁻¹) CO₂ and at two levels of N nutrition in a mesocosm experiment. Pulse ¹⁵N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and ¹⁵N recovery irrespective of N and CO₂ treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO₂, but the pattern of Fraxinus- and microbial-N pool response to N and CO₂ varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, ¹⁵N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO₂ treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of ¹⁵N recovery were exacerbated by N addition. Contrary to expectations, elevated CO₂ did not increase plant-microbe competition. Nevertheless, microbial ¹⁵N recovery showed a small positive increase in the high CO₂ treatment. Overall, elevated CO₂ and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO₂ or N-induced modifications in N competition between plant and soil compartments.     

Decomposition of organic matter in peat soil in a minerotrophic mire

Decomposition of organic materials, oxygen consumption, and carbon dioxide emission were investigated in Masukata mire, a small minerotrophic mire in central Japan. We selected three dominant community types in the mire, a Sphagnum palustre community, a Phragmites australis community, and an Alnus japonica community, for the decomposition study sites. Decomposition rates were measured in the field by examining mass loss of peat and cellulose for 6 months. The oxygen consumption rate was measured in the field using a closed chamber equipped with an oxygen electrode. The carbon dioxide emission rate of the peat was measured by an infrared gas analyser in the laboratory under controlled conditions. Results of these measurements were tested by correlation analysis. The rate of mass loss of peat positively correlated with the CO₂ emission rate. The cellulose decomposition rate showed significant differences among community types, and it had significant positive correlation with the oxygen consumption rate. Although oxygen consumption measurement is not generally used to estimate peatland soil respiration, the oxygen consumption method can be used for predicting long-term decomposition rate according to different vegetation types within a short time.     

A comparison of peat properties in intact,afforested and restored raised and blanket bogs

Recognition of peatlands as a key natural store of terrestrial carbon has led to new initiatives to protect and restore them. Some afforested bogs are being clear-felled and restored (forest-to-bog restoration) to recover pre-afforestation ecosystem function. However, little is known about differences in the peat properties between intact, afforested and restored bogs. A stratified random sampling procedure was used to take 122 peat cores from three separate microforms associated with intact (hollows; hummocks; lawns), afforested and restored bogs (furrows; original surface; ridges) at two raised and two blanket bog locations in Scotland. Common physical and chemical peat properties at eight depths were measured in the laboratory. Differences in bulk density, moisture and carbon content between the afforested (mean = 0.103 g cm⁻³, 87.8% and 50.9%, respectively), intact (mean = 0.091 g cm⁻³, 90.3% and 51.3%, respectively) and restored bogs (mean = 0.095 g cm⁻³, 89.7% and 51.1%, respectively) were small despite their statistical significance. The pH was significantly lower in the afforested (mean = 4.26) and restored bogs (mean = 4.29) than the intact bogs (mean = 4.39), whereas electrical conductivity was significantly higher (mean: afforested = 34.2, restored = 38.0, intact = 25.3 μS cm⁻¹). While significant differences were found between treatments, effect sizes were mainly small, and greater differences in pH, electrical conductivity, specific yield and hydraulic conductivity existed between the different intact bogs. Therefore, interactions between geographic location and land management need to be considered when interpreting the impacts of land-use change on peatland properties and functioning.     

Boreal pine forest floor biogenic volatile organic compound emissions peak in early summer and autumn

In boreal forests, canopy-scale emissions of biogenic volatile organic compounds (BVOCs) are rather well characterised, but knowledge of ecosystem-scale BVOC emissions is still inadequate. We used adsorbent tubes to measure BVOCs from a boreal Scots pine (Pinus sylvestris L.) forest floor in southern Finland and analysed the compounds with a gas chromatograph-mass spectrometer. The most abundant compound group was the monoterpenes (averaging 5.04 μg m⁻² h⁻¹), in which α-pinene, Δ³-carene and camphene contributed over 90% of the emissions. Emissions of other terpenoids (isoprene and sesquiterpenes) were low (averaging 0.05 and 0.04 μg m⁻² h⁻¹, respectively). BVOC emissions from the forest floor varied seasonally, peaking in early summer and autumn, with most of the compounds following similar patterns. The emission pattern was sustained throughout the measurement period, suggesting that the main sources of the emissions remained more or less stable. We compared the BVOC fluxes with environmental parameters such as temperature, precipitation and PAR, and with fluxes of other trace gases (CO₂, CH₄, N₂O), as well as with ground vegetation photosynthesis and with litter input. Several of these parameters were correlated with the presence of BVOCs. The sources of soil BVOC emissions are very poorly understood, but our results suggest, that changes in litter quantity and quality, soil microbial activity and the physiological stages of plants are linked with changes in BVOC fluxes.