Fine-root growth in a forested bog is seasonally dynamic,but shallowly distributed in nutrient-poor peat

Background and aims

Fine roots contribute to ecosystem carbon, water, and nutrient fluxes through resource acquisition, respiration, exudation, and turnover, but are understudied in peatlands. We aimed to determine how the amount and timing of fine-root growth in a forested, ombrotrophic bog varied across gradients of vegetation density, peat microtopography, and changes in environmental conditions across the growing season and throughout the peat profile.

Methods

We quantified fine-root peak standing crop and growth using non-destructive minirhizotron technology over a two-year period, focusing on the dominant woody species in the bog: Picea mariana, Larix laricina, Rhododendron groenlandicum, and Chamaedaphne calyculata.

Results

The fine roots of trees and shrubs were concentrated in raised hummock microtopography, with more tree roots associated with greater tree densities and a unimodal peak in shrub roots at intermediate tree densities. Fine-root growth tended to be seasonally dynamic, but shallowly distributed, in a thin layer of nutrient-poor, aerobic peat above the growing season water table level.

Conclusions

The dynamics and distribution of fine roots in this forested ombrotrophic bog varied across space and time in response to biological, edaphic, and climatic conditions, and we expect these relationships to be sensitive to projected environmental changes in northern peatlands.

     

Actinobacteria community structure in the peat profile of boreal bogs follows a variation in the microtopographical gradient similar to vegetation

Background and aims

Boreal mires are globally important carbon stores. In nutrient poor mires, i.e. bogs, vegetation is highly structured between hydrologically different microforms: hummocks, lawns and hollows. We studied whether the vegetation structure in four boreal bogs is reflected in the actinobacterial decomposer communities.

Methods

Vegetation was determined by projection cover of plant species. Actinobacteria were detected from peat profiles at depths of 0–20, 20–40 and 40–60 cm by molecular methods. The data was analysed by a hierarchical set of ordinations.

Results

The inter-bog variation in plant communities was insignificant, while the intra-bog differences (microforms) at each bog were highly significant. This variation was reflected in actinobacteria communities in the two upper peat layers. The deepest peat layer (40–60 cm) showed some inter-bog differences, possibly due to the different history of the bogs.

Conclusions

Our results show that the actinobacteria communities reflect the variation in bog vegetation, which in turn is tied to hydrological conditions. The possible alterations in the water level caused by changing climate are likely to cause predictable changes in vegetation and microbial communities in bog ecosystems.     

Life-cycle climate impacts of peat fuel: calculation methods and methodological challenges

Purpose

There has been lively debate, especially in Finland and Sweden, on the climate impacts of peat fuel. Previous studies of peat fuel's life-cycle climate impacts were controversial in their interpretation. The aim of this paper is conclusive examination of the issues of LCA methodology, derived from critical review of previous studies and recalculation based on the latest knowledge of greenhouse gas balances related to peat fuel’s utilisation and the radiative forcing impacts of greenhouse gases.

Methods

The most recent findings on emissions and the gas fluxes between soil, vegetation and atmosphere were used in calculation of the life-cycle climate impacts of the various peat fuel utilisation chains by means of LCA methodology. In the main, the calculation methods and rules were the same as in the previous studies, with the aim being to distinguish the impact of peat fuel’s utilisation from that of the natural or semi-natural situation. A dynamic method was employed for assessing changes in radiative forcing. The results of alternative peat fuel utilisation chains were compared to the corresponding result for coal.

Results

There are many steps in peat fuel LCA, where different assumptions lead to different outcomes. Determining the functional unit, reference situations and system boundaries, as well as the emission calculation methods, is important from this point of view. Determination of the initial reference situation emerged as one of the critical points in the calculations. Time scale can strongly affect the final outcomes in a study where effects of long-term land-use change are considered.

Conclusions

Each peatland area is unique. The higher the greenhouse gas emissions in the initial reference situation, the greater is the climate impact of the area and the more suitable the area is for peat extraction. The study showed that more greenhouse gas flux measurements are needed, for better assessment of the climate impacts of different potential peat extraction sites. Climate change mitigation requires quick actions, and uncertainties related to emissions are higher for longer time spans. Therefore, it can be concluded that a perspective spanning more than 100 years is inappropriate in peat fuel's life-cycle climate impact assessments.     

Ecosystem respiration in a heterogeneous temperate peatland and its sensitivity to peat temperature and water table depth

Background and aims

Ecosystem respiration (R _eco ) is controlled by thermal and hydrologic regimes, but their relative importance in defining the CO₂ emissions in peatlands seems to be site specific. The aim of the paper is to investigate the sensitivity of R _eco to variations in temperature and water table depth (WTD) in a wet, geogenous temperate peatland with a wide variety of vegetation community groups.

Methods

The CO₂ fluxes were measured using chambers. Measurements were made at four microsites with different vegetation communities and peat moisture and temperature conditions every 3 to 4 weeks during the period 2008–2009, 2 years with contrasting WTD patterns. Models were used to examine the relative response of each microsite to variations in peat temperature and WTD and used to estimate annual total R _eco .

Results

Temporal variations in R _eco were strongly related to peat temperature at the 5 cm depth. However, two of the microsites did not show any significant change in this relationship while two others showed contrasting responses including an increase and decrease in temperature sensitivity with deeper WTD. Average R _eco varied among the microsites and tended to be greatest for those with greatest leaf area which also positively correlated with deeper WTD, ash content and degree of peat decomposition at 20 cm. A combined temperature and WTD model explained up to 94 % of the temporal variation in daily average R _eco and was used to show that on an annual basis, R _eco was between 5 and 18 % greater in the warmer year with deeper WTD.

Conclusion

Microsite-specific responses were related to differences in vegetation and peat characteristics among microsites. R _eco may have remained insensitive to WTD variations at one microsite due to the dominance of autotrophic respiration from abundant sedge biomass. At a Sphagnum-dominated microsite, a lack of response may have been due to relatively small variations in WTD that did not greatly influence microbial respiration or due to offsets between decreasing and increasing respiration rates in near-surface and deeper peat. The microsite with the most recalcitrant peat had reduced R _eco sensitivity to temperature under more aerobic conditions while another microsite showed the opposite response, perhaps due to less nutrient availability during the wet year. Ultimately, micro-site specific models with both soil temperature and WTD as explanatory variables described temporal variations in R _eco and highlighted the significant spatial variations in respiration rates that may occur within a single wetland.     

Soil nutrients under shrub hummocks and debris mounds two decades after ploughing

Aims

Shrub removal by ploughing has been used widely to reduce the effects of shrub encroachment into open woodlands and grasslands. Our aim was to demonstrate that soil chemical properties varied markedly among three patch types (shrub hummock, debris mound, interspace) which varied in age, almost two decades after shrub removal by ploughing.

Methods

We compared changes in nutrients under 1) young post-ploughing recruits and mature, unploughed shrubs, 2) mature and recently formed debris mounds and 3) ploughed and recovering interspaces at three depths.

Results

Irrespective of their age, nutrient concentrations were greater under shrub hummocks and debris mounds than in the interspaces at two sites. Soil in mature shrub hummocks generally had greater levels of labile carbon and nitrogen (total, mineral, mineralisable), but results varied between sites. There were a few, sometimes inconsistent, effects of ploughing on nutrients under debris mounds, and no differences between the interspaces two decades after ploughing. Nutrient effects were most marked in the top 15 cm of the soil, diminishing rapidly with depth.

Conclusions

Our results reinforce the importance of hummocks and mounds as resource sinks and indicate the long-lasting effects of disturbances such as ploughing on soil nutrient pools.     

Rhizobial counts in peat inoculants vary amongst legume inoculant groups at manufacture and with storage: implications for quality standards

Background and aims

Inoculation of legumes at sowing with rhizobia has arguably been one of the most cost-effective practices in modern agriculture. Critical aspects of inoculant quality are rhizobial counts at manufacture/registration and shelf (product) life.

Methods

In order to re-evaluate the Australian standards for peat-based inoculants, we assessed numbers of rhizobia (rhizobial counts) and presence of contaminants in 1,234 individual packets of peat–based inoculants from 13 different inoculant groups that were either freshly manufactured or had been stored at 4 °C for up to 38 months to determine (a) rates of decline of rhizobial populations, and (b) effects of presence of contaminants on rhizobial populations. We also assessed effects of inoculant age on survival of the rhizobia during and immediately after inoculation of polyethylene beads.

Results

Rhizobial populations in the peat inoculants at manufacture and decline rates varied substantially amongst the 13 inoculant groups. The most stable were Sinorhizobium, Bradyrhizobium and Mesorhizobium with Rhizobium, particularly R. leguminosarum bv. trifolii the least stable. The presence of contaminants at the 10^?6 level of dilution, i.e. >log 6.7 g^?1 peat, reduced rhizobial numbers in the stored inoculants by an average of 37 %. Survival on beads following inoculation improved 2–3 fold with increasing age of inoculant.

Conclusions

We concluded that the Australian standards for peat-based rhizobial inoculants should be reassessed to account for the large differences amongst the groups in counts at manufacture and survival rates during storage. Key recommendations are to increase expiry counts from log 8.0 to log 8.7 rhizobia g^?1 peat and to have four levels of inoculant shelf life ranging from 12 months to 3 years.     

Mineralization rates of peat from eroding peat islands in reservoirs

Reservoirs are sources of greenhouses gases to the atmosphere, primarily due to organic carbon mineralization in flooded plants and soils to carbon dioxide (CO₂) and methane (CH₄). Floating peat islands are common in reservoirs that inundated peatlands. These islands can decompose on mass, or small pieces of peat can erode from islands to decompose in the water column or on the bottom of reservoirs. Here we used large 450 liter sealed enclosures to measure mineralization rates of small peat pieces and larger peat blocks collected from floating peat islands. Mineralization rates were calculated by quantifying dissolved inorganic carbon (DIC), CO₂ and CH₄ accumulation within the water and headspace of the enclosures over time. We found that peat did decompose under water, but rates of mineralization of peat pieces were not different than rates of mineralization of larger peat blocks. Mineralization rates ranged between 59 and l40 g C g^–1 d^–1. Peat pieces acidified the water, shifting the bicarbonate equilibrium to almost exclusively dissolved CO₂, which was then readily able to flux to the atmosphere. We estimated that 2.4–5.6% of peat carbon was mineralized annually, suggesting that fluxes of CO₂ and CH₄ from reservoirs that flood peatlands could last at minimum 18–42 years from this carbon source alone.     

Carbon fluxes from a tropical peat swamp forest floor

A tropical ombrotrophic peatland ecosystem is one of the largest terrestrial carbon stores. Flux rates of carbon dioxide (CO₂) and methane (CH₄) were studied at various peat water table depths in a mixed‐type peat swamp forest floor in Central Kalimantan, Indonesia. Temporary gas fluxes on microtopographically differing hummock and hollow peat surfaces were combined with peat water table data to produce annual cumulative flux estimates. Hummocks formed mainly from living and dead tree roots and decaying debris maintained a relatively steady CO₂ emission rate regardless of the water table position in peat. In nearly vegetation‐free hollows, CO₂ emission rates were progressively smaller as the water table rose towards the peat surface. Methane emissions from the peat surface remained small and were detected only in water‐saturated peat. By applying long‐term peat water table data, annual gas emissions from the peat swamp forest floor were estimated to be 3493±316 g CO₂ m^?2 and less than 1.36±0.57 g CH₄ m^?2. On the basis of the carbon emitted, CO₂ is clearly a more important greenhouse gas than CH₄. CO₂ emissions from peat are the highest during the dry season, when the oxic peat layer is at its thickest because of water table lowering.     

Interpreting the ash trend within ombrotrophic bog profiles: atmospheric dust depositions vs. mineralization processes. The Etang de la Gruère case study

Introduction

Ash peaks along ombrotrophic bog profiles may arise from several different processes. In a recent paper, Leifeld and co-authors (Plant Soil 341:349–361, 2011) argued that ash peaks along the Etang de la Gruère (EGr) peat bog profiles are signs of previous periods of higher peat decomposition rather that an indication of periods of elevated dust inputs.

Aims and methods

Here we question the approach and scrutinize results using published data on several peat cores from EGr, demonstrating that peaks in ash content at EGr are very reproducible when cores are carefully collected (e.g., using the Wardenaar corer), and age dated (using ²¹⁰Pb and ¹⁴C).

Results

Data clearly show that variations in ash content along bog profiles cannot be attributed simply, or exclusively, to differences in organic matter mineralization rate, and that averaging the ash contents and normalizing to a single ash peak leads to losses in valuable information and defeats the purpose of detailed paleoenvironmental reconstructions.

Conclusions

Comparing results obtained using sensitive spectroscopic and isotopic tools with the ash content profiles at EGr shows clearly that the distribution of ash and/or acid-insoluble ash cannot be used as a surrogate for the intensity of processes including organic matter mineralization, decomposition and/or humification.     

Wheat root diversity and root functional characterization

Background and Aims

Under limited moisture conditions, roots can play an outstanding role with respect to yield stability by effective absorption of water from soil. A targeted integration of root traits into plant breeding programs requires knowledge on the existing root diversity and access to easy and cost-effective methods. This study aimed to assess wheat root diversity, root properties in relation to water regime, and the efficiency of root capacitance for in situ screening.

Methods

Root morphological, anatomical properties and root capacitance of wheat species from different ploidy levels were studied under field conditions in 2 years contrasting in water regime. Soil water content was weekly measured.

Results

Significant genotypic differences were observed for most root traits. The investigated genotypes exploited different strategies to maximize soil water depletion, e.g. high topsoil root length density, low tissue mass density, high specific root length, deep rooting and looser xylem vessels. Multivariate statistics of root traits revealed an acceptable genotypic differentiation according to regional origin, genetics and capacity to extract soil water.

Conclusions

Under supply-driven environments, dehydration avoidance via water uptake maximization can be achieved through high topsoil rooting density. In this regard, root capacitance can be useful for in situ screening.     

Topsoil organic matter properties in contrasted hedgerow vegetation types

Background and aims

Hedges, semi-natural landscape components, have the ability to integrate both agronomic and environmental functions and to provide several ecosystem services. The aim of this study was to test whether hedgerow vegetation is a determinant of soil organic matter properties in ancient agricultural lands.

Methods

We complemented cluster analysis and ordination to determine the extent to which two types of hedges that were distinct in character-plant species also differed between each other in concentration and composition of two major constituents of soil organic matter, namely humic substances and dissolved organic matter.

Results

The two types of hedges were associated with significant differences in humic carbon content, hormone-like activity and molecular size of humic substances, which, in general, were more similar to those typical of forest than of agricultural soils. Moreover, we detected between-group differences in several phenolic acids.

Conclusions

Variation of the topsoil biochemical properties of hedges may be explained by variation in their vegetation characteristics, similar to other ecosystems. Spontaneous vegetation in hedges perform an important role in controlling the variability of surface soil properties that influence the evolution of soil organic matter and nutrient availability in agricultural lands.     

Effect of wheat phosphorus status on leaf surface properties and permeability to foliar-applied phosphorus

Aims

This study aimed to analyse the effect of phosphorus (P) nutritional status on wheat leaf surface properties, in relation to foliar P absorption and translocation.

Methods

Plants of Triticum aestivum cv. Axe were grown with three rates of root P supply (equivalent to 24, 8 and 0 kg P ha^?1) under controlled conditions. Foliar P treatments were applied and the rate of drop retention, P absorption and translocation was measured. Adaxial and abaxial leaf surfaces were analysed by scanning and transmission electron microscopy. The contact angles, surface free energy and work-of-adhesion for water were determined.

Results

Wheat leaves are markedly non-wettable, the abaxial leaf side having some degree of water drop adhesion versus the strong repulsion of water drops by the adaxial side. The total leaf area, stomatal and trichome densities, cuticle thickness and contact angles decreased with P deficiency, while the work-of-adhesion for water increased. Phosphorous deficient plants failed to absorb the foliar-applied P.

Conclusions

Phosphorous deficiency altered the surface structure and functioning of wheat leaves, which became more wettable and had a higher degree of water drop adhesion, but turned less permeable to foliar-applied P. The results obtained are discussed within an agronomic and eco-physiological context.     

Molecular simulation study of PAMAM dendrimer composite membranes

Pure polysulfone (PSF) and its composites with chitosan (CST), hyaluronic acid (HA), conventional poly(amidoamine), and hydroxyl poly(amidoamine) dendrimers as the membranes for separation of the gases, methane, carbon dioxide, hydrogen sulfide, nitrogen, and oxygen have been studied by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The transport properties (solubility, diffusivity, and permeability) of pure and gas mixtures in the membranes were calculated and the results of the simulations were compared with the available experimental data. The simulated structural properties of the pure and composite PSF membranes including occupied volume, free volume, surface area, fractional free volume (FFV), and radius of gyration (R _g ) were evaluated and their effects on the separability of the gases by the membranes were analyzed and interpreted by the obtained results.

Biochemical determinants of litter quality in 15 species of <Emphasis Type="Italic">Sphagnum</Emphasis>

Background and aims

Sphagnum mosses are ecosystem engineers that create and maintain boreal peatlands. With unique biochemistry, waterlogging and acidifying capacities, they build up meters-thick layers of peat, reducing competition and impeding decomposition. We quantify within-genus differences in biochemical composition to make inferences about decay rates, related to hummock–hollow and fen–bog gradients and to phylogeny.

Methods

We sampled litter from 15 Sphagnum species, abundant over the whole northern hemisphere. We used regression and Principal Components Analysis (PCA) to evaluate general relationships between litter quality parameters and decay rates measured under laboratory and field conditions.

Results

Both concentrations of the polysaccharide sphagnan and the soluble phenolics were positively correlated with intrinsic decay resistance, however, so were the previously understudied lignin-like phenolics. More resistant litter had more of all the important metabolites; consequently, PC1 scores were related to lab mass loss (R²?=?0.57). There was no such relationship with field mass loss, which is also affected by the environment. PCA also revealed that metabolites clearly group Sphagnum sections (subgenera).

Conclusions

We suggest that the commonly stated growth-decomposition trade-off is largely due to litter quality. We show a strong phylogenetic control on Sphagnum metabolites, but their effects on decay are affected by nutrient availability in the habitat.

     

CO2 efflux from cleared mangrove peat

Background

CO₂ emissions from cleared mangrove areas may be substantial, increasing the costs of continued losses of these ecosystems, particularly in mangroves that have highly organic soils.

Methodology/Principal Findings

We measured CO₂ efflux from mangrove soils that had been cleared for up to 20 years on the islands of Twin Cays, Belize. We also disturbed these cleared peat soils to assess what disturbance of soils after clearing may have on CO₂ efflux. CO₂ efflux from soils declines from time of clearing from ∼10 600 tonnes km⁻² year⁻¹ in the first year to 3000 tonnes km² year⁻¹ after 20 years since clearing. Disturbing peat leads to short term increases in CO₂ efflux (27 umol m⁻² s⁻¹), but this had returned to baseline levels within 2 days.

Conclusions/Significance

Deforesting mangroves that grow on peat soils results in CO₂ emissions that are comparable to rates estimated for peat collapse in other tropical ecosystems. Preventing deforestation presents an opportunity for countries to benefit from carbon payments for preservation of threatened carbon stocks.     

Soil drainage and phosphorus depletion contribute to retrogressive succession along a New Zealand chronosequence

Background and aims

Models of retrogressive succession have emphasised the role of phosphorus (P) depletion in driving biomass loss on surfaces of increasing geologic age, but the influence of impeded drainage on old surfaces has received much less attention. We tested whether poor drainage contributed to changes in ecosystem properties along a 291,000-year chronosequence in New Zealand (the Waitutu chronosequence).

Methods

Soil and ecosystem properties were measured at 24 evenly distributed points within each of eight 1.5 ha plots located on young, intermediate and old surfaces. Regression analyses tested whether drainage, in addition to P, affected ecosystem functioning. A complementary fertilization experiment tested whether P was indeed limiting on the most nutrient-depleted sites.

Results

Most phosphorus depletion occurred in the early stages of pedogenesis (within 24,000 years), and the older surfaces were similar in soil-P contents, whereas drainage was initially good but became increasingly impeded with surface age. In the fertilizer experiment, species showed positive responses to both nitrogen (N) and P addition on the oldest surfaces, supporting Walker and Syer’s model. However, water table depth was also found to be strongly correlated with plant species composition, forest basal area, light transmission, and litter decomposition when comparisons were made across sites, emphasising that it too has strong influences on ecosystem processes.

Conclusions

Poor drainage influences the process of retrogressive succession along the Waitutu chronosequence. We discuss the implications of our work with regard to other chronosequences, suggesting that topography is likely to have strong influences on retrogressive processes.     

Legume inoculant application methods: effects on nodulation patterns,nitrogen fixation,crop growth and yield in narrow-leaf lupin and faba bean

Aims

Plant growth forms can influence carbon cycling, particularly in carbon-rich ecosystems like northern peatlands; however, mechanistic evidence of this relationship is limited. Our aim was to determine if northern peatland plant growth forms alter belowground dissolved carbon chemistry and enhance carbon release through stimulated microbial metabolism.

Methods

We used replicated, peat monoliths populated exclusively by Sphagnum mosses, graminoids, or bare peat and quantified changes in belowground dissolved organic carbon chemistry, microbial metabolism, as well as respired CO₂.

Results

The graminoid growth form was significantly distinct in belowground dissolved organic carbon chemistry with carbon compound lability 20 % and 11 % greater than bare peat and Sphagnum moss respectively. The labile dissolved organic carbon stimulated the microbial community, as indicated by greater microbial metabolic activity and richness values in conjunction with 50 % higher respired CO₂ fluxes under the graminoid treatment.

Conclusions

Our results provide mechanistic evidence that peatland plant growth forms can drive carbon cycling processes by altering dissolved organic carbon chemistry to prompt cascading effects on the microbial community and carbon release — trends suggestive of microbial priming effects. Should climate change increase graminoid prevalence at the expense of Sphagnum moss northern peatland carbon store stability may be threatened by this mechanism.

     

Impacts of peat bulk density,ash deposition and rainwater chemistry on establishment of peatland mosses

Background and aims

Peatland moss communities play an important role in ecosystem function. Drivers such as fire and atmospheric pollution have the capacity to influence mosses via multiple pathways. Here, we investigate physical and chemical processes which may influence establishment and growth of three key moss species in peatlands.

Methods

A controlled factorial experiment investigated the effects of different peat bulk density, ash deposition and rainwater chemistry treatments on the growth of Sphagnum capillifolium, S. fallax and Campylopus introflexus.

Results

Higher peat bulk density limited growth of both Sphagnum species. S. capillifolium and C. introflexus responded positively to ash deposition. Less polluted rain limited growth of C. introflexus. Biomass was well correlated with percentage cover in all three species.

Conclusions

Peat bulk density increases caused by fire or drainage can limit Sphagnum establishment and growth, potentially threatening peatland function. Ash inputs may have direct benefits for some Sphagnum species, but are also likely to increase competition from other bryophytes and vascular plants which may offset positive effects. Rainwater pollution may similarly increase competition to Sphagnum, and could enhance positive effects of ash addition on C. introflexus growth. Finally, cover can provide a useful approximation of biomass where destructive sampling is undesirable.

     

Temporal variation in microbial and plant biomass during summer in a Mediterranean high-mountain dry grassland

Aims

We assessed the temporal changes on microbial biomass in relation to changes in soil moisture, dissolved organic carbon and plant biomass during the summer season in a Mediterranean high-mountain grassland.

Methods

Temporal variations were tested by two-way ANOVA. The relationships among microbial biomass, plant biomass, soil water content, soil organic carbon, dissolved organic carbon and total soil nitrogen during the summer season were assessed by means of structural equation modeling.

Results

Microbial biomass did not show variation, while dissolved organic carbon and root biomass decreased throughout the summer. Aboveground plant biomass peaked in the middle of the summer, when soil water content was at its minimum. Soil water content directly and negatively affected soil microbial biomass, and positively affected dissolved organic carbon. Moreover soil microbial biomass and dissolved organic carbon were negatively related. Plant biomass effects on soil microbial biomass were driven by root biomass, which indirectly affected soil microbial biomass through effects on soil organic carbon and soil nitrogen.

Conclusions

The temporal dynamic of microbial biomass during the summer season appeared to differ from previous observations in temperate alpine communities, and indicated the drought resistance of the microbial community during the summer in Mediterranean high-mountain grasslands. During the dry period, microbial biomass may play an alternative role in soil carbon conservation.