Dr Deborah Pearce

Cation availability in peat may limit CH(4) production and microbial activity and thereby impact on rates of organic matter accumulation and the chemical character of the peat. We quantify total, soluble, and exchangeable cation concentrations, Exchange Site Saturation Levels (ESSLs) and organic fractions in bog-peat profiles and compare these with fen peat. Total and soluble cation concentrations are not correlated and these and exchangeable cation concentrations are lower in bog than fen peat. In all sites these vary with depth and the distribution patterns of individual cations are unique. This is explained by variation in ESSL, which is negatively correlated with Cation Exchange Capacity (CEC). Total cation concentrations in bog peat are higher in the top and bottom fractions than in the middle. Soluble concentrations in surface bog peat are low, because cations are trapped due to low ESSL This does not occur in fen peat, with lower CEC and higher ESSL CEC is related to total organic matter content, not just to Sphagnum, which has been invoked as the explanatory variable of high CEC in peat bogs. There is a complexity in the mechanisms controlling cation availability in peat and we suggest that total, soluble and exchangeable cation fractions need to be taken into account in studies of cation limitation of microbial activity in organic soils. CEC may also chelate exo-enzymes, further inhibiting decay processes.

Peat forming wetlands are globally important sources of the greenhouse gas CH(4). The variability of flux recordings from peatlands is however considerable and the distribution of CH(4) below the water table poorly described. Surface peat (0-500 mm below the water table) is responsible for the bulk of emissions and a localised region of intense CH(4) concentration may exist within this region but the structure of peat and presence of gas bubbles make the determination of in situ gas distributions problematic. We report on the in situ distribution and concentrations of CH(4), CO(2) and O(2) in surface bog peat cores using Quadrupole Mass Spectrometry and relate this to peat physical structure. Replicate cores collected in spring and autumn from both hollows and hummocks are used (n = 10). CH(4) recorded in almost every profile was localised in intense peaks reaching concentrations up to 350 mu M at depths where O(2) was absent. Each CH(4) peak had a coincident CO(2) peak with a minimum mean ratio of similar to 20:1 (CO(2):CH(4)) and we found more CH(4) beneath hollows than hummocks. In statistical comparisons CH(4) concentration and distribution differed significantly between profiles for each depth. We demonstrate that variability found within a single core is at least as great as that between cores collected across the bog. The distribution of CH(4) was negatively correlated with bulk density and in some cases the location of roots matched those of intense CH(4) concentration where bubbles had formed and been trapped. Our comparisons suggest variability in gas distribution is caused by a heterogenous peat structure that controls the movement of gas bubbles and contains localised hotspots of gas production. The small and fine root systems of vascular plants on the peatland surface may cause high levels of methanogenic activity in their vicinity and also represent a physical barrier capable of trapping CH(4) bubbles.

The high cation exchange capacity (CEC) of Sphagnum holocellulose may prevent bog peat decay by binding cations necessary for microbial activity. Sphagnum hollocellulose was extracted and added to peat to investigate the effect of increasing CEC on microbial activity. Holocellulose may also act as a carbon source. Thus the effect of different carbon sources was also investigated and holocellulose was added in combination with glucose. PbCl(2) was added to peat to release cations bound by cation exchange singly and combined with a carbon source (glucose) or holocellulose. Carbon sources stimulated both CO(2) and CH(4) production. Sphagnum holocellulose also acted as a carbon source obscuring the effect it may have had through binding cations. PbCl(2) increased cations measured in peat confirming that a large pool of essential cations is bound to peat. The cation release stimulated CH(4) production when Pb was added alone but not in experiments when Pb was combined to a carbon source. Treatments decreased pH but experiments involving changing the pH showed that increasing the pH, and not decreasing it, stimulated CH(4) production. Although there is some evidence that the high CEC of bog peat may have a role in preventing decay by sequestering essential cations; the lack of a carbon substrate is more important. We discuss these findings in relation to increasing N deposition and elevated CO(2) concentrations, and the implications for carbon storage by peatlands.

We examined the effect of cation treatments on methanogenic activity and nutrient release from exchange sites in raised bog and fen peats. Treatments consisted of cation chloride solutions (MgCl2, AlCl3 and PbCl2) applied individually. In raised bog peat Al3+ and Pb2+ increased CH4 production. Acorrelation was found between CH4 production and the amount of micro- and macronutrient cations released by the treatments. In calcareous fen peat, such a stimulation was also found, but there was no correlation between CH4 production and micro and macronutrient release. Direct nutrient and pH effects could not account for these observations. Thus the results support the hypothesis that the methanogenic community in the raised bog is limited by the availability of mineral nutrients and/or inactivity of exo-enzymes, both of which are bound onto exchange sites.

The quantification of greenhouse gas sources and sinks is important to understanding the impact of climate change. Methane (CH4) is a potent greenhouse gas, which, on a global scale, is released largely as a product of anaerobic microbial decomposition and predominantly from wetlands. A zone of intense CH4 production just below the water table is thought to contribute significantly to the overall flux from peat bogs. We describe the use of membrane inlet quadrupole mass spectrometry (QMS) to confirm the existence of bubbles, their gaseous concentrations and their localization at a fine spatial resolution within intact peat cores. We use the distribution of the noble gas argon (Ar) and the distinct QMS responses to dissolved and gaseous (bubble) phases to identify trapped bubbles with a resolution of 0.6 mm. Bubbles with CH4 concentrations of up to 20 kPa were widely distributed in the upper 300 mm of the cores with ˆ¼11% of all profiles comprising bubbles. The dissolved concentrations responsible for the bubbles were on average 83±80 μm, indicating lower concentrations relative to other QMS studies. We suggest that if the distinction between dissolved and gaseous phases is not made in studies of CH4 within peat profiles then the prominence of bubbles is likely to result in overestimates of dissolved CH4 concentrations. Fluxes of CH4 from peat as a result of drawdown or other perturbation are likely to be large, rapid and short lived because of bubble burst, and also larger than from peat without bubbles. We suggest that the dynamics of fluxes need to be modelled taking into account both gaseous and dissolved phases. Estimates of potential fluxes that assume CH4 is dissolved are likely to overestimate fluxes if the gaseous phase has not been taken into account.

The major bog systems in northern areas are dominated by Sphagnum species, the partially decomposed remains of which form the bulk of deep peat. By adding mono- and di-valent cations to deep peat cores (2.0-2.5 m) and measuring CH4 and CO2 concentrations in the manipulated peat cores using quadrupole mass spectrometry (QMS) we demonstrate that the lack of availability to microorganisms of essential cations is limiting decay in deep peat. The cations with the highest binding strength displaced the most cations and stimulated decay. Decay in deep peat cores was also stimulated by a C source (acetate), but not by NH4+. The addition of cations and acetate resulted in a less than additive stimulation of decay. The stimulatory effect of acetate and copper decreased in the presence of ammonium. The addition of EDTA to surface bog peat (where cations are conserved) decreased decay rates in surface peat (0.0-0.5 m) to that of the untreated deep peat (2.0-2.5 m). Deep peat was unaffected by treatment with EDTA. The effect of adding Cu2+ differed with the depth from which the peat was collected. Cu2+ did not stimulate decay in surface bog peat (0.0-1.5 m) but stimulated decay in peat from 1.5-3.5 m. Below 3.5 m to the bottom of the profile (5.0 m) no positive effect was observed. By comparing deep peat with surface peat we have shown that cation limitation because of high cation exchange capacity is specific to the main mass of deep bog peat and may explain differences in decay rates between anoxic surface peat and deep peat. (C) 2003 Elsevier Ltd. All rights reserved.

Aerenchyma formation in roots of maize (Zea mays L.) involves programmed death of cortical cells that is promoted by exogenous ethylene (1 muL L-1) or by endogenous ethylene produced in response to external oxygen shortage (3%, v/v). In this study, evidence that degeneration of the cell wall accompanies apoptotic-like changes previously observed in the cytoplasm and nucleus (Gunawardena et al. Planta 212, 205-214, 2001), has been sought by examining de-esterified pectins (revealed by monoclonal antibody JIM 5), and esterified pectins (revealed toy monoclonal antibody JIM 7). In controls, de-esterified wall pectins were found at the vertices of triangular junctions between cortical cells (untreated roots). Esterified pectins in control roots were present in the three walls bounding triangular cell-to-cell junctions. After treatment with 3% oxygen or 1 muL L-1 ethylene, this pattern was lost, but walls surrounding aerenchyma gas spaces became strongly stained. The results showed that cell wall changes commenced within 0.5 d and evidently were initiated by ethylene in parallel with cytoplasmic and nucleoplasmic events associated with classic intracellular processes of programmed cell death.

We made an experiment on a 30 cm. diameter core of Sphagnum-dominated vegetation and peat to estimate the parameters controlling methane oxidation during movement to the ambient air: 13 CH4 was added at the water table, and excess 13 CO2 appeared in the gas space above the core. At 20 degreesC in otherwise undisturbed conditions, similar to 22% of CH4 was oxidized to CO2 during passage up through the overlying 10-cm thick unsaturated peat and plants. We simulated the experiment, with seven parameters: transfer coefficients in water, in the gas phase, and through the container wall; the rate of CH4 and Of CO2 generation; and the two parameters of a hyperbolic relation between CH4 concentration and the rate of CH4 oxidation. We optimized these parameters to fit the experimental results, and then were able to generalize to any temperature (0 degrees -25 degreesC) and any depth (0-55 cm) of water table. Changing temperature has important effects on the proportion of CH4 oxidized.

The top 5-50 cm of a peat deposit above the water table are predominantly-oxic while below that the peat is anoxic. The concentrations of CH4 and CO2 in the peat below 50 cm do not change with the seasons. The concentrations are greatest at or near the base of the peat and decrease quadratically upwards, consistent with a gas production rate (CH4 + CO2) of 0.03 mu mol cm(-3) a(-1) and movement by diffusion. The upward efflux of CH4, calculated from the concentration profile in deep peat, is 1, and of CO2 is 17 mu mol m(-2) h(-1). Just below the water table there is a small peak in CH4 concentration. The peak concentrations are greater in summer than in winter. This indicates a second, seasonal and local, but not yet quantified source of CH4. Effluxes of CH4 from the peatland surface range from ordinary summer maxima of about 200 down to winter values less than 10 mu mol m(-2) h(-1), and at times negative values. The efflux from hummocks is usually about a third of that from hollows. These results indicate that methane oxidation may be important in hummocks.