Source and age of carbon in peatland surface waters: new insights from 14C analysis

Peatlands are a significant source of carbon to the aquatic environment which is increasingly being recognised as an important flux pathway (both lateral and vertical) in total landscape carbon budgets. Determining the source and age of the carbon (in its various forms) is a key step to understanding the stability of peatland systems as well as the connectivity between the soil carbon pool and the freshwater environment. Novel analytical and sampling methods using molecular sieves have been developed for (1) within-stream, in situ sampling of CO2 in the field and (2) for the removal/separation of CO2 in the laboratory prior to 14C analysis of CH4. Here we present dual isotope (δ13C and 14C) data from freshwater systems in UK and Finnish peatlands to show that significant differences exist in the source and age of CO2, DOC (dissolved organic carbon) and POC (particulate organic carbon). Individual peatlands clearly differ in terms of their isotopic freshwater signature, suggesting that carbon cycling may be “tighter” in some systems compared to others. We have also measured the isotopic signature of different C species in peatland pipes, which appear to be able to tap carbon from different peat depths. This suggests that carbon cycling and transport within “piped-peatlands” may be more complex than previously thought. Some of our most recent work has focussed on the development of a method to measure the 14C component of CH4 in freshwaters. Initial results suggest that CH4 in peatland streams is significantly older than CO2 and derived from a much deeper source. We have also shown that the age (but not the source) of dissolved CO2 changes over the hydrological year in response to seasonal changes in discharge and temperature. Radiocarbon measurements in the peat-riparian-stream system suggest that a significant degree of connectivity exists in terms of C transport and cycling, although the degree of connectivity differs for individual C species. In summary, 14C analysis of peatland surface waters reveals multiple sources and ages for CO2, CH4, DOC and POC with different ages characterising individual peatlands. This implies that carbon transport from peat to stream is more complex than previously thought. Dual isotope (δ13C and 14C) analysis of carbon in its various aquatic forms is clearly a powerful tool in developing a better understanding of the functioning and stability of carbon-rich landscapes

An assessment of chamber 14C methodologies for sampling aquatic CO2 evasion

The development of new methods to directly measure the radiocarbon age of dissolved and evaded aquatic carbon dioxide has enhanced our ability to understand carbon transport and cycling in the soil–water–atmosphere system. One of the methods involves collecting enough carbon dioxide for radiocarbon dating by allowing carbon dioxide to outgas from the water surface into an enclosed floating chamber, with the gas subsequently trapped onto a zeolite molecular sieve cartridge. There are, however, several different methodological approaches that can be used for the collection of floating chamber samples, and it is currently unknown whether these different approaches influence the isotopic (stable carbon and radiocarbon) composition of the measured sample. Here, we evaluate four different floating chamber approaches and compare the stable and radiocarbon composition of the evaded carbon dioxide. Chamber conditions varied considerably with the different methodologies, with for example, maximum chamber CO2 concentration ranging from approximately 400–6,300 ppm during sampling. Despite the varying chamber conditions, our results indicate no significant differences in the 14C age of evasion (range: 1,276–1,364 years before present) with any of the methodological approaches (in chambers where atmospheric carbon dioxide had been excluded). This confirms the methodologies are both robust and widely applicable

Outlook and appraisal [June 1993]

First quarter GDP figures signal the end of the recession. Recent falls in unemployment and rising house prices should stimulate greater expenditure from consumers now that the threat of redundancy has receded

Quantifying the predictability of a predictand: demonstrating the diverse roles of serial dependence in the estimation of forecast skill

Predictability varies. In geophysical systems, and related mathematical dynamical systems, variations are often expressed as serial dependence in the skill with which the system is, or can be, predicted. It is well known, of course, that estimation is more complicated in cases where the time series sample in‐hand does not reflect an independent from the target population; failure to account for this results in erroneous estimates both of the skill of the forecast system and of the statistical uncertainty in the estimated skill. This effect need not be indicated in the time series of the predictand; specifically: it is proven by example that linear correlation in the predictand is neither necessary nor sufficient to identify misestimation. Wilks [Quarterly Journal of the Royal Meteorological Society 136, 2109 (2010)] has shown that temporal correlations in forecast skill give rise to biased estimates of skill of a forecast system, and made progress on accounting for this effect in probability‐of‐precipitation forecasts. Related effects are explored in probability density forecasts of a continuous target in three different dynamical systems (demonstrating that linear correlation in the predictand is neither necessary nor sufficient), and a simple procedure is presented as a straightforward, good practice test for the effect when estimating the skill of forecast system

Carbon concentrations in natural and restoration pools in blanket peatlands

Open-water perennial pools are common natural features of peatlands globally, and peatland restoration often results in new pool creation, yet the concentrations of different forms of aquatic carbon (C) in natural and artificial restoration pools are not well studied. We compared carbon concentrations in both natural pools and restoration pools (4–15 years old) on three blanket peatlands in northern Scotland. At all sites, restoration pools were more acidic and had mean dissolved organic carbon (DOC) concentrations in restoration pools of 23, 22, and 31 mg L−1 compared with natural pool means of 11, 11 and 15 mg L−1 respectively across the three sites. Restoration pools had a greater fulvic acid prevalence than the natural pools and their DOC was more aromatic. Restoration pools were supersaturated with dissolved CO2 at around 10 times atmospheric levels, whereas for natural pools, CO2 concentrations were just above atmospheric levels. Dissolved CH4 concentrations were not different between pool types, but were ~200 times higher than atmospheric levels. Regular sampling at one of the peatland sites over 2.5 years showed that particulate organic carbon (POC) concentrations were generally below 7 mg L−1 except during the warm, dry summer of 2013. At this regularly-sampled site, natural pools were found to process DOC so that mean pool outflow concentrations in overland flow were significantly lower than mean inflow DOC concentrations. Such an effect was not found for the restoration pools. Soil solution and pool water chemistry, and relationships between DOC and CO2 concentrations suggest that different processes are controlling the transformation of C, and therefore the form and amount of C, in natural pools compared to restoration pools

Refining the role of phenology in regulating gross ecosystem productivity across European peatlands

Abstract The role of plant phenology as regulator for gross ecosystem productivity (GEP) in peatlands is empirically not well constrained. This is because proxies to track vegetation development with daily coverage at the ecosystem scale have only recently become available and the lack of such data has hampered the disentangling of biotic and abiotic effects. This study aimed at unraveling the mechanisms that regulate the seasonal variation in GEP across a network of eight European peatlands. Therefore, we described phenology with canopy greenness derived from digital repeat photography and disentangled the effects of radiation, temperature and phenology on GEP with commonality analysis and structural equation modeling. The resulting relational network could not only delineate direct effects but also accounted for possible effect combinations such as interdependencies (mediation) and interactions (moderation). We found that peatland GEP was controlled by the same mechanisms across all sites: phenology constituted a key predictor for the seasonal variation in GEP and further acted as distinct mediator for temperature and radiation effects on GEP. In particular, the effect of air temperature on GEP was fully mediated through phenology, implying that direct temperature effects representing the thermoregulation of photosynthesis were negligible. The tight coupling between temperature, phenology and GEP applied especially to high latitude and high altitude peatlands and during phenological transition phases. Our study highlights the importance of phenological effects when evaluating the future response of peatland GEP to climate change. Climate change will affect peatland GEP especially through changing temperature patterns during plant-phenologically sensitive phases in high latitude and high altitude regions.Peer reviewe

Peatland carbon production and transport: the role of the riparian zone

Northern peatlands are an important carbon store in which the carbon cycling is intrinsically linked to hydrological state. Changes in climate and land management can alter the hydrology of peatlands, with artificial drainage (and its subsequent remediation) a significant driver of hydrochemical change across UK peatlands. With fluxes associated with the aquatic pathway (Particulate Organic Carbon (POC), Dissolved Organic Carbon (DOC)), and dissolved CO2 and CH4) representing ~30% of Net Ecosystem Exchange (NEE) (Dinsmore et al. 2010), changes in peatland hydrology have the potential to significantly alter catchment carbon balances. Riparian soils occur in a key transition zone between the peatland and the stream, and have the potential to modify carbon transport pathways (Lyon et al. 2011), while being a hotspot for CH4 emissions (Dinsmore et al. 2009). It is therefore important at the catchment scale despite its limited spatial extent. This research aims to investigate the production, retention, transformation and transport of aquatic carbon species within the riparian zone. Instrumented nested piezometers have been installed in a peatland-riparian zone-stream transect of an ombrotrophic peatland near Edinburgh, UK. The experimental setup is replicated in a shallow peat site (~0.5 m peat depth) and a deeper peat site (~2 m depth) with piezometers installed at two depths corresponding to the surface and subsurface soils. At these sites a range of hydrochemical parameters (electrical conductivity, pH, temperature, water table and stream height) are combined with continuous CO2 measurements using Vaisala© non-dispersive infra-red (NDIR) sensors, weekly headspace measurements of dissolved CO2 and CH4 and water samples analysed for POC, DOC and DIC in both the piezometers and the stream. This setup provides high temporal resolution data to investigate diurnal cycling and storm event scale processes which are often shorter than routine weekly sampling would allow. Measurements are being made for at least 12 months to allow seasonal effects to be investigated under a wide range of hydrological conditions. This study is unique in its deployment of multiple NDIR CO2 sensors across the peat-stream interface. The results of 12 months monitoring will be presented and linked with carbon concentration-discharge relationships. These measurements will also be combined with hydrological tracer experiments to investigate flow paths through the riparian zone. Overall this study aims to provide new insights into both the hydrological and biogeochemical processes occurring in peatland riparian zones and to assess their importance for peatland carbon budgets. Dinsmore, K.J., Skiba, U.M., Billett, M.F., Rees, R.M. & Drewer, J., 2009. Spatial and temporal variability in CH4 and N2O fluxes from a Scottish ombrotrophic peatland: Implications for modelling and up-scaling. Soil Biology and Biochemistry 41(6): 1315-1323. Dinsmore, K.J., Billett, M.F., Skiba, U.M., Rees, R.M., Drewer, J & Helftner, C., 2010. Role of the aquatic pathway in the carbon and greenhouse gas budgets of a peatland catchment. Global Change Biology 16(10): 2750-2762. Lyon, S.W., Grabs, T., Laudon, H., Bishop, K.H. & Seibert, J., 2011. Variability of groundwater levels and total organic carbon in the riparian zone of a boreal catchment. J. Geophys. Res 116(G1): G01020