Particulate organic carbon (POC) export from soil and vegetation in temperate mountain regions

In assessing the significance of terrestrial particulate organic carbon (POC) export in the global carbon cycle, it is essential to be able to predict the POC yield and its make-up (broadly, fossil versus non-fossil) from any given setting. Because mountains vastly dominate the physical erosion load, an understanding of the processes operating in mountains of different kinds, and what controls them, is necessary. In particular, the dynamics of POC harvest in temperate forested uplands are poorly constrained, despite the large area covered by these zones. C and N concentration and isotopic composition data (for both riverine suspended sediment and carbon stores) are presented from two contrasting temperate mountain regions with vast stocks of soil organic carbon. In the northern Swiss Alps, as discharge increases, POC is initially diluted by lithic material through in-channel clearing, but beyond a threshold POC is added. This happens under moderate flow conditions when hillslopes are activated and rain- induced overland flow delivers soil POC to channels. As a result, the proportion of non-fossil POC increases significantly as discharge and suspended sediment load increase. In contrast to the Swiss Alps, overland flow occurs rarely in the Oregon Cascades and Coast Range. There, hillslope soil is decoupled from the channel, due largely to riparian vegetation that both prevents extensive mobilisation and traps sediment before it reaches the stream. Where channels are aggrading, there is no other input mechanism for soil or bedrock, resulting in very low total sediment and POC yields (and correspondingly high POC concentrations). In the Coast Range, with largely sedimentary rather than volcanic substrate, there is some evidence for hillslope soil mobilisation, but not (under moderate meteorological conditions) on the scale observed in Switzerland. Instead, nearly all POC exported comes from vegetation. Initial dilution of POC through in-channel clearing is still evident, and without subsequent activation of the soil reservoir, Oregon’s POC export (per unit area) is around an order of magnitude less than the Swiss system

Influence of land-use on the dynamics, quantity and composition of the organic matter transported across estuaries

The flux of terrigenous organic carbon across estuaries is an important and changing component of the global carbon cycle, but it is poorly understood. It has been proposed that estuaries can act either as a transporter of terrestrial dissolved organic carbon (DOC) to the ocean or as a reactor system in which DOC can be buried or transformed into carbon dioxide and released to the atmosphere. However, there is no clear understanding of the factors that drive estuaries to behave in one way or the other. Here we present the results from a study conducted in thirteen British estuaries which drain catchments of diverse land-uses under different hydrological conditions. Our data show that land-use influences the composition of the dissolved organic matter (DOM), the mixing dynamics of DOC and the quantity of DOC exported off the estuaries. Estuaries, whose catchments are less intensively managed and represent more natural ecosystems (average proportion of arable and (sub)-urban land-use ~12 %), contain a higher proportion of biologically-refractory “humic-like” DOM, which is transported conservatively across the salinity gradient. In contrast, estuaries whose catchments are more intensively managed (average proportion of arable and (sub)-urban land-use ~32 %) contain a high fraction of “protein-like” DOM which is transported non-conservatively, and thus suggest the existence of additions and removal processes across the salinity gradient. Furthermore, estuaries with more intensively managed catchments tend to export more DOC to coastal areas than they receive from rivers. Our results indicate that future changes in land-use have the potential to alter aquatic fluxes of terrigenous DOM and the fate of the constituent carbon

Application of the Occupational Sitting and Physical Activity Questionnaire (OSPAQ) to office based workers

Background The workplace is a setting where sedentary behaviour is highly prevalent. Accurately measuring physical activity and sedentary behaviour is crucial to assess the impact of behavioural change interventions. This study aimed to evaluate the reliability and criterion validity of the Occupational Sitting and Physical Activity Questionnaire (OSPAQ) and compare with data collected by accelerometers. Methods A test-retest study was undertaken on 99 participants using the OSPAQ. Data were then compared to accelerometer records of 41 participants. Reliability was assessed by paired t-test and intra-class correlations (ICC) via a two-way mixed model based on absolute agreement. Difference and agreement were measured by comparison of mean self-reported data with accelerometer data using the Pearson’s correlation coefficient and Bland-Altman plots. Results The ICCs for minutes spent sitting (0.66), standing (0.83) and walking (0.77) showed moderate to strong test-retest reliability. No significant differences were found between the repeated measurements taken seven days apart. Correlations with the accelerometer readings were moderate. The Bland-Altman plots showed moderate agreement for standing time and walking time but systematic variation for sedentary time. Conclusion The OSPAQ appears to have acceptable reliability and validity measurement properties for application in the office workplace setting

Sources, composition, and export of particulate organic matter across British estuaries

Estuaries receive and process a large amount of particulate organic carbon (POC) prior to its export into coastal waters. Studying the origin of this POC is key to understanding the fate of POC and the role of estuaries in the global carbon cycle. Here, we evaluated the concentrations of POC, as well as particulate organic nitrogen (PON), and used stable carbon and nitrogen isotopes to assess their sources across 13 contrasting British estuaries during five different sampling campaigns over 1 year. We found a high variability in POC and PON concentrations across the salinity gradient, reflecting inputs, and losses of organic material within the estuaries. Catchment land cover appeared to influence the contribution of POC to the total organic carbon flux from the estuary to coastal waters, with POC contributions >36% in estuaries draining catchments with a high percentage of urban/suburban land, and <11% in estuaries draining catchments with a high peatland cover. There was no seasonal pattern in the isotopic composition of POC and PON, suggesting similar sources for each estuary over time. Carbon isotopic ratios were depleted (−26.7 ± 0.42‰, average ± sd) at the lowest salinity waters, indicating mainly terrigenous POC (TPOC). Applying a two-source mixing model, we observed high variability in the contribution of TPOC at the highest salinity waters between estuaries, with a median value of 57%. Our results indicate a large transport of terrigenous organic carbon into coastal waters, where it may be buried, remineralized, or transported offshore

Pb reactivity in soils using 204Pb stable isotope dilution

Stable isotope dilution has been used to quantify the ‘reactivity’ of lead in a variety of contaminated soils. ‘Reactivity’ or ‘lability’ of lead is difficult to measure using traditional methods. This study used isotopic exchangeability, as a proxy for Pb reactivity, by determining the extent to which an added 204Pb spike mixed with the native soil Pb pool. After spike equilibration, the 204Pb/208Pb atom ratio in the solution phase of equilibrated soil suspensions was measured by ICP-MS to determine the isotopically exchangeable Pb or ‘E-value’ for the soil: Method development involved testing post-spike equilibration times and electrolyte composition to determine the most robust method. This method was then tested on five soils with different contamination histories to demonstrate the range of application of the method. The soils tested were historically contaminated from a range of sources, including petrogenic Pb, Pb/Zn minespoil, sewage sludge application and C19th urban waste disposal. 0.05 M EDTA is a commonly used extractant to estimate the reactive pool of metal in soils. Our results however suggest that the use of this extractant is not viable for Pb. Increasing EDTA concentrations appear to convert non-labile Pb to isotopically-exchangeable forms. Nevertheless, the reactive pool of soil Pb may be larger than intuitively expected. For example, in soils contaminated with Pb/Zn minespoil ~38% Pb was isotopically exchangeable

The use of 204Pb stable isotope dilution to measure lead lability in soils

Isotopic dilution was used to quantify the mobility or ‘reactivity’ of lead in a range of contaminated soils. Lead is a common contaminant in soils and the mobility of lead is not easily measured, but is an important factor in the prediction of toxicity and risk assessment. The technique developed here uses 204Pb to spike soils and thereby alter the ratio of abundance in the reactive or ‘labile’ pool of soil Pb. The atom ratio 204Pb/208Pb in the solution phase of equilibrated soil suspensions is then measured by ICPMS to resolve the total amount of isotopically exchangeable (Labile) Pb present. The method development involved testing several pre- and post-spike equilibration times, and electrolyte concentrations to provide the most robust method. The final method was then tested on five soils with different contamination histories to demonstrate the range of application of this method. The soils tested had been historically contaminated from a range of sources, including petrogenic Pb, Pb/Zn minespoil, sewage sludge, C19th urban waste and artificial contamination as Pb(NO3)2 salt

Forests, rain and runoff : particulate organic carbon in the Pacific North-West and its impact on the Earth's thermostat

Export and burial of carbon recently fixed from the atmosphere by plants and soils (as opposed to fossil carbon eroded from bedrock) transfers carbon dioxide from the atmosphere into geological storage. Recent studies suggest that storm-driven erosion of terrestrial biomass (principally through large, deep-seated landslides) can effectively sequester carbon in tectonically and climatically extreme regimes. However, as the contribution of more typical continental terrain remains poorly constrained, it is difficult to evaluate the importance of biomass erosion on a global scale. Moreover, there is insufficient understanding of the processes which mobilise particulate organic matter (POM), its sources and initial pathways and their variation under different hydrologic conditions. We address these issues in the temperate montane forests of Oregon’s Coast and Cascade Ranges. We have obtained the C and N concentrations and stable isotope ratios of ~200 samples of POM in riverine suspended sediment from four watersheds, varying in size, geology, climate and vegetation. According to our measurements, the riverine POM comes from multiple carbon stores, which are characterised by distinct values of N/C, δ13C and δ15N, and is mixed during mobilisation and transport. Considerable amounts of POM are mobilised under moderate flow conditions (occurring several times a year), which do not trigger major landslides. Instead, runoff erosion is the principal mobilisation mechanism. We compare the composition of POM in Oregon headwaters to a similar dataset from the Swiss Alps, showing that suspended POM from each location contains both non-fossil and fossil components. Moreover, we show that vegetation is the primary non-fossil end member for the Oregon samples. This contrasts with the Swiss location, where all standing biomass appears to be homogenised through a soil ‘window’ before erosion. Our findings demonstrate the potential for significant export of POM (in particular, non-fossil POM) from steep uplands wherever there is rain on soil and vegetation. Given the vast area meeting these conditions, we suggest that erosion of continental biomass may be more important to the global carbon cycle than previously thought, and explore potential feedbacks in the Earth’s climate system in light of the links between POM transfer, runoff and rainfall