K.J. Hargreaves

Linking land-atmosphere-stream carbon fluxes in a lowland peatland system

Any change in the ability of northern peatlands to act as a sink for atmospheric CO2 will play a crucial part in the response of the Earth system to global warming. We argue that a true assessment of the sink-source relationships of peatland ecosystems requires that losses of C in drainage waters be included when determining annual net C uptake, thus connecting measurements of stream C fluxes with those made at the land surface-atmosphere interface. This was done by combining estimates of net ecosystem exchange (NEE) with stream water measurements of TOC, DIC, and gaseous C loss, in a 335-ha lowland temperate peatland catchment (55°48.80′N, 03°14.40′W) in central Scotland over a 2-year period (1996–1998). Mean annual downstream C flux was 304 (±62) kg C ha−1 yr−1, of which total organic carbon (TOC) contributed 93%, the remainder being dissolved inorganic carbon (DIC) and free CO2. At the catchment outlet evasion loss of CO2 from the stream surface was estimated to be an additional 46 kg C ha−1 yr−1. Over the study period, NEE of CO2-C resulted in a flux from the atmosphere to the land surface of 278 (±25) kg C ha−1 yr−1. Net C loss in drainage water, including both the downstream flux and CO2 evasion from the stream surface to the atmosphere, was therefore greater or equal to the net annual C uptake as a result of photosynthesis/respiration at the land surface. By combining these and other flux terms, the overall C mass balance suggests that this system was either acting as a terrestrial C source or was C neutral.

Methane emission rates from a northern wetland; response to temperature, water table and transport

Static chamber measurements of CH4 flux were made from a range of micro-environments in an area of blanket bog in Northern Scotland. CH4 flux covered a wide range, the largest rate of CH4 emission, at 175.6 μmol m−2 h−1, was observed in pool areas through the vascular plant Menyanthes trifoliata. Investigations into the response of net CH4 emission rates to temperature and water table were carried out under semi-natural conditions on 45 large peat monoliths, maintained in open-top chambers, over a three-year period. The mean rate of CH4 emission at 10°C was an order of magnitude larger from pool monoliths (surface water table) at 78.0 μmol m−2 h−1, than from hummock monoliths (water table 15 cm below surface) at 8.4 μmol m−2 h−1. Rates of CH4 emission showed a positive linear response to increasing temperature from pool and lawn monoliths with activation energies of 74.3 and 79.5 kJ mol−1 and Q10 values of 3.0 and 3.3, respectively. When conditions of temperature, water table, light and humidity were controlled pool cores showed an exponential increase in CH4 emission rates between 5 and 30°C.

Fluxes of nitric and nitrous oxides from agricultural soils in a cool temperate climate

Fluxes of NO and N2O from sandy loam soils cropped with winter wheat and a clay loam soil under ryegrass, with and without the addition of NH4NO3 fertilizer, were measured using static and dynamic chamber methods. Nitric oxide fluxes ranged from −0.3 (deposition) to 6.9 (emission) ng NO-N m−2 s−1. The corresponding N2O flux ranged from 0 to 91 (emission) ng N2O-N m−2 s−1. The NO flux was temperature dependent. Activation energies ranged from 40 to 81 kJ mol−1. Nitric oxide and N2O fluxes increased linearly with soil available nitrogen (NH4 + NO3). Emissions of NO and N2O were not detectable from unfertilized ryegrass plots. Instead, nitric oxide was absorbed by the soil and vegetation at a maximum rate of 0.31 ng NO-N m−2 s−1. The aeration state of the soil controlled the relative rates of NO and N2O emission. Nitric oxide was the major gas emitted from well aerated soils, conditions that favour nitrification. The NO/N2O emission ratio was >100 for the coarse-textured sandy loam soil and the clay loam soil only during low rainfall periods. Nitrous oxide was the major gas emitted from less aerated soils, conditions that allowed denitrification to occur. The NO/N2O emission ratio was <0.001 for the clay loam soil when rainfall was high and soils were wet. Extrapolation to the U.K. situation showed that agricultural land may account for 2–6% of the total annual NOx emission and for 16–64% of the total annual N2O emission in the U.K.

Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.

Occurrence and formation of nitrated phenols in and out of cloud

In this study, the concentrations of phenol, four nitrated phenols, their precursors and reactants in air and cloud water, are presented. The concentrations in air and cloud water were measured simultaneously at the summit of Great Dun Fell (GDF). The measured concentrations were compared with emission data, leading to the conclusion, that in particular dinitrophenols are formed by atmospheric reactions, while car exhaust accounts to a significant extent for the mononitrophenols observed. The experimental results point to a formation of dinitrophenols in the liquid phase (cloud droplets). This is corroborated by flow tube experiments which show that phenol in aqueous solution reacts with N2O5 and ClNO2 to form nitrophenols.

Soil environmental variables affecting the flux of methane from a range of forest, moorland and agricultural Soils

Measurements of the net methane exchange over a range of forest, moorland, and agricultural soils in Scotland were made during the period April to June 1994 and 1995. Fluxes of CH4 ranged from oxidation −12.3 to an emission of 6.8 ng m−2 s−1. The balance between CH4 oxidation and emission depended on the physical conditions of the soil, primarily soil moisture. The largest oxidation rates were found in the mineral forest soils, and CH4 emission was observed in several peat soils. The smallest oxidation rate was observed in an agricultural soil. The relationship between CH4 flux and soil moisture observed in peats (FluxCH4 = 0.023 × %H2O (dry weight) − 7.44, p > 0.05) was such that CH4 oxidation was observed at soil moistures less than 325%( ± 80%). CH4 emission was found at soil moistures exceeding this value. A large range of CH4 oxidation rates were observed over a small soil moisture range in the mineral soils. CH4 oxidation in mineral soils was negatively correlated with soil bulk density (FluxCH4 = −37.35 × bulk density (g cm−3) + 48.83, p > 0.05). Increased nitrogen loading of the soil due to N fixation, atmospheric deposition of N, and fertilisation, were consistently associated with decreases in the soil sink for CH4, typically in the range 50 to 80%, on a range of soil types and land uses.

Nitrous oxide emission from an agricultural field : comparison between measurements by flux chamber and micrometerological techniques

The soil in a drained fjord area, reclaimed for arable farming, produced N2O mainly at 75–105 cm depth, just above the ground water level. Surface emissions of N2O were measured from discrete small areas by closed and open-flow chamber methods, using gas chromatographic analysis and over larger areas by integrative methods: flux gradient (analysis by FTIR), conditional sampling (analysis by TDLAS), and eddy covariance (analysis by TDLAS). The mean emission of N2O as determined by chamber procedures during a 9-day campaign was 162–202 μg N2ONm−2h−1 from a wheat stubble and 328–467 μg N2ONm−2 h−1 from a carrot field. The integrative approaches gave N2O emissions of 149–495 μg N2ONm−2 h−1, i.e. a range similar to those determined with the chamber methods. Wind direction affected the comparison of chamber and integrative methods because of patchiness of the N2O emission over the area. When a uniform area with a single type of vegetation had a dominant effect on the N2O gradient at the sampling mast, the temporal variation in N2O emission determined by the flux gradient/FTIR method and chamber methods was very similar, with differences of only 18% or less in mean N2O emission, well below the variation encountered with the chamber methods themselves. A detailed comparison of FTIR gradient and chamber data taking into account the precise emission footprint showed good agreement. It is concluded that there was no bias between the different approaches used to measure the N2O emission and that the precision of the measurements was determined by the spatial variability of the N2O emission at the site and the variability inherent in the individual techniques. These results confirm that measurements of N2O emissions from different ecosystems obtained by the different methods can be meaningfully compared.

Measurements of CH4 and N2O fluxes at the landscape scale using micrometeorological methods

Flux gradient, eddy covariance and relaxed eddy accumulation methods were applied to measure CH4 and N2O emissions from peatlands and arable land respectively. Measurements of N2O emission by eddy covariance using tunable diode laser spectroscopy provided fluxes ranging from 2 to 60 µ mol N2O m-2 h-1 with a mean value of 22 µ mol N2O m-2 h-1 from 320 h of continuous measurements. Fluxes of CH4 measured above peatland in Caithness (U.K.) during May and June 1993 by eddy covariance and relaxed eddy accumulation methods were in the range 70 to 120 µ mol CH4 m-2 h-1 with means of 14.7 µ mol CH4 m -2 h-1 and 22.7 µ mol CH4 m-2 h-1 respectively. Emissions of CH4 from peatland changed with water table depth and soil temperature; increasing from 25 |Amol CH4 m-2 h-1 at 5% pool area to 50 p.mol CH4 m-2 h-1 with 30% within the flux footprint occupied by pools. A temperature response of 4.9 (xmol CH4 m-2 h-1 °C-1 in the range 6-12 °C was also observed. The close similarity in average CH4 emission fluxes reported for wetlands in Caithness, Hudson Bay and Alaska in the range 11 to 40 jamol CH4 m-2 h-1 suggests that earlier estimates of CH4 emission from high latitude wetlands were too large or that the area of high latitudes contributing to CH4 emission has been seriously underestimated.

The exchange of nitric oxide, nitrogen dioxide and ozone between pasture and the atmosphere.

Fluxes of NO, NO2 and O3 were determined over a drained marshland pasture in south-east England by using flux-gradient techniques. Nitric oxide was found to be emitted at rates of up to 40 ng m(-2) s(-1), the rate of emission being related to the magnitude of the eddy diffusivity. Nitrogen dioxide deposited at rates of up to 90 ng m(-2) s(-1) under the control of stomatal resistance, a clear diurnal cycle being observed. Minimum canopy resistance was of the order of 80 s m(-1). Ozone deposition was also controlled by stomatal resistance, the minimum canopy resistance being around 100 s m(-1) and fluxes reaching a maximum of 220 ng m(-2) s(-1). Corrections made to NO and NO2 fluxes to compensate for chemical reactions showed flux divergences of the order of 30% for NO and NO2, but these were not statistically significantly different from the measured fluxes. The pasture was found to be a net sink for nitrogen in the form of NOx.

Advances in micrometeorological methods for the measurement and interpretation of gas and particle nitrogen fluxes

The application of micrometeorology for flux measurements of nitrogen species between terrestrial ecosystems and the atmosphere and some of their main limitations are reviewed. New methods which are gaining rapid acceptance such as relaxed eddy accumulation are also described. A new development to provide long term average fluxes by time averaged gradients is shown to yield long-term average NH3 fluxes over moorland within 10% of values obtained using continuous wet denuder methods and at less than 10% of the cost. The use of mass balance methods to quantify fluxes at the plot, landscape and regional scale are described, and show that in suitable conditions and for some countries, methods to check national inventories of radiatively active gases are now available.