Overriding water table control on managed peatland greenhouse gas emissions.

Global peatlands store more carbon than is naturally present in the atmosphere1,2. However, many peatlands are under pressure from drainage-based agriculture, plantation development and fire, with the equivalent of around 3 per cent of all anthropogenic greenhouse gases emitted from drained peatland3-5. Efforts to curb such emissions are intensifying through the conservation of undrained peatlands and re-wetting of drained systems6. Here we report eddy covariance data for carbon dioxide from 16 locations and static chamber measurements for methane from 41 locations in the UK and Ireland. We combine these with published data from sites across all major peatland biomes. We find that the mean annual effective water table depth (WTDe; that is, the average depth of the aerated peat layer) overrides all other ecosystem- and management-related controls on greenhouse gas fluxes. We estimate that every 10 centimetres of reduction in WTDe could reduce the net warming impact of CO2 and CH4 emissions (100-year global warming potentials) by the equivalent of at least 3 tonnes of CO2 per hectare per year, until WTDe is less than 30 centimetres. Raising water levels further would continue to have a net cooling effect until WTDe is within 10 centimetres of the surface. Our results suggest that greenhouse gas emissions from peatlands drained for agriculture could be greatly reduced without necessarily halting their productive use. Halving WTDe in all drained agricultural peatlands, for example, could reduce emissions by the equivalent of over 1 per cent of global anthropogenic emissions.

Headwater gas exchange quantified from O2 mass balances at the reach scale.

Headwater streams are important in the carbon cycle and there is a need to better parametrize and quantify exchange of carbon-relevant gases. Thus, we characterized variability in the gas exchange coefficient (k 2) and dissolved oxygen (O2) gas transfer velocity (k) in two lowland headwaters of the River Avon (UK). The traditional one-station open-water method was complemented by in situ quantification of riverine sources and sinks of O2 (i.e., groundwater inflow, photosynthesis, and respiration in both the water column and benthic compartment) enabling direct hourly estimates of k 2 at the reach-scale (~ 150 m) without relying on the nighttime regression method. Obtained k 2 values ranged from 0.001 h-1 to 0.600 h-1. Average daytime k 2 were a factor two higher than values at night, likely due to diel changes in water temperature and wind. Temperature contributed up to 46% of the variability in k on an hourly scale, but clustering temperature incrementally strengthened the statistical relationship. Our analysis suggested that k variability is aligned with dominant temperature trends rather than with short-term changes. Similarly, wind correlation with k increased when clustering wind speeds in increments correspondent with dominant variations (1 m s-1). Time scale is thus an important consideration when resolving physical drivers of gas exchange. Mean estimates of k 600 from recent parametrizations proposed for upscaling, when applied to the settings of this study, were found to be in agreement with our independent O2 budget assessment (within < 10%), adding further support to the validity of upscaling efforts aiming at quantifying large-scale riverine gas emissions.

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in a temperate re-connected floodplain.

The relative magnitudes of, and factors controlling, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in the soil of a re-connected temperate floodplain divided into four different land management zones (grazing grassland, hay meadow, fritillary meadow and a buffer zone). Soil samples were collected from each zone to measure their respective potentials for nitrate attenuation using 15N both at the surface and at depth in the soil column and additional samples were collected to measure the lability of the organic carbon. Denitrification capacity ranged between 0.4 and 4.2 (µmol N g(-1) dry soil d(-1)) across the floodplain topsoil and DNRA capacity was an order of magnitude lower (0.01-0.71 µmol N g(-1) d(-1)). Land management practice had a significant effect on denitrification but no significant effects were apparent for DNRA. In this nitrogen-rich landscape, spatial heterogeneity in denitrification was explained by differences in lability and the magnitude of organic carbon associated with different management practices (mowing and grazing). The lability of organic carbon was significantly higher in grazing grassland in comparison to other ungrazed areas of the floodplain, and consequently denitrification capacity was also highest in this area. Our results indicate that bacteria capable of DNRA do survive in frequently flooded riparian zones, and to a limited extent, compete with denitrification for nitrate, acting to retain and recycle nitrogen in the floodplain. Exponential declines in both denitrification and DNRA capacity with depth in the floodplain soils of a hay meadow and buffer zone were controlled primarily by the organic carbon content of the soils. Furthermore, grazing could be employed in re-connected, temperate floodplains to enhance the potential for nitrate removal from floodwaters via denitrification.