P and K additions enhance canopy N retention and accelerate the associated leaching

This study evaluated the interactive effects of combined phosphorus (P) and potassium (K) additions on canopy nitrogen (N) retention (CNR) and subsequent canopy leaching at a long-term N manipulation site on Whim bog in south Scotland. Ambient deposition is 8 kg N ha-1 year-1 and an additional 8, 24, and 56 kg N ha-1 year-1 of either ammonium (NH4+) or nitrate (NO3-) with or without P and K has been applied over 11 years. Throughfall N deposition below Calluna vulgaris and foliar N and P concentrations were assessed. Results showed that 60% for low dose and 53% for high dose of NO3- contrasting with 80% for low dose and 38% for high dose of NH4+ onto Calluna was retained by Calluna canopy. The CNR was enhanced by P and K addition in which 84% of NO3 - and 83% of NH4+ for high dose were retained. CNR for NO3- increased the canopy leaching of dissolved organic N (DON) and associated organic anions. NH4+ retention increased canopy leaching of magnesium and calcium through ion exchange. Even over 11-years N exposure without P and K, foliage 29 N:P ratio of Calluna did not increase, suggesting that N exposure did not lead to N saturation of Calluna at Whim bog. Our study concluded that increases in P and K availability enhance CNR of Calluna, but accelerate the associated canopy leaching of DON and base cations, depending on foliar N status

Evaluation of isoprene light response curves for bryophyte-dominated ecosystems and implications for atmospheric composition

Isoprene is emitted from numerous plant species in response to light and temperature and parameterisations of these relationships, based on observations from a few vascular plant species, have been shown to be broadly applicable to many different vegetation types. Here, we investigate their performance when applied to an ecosystem dominated by bryophytes. Over a six-week period, emissions of isoprene were measured above a Scottish peat bog. The light response derived on the basis of both canopy-scale flux and whole-plant enclosure measurements, deviated from the classical response, showing no sign of saturation within the observed range. We attribute this response to the canopy architecture of moss hummocks, which may attenuate light differently compared to a grass canopy. Both existing big-leaf and canopy-level emission algorithms, developed for vascular plants but commonly used for moorland vegetation, failed to replicate the observed fluxes, overestimating at low light intensities (<1000 μmol m−2 s−1 photosynthetically active radiation) and underestimating during daytime clear sky conditions. The light response was optimised for bryophyte-dominated ecosystems using measured fluxes and incorporated into the EMEP4UK chemical transport model and applied exclusively to moorland. The revised parameterisation resulted in a small reduction in the average annual isoprene emissions in the northern latitudes (5%), but peak isoprene emissions and concentrations increased by up to a factor of two. Yet, no significant change in average or maximum surface ozone concentrations was observed, reflecting that the northern latitudes are in a chemical regime that is strongly NOx limited, in part due to the spatial segregation with the urban sources of NOx. We conclude that, the anticipated increase in isoprene emissions from the northern latitudes in response to climate change is unlikely to contribute towards ozone-related air quality issues, as long as NOx pollution does not increase. However, the non-saturating light response may be equally applicable to non-vascular plants elsewhere, including in the tropics

The Importance of Capturing Local Measurement-Driven Adjustment of Modelled <i>j</i>(NO₂)

Accurate photolysis rate constants are essential for simulation of local air quality but their values can vary substantially with changes in local meteorological and surface conditions. This study demonstrates the use of local radiometer measurements for capturing via hourly measurement-driven adjustment factors (MDAF) the temporal resolution needed to adjust clear-sky or cloud-free model estimates of j(NO2). Measurements simultaneously at two sites in the UK (Auchencorth Moss and Manchester) showed that TUV (v5.3) model estimates of j(NO2)↓ in cloud-free conditions (used as an example of modelled j-values) were, on average, approximately 45% larger than measured j(NO2)↓, which would lead to substantial model bias in the absence of local adjustment. At Auchencorth Moss, MDAF values based on 4π and 2π radiometer inlets generally agreed very well with each other (<6% average difference). However, under conditions of particularly high surface albedo (such as snow cover), increased upwelling local diffuse radiation yielded an MDAF derived using total radiation (sum of ↓ and ↑ components) ~40% larger than the MDAF derived using only ↓ radiation. The study has demonstrated: (1) the magnitude of potential impact of local conditions—principally cloud cover, but also changes in surface albedo—on assumed j-values; (2) that whilst annual mean MDAF values are similar at Auchencorth Moss and Manchester, there is no contemporaneous correlation between them at hourly resolution; hence MDAF values derived at one site cannot readily be applied at another site. These data illustrate the need to routinely deploy long-term radiometer measurements alongside compositional measurements to support atmospheric chemistry modelling

Nitrous oxide emissions from a peatbog after 13 years of experimental nitrogen deposition

Nitrogen deposition was experimentally increased on a Scottish peatbog over a period of 13 years (2002–2015). Nitrogen was applied in three forms, NH3 gas, NH4Cl solution, and NaNO3 solution, at rates ranging from 8 (ambient) to 64 kg N ha−1 yr−1, and higher near the NH3 fumigation source. An automated system was used to apply the nitrogen, such that the deposition was realistic in terms of rates and high frequency of deposition events. We measured the response of nitrous oxide (N2O) flux to the increased nitrogen input. Prior expectations, based on the IPCC default emission factor, were that 1 % of the added nitrogen would be emitted as N2O. In the plots treated with NH4+ and NO3− solution, no response was seen, and there was a tendency for N2O fluxes to be reduced by additional nitrogen, though this was not significant. Areas subjected to high NH3 emitted more N2O than expected, up to 8.5 % of the added nitrogen. Differences in the response are related to the impact of the nitrogen treatments on the vegetation. In the NH4+ and NO3− treatments, all the additional nitrogen is effectively immobilised in the vegetation and top 10 cm of peat. In the NH3 treatment, much of the vegetation was killed off by high doses of NH3, and the nitrogen was presumably more available to denitrifying bacteria. The design of the wet and dry experimental treatments meant that they differed in statistical power, and we are less likely to detect an effect of the NH4+ and NO3− treatments, though they avoid issues of pseudo-replication

Sphagnum can ‘filter’ N deposition, but effects on the plant and pore water depend on the N form

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20 kg N ha− 1 yr− 1) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11 years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8 kg N ha− 1 yr− 1, an additional 8, 24, and 56 kg N ha− 1 yr− 1 of either ammonium (NH4+) or nitrate (NO3−) has been applied for 11 years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32 kg N ha− 1 yr− 1 even after 11 years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56 kg N ha− 1 yr− 1 of NH4+ addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH4+ dominates wet deposition for high inputs (56 kg N ha− 1 yr− 1). The N filter function in response to NO3− uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH4+ uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO3− and NH4+, may lead to species change

Long-term interactive effects of N addition with P and K availability on N status of Sphagnum

Little information exists concerning the long-term interactive effect of nitrogen (N) addition with phosphorus (P) and potassium (K) on Sphagnum N status. This study was conducted as part of a long-term N manipulation on Whim bog in south Scotland to evaluate the long-term alleviation effects of phosphorus (P) and potassium (K) on N saturation of Sphagnum (S. capillifolium). On this ombrotrophic peatland, where ambient deposition was 8 kg N ha−1 yr−1, 56 kg N ha−1 yr−1 of either ammonium (NH4+, Nred) or nitrate (NO3−, Nox) with and without P and K, were added over 11 years. Nutrient concentrations of Sphagnum stem and capitulum, and pore water quality of the Sphagnum layer were assessed. The N-saturated Sphagnum caused by long-term (11 years) and high doses (56 kg N ha−1 yr−1) of reduced N was not completely ameliorated by P and K addition; N concentrations in Sphagnum capitula for Nred 56 PK were comparable with those for Nred 56, although N concentrations in Sphagnum stems for Nred 56 PK were lower than those for Nred 56. While dissolved inorganic nitrogen (DIN) concentrations in pore water for Nred 56 PK were not different from Nred 56, they were lower for Nox 56 PK than for Nox 56 whose stage of N saturation had not advanced compared to Nred 56. These results indicate that increasing P and K availability has only a limited amelioration effect on the N assimilation of Sphagnum at an advanced stage of N saturation. This study concluded that over the long-term P and K additions will not offset the N saturation of Sphagnum

Assessing the bias of molybdenum catalytic conversion in the measurement of NO2 in rural air quality networks

The measurement method of NO2 with continuous analysers is specified for EU Ambient Air Quality Directive compliance reporting, which provides a consistent methodology and concurrent NO measurements (85/203/EEC-NO2). While the established method of measurement of NO2, following conversion of NO2 to NO using a molybdenum-conversion process, has known interference uncertainties (due to conversion of other oxidised nitrogen (NOy) chemicals, the consistency and traceability of compliance measurement is important. This study compared three continuous NO2 analyser instruments: a Thermo-NOx molybdenum convertor chemiluminescence analyser (Model 42C, ThermoFisher Scientific Inc., MA, USA), a photolytic chemiluminescence analyser (T200UP, Teledyne Technologies Inc., San Diego, USA) and a Cavity Attenuated Phase Shift (CAPS) analyser (T500U, Teledyne Technologies Inc., CA, USA). The instruments were run for over a year at the Auchencorth Moss long-term peatland monitoring site (Southeast Scotland) which is a low NOx atmosphere away from sources. NOy and NHx chemicals were also measured concurrently. This study concludes that there is a strong artefact in molybdenum catalyst chemiluminescent instruments as a result of unselective catalysis of airborne NOy compounds that causes an overestimate of NO2 measured in the atmosphere. The observed artefact in concentration measurements is likely to be observed at the entire UK scale as almost the entirety of the rural air network relies on molybdenum catalyst instruments. We therefore recommend that molybdenum catalyst instruments should be phased out and replaced in air quality monitoring networks with molecule specific (spectroscopy) instrumentation (equivalent in cost, such as those described in this study) that do not suffer from the same measurement artefacts

Validation of ammonia diffusive and pumped samplers in a controlled atmosphere test facility using traceable Primary Standard Gas Mixtures

We report the determination of ammonia (NH3) diffusive sampling rates for six different designs of commercial diffusive samplers (CEH ALPHA sampler, Gradko diffusion tube, Gradko DIFRAM-400, Passam ammonia sampler,and ICS Maugeri Radiello radial sampler (blue and white turbulence barriers)), together with the validation test results for a pumped sampler (CEH DELTA denuder). The devices were all exposed in the UK's National Physical Laboratory's (NPL) controlled atmosphere test facility (CATFAC). For each of the seven diffusive sampler exposure tests there were traceable concentrations of ammonia (in the range 3–25 μgm−3) generated under well-defined conditions of temperature, relative humidity and wind speed, which are applicable to a variety of ambient monitoring environments. The sampler exposure time at each concentration was 28 days, except for the radial devices, which were exposed for 14 days. The work relied on the dilution of newly developed stable Primary Standard Gas Mixtures (PSMs) prepared by gravimetry in passivated gas cylinders as a method of improving the metrological traceability of ammonia measurements. The exposed diffusive samplers were sent blind to the participants for analysis and the reported NH3 concentrations were then compared against the known reference concentration. From the results for each sampler type a diffusive sampling rate was calculated and compared against the rate used routinely by the participants. Some measurement results were in good agreement with the known traceable reference concentration (particularly for one diffusive sampler design (ALPHA)), while other devices exhibited over-reading and under-reading (each with a clear bias). The new diffusive sampling rates determined in the laboratory study were then applied to measurements in a field comparison campaign, and this was found to deliver an improvement in agreement between the different devices deployed

Evaluation study of the suitability of instrumentation to measure ambient NH3 concentrations under field conditions

The uncertainties in emissions of ammonia (NH3) in Europe are large, partially due to the difficulty in monitoring of ambient concentrations due to its sticky nature. In the European Monitoring and Evaluation Program (EMEP) the current recommended guidelines to measure NH3 are by coated annular denuders with offline analysis. This method, however, is no longer used in most European countries and each one has taken a different strategy to monitor atmospheric ammonia due to the increase of commercial NH3 monitoring instrumentation available over the last 20 years. In June 2014, a 3 year project funded under the European Metrology Research Programme, “Metrology for Ammonia in Ambient Air” (MetNH3), started with the aim to develop metrological traceability for the measurement of NH3 in air from primary gas mixtures and instrumental standards to field application. This study presents the results from the field intercomparison (15 instruments) which was held in South East Scotland in August 2016 over an intensively managed grassland. The study compared active sampling methods to a meteorological traceable method which was developed during the project with the aim to produce a series of guidelines for ambient NH3 measurements. Preliminary results highlight both the importance of inlets and management of relative humidity in the measurement of ambient NH3 and of the requirement to carry out frequent intercomparison of NH3 instrumentation. Overall, it would be recommended from this study that a WMO-GAW world centre for NH3 would be established and support integration of standards into both routine and research measurements