Pollutant swapping in constructed agricultural wetlands

Diffuse agricultural pollution presents a major challenge to global water quality management, requiring the adoption of new land management practices such as constructed agricultural wetlands. These wetlands, promoted in agri-environment schemes, may effectively intercept rainfall-mobilised phosphorus (P), nitrogen (N) and carbon (C). However, wetlands may potentially facilitate ‘pollutant swapping’: the transfer of one form or pathway of pollution for another, as a result of mitigation efforts. Retained pollutants may be remobilised through solubilisation or as the greenhouse gases (GHGs): methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). Therefore this research examines the potential for agricultural wetlands to ‘swap’ local improvements in water quality, for (1) increased pollution to groundwaters and (2) to the atmosphere. GHG exchanges from an agricultural wetland (area 0.032 ha) in Cumbria, UK were monitored over an 18 month period, using floating gas chambers, ebullition traps and diffusive gas exchange models. While the wetland was a net sink of particulate C and N, mean net releases of CO2 (2249 – 5724 mg m-2 d -1 ), N2O (0.93 – 2.04 mg m-2 d -1 ) and CH4 (169 - 456 mg m -2 d -1 ) were significantly greater than those from adjacent riparian land. Wetland releases of CH4 were most significant in terms of potential atmospheric impact compared to other wetland GHG releases. Shallow groundwater samples extracted from a piezometer network surrounding the study site, illustrated that retained sediments acted as a source of NH4-N and DOC to surface and local groundwaters but mitigated leaching and outward transport of NO3-N to surface and groundwaters. Field and laboratory microcosm experiments demonstrated that pollutant swapping of GHGs and nutrients may be increased during periods of reduced water oxygen content associated with eutrophic conditions. In wetland designs with water depths >0.5 m, anoxic conditions may perpetuate in lower water column zones and facilitate increased CH4 and NH4-N production and storage. Additionally, microcosm studies identified that disturbance of bottom sediments by stormflow may elicit heightened GHG and nutrient releases. Therefore the net impact of wetland construction in catchments may need reconsiderations, with respect to the potentially detrimental effects on water and the atmosphere. However upscaling of observations suggests that wetland implementation in the UK is unlikely to significantly increase GHG budgets. Use of shallower wetlands with vegetation or inlet baffles may reduce CH4 emissions by encouraging oxidation and protecting sediments from storm flows

Pollutant swapping: greenhouse gas emissions from wetland systems constructed to mitigate agricultural pollution

Diffuse (non-point) water pollution from agricultural land continues to challenge water quality management, requiring the adoption of new land management practices. The use of constructed agricultural wetlands is one such practice, designed to trap multiple pollutants mobilised by rainfall prior to them reaching receiving water. Through capturing and storing pollutants in bottom sediments, it could be hypothesised that the abundance of nutrients stored in the anoxic conditions commonly found in these zones may lead to pollutant swapping. Under these circumstances, trapped material may undergo biogeochemical cycling to change chemical or physical form and thereby become more problematic or mobile within the environment. Thus, constructed agricultural wetlands designed to mitigate against one form of pollution may in fact offset the created benefits by ‘swapping’ this pollution into other forms and pathways, such as through release to the atmosphere. Pollutant swapping to the atmosphere has been noted in analogous wetland systems designed to treat municipal and industrial wastewaters, with significant fluxes of CO2, CH4 and N2O being recorded in some cases. However the small size, low level of engineering and variable nutrient/sediment inputs which are features of constructed agricultural wetlands, means that this knowledge is not directly transferable. Therefore, more information is required when assessing whether a wetland’s potential to act as hotspot for pollution swapping outweighs its potential to act as a mitigation tool for surface water pollution. Here we present results from an on-going monitoring study at a trial agricultural wetland located in small a mixed-use catchment in Cumbria, UK. Estimates were made of CH4, CO2 and N2O flux from the wetland surface using adapted floating static chambers, which were then directly compared with fluxes from an undisturbed riparian zone. Results indicate that while greenhouse gas flux from the wetland may be significant, the impacts of this may be greatly diminished when considering wetland size in relation to catchment area. As such, this increased understanding will be valuable when considering the implications of rural land use management for water quality improvement. This knowledge could also be applied to further enhancing our knowledge of gas regional/global gas emissions from freshwater systems, which at the moment are poorly constrained

The effect of water oxygen content on the production of greenhouse gases from shallow pond sediments

Shallow lakes and ponds, including those commonly found in agricultural landscapes are often only a few metres deep, with surface areas <1ha. Despite this, landscapes may contain a high number of these ponds, amounting to a considerable cumulative surface area. Many of these features, both naturally formed and man-made, receive and trap runoff with high nutrient and sediment loadings. As such, the potential for the production of greenhouse gases (GHGs) through biogeochemical cycling in the pond sediments may be significant. Furthermore, the abundance of available nutrients coupled with the shallow physical characteristics of these systems, mean that short, irregular eutrophic episodes during the summer are common, causing large fluctuations in th oxygen content of the overlying water column. The oxygen content of the water column is often cited as key factor in the production of GHGs in large lake and reservoir systems. Given the limited research focusing on shallow ponds/lakes, and potential for these systems to be important sources of GHGs, the impacts of variable water oxygen content should be investigated. Here we present the results from a sediment microcosm experiment utilising sediment cores from an agricultural pond system in Cumbria, UK. Intact sediment cores were incubated in the dark at in-situ temperature and continuously fed with filtered pond water for 2 weeks. During this time the oxygen content of the water was manipulated between fully oxygenated and anaerobic. Measurements of GHG release were based on calculated dissolved gas concentrations present in the water columns of these cores. Results indicated that during times of water column anoxia, production of methane and carbon dioxide increased significantly, despite the presence of substantial quantities of nitrate in the water columns. No change in N2O production was detected. These results indicate that while representing a significant cumulative carbon store in agricultural landscapes, shallow pond and lake systems can contribute to emission of GHGs. Furthermore, the physical and ecological characteristics of these systems have the potential to significantly increase the quantity of gas produced. This understanding will be valuable when constraining both freshwater and agricultural GHG budgets

Assessing uncertainty and complexity in regional-scale crop model simulations

Crop models are imperfect approximations to real world interactions between biotic and abiotic factors. In some situations, the uncertainties associated with choices in model structure, model inputs and parameters can exceed the spatiotemporal variability of simulated yields, thus limiting predictability. For Indian groundnut, we used the General Large Area Model for annual crops (GLAM) with an existing framework to decompose uncertainty, to first understand how skill changes with added model complexity, and then to determine the relevant uncertainty sources in yield and other prognostic variables (total biomass, leaf area index and harvest index). We developed an ensemble of simulations by perturbing GLAM parameters using two different input meteorology datasets, and two model versions that differ in the complexity with which they account for assimilation. We found that added complexity improved model skill, as measured by changes in the root mean squared error (RMSE), by 5-10% in specific pockets of western, central and southern India, but that 85% of the groundnut growing area either did not show improved skill or showed decreased skill from such added complexity. Thus, adding complexity or using overly complex models at regional or global scales should be exercised with caution. Uncertainty analysis indicated that, in situations where soil and air moisture dynamics are the major determinants of productivity, predictability in yield is high. Where uncertainty for yield is high, the choice of weather input data was found critical for reducing uncertainty. However, for other prognostic variables (including leaf area index, total biomass and the harvest index) parametric uncertainty was generally the most important source, with a contribution of up to 90% in some cases, suggesting that regional-scale data additional to yield to constrain model parameters is needed. Our study provides further evidence that regional-scale studies should explicitly quantify multiple uncertainty sources

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum