Karen E. Dobbie

General CH4 oxidation model and comparisons of CH4 Oxidation in natural and managed systems

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%).

Comparison of CH4 oxidation rates in woodland, arable and set aside soils

CH4 fluxes and various soil properties were measured over three successive years at a field site on a loamy sand soil in eastern Scotland, to determine which factors influence CH4 oxidation rate. This site included three adjacent areas with contrasting land use: woodland, arable land and set aside land. The CH4 oxidation rates in the arable soil were less than half the corresponding rates in the woodland soil. The CH4 oxidation rates in the set aside soil were even lower, indicating that there is no immediate recovery when cultivation and fertilisation are abandoned. In the woodland and set aside soils, a seasonal variation in CH4 oxidation rate was found, but in the arable soil there was no such trend. The CH4 oxidation rate was negatively correlated with soil moisture content (P < 0.001) in the woodland soil and positively correlated with soil temperature (P < 0.001) in the set aside soil. In the arable soil, CH4 oxidation rate was related to moisture content only in dry summer conditions, when the relationship was positive (P < 0.001). These relationships suggest that CH4 oxidation was controlled partly by diffusion and partly by biological activity. A negative correlation was found between soil ammonium concentration and CH4 oxidation rate in the woodland soil (P < 0.001), indicating that ammonium inhibited CH4 oxidation in that environment.

Impact of different forms of N fertilizer on N2O emissions from intensive grassland

Nitrous oxide (N2O) emissions were measured over two years from an intensively managed grassland site in the UK. Emissions from ammonium nitrate (AN) and urea (UR) were compared to those from urea modified by various inhibitors (a nitrification inhibitor, UR(N), a urease inhibitor, UR(U), and both inhibitors together, SU), as well as a controlled release urea (CR). N2O fluxes varied through time and between treatments. The differences between the treatments were not consistent throughout the year. After the spring and early summer fertilizer applications, fluxes from AN plots were greater than fluxes from UR plots, e.g. the cumulative fluxes for one month after N application in June 1999 were 5.2 ± 1.1 kg N2O-N ha−1 from the AN plots, compared to 1.4 ± 1.0 kg N2O-N ha−1 from the UR plots. However, after the late summer application, there was no difference between the two treatments, e.g. cumulative fluxes for the month following N application in August 2000 were 3.3 ± 0.7 kg N2O-N ha−1 from the AN plots and 2.9 ± 1.1 kg N2O-N ha−1 from the UR plots. After all N applications, fluxes from the UR(N) plots were much less than those from either the AN or the UR plots, e.g. 0.2 ± 0.1 kg N2O-N ha−1 in June 1999 and 1.1 ± 0.3 kg N2O-N ha−1 in August 2000. Combining the results of this experiment with earlier work showed that there was a greater N2O emission response to rainfall around the time of fertilizer application in the AN plots than in the UR plots. It was concluded that there is scope for reducing N2O emissions from N-fertilized grassland by applying UR instead of AN to wet soils in cool conditions, e.g. when grass growth begins in spring. Applying UR with a nitrification inhibitor could cut emissions further.

Slow increase in rate of methane oxidation in soils with time, following land use change from arable agriculture to woodland

In a successional range of sites on former arable land in Denmark and Scotland, CH4 oxidation rates took more than 100 y to reach pre-cultivation level. During the first 2–5 y following abandonment of agriculture, CH4 oxidation decreased slightly, eventually followed by an increase from 5–15 μg CH4 m−2 h−1 to a substantially higher rate of 100–150 μg CH4 m−2 h−1 in the oldest (200 y) woodlands.

Evaluating annual nitrous oxide fluxes at the ecosystem scale

Evaluation of N2O flux has been one of the most problematic topics in environmental biogeochemistry over the last 10–15 years. Early ideas that we should be able to use the large body of existing research on terrestrial N cycling to predict patterns of N2O flux at the ecosystem scale have been hard to prove due to extreme temporal and spatial variability in flux. The vast majority of the N2O flux measurement and modeling activity that has taken place has been process level and field scale, i.e., measurement, analysis and modeling of hourly and daily fluxes with chambers deployed in field plots. It has been very difficult to establish strong predictive relationships between these hourly and daily fluxes and field-scale parameters such as temperature, soil moisture, and soil inorganic N concentrations. In this study, we addressed the question of whether we can increase our predictive understanding of N2O fluxes by examining relationships between flux and environmental parameters at larger spatial and temporal scales, i.e., to explore relationships between annual rather than hourly or daily fluxes and ecosystem-scale variables such as plant community and soil type and annual climate rather than field-scale variables such as soil moisture and temperature. We addressed this question by examining existing data on annual fluxes from temperate forest, cropland, and rangeland ecosystems, analyzing both multiyear data sets from individual sites as well as cross-site comparison of single annual flux values from multiple sites. Results suggest that there are indeed coherent patterns in annual N2O flux at the ecosystem scale in forest, cropland, and rangeland ecosystems but that these patterns vary by region and only emerge with continuous (at least daily) flux measurements over multiple years. An ecosystem approach to evaluating N2O fluxes will be useful for regional and global modeling and for computation of national N2O flux inventories for regulatory purposes but only if measurement programs are comprehensive and continuous.

Novel use of ochre from mine water treatment plants to reduce point and diffuse phosphorus pollution

Treatment of polluting discharges from abandoned mines is producing large quantities of ochre (mainly iron (III) oxides) for which no major end-use has yet been identified. Newcastle and Edinburgh Universities are currently conducting research to develop and test novel field-scale methods for use of ochre for phosphorus removal from sewage effluent and land drainage. Phosphorus pollution is a serious threat to the water environment in industrialised countries, causing eutrophication, algal blooms, fish kills and loss of water resources. Our prior experiments have demonstrated that ochre is an excellent adsorbent of phosphorus from solution. The ongoing research will build upon this preliminary work to develop valuable uses for a low-value by-product of mine water treatment, with benefits for the mining and water industries and the water environment as a whole. The project will assess the performance of ochre for phosphorus removal in three settings: constructed wetlands for sewage effluent treatment; addition of powdered ochre to polluted standing waters (e.g., sewage treatment tanks, septic tanks) so that phosphorus is stripped out as the ochre settles; and treatment of agricultural drainage with ochre-filled filter and dosing units. We will also examine the fate of spent ochre from these applications, when the material is saturated with phosphorus, to assess its performance and environmental acceptability as a slow-release fertiliser and thereby develop a total use cycle for ochre. The paper presents results that demonstrate the potential of ochre for phosphorus removal and discusses current research into this method of ochre use.