Diane Hope

A review of the export of carbon in river water: Fluxes and processes

This review summarizes data on exports of carbon from a large number of temperate and boreal catchments in North America, Europe and New Zealand. Organic carbon losses, usually dominated by dissolved organic matter, show relatively little variation, most catchments exporting between 10 and 100 kg C ha(-1) yr(-1). Inorganic carbon exports occur at a similar rate. However, a lack of information on the flux of particulate organic carbon and dissolved CO2 is highlighted, particularly for rivers in Europe. Processes regulating the flux of organic carbon to streams and its subsequent fate in-stream are reviewed, along with the effects of land use and acidification on these processes. The size of the global riverine flux of carbon in relation to the global carbon cycle and the possible effects of environmental change on the export of carbon in rivers are considered.

Socioeconomics drive urban plant diversity

Spatial variation in plant diversity has been attributed to heterogeneity in resource availability for many ecosystems. However, urbanization has resulted in entire landscapes that are now occupied by plant communities wholly created by humans, in which diversity may reflect social, economic, and cultural influences in addition to those recognized by traditional ecological theory. Here we use data from a probability-based survey to explore the variation in plant diversity across a large metropolitan area using spatial statistical analyses that incorporate biotic, abiotic, and human variables. Our prediction for the city was that land use, along with distance from urban center, would replace the dominantly geomorphic controls on spatial variation in plant diversity in the surrounding undeveloped Sonoran desert. However, in addition to elevation and current and former land use, family income and housing age best explained the observed variation in plant diversity across the city. We conclude that a functional relationship, which we term the “luxury effect,” may link human resource abundance (wealth) and plant diversity in urban ecosystems. This connection may be influenced by education, institutional control, and culture, and merits further study.

Nitrogen Emissions, Deposition, and Monitoring in the Western United States

Abstract Nitrogen (N) deposition in the western United States ranges from 1 to 4 kilograms (kg) per hectare (ha) per year over much of the region to as high as 30 to 90 kg per ha per year downwind of major urban and agricultural areas. Primary N emissions sources are transportation, agriculture, and industry. Emissions of N as ammonia are about 50% as great as emissions of N as nitrogen oxides. An unknown amount of N deposition to the West Coast originates from Asia. Nitrogen deposition has increased in the West because of rapid increases in urbanization, population, distance driven, and large concentrated animal feeding operations. Studies of ecological effects suggest that emissions reductions are needed to protect sensitive ecosystem components. Deposition rates are unknown for most areas in the West, although reasonable estimates are available for sites in California, the Colorado Front Range, and central Arizona. National monitoring networks provide long-term wet deposition data and, more recently, estimated dry deposition data at remote sites. However, there is little information for many areas near emissions sources.

The relationship between dissolved organic carbon in stream water and soil organic carbon pools at different spatial scales

The relationship between stream water DOC concentrations and soil organic C pools was investigated at a range of spatial scales in subcatchments of the River Dee system in north-east Scotland. Catchment percentage peat cover and soil C pools, calculated using local, national and international soils databases, were related to mean DOC concentrations in streams draining small- ( 700 m) catchments, suggesting that disturbance and land use may have a small effect on DOC concentration. Our results therefore suggest that the relationship between stream water DOC concentration and catchment soil C pools exists at a range of spatial scales and this relationship appears to be sufficiently robust to be used to predict the effects of changes in catchment soil C storage on stream water DOC concentration. Copyright

Urban Ecology as an Interdisciplinary Field: Differences in the use of “Urban” Between the Social and Natural Sciences

Though there is a growing appreciation of the importance of research on urban ecosystems, the question of what constitutes an urban ecosystem remains. Although a human-dominated ecosystem is sometimes considered to be an accurate description of an urban ecosystem, describing an ecosystem as human-dominated does not adequately take into account the history of development, sphere of influence, and potential impacts required in order to understand the true nature of an urban ecosystem. While recognizing that no single definition of “urban” is possible or even necessary, we explore the importance of attaching an interdisciplinary, quantitative, and considered description of an urban ecosystem such that projects and findings are easier to compare, repeat, and build upon. Natural science research about urban ecosystems, particularly in the field of ecology, often includes only a tacit assumption about what urban means. Following the lead of a more developed social science literature on urban issues, we make suggestions towards a consistent, quantitative description of urban that would take into account the dynamic and heterogeneous physical and social characteristics of an urban ecosystem. We provide case studies that illustrate how social and natural scientists might collaborate in research where a more clearly understood definition of “urban” would be desirable.

Nitrogen Balance for the Central Arizona–Phoenix (CAP) Ecosystem

A detailed fixed nitrogen (N) mass balance was constructed for the Central Arizona–Phoenix (CAP) ecosystem. Input of fixed N input to the ecosystem was 98 Gg y−1. Of this, humans deliberately imported or mediated the fixation of 51 Gg N y−1; combustion processes added another 36 Gg y−1. Fixation by desert plants, wet deposition, and surface water input accounted for 11% of total N input. Total fixed N output was 78 Gg N y−1, a large component of which was gaseous N products of combustion and denitrification. Computed accumulation of N was 21 Gg y−1 (total input minus total output) or alternatively, 17 Gg y−1 (summing individual accumulation fluxes). Key uncertainties include dry deposition of atmospheric N and changes in soil storage. Inputs to the urban and agricultural components of the ecosystem were an order of magnitude higher than inputs to the desert. Human hydrologic modifications in this ecosystem promote the accumulation and volatilization of N while keeping riverine export low (3% of input). Interplay among the form and amount of N inputs, edaphic and climatic characteristics of the system, hydrologic modifications, and deliberate efforts to reduce N pollution controls the fate of N in human-dominated ecosystems.

EXPORTS OF ORGANIC CARBON IN BRITISH RIVERS

This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7–103·5 kg ha−1 year−1, with a median flux of 31·9 kg ha−1 year−1 in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two–thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change.

Sources of organic and inorganic carbon in a headwater stream: evidence from carbon isotope studies

A combination of stable isotope studies and 14Cdating were used to identify the main sources andprocesses controlling streamwater DOC and TIC in atemperate non-forested watershed. δ13Cvalues for terrestrial (−24.9 to −29.1‰) and aquatic(−30.5 to −33.5‰) plants were similar to valuesreported in the literature for similar ecosystems.δ13C values for DOC in soil solution andstreamwater were consistent with soil and terrestrialvegetation, indicating that the terrestrial ecosystemis the dominant source of aquatic DOC in thiswatershed. δ13C values of soil atmosphereCO2 (−17.2 to −25.2‰) were slightly lessnegative than would be expected for production viaaerobic soil microbial decomposition and rootrespiration. There was a close correspondence betweenδ13C values (−15.5 to −21.5‰) forstreamwater TIC and soil atmospheric CO2 in thecentral part of the catchment where the stream drainsCO2-rich peats. 14C dating showed thatalthough peat has been accumulating in the watershedfor at least 2700 years, DOC in soil pore water andstreamwater contains carbon of predominantly recentorigin (post-AD 1955).

Bonding and Bridging Understanding the Relationship between Social Capital and Civic Action

This study investigates the relationship between social connections and collective civic action. Measuring social capital in eight Phoenix, Arizona, neighborhoods allowed the authors to determine that individuals with strong social bonding (i.e., association and trust among neighbors) are more likely to take civic action. However, while social capital lessens the relationship between an individual’s social status and the likelihood of taking action, it does not eliminate the positive relationship. The analysis also suggests that bonding and bridging are distinct forms of social capital that have some different antecedents

A method for measuring free CO2 in upland streamwater using headspace analysis

Abstract Existing titration-based methods for the measurement of dissolved free CO 2 are indirect and require the measurement of a number of other determinands (e.g. pH); they may underestimate free CO 2 concentrations, because analysis is carried out frequently in an open vessel from which some free CO 2 may be lost prior to measurement. Here, a method of headspace analysis is described; this minimises CO 2 loss and provides a more direct technique for determining free CO 2 in low ionic strength, organic-rich upland streamwaters. A sample of streamwater is collected in a sealed flask and a headspace is created by pumping out a known volume of sample, replacing it with CO 2 -scrubbed air. After equilibration of CO 2 between the remaining water and the headspace, the concentration of CO 2 in the headspace is measured using an Infra Red Gas Analyser. The concentration of free CO 2 in the original sample is then calculated using Henrys law. This method measured free CO 2 in standard solutions containing 1–10 mg l −1 free CO 2 to within 0.1 mg l −1 . The method was used to measure free CO 2 in streamwater from 19 sites on the River Dee in north-east Scotland and the results were compared with those reported for streams elsewhere. Free CO 2 concentrations measured by headspace analysis were significantly higher than those found using acidimetric titration.