Lawrence A. Baker

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.

Urbanization and warming of Phoenix (Arizona, USA): Impacts, feedbacks and mitigation

This paper examines the impacts, feedbacks, and mitigation of the urban heat island in Phoenix, Arizona (USA). At Sky Harbor Airport, urbanization has increased the nighttime minimum temperature by 5°C and the average daily temperatures by 3.1°C. Urban warming has increased the number of “misery hours per day” for humans, which may have important social consequences. Other impacts include (1) increased energy consumption for heating and cooling of buildings, (2) increased heat stress (but decreased cold stress) for plants, (3) reduced quality of cotton fiber and reduced dairy production on the urban fringe, and (4) a broadening of the seasonal thermal window for arthropods. Climate feedback loops associated with evapotranspiration, energy production and consumption associated with increased air conditioning demand, and land conversion are discussed. Urban planning and design policy could be redesigned to mitigate urban warming, and several cities in the region are incorporating concerns regarding urban warming into planning codes and practices. The issue is timely and important, because most of the worlds human population growth over the next 30 years will occur in cities in warm climates.

Introduction to nonpoint source pollution in the United States and prospects for wetland use

Abstract Nonpoint source (NPS) pollution is the major cause of impairment of US surface waters. The dominant source of NPS pollution is agricultural activity, and “traditional” pollutants — nutrients, sediments, and pathogens — are the main detrimental constituents. Erosion from cropland has been declining and is expected to decline further in the 1990s, but it is unclear how this will translate into changes in sediment yields in streams. Pollution by nitrogen is of particular concern in eutrophication of estuaries, as a contaminant of groundwater and as an acidifying agent in atmospheric deposition. Nitrogen fertilizer and emissions of nitrous oxides are major contributors to the problem. The outlook on pesticides is mixed: bans on organochlorine pesticides in the 1970s have resulted in decreasing concentrations in fish tissue; however, herbicides are now a problem for some surface and groundwater sources of drinking water, especially in the Upper Midwest. Metals in NPS pollution are primarily a concern in mining areas and in urban runoff. Declining use of leaded gasoline has resulted in decreased lead in fish tissues, sediments, and surface waters around the nation. New directions in controlling NPS pollution include: (1) a greater emphasis on risk assessment, (2) a move toward regulatory or quasi-regulatory approaches, and (3) a trend toward source reduction. The potential for using wetlands tto control agricultural NPS pollution is discussed by contrasting cropland runoff with secondary wastewater effluent.

Experimental acidification of Little Rock Lake, Wisconsin

The controlled acidification of a two-basin lake is described. The lake was divided by a vinyl curtain in 1984; acidification of one basin began in 1985. Target pH values of 5.5, 5.0 and 4.5 are planned for 2-yr increments. Biotic and chemical responses and internal alkalinity generation are being studied. Baseline studies, initial results at pH 5.5, and predictions of lake responses to acidification are described.

Carbon, nitrogen, and phosphorus fluxes in household ecosystems in the Minneapolis‐Saint Paul, Minnesota, urban region

Rapid worldwide urbanization calls for a better understanding of the biogeochemical cycling of those macroelements that have large environmental impacts in cities. This study, part of the Twin Cities Household Ecosystem Project, quantified fluxes of carbon (C), nitrogen (N), and phosphorus (P) at the scale of individual households in the Minneapolis-Saint Paul metropolitan area in Minnesota, USA. We estimated input and output fluxes associated with several components of household activities including air and motor vehicle travel, food consumption, home energy use, landscape, pets, and paper and plastic use for 360 owner-occupied, stand-alone households. A few component fluxes dominated total input fluxes of elements. For instance, air and motor vehicle transportation, together with home energy use, accounted for 85% of total C consumption and emissions. All total and component fluxes were skewed to varying degrees, suggesting that policies targeting disproportionately high fluxes could be an effective and efficient way to reduce pollution. For example, 20% of households contributed 75% of air travel emissions and 40% of motor vehicle emissions. Home energy use was more nearly normally distributed. Nitrogen fluxes were dominated by human diet and lawn fertilizer applications, which together accounted for 65% of total household N inputs. The majority of P inputs were associated with human diet, use of detergents, and pet food. A large portion of the variation among household fluxes of C, N, and P was related to a few biophysical variables. A better understanding of the biophysical, demographic, and behavioral drivers of household activities that contribute to C, N, and P fluxes is pivotal for developing accurate urban biogeochemical models and for informing policies aimed at reducing sources of pollution in urban ecosystems.

DESIGN CONSIDERATIONS AND APPLICATIONS FOR WETLAND TREATMENT OF HIGH-NITRATE WATERS

Nitrate contamination is a serious problem worldwide. By providing an ample supply of carbon and an anaerobic environment, wetlands are a valuable low technology for treating nitrate-contaminated waters with low organic carbon concentrations. Denitrification is apparently limited by the C:N ratio, with ratios >5:1 resulting in >90% nitrate removal efficiencies. The denitrification rate constant, V NO3 , varies in direct proportion to carbon supply. Several novel or emerging applications of wetlands include renovation of nitrate-contaminated aquifers (a pump-and-treat strategy), denitrification of nitrified sewage effluents, and treatment of irrigation return flows. Treatment of dual sources is also discussed. In arid regions with limited supplies of high quality water, nitrate treatment wetlands may play a significant role in the development of water resources.

Model of Internal Alkalinity Generation: Sulfate Retention Component

Internal alkalinity generation is modeled by an input-output approach in which equations to describe budgets for sulfate, nitrate, ammonium, and base cations are linked to an alkalinity budget equation. Calibration of the sulfate model using ion budgets for 14 softwater lakes shows that the sulfate sink coefficient is reasonably uniform (mean = 0.46 m yr−1) and can be used to predict sulfate retention. Model predictions show that internal sulfate sinks are needed to correctly predict lakewater [SO42−] and that in-lake sulfate sinks can account for over 50% of input. For experimentally acidified Little Rock Lake, Wisconsin, the sulfate model predicts 90% recovery of [SO42−] 13 years after acid additions stop.

Sources and transport of organic carbon in an Arizona river-reservoir system

Abstract Sources and transport of organic carbon were studied in two large, Sonoran desert watersheds (Salt and Verde rivers) and a two-reservoir system on the Verde River. Total organic carbon (TOC) concentrations in the unregulated rivers above the reservoirs did not follow a simple relationship with flow. TOC concentrations usually declined during spring runoff and reached low concentrations (1–3 mg L −1 ) by early summer. In most years, distinct peaks (10–30 mg L −1 ) occurred in late summer, coincident with small but abrupt increases in flow associated with the first monsoon rains. A two-reservoir mass balance showed that 72% of the particulate organic carbon (POC) input was retained, probably due largely to simple sedimentation. Production of dissolved organic carbon (DOC) within the reservoir was 41% of the inflow loading, even though the reservoirs accounted for only 0.14% of the watershed area. Reservoir DOC production comprised a large fraction of total watershed production because upstream DOC production was extremely low (0.2 g C m −2 yr −1 ), the reservoirs were moderately productive, and water residence times were fairly long during most of the year. We postulate that reservoirs are major contributors to total watershed DOC production in arid regions.

Total Value of Phosphorus Recovery

Phosphorus (P) is a critical, geographically concentrated, nonrenewable resource necessary to support global food production. In excess (e.g., due to runoff or wastewater discharges), P is also a primary cause of eutrophication. To reconcile the simultaneous shortage and overabundance of P, lost P flows must be recovered and reused, alongside improvements in P-use efficiency. While this motivation is increasingly being recognized, little P recovery is practiced today, as recovered P generally cannot compete with the relatively low cost of mined P. Therefore, P is often captured to prevent its release into the environment without beneficial recovery and reuse. However, additional incentives for P recovery emerge when accounting for the total value of P recovery. This article provides a comprehensive overview of the range of benefits of recovering P from waste streams, i.e., the total value of recovering P. This approach accounts for P products, as well as other assets that are associated with P and can be recovered in parallel, such as energy, nitrogen, metals and minerals, and water. Additionally, P recovery provides valuable services to society and the environment by protecting and improving environmental quality, enhancing efficiency of waste treatment facilities, and improving food security and social equity. The needs to make P recovery a reality are also discussed, including business models, bottlenecks, and policy and education strategies.

The residential landscape: fluxes of elements and the role of household decisions

We assessed biogeochemical cycling of elements through residential household landscapes to evaluate the importance of annual to decadal household-level decisions for element fluxes that contribute to urban and regional pollution. We combined a mailed survey, vegetation measurements, and allometric and biogeochemical models to estimate fluxes and accumulation of carbon (C), nitrogen (N), and phosphorus (P) in landscapes of 360 single-family homes in the Minneapolis-Saint Paul, Minnesota metropolitan area. Carbon inputs and accumulation were strongly influenced by the presence of trees on the property. Nitrogen inputs to the landscape exceeded estimated ecosystem demand for N on average by 51% and were dominated by N fertilizer application. Because Minnesota state law restricts the use of P fertilizer, pet waste was responsible for 84% of P inputs to the landscape. The results have implications for understanding sources of urban pollution and the potential flexibility (i.e., the potential for change) in household behaviors such as tree planting, fertilization, and pet waste management that contribute to such pollution.