Robert C. Harriss

Review and assessment of methane emissions from wetlands

The number of emission measurements of methane (CH4) to the atmosphere has increased greatly in recent years, as recognition of its atmospheric chemical and radiative importance becomes widespread. In this report, we review progress on estimating and understanding both the magnitude of, and controls on, emissions of CH4 from natural wetlands. We also calculate global wetland CH4 emissions using this extensive flux data base and the wetland areas compiled and published by Matthews and Fung (1987). Tropical regions (20° N-30° S) were calculated to release 66 TgCH4/yr, 60% of the total wetland emission of 109 Tg/yr. Flux data from tropical wetlands, reported only within the last four years, are currently restricted in geographic coverage. Additional data from other regions will be required to confirm these calculated large emissions. Although emissions from subtropical and temperate wetlands (45° N-20° N and 30° S-50° S) were relatively low at 5 Tg/yr, the process-oriented focus of most of the research in this region suggests that work at these latitudes may serve as models to examine controls and possible uncertainties in estimating fluxes. These types of efforts are frequently not possible in more remote, globally significant wetlands. Northern wetlands (north of 45° N) were calculated to release a total of 38 TgCH4/yr (34% of total flux); 34 Tg/yr from wet soils and 4 Tg/yr from relatively dry tundra. These latitudes have been the focus of recent intensive research. Significant differences between the relatively large flux data bases accumulated in the two primary measurement areas, northern Minnesota and the Hudson Bay Lowlands of Canada, indicate that extrapolation from one wetland region to another may be subject to considerable error. Global emissions were also compared to fluxes calculated using the wetland areas published by Aselmann and Crutzen (1989) in an effort to assess uncertainties due to wetland area estimates. Further refinement of wetland CH4 emissions awaits flux measurements from large areas currently lacking data, particularly in the tropics and the Siberian Lowlands, more realistic assessments of seasonal active periods, and accurate, up-to-date habitat classification and measurement.

Global Patterns in Human Consumption of Net Primary Production

The human population and its consumption profoundly affect the Earths ecosystems. A particularly compelling measure of humanitys cumulative impact is the fraction of the planets net primary production that we appropriate for our own use. Net primary production—the net amount of solar energy converted to plant organic matter through photosynthesis—can be measured in units of elemental carbon and represents the primary food energy source for the worlds ecosystems. Human appropriation of net primary production, apart from leaving less for other species to use, alters the composition of the atmosphere, levels of biodiversity, energy flows within food webs and the provision of important ecosystem services. Here we present a global map showing the amount of net primary production required by humans and compare it to the total amount generated on the landscape. We then derive a spatial balance sheet of net primary production ‘supply’ and ‘demand’ for the world. We show that human appropriation of net primary production varies spatially from almost zero to many times the local primary production. These analyses reveal the uneven footprint of human consumption and related environmental impacts, indicate the degree to which human populations depend on net primary production ‘imports’ and suggest policy options for slowing future growth of human appropriation of net primary production.

Modeling carbon biogeochemistry in agricultural soils

An existing model of C and N dynamics in soils was supplemented with a plant growth submodel and cropping practice routines (fertilization, irrigation, tillage, crop rotation, and manure amendments) to study the biogeochemistry of soil carbon in arable lands. The new model was validated against field results for short-term (1–9 years) decomposition experiments, the seasonal pattern of soil CO2 respiration, and long-term (100 years) soil carbon storage dynamics. A series of sensitivity runs investigated the impact of varying agricultural practices on soil organic carbon (SOC) sequestration. The tests were simulated for corn (maize) plots over a range of soil and climate conditions typical of the United States. The largest carbon sequestration occurred with manure additions; the results were very sensitive to soil texture (more clay led to greater sequestration). Increased N fertilization generally enhanced carbon sequestration, but the results were sensitive to soil texture, initial soil carbon content, and annual precipitation. Reduced tillage also generally (but not always) increased SOC content, though the results were very sensitive to soil texture, initial SOC content, and annual precipitation. A series of long-term simulations investigated the SOC equilibrium for various agricultural practices, soil and climate conditions, and crop rotations. Equilibrium SOC content increased with decreasing temperatures, increasing clay content, enhanced N fertilization, manure amendments, and crops with higher residue yield. Time to equilibrium appears to be one hundred to several hundred years. In all cases, equilibration time was longer for increasing SOC content than for decreasing SOC content. Efforts to enhance carbon sequestration in agricultural soils would do well to focus on those specific areas and agricultural practices with the greatest potential for increasing soil carbon content.

Quantifying the effect of oxidation on landfill methane emissions

Field, laboratory, and computer modeling methods were utilized to quantitatively assess the capability of aerobic microorganisms to oxidize landfill-derived methane (CH4) in cover soils. The investigated municipal landfill, located in Nashua, New Hampshire, was operating without gas controls of any type at the time of sample collection. Soil samples from locations of CH4 flux to the atmosphere were returned to the laboratory and subjected to incubation experiments to quantify the response of oxidation in these soils to temperature, soil moisture, in situ CH4 mixing ratio, soil depth, and oxygen. The mathematical representations of the observed oxidation reponses were combined with measured and predicted soil characteristics in a computer model to predict the rate of CH4 oxidation in the soils at the locations of the measured fluxes described by Czepiel et al. [this issue]. The estimated whole landfill oxidation rate at the time of the flux measurements in October 1994 was 20%. Local air temperature and precipitation data were then used in conjunction with an existing soil climate model to estimate an annual whole landfill oxidation rate in 1994 of 10%.

Methane emissions from tundra environments in the Yukon‐Kuskokwim delta, Alaska

Over a 6-week period from July 3 to August 10, 1988, we made measurements of the flux of methane by enclosure techniques from major tundra environments in the Yukon-Kuskokwim Delta of Alaska (60°45′N). Emissions from wet meadow tundra averaged 144 ± 31 mg CH4/m2/d and ranged from 15.6 to 426 mg CH4/m2/d, varying with soil moisture and temperature. Flux from the drier upland tundra was approximately 2 orders of magnitude lower and averaged 2.3 ± 1.1 mg CH4/m2/d. Consumption of ambient levels of methane was sporadically measured at these drier sites, and emissions ranged between −2.1 and 18.1 mg CH4/m2/d. Tundra lakes emit methane from the open water surface as well as from fringing aquatic vegetation. The presence of vegetation significantly enhanced flux over open water rates. Average fluxes from a variety of sites with vegetation ranged between 62.7 and 153.5 mg CH4/m2/d. Calculated diffusive fluxes from open water varied with lake size, the large lakes emitting 3.8 mg CH4/m2/d and small lakes emitting an average of 77 mg CH4/m2/d. An updated estimate of global emissions from tundra indicates an annual flux of approximately 11 ± 3 Tg CH4.

Methane emissions along a salt marsh salinity gradient

The seasonal flux of methane to the atmosphere was measured at three salt marsh sites along a tidal creek. Average soil salinities at the sites ranged from 5 to 17 ppt and fluxes ranged from below detection limits (less than 0.3 mgCH4 m-2 d-1) to 259 mgCH4 m-2 d-1. Annual flux to the atmosphere was 5.6 gCH4 m-2 from the most saline site, 22.4 gCH4 m-2 from the intermediate site, and 18.2 gCH4 m-2 from the freshest of the three sites. Regression of the amount of methane in the soil with flux indicates that changes in this soil methane can account for 64% of the observed variation in flux. Data on pore water distributions of sulfate suggests that the activity of sulfate reducing bacteria is a primary control on methane flux in these transitional environments. Results indicate that relatively high emissions of methane from salt marshes can occur at soil salinities up to approximately 13 ppt. When these data are combined with other tidal marsh studies, annual CH4 flux to the atmosphere shows a strong negative correlation with the long term average soil salinity over a range from essentially fresh water to 26 ppt.

Model estimates of nitrous oxide emissions from agricultural lands in the United States

The Denitrification-Decomposition (DNDC) model was used to elucidate the role of climate, soil properties, and farming practices in determining spatial and temporal variations in the production and emission of nitrous oxide (N2O) from agriculture in the United States. Sensitivity studies documented possible causes of annual variability in N2O flux for a simulated Iowa corn-growing soil. The 37 scenarios tested indicated that soil tillage and nitrate pollution in rainfall may be especially significant anthropogenic factors which have increased N2O emissions from soils in the United States. Feedbacks to climate change and biogeochemical manipulation of agricultural soil reflect complex interactions between the nitrogen and carbon cycles. A 20% increase in annual average temperature in °C produced a 33% increase in N2O emissions. Manure applications to Iowa corn crops enhanced carbon storage in soils, but also increased N2O emissions. A DNDC simulation of annual N2O emissions from all crop and pasture lands in the United States indicated that the value lies in the range 0.9–1.2 TgN. Soil tillage and fertilizer use were the most important farming practices contributing to enhanced N2O emissions at the national scale. Soil organic matter and climate variables were the primary determinants of spatial variability in N2O emissions. Our results suggest that the United States Government, and possibly the Intergovernmental Panel on Climatic Change (IPCC), have underestimated the importance of agriculture as a national and global source of atmospheric N2O. The coupled nature of the nitrogen and carbon cycles in soils results in complex feedbacks which complicate the formulation of strategies to reduce the global warming potential of greenhouse gas emissions from agriculture.

Nitrous Oxide Emissions from Municipal Wastewater Treatment

Nitrous oxide (N 2 0) emissions from primary and secondary wastewater treatment processes were measured during spring and summer 1993 in Durham, NH. The most significant emissions occurred during secondary aeration. Dissolved N 2 0 generated as a result of denitrification during primary settling was stripped from the liquid during mechanical aeration. Emission factors derived from our field measurements included per capita emissions of 3.2 g of N 2 0 person -1 yr -1 and flow based emissions of 1.6 x 10 -6 of N 2 0 (L of wastewater) -1 .

Deforestation in Costa Rica: A Quantitative Analysis Using Remote Sensing Imagery1

Accurate estimates of forest cover and forest fragmentation are critical for developing countries such as Costa Rica, which holds four to five percent of the world’s plant and bird species. We estimated forest cover for Costa Rica using Landsat 5 Thematic Mapper satellite scenes acquired between 1986 and 1991. In 1991, 29 percent (ca 14,000 km2) of the land cover of Costa Rica was closed forest cover; of that forested area, ca 30 percent is protected by national conservation policies. Forest loss in a study area representing ca 50 percent of Costa Rica’s territory during a five-year period (1986–1991) was 2250 km2, and the estimated deforestation rate was ca 450 km2/yr, or ca 4.2 percent/yr, of remaining forest cover. Forests are almost completely eliminated from the Tropical Moist Forest and Premontane Moist Forest life zones, and the level of fragmentation of remaining forests may be more advanced than previously thought.

Summertime photochemistry of the troposphere at high northern latitudes

The budgets of O3, NOx (NO+NO2), reactive nitrogen (NOy), and acetic acid in the 0–6 km column over western Alaska in summer are examined by photochemical modeling of aircraft and ground-based measurements from the Arctic Boundary Layer Expedition (ABLE 3A). It is found that concentrations of O3 in the region are regulated mainly by input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The concentrations of NOx (10–50 ppt) are sufficiently high to slow down O3 photochemical loss appreciably relative to a NOx-free atmosphere; if no NOx were present, the lifetime of O3 in the 0–6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NOx present in the Arctic troposphere have thus a major impact on the regional O3 budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NOx below 4-km altitude, but for only 20% at 6-km altitude. Decomposition of other organic nitrates might supply the missing source of NOx. The lifetime of NOy, in the ABLE 3A flight region is estimated at 29 days, implying that organic nitrate precursors of NOx could be supplied from distant sources including fossil fuel combustion at northern mid-latitudes. Biomass fire plumes sampled during ABLE 3A were only marginally enriched in O3; this observation is attributed in part to low NOx emissions in the fires, and in part to rapid conversion of NOx to PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional O3 budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH3CO3 with HO2 and CH3O2. There remains a major unidentified source of acetic acid in the atmosphere.