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Summary Despite decades of research to define optimal chamber design and deployment protocol for measuring gas exchange between the Earth's surface and the atmosphere, controversy still surrounds the procedures for applying this method. Using a numerical simulation model we demonstrated that (i) all non‐steady‐state chambers should include a properly sized and properly located vent tube; (ii) even seemingly trivial leakiness of the seals between elements of a multiple‐component chamber results in significant risk of measurement error; (iii) a leaking seal is a poor substitute for a properly designed vent tube, because the shorter path length through the seal supports much greater diffusive gas loss per unit of conductance to mass flow; (iv) the depth to which chamber walls must be inserted to minimize gas loss by lateral diffusion is smaller than is customary in fine‐textured, wet or compact soil, but much larger than is customary in highly porous soils, and (v) repetitive sampling at the same location is not a major source of error when using non‐steady‐state chambers. Finally, we discuss problems associated with computing the flux of a gas from the non‐linear increase in its concentration in the headspace of a non‐steady‐state chamber.

Methane flux was measured in situ in the Alaska Arctic tundra to assess the magnitude and controls on spatial variability of emissions. A total of 122 measurements were made at 57 spatially independent sites across the Alaska North Slope during the summer of 1987. Variability in rates of emissions was similar in magnitude on local and regional scales, ranging from 0 to 286.5 mg CH4 m−2 d−1 overall and often varying across two orders of magnitude within 0.5‐m distances. Primary control on rates of emissions was determined by the substrate and the position of the water table relative to the surface. Secondary controls were defined by the substrate temperature and the type and quantity of vegetation participating in the plant‐mediated release of CH4 to the atmosphere. Emission rates in the Arctic Foothills ranged from 0.2 mg CH4 m−2 d−1 for tussock tundra to 55.3 mg CH4 m−2 d−1 over wet meadows. Within the Arctic Coastal Plain, rates of emissions were highest on inundated terrestrial sites (72.2 mg CH4 m−2 d−1), decreasing nearly 12 fold on comparable sites where the water table was 5 cm or more below the surface (6.1 mg CH4 m−2 d−1). Emission rates increased linearly with substrate temperatures at 10‐cm depth, increasing nearly ninefold over the 6°C temperature range observed. Plant mediated release of CH4 to the atmosphere was directly proportional to green leaf area and represented 92–98% of the total emission rates over vegetated sites. Comparisons between boreal studies reflect similarities in environmental controls on emissions at local‐to‐regional scales and demonstrate the sensitivity of regional to global estimates to sampling bias. These results suggest that current published emissions rates may have overestimated the contribution of boreal ecosystems to the global CH4 budget by several fold.

1. The sources of nitrogen for phytoplankton were determined for a bloom-prone lake as a means of assessing the hypothesis that cyanobacteria dominate in eutrophic lakes because of their ability to fix nitrogen when the nitrogen : phosphorous (N : P) supply ratio is low and nitrogen a limiting resource. 2. Nitrogen fixation rates, estimated through acetylene reduction with 15 N calibration, were compared with 15 N-tracer estimates of ammonium and nitrate uptake monthly during the ice-free season of 1999. In addition, the natural N stable isotope composition of phytoplankton, nitrate and ammonium were measured biweekly and the contribution of N 2 to the phytoplankton signature estimated with a mixing model. 3. Although cyanobacteria made up 81-98% of phytoplankton biomass during summer and autumn, both assays suggested minimal N acquisition through fixation (<9% for the in-situ incubations; <2% for stable isotope analysis). Phytoplankton acquired N primarily as ammonium (82-98%), and secondarily as nitrate (15-18% in spring and autumn, but <5% in summer). Heterocyst densities of <3 per 100 fixer cells confirmed low reliance on fixation. 4. The lake showed symptoms of both light and nitrogen limitation. Cyanobacteria may have dominated by monopolizing benthic sources of ammonium, or by forming surface scums that shaded other algae.