Over the past 6 years (1988–1993), we have examined the effects of soil temperature, soil moisture, site fertility, and nitrogen fertilization on the consumption of atmospheric CH4 by temperate forest soils located at the Harvard Forest in Petersham, Massachusetts. We found that soil temperature is an important controller of CH4 consumption at temperatures between −5° and 10°C but had no effect on CH4 consumption at temperatures between 10° and 20°C. Soil moisture exerts strong control on CH4 consumption over a range of 60 to 100% water‐filled pore space (% WFPS). As moisture increased from 60 to 100% WFPS, CH4 consumption decreased from 0.1 to 0 mg CH4‐C m−2 h−1 because of gas transport limitations. At 20 to 60% WFPS, site fertility was a strong controller of CH4 consumption. High‐fertility sites had 2 to 3 times greater CH4 consumption rates than low‐fertility sites. Nitrogen‐fertilized soils (50 and 150 kg NH4NO3‐N ha−1 yr−1 ) had annually averaged CH4 consumption rates that were 15 to 64% lower than annually averaged CH4 consumption by control soils. The decrease in CH4 consumption was related to both the years of application and quantity of nitrogen fertilizer added to these soils.

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Input-output budgets for dissolved inorganic nitrogen (DIN) are summarized for 24 small watersheds at 15 locations in the northeastern United States. The study watersheds are completely forested, free of recent physical disturbances, and span a geographical region bounded by West Virginia on the south and west, and Maine on the north and east. Total N budgets are not presented; however, fluxes of inorganic N in precipitation and streamwater dominate inputs and outputs of N at these watersheds. The range in inputs of DIN in wet-only precipitation from nearby National Atmospheric Deposition Program (NADP) sites was 2.7 to 8.1 kg N ha-' yr-' (mean = 6.4 kg N ha-' yr-' ; median = 7.0 kg N ha-' yr-'). Outputs of DIN in streamwater ranged from 0.1 to 5.7 kg N ha-' yr-' (mean = 2.0 kg N ha-' yr-'; median = 1.7 kg N ha-' yr-'1. Precipitation inputs of DIN exceeded outputs in streamwater at all watersheds, with net retention of DIN ranging from 1.2 to 7.3 kg N ha-' yr-' (mean = 4.4 kg N ha-' yr-l; median = 4.6 kg N ha-l yr-'1. Outputs of DIN in streamwater were predominantly NO3-N (mean = 89%; median = 94%). Wet deposition of DIN was not significantly related to DIN outputs in streamwater for these watersheds. Watershed characteristics such as hydrology, vegetation type, and land-use history affect DIN losses and may mask any relationship between inputs and outputs. Consequently, these factors need to be included in the development of indices and simulation models for predicting 'nitrogen saturation' and other ecological processes.

To determine factors controlling the carbon dynamics of an intensively managed landscape, we measured net CO 2 exchange with the atmosphere using eddy covariance and soil CO 2 fluxes using static chambers along a chronosequence of slash pine (Pinus elliottii var. elliottii) plantations consisting of a recent clearcut, a mid-rotation (10-yr-old) stand, and a rotation-aged (24-yr-old) stand. Daytime net ecosystem exchange of CO 2 (NEE day ) at the clearcut was not significantly different than zero during the growing season of the first year following harvest and reached levels that were ϳ40% of those at the older stands during the second growing season. NEE day was similar at the mid-rotation and rotation-aged sites, reflecting the similar leaf areas of these stands. Nighttime net ecosystem exchange of CO 2 (NEE night ) was an exponential function of air or soil temperature at all sites. However, low decomposition rates of litter and flooding of the site following harvest likely constrained NEE night at the clearcut, and drought affected rates at the mid-rotation site. Annual net ecosystem exchange of CO 2 (NEE yr ) was estimated at Ϫ1269 and Ϫ882 g C·m Ϫ2 ·yr Ϫ1 at the clearcut, and 576 and 603 g C·m Ϫ2 ·yr Ϫ1 at the mid-rotation stand in 1998 and 1999, respectively. For comparison, NEE yr was 741 and 610 g C·m Ϫ2 ·yr Ϫ1 at the rotationaged stand in 1996 and 1997, respectively. In contrast, annual ecosystem respiration (R eco ) was similar in magnitude at all sites during all years. Although R eco is similar in magnitude, NEE yr is highly dynamic across this intensively managed landscape, with a maximum range of ϳ2000 g C·m Ϫ2 ·yr Ϫ1 . This range exceeds that across all the sites in both the Ameriflux and Euroflux networks and illustrates the need to include the range of stand ages and disturbance histories in landscape-to regional-scale flux estimates.

Dry deposition of speciated mercury, i.e., gaseous oxidized mercury (GOM), particulate-bound mercury (PBM), and gaseous elemental mercury (GEM), was estimated for the year 2008–2009 at 19 monitoring locations in eastern and central North America. Dry deposition estimates were obtained by combining monitored two- to four-hourly speciated ambient concentrations with modeled hourly dry deposition velocities (<i>V</i>_d) calculated using forecasted meteorology. Annual dry deposition of GOM+PBM was estimated to be in the range of 0.4 to 8.1 μg m⁻² at these locations with GOM deposition being mostly five to ten times higher than PBM deposition, due to their different modeled <i>V</i>_d values. Net annual GEM dry deposition was estimated to be in the range of 5 to 26 μg m⁻² at 18 sites and 33 μg m⁻² at one site. The estimated dry deposition agrees very well with limited surrogate-surface dry deposition measurements of GOM and PBM, and also agrees with litterfall mercury measurements conducted at multiple locations in eastern and central North America. This study suggests that GEM contributes much more than GOM+PBM to the total dry deposition at the majority of the sites considered here; the only exception is at locations close to significant point sources where GEM and GOM+PBM contribute equally to the total dry deposition. The relative magnitude of the speciated dry deposition and their good comparisons with litterfall deposition suggest that mercury in litterfall originates primarily from GEM, which is consistent with the limited number of previous field studies. The study also supports previous analyses suggesting that total dry deposition of mercury is equal to, if not more important than, wet deposition of mercury on a regional scale in eastern North America

We conducted soil moisture manipulation experiments in a red pine (Pinusresinosa Ait.) plantation at the Harvard Forest (Petersham, Mass.) in August 1992 and May 1993. To manipulate soil moisture, we added 10 cm of groundwater to 1-m2 plots and allowed the soils to dry down to their pretreatment moisture contents. We measured methane (CH4) flux, soil moisture, and temperature prior to and after the water addition. Soils in both the control and watered plots were usually sinks for atmospheric CH4. Average consumption rates by control soils ranged from 0.12 to 0.17 mg CH4-C•m−2•h−1. Methane consumption rates by watered soils ranged from 0 to 0.12 mg CH4-C•m−2•h−1 and were inversely related to the moisture content of the upper 10 cm of mineral soil. Linear regression between soil moisture and CH4 consumption explained 78% of the variability (CH4 consumption = 0.001 75 (percent water filled pore space)–0.1957). Using this empirical relationship, we predicted CH4 consumption by soils at three other locations in the Harvard Forest, which agreed closely (r2 = 0.7574) with rates measured in the spring, summer, and fall of 1988–1992. Results from our study suggest that soil moisture is a good predictor of methane uptake by these forest soils and may be used to predict how future changes in soil moisture resulting from alterations in regional precipitation patterns will affect the strength of this terrestrial CH4 sink.

During June 1986, eight systems for measuring vapor phase and four for measuring particulate phase concentrations of formic acid (HCOOH) and acetic acid (CH3COOH) were intercompared in central Virginia. HCOOH and CH3COOH vapors were sampled by condensate, mist, Chromosorb 103 GC resin, NaOH-coated annular denuders, NaOH impregnated quartz filters, K2CO3 and Na2CO3 impregnated cellulose filters, and Nylasorb membranes. Atmospheric aerosol was collected on Teflon and Nuclepore filters using both hi-vol and 10-vol systems to measure particulate phase concentrations. Samples were collected during 31 discrete day and night intervals of 0.5-2 hour duration over a 4-day period. Performance of the mist chamber and K2CO3 impregnated filter techniques were also evaluated using zero air and ambient air spiked with HCOOHg, CH3COOHg , and formaldehyde (CH2Og) from permeation sources. Results of this intercomparison show significant systematic and episodic artifacts among many currently deployed measurement systems for HCOOHg and CH3COOHg. The spiking experiments revealed no significant interferences for the mist chamber technique and results generated by the mist chamber and denuder techniques were statistically indistinguishable. The condensate technique showed general agreement with the mist chamber and denuder methods, but episodic bias between these systems was inferred from large and significant differences observed during the first day of sampling. Nylasorb membranes are unacceptable for collecting carboxylic acid vapors as they did not retain HCOOHg and CH3COOHg quantitatively.Strong base impregnated filter and GC resin sampling techniques are prone to large positive interferences apparently resulting, in part, from reactions involving CH2Og to generate HCOOH and CH3COOH subsequent to collection. Significant bias presumably associated with differences in postcollection handling was observed for particulate phase measurements by participating groups. Analytical bias did not contribute significantly to differences in vapor and particulate phase measurements.

Dry deposition of atmospheric mercury (Hg) to various land covers surrounding 24 sites in North America was estimated for the years 2009 to 2014. Depending on location, multiyear mean annual Hg dry deposition was estimated to range from 5.1 to 23.8 μg m yr to forested canopies, 2.6 to 20.8 μg m yr to nonforest vegetated canopies, 2.4 to 11.2 μg m yr to urban and built up land covers, and 1.0 to 3.2 μg m yr to water surfaces. In the rural or remote environment in North America, annual Hg dry deposition to vegetated surfaces is dominated by leaf uptake of gaseous elemental mercury (GEM), contrary to what was commonly assumed in earlier studies which frequently omitted GEM dry deposition as an important process. Dry deposition exceeded wet deposition by a large margin in all of the seasons except in the summer at the majority of the sites. GEM dry deposition over vegetated surfaces will not decrease at the same pace, and sometimes may even increase with decreasing anthropogenic emissions, suggesting that Hg emission reductions should be a long-term policy sustained by global cooperation.