Response of atmospheric methane consumption by maine forest soils to exogenous aluminum salts

Abstract

Atmospheric methane consumption by Maine forest soils was inhibited by additions of environmentally relevant levels of aluminum. Aluminum chloride was more inhibitory than nitrate or sulfate salts, but its effect was comparable to that of a chelated form of aluminum. Inhibition could be explained in part by the lower soil pH values which resulted from aluminum addition. However, significantly greater inhibition by aluminum than by mineral acids at equivalent soil pH values indicated that inhibition also resulted from direct effects of aluminum per se. The extent of inhibition by exogenous aluminum increased with increasing methane concentration for soils incubated in vitro. At methane concentrations of >10 ppm, inhibition could be observed when aluminum chloride was added at concentrations as low as 10 nmol g (fresh weight) of soil(-1). These results suggest that widespread acidification of soils and aluminum mobilization due to acid precipitation may exacerbate inhibition of atmospheric methane consumption due to changes in other parameters and increase the contribution of methane to global warming.

FIG. 1

(A) Atmospheric methane uptake rate constants for soils amended with various concentrations of aluminum sulfate (●) or sulfuric acid (○). Data are means of triplicate determinations ± 1 standard error. (B) Plot of soil pH versus amount of proton equivalent added for aluminum sulfate (●) or sulfuric acid (○). Note that the molar ratio of proton equivalents is 3:1 for aluminum additions. (C) Plot of methane uptake rate constants for aluminum sulfate (●) or sulfuric acid (○) versus soil pH.

FIG. 2

Methane uptake rates for various aluminum salt additions (1 μmol of Al³⁺ gfw⁻¹). Soils were incubated with an initial headspace methane concentration of 100 ppm. Data are means of triplicate determinations ± 1 standard error.

FIG. 3

Percent inhibition of methane consumption by AlCl₃ (2 μmol of Al³⁺ gfw⁻¹) for soils incubated with various methane concentrations (atmospheric concentration to 10,000 ppm). Percent inhibition was determined from the ratio of methane uptake for aluminum-treated soils to methane uptake for untreated soils at each methane concentration. Data are means of triplicate determinations ± 1 standard error.

FIG. 4

(A) Methane uptake rate constants for soils incubated with atmospheric methane after addition of AlCl₃, aluminum citrate, or citrate at a concentration of 0.1 μmol of Al³⁺ gfw⁻¹ (open bars) or 1.6 μmol of Al³⁺ gfw⁻¹ (solid bars). Note that the citrate concentrations are one-third those of aluminum. Data are means of triplicate determinations ± 1 standard error. (B) Same as panel A, but soils were incubated with an initial headspace methane concentration of 64 ppm. Ctrl, control; Al-Cit, aluminum citrate.

FIG. 5

V_max (○) and apparent K_m (app K_m) (●) as a function of added aluminum chloride. Data are means of triplicate determinations ± 1 standard error.

FIG. 6

V_max (○) and apparent K_m (app K_m) (●) as a function of added NH₄Cl. Data are means of triplicate determinations ± 1 standard error.

FIG. 7

Ammonium concentrations in aqueous extracts (1 ml of deionized water gfw⁻¹) for soils incubated with 1 μmol of aluminum salts gfw⁻¹ or 3 μmol of acids gfw⁻¹. Data are means of triplicate determinations ± 1 standard error. Ctrl, control.