Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils

The short-term (24h) and: medium-term (30 day) influence of N salts (NH4Cl, NaNO3 and NaNO2) and a non-N salt (NaCl) on first-order rate constants, k (h(-1)) and thresholds (C-Th) for atmospheric CH4 oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 mu mol g(-1) dry weight (dw) soil. Additions of NH4Cl, NaNO3 and NaCl to 0.5 mu mol g(-1) dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH4Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH4+-N (less than or equal to 1 mu mol g(-1)dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH4 oxidation by exogenous NH4+-N or NH4+-N released through cation exchange. The decrease in k was consistently less for NaNO3 than for NH4Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl- counteranion. Thresholds for CH4 oxidation were less sensitive to salt addition than k for these three salts, as significant increases in C-Th relative to controls were only observed at concentrations greater than or equal to 1.0 mu mol g(-1) dw soil. Both soils were more sensitive to NaNO2 than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 mu mol NaNO2 g(-1) dw soil that was clearly attributable to NO;. Soils showed no recovery from NaCl, NH4+-N or NaNO3 addition with respect to atmospheric CH4 oxidation after 30days. However, soils amended with NaNO2 to 1.0 mu mol NaNO2 g(-1) dw showed values of k that were not significantly different from controls. Recovery of CH4-oxidizing ability was due to complete oxidation of NO2--N to NO3--N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH4 oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH4 budget.