Methane fluxes on agricultural and forested boreal organic soils

Abstract

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH 4 . Surprisingly, the agricultural soils emitted CH 4 during a warm dry summer. The CH 4 oxidation capacity and CH 4 uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH 4 during a dry summer. Despite periods of CH 4 emission, the agricultural soils were annual sinks for CH 4 , with uptake rate of CH 4 ‐C varying from 0.1 to 3.7 kg ha −1 yr −1 . The forested soil had a methane uptake rate of 3.9 kg CH 4 ‐C ha −1 yr −1 . All the soils acted as sinks for CH 4 during winter, which contributed up to half of the annual CH 4 uptake. The capacity of soils to transport gases did not explain the larger CH 4 uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH 4 uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH 4 uptake. The results suggest that CH 4 oxidation in agricultural organic soil is more sensitive to soil drying than CH 4 oxidation in forested organic soil.