Mineralization of detritus and oxidation of methane in acid boreal coniferous forest soils: seasonal and vertical distribution and effects of clear-cut

Abstract

Rates of soil carbon mineralization and CH 4 oxidation potential were measured in the different layers of the podsolized soils (20 cm topsoil) of managed boreal and Pinus sylvestris (age 38 years) and Picea abies (stand age 120 years) forests for eight seasons during the years 1997–1999. Endogenous CO 2 evolution was highest in October both in the P. sylvestris (1800–2200 mg CO 2–C m −2 d −1) and in the P. abies (2900–3000 mg CO 2–C m −2 d −1) forest soils and located mainly in the humus layer, when measured at 7 °C, i.e. close to the annual average soil temperature. The turnover rates of organic carbon were similar in the lower part of the illuvial layer and in the humus, annual mean of 0.027–0.028% C d −1 in the P. sylvestris and P. abies forests, indicating the presence of easily available sources of carbon deeper in the soil. The cumulative annual CO 2 evolution at in situ temperature was calculated as 430–440 g CO 2–C m −2 in the P. sylvestris and 630–650 g CO 2–C m −2 in the P. abies forest soil. Nineteen to twenty-three percent of this occurred during the cold half of the year. When the P. abies forest was clear-cut in 1998 the mineralizing activities of the eluvial and illuvial layers translocated into the humus layer, but the cumulative total CO 2 evolution per m 2 remained unchanged. Clear-cutting increased soil temperature dependence on the air temperature. The sources of the annual CO 2 evolution were above-ground litter (18–26%), and roots (25%) related to the direct photosynthetic activity of the trees in the eluvial and illuvial layers. CH 4 oxidation potential (at 200 ppm CH 4) of the podsolized conifer forest soils ranged from 12 to 22 mg CH 4–C m −2 d −1 (annual mean) in the P. sylvestris and the P. abies forests. The activity mainly located (50–90%) in the illuvial horizons in all seasons. In the upper, 2–3 cm zone, of the illuvial layer, immediately below the eluvial layer, methane oxidation potential was found 5–10 fold higher than in adjacent soil above or below. CH 4 oxidation potential of (g of soil organic carbon) in the forest soils increased 100 fold ( P. abies) or 5–50 fold ( P. sylvestris) with increasing depth. Ten to twenty-four percent of the annual CH 4 oxidation potential at in situ soil temperature was contributed by the cold half of the year similarly in both forests. Clear-cutting the P. abies stand decreased the potential for CH 4 oxidation at in situ temperature by 40% (to 1.8 g CH 4–C m −2 year −1).