Carbon storage of microbes and roots and the flux of CO2 across a moisture gradient

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Abstract
Living root biomass (tree roots 2 mm, tree roots >2 mm, and dwarf shrub roots), dead root biomass, microbial biomass carbon (Cmic), and soil surface CO2 flux were investigated in a northern boreal mixed forest. Three study sites were established along a 100-m gradient with differing moisture and soil texture. The amount of total root biomass did not differ across the moisture gradient. The amount of living root biomass averaged 1.46 kg·m-2, and 1.04 kg·m-2 was found as dead roots. Half of living root biomass was fine roots (diameter 2 mm), and of this fraction, 75% was fine roots of trees. The total amount of Cmic was lower on the dry site (78.5 g C·m-2) compared with the intermediate and mesic sites (95.3 g C·m-2). The Cmic to organic C ratio was 1.4% in the L+F+H horizon, and the ratio increased with increasing depth to 4.1% in the C horizon, which indicated that microbes in the deeper soil horizons decomposed recently deposited root litter or exudates. The storage of carbon in fine roots was four times more than that in Cmic. The soil surface CO2 flux fluctuated seasonally but was similar across the gradient.

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