Black Spruce Soils Accumulate More Uncomplexed Organic Matter than Aspen Soils

Abstract

Improving knowledge on the dynamics and maintenance of the boreal soil's C pool is of particular importance in response to climate change concerns. We hypothesized that different forest types (black spruce, trembling aspen, and mixedwood) found on a similar site type differentially affect soil organic carbon (SOC) distribution among physical fractions. The surface mineral soil (0-15 cm) of 24 plots differing in forest composition was sampled in forested Hapludalfs of the Abitibi-James Bay region, Canada. The soil was first separated into three water-stable aggregate size fractions (> 1000, 1000-250, and mixedwood (41.9) > trembling aspen (34.7). While similar amounts of SOC (∼30 Mg C ha ―1 ) were found in the S&C, more SOC was found in the less protected fractions (i.e., uncomplexed organic matter, UOM: LF and iPOM) under black spruce than under trembling aspen, the mixedwood being intermediate. This higher accumulation of UOM under black spruce suggests a slower C turnover that is probably induced by the low-quality C inputs and environmental constraints to decomposition found in these forests. These differences in the amounts of SOC stored within soil physical fractions might have strong repercussions on the SOC budget of the boreal forest of eastern Canada under climate change.