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 ot 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 Hapludalfe of the Abitibi-James Bay region, Canada. The soil was first separated into three water-stable aggregate size fractions (>1000, 1000-250, and <250 [un) by wet sieving, followed by a density flotation (Nal: 1.7 g cm"') and a dispersion (with glass beads) to isolate the free light fraction (LF), die intraa^regate partieulate organic matter (iPOM) and the silt plus clay fraction (S&C). According to mixed linear models, whole SOC contents (in Mg C ha"' ) decreased in the following order: black spruce (46.3) > mixedwood (41.9) > trembling aspen (34.7). While similar amounts of SOC (~30 MgC ha"') 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. Tliis 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 dimate change.Abbreviations: CEC, cation exchange capacity; HF, heavy fraction; iPOM, intraaggregate partieulate organic matter Fraction; LF, light fraction; S&C, silt plus clay fraction; SOC, soil organic carbon; UOM, uncomplexed organic matter. B oreal soils hold one of the largest pools of terrestrial C in the form of SOC, storing around three times more SOC than temperate or tropical biomes (Lai, 2005). Accordingly, any variation in the size and turnover rate of this major C pool could alter atmospheric COj concentration and global climate.The residence time of C in soils is highly variable, ranging from a few days to thousands of years (von Lützow et al., 2006). Soil organic C stabilization and accumulation depend on a variety of factors, including C input rate and quality, and soil microclimatic conditions such as temperature and moisture that greatly affect the C output rate, that is, decomposition (Lorenz and Lai, 2010). Stabilization of SOC also occurs when organic matter and mineral particles are mixed together to form organo-mineral complexes. Tisdall and Oades (1982) conceptualized a hierarchical structure for soil aggregation that involves a distribution of organic matter into differently sized aggregates with varying degrees of stability.Interactions between organic matter, roots, microbes, and mineral particles generate macroaggregates (>250 [J^m) that contain C in various soil ...

Soil Organic Carbon Storage and Stability in the Aspen-Conifer Ecotone in Montane Forests in Utah, USA

“…The relative distribution of SOC among the different fractions (expressed by the percent of SOC) was similar across the vegetation types in plots and transects, probably due to the high variability in LF content in our sites. These results somewhat contradict previous observations from montane semi-arid [26] and boreal aspen-conifer forests [30], where aspen stands had a significantly higher proportion of SOC in the MoM fraction than mixed and conifer stands. Table 4.…”

“…), while mineral-associated SOC content was similar across the pine-oak ecotone. Similarly, Laganiè re et al [30] did not find differences in mineral-associated SOC between aspen and black spruce (Picea mariana (Mill.) Britton et al), but reported more SOC in less protected fractions under black spruce.…”

“…This was somewhat unexpected, as LF is considered responsive to changes in overstory species and land use [7]. In their aspen-conifer comparison, Laganiere et al [30] and Woldeselassie et al [26] had previously found a higher proportion of unprotected SOC under conifer stands than under aspen stands. The two methods used to characterize relatively labile SOC, i.e., decomposability (long-term incubations) and solubility (hot water extractions) were positively, albeit weakly, correlated.…”

“…A shift towards mixed and conifer-dominated stands could thus modify SOC dynamics and potentially reduce long-term SOC storage (i.e., SOC sequestration potential). Several studies have addressed the properties of SOC under mixed aspen-conifer stands in the boreal climate [15,30], but studies of SOC storage and stability in mixed stands in semi-arid climates are largely missing. In particular, we do not know whether changes in SOC properties occur gradually or abruptly at critical composition thresholds.…”

Land use effects on soil organic carbon sequestration in calcareous Leptosols in former pastureland – a case study from the Tatra Mountains (Poland)

“…Some data on differences in mechanisms associated with soil organic matter (SOM) stabilization in forest soils were provided by Laganière et al (2011). There are few data available for mountain soils, and our understanding of the effect of various environmental factors on SOM turnover is limited Budge et al, 2011;Martinsen et al, 2011).…”