Abstract
To assess the potential impact of conifer encroachment on soil organic carbon (SOC) dynamics and storage in montane aspen-conifer forests from the interior western US, we sampled mineral soils (0–15 cm) across the aspen-conifer ecotones in southern and northern Utah and quantified total SOC stocks, stable SOC (i.e., mineral-associated SOC (MoM)), labile SOC (i.e., light fraction (LF), decomposable (CO2 release during long-term aerobic incubations) and soluble SOC (hot water extractable organic carbon (HWEOC)). Total SOC storage (47.0 ± 16.5 Mg C ha−1) and labile SOC as LF (14.0 ± 7.10 Mg C ha−1), SOC decomposability (cumulative released CO2-C of 5.6 ± 3.8 g C g−1 soil) or HWEOC (0.6 ± 0.6 mg C g−1 soil) did not differ substantially with vegetation type, although a slight increase in HWEOC was observed with increasing conifer in the overstory. There were statistically significant differences (p = 0.035) in stable MoM storage, which was higher under aspen (31.2 ± 15.1 Mg C ha−1) than under conifer (22.8 ± 9.0 Mg C ha−1), with intermediate values under mixed (25.7 ± 8.8 Mg C ha−1). Texture had the greatest impact on SOC distribution among labile and stable fractions, with increasing stabilization in MoM and decreasing bio-availability of SOC with increasing silt + clay content. Only at lower silt + clay contents (40%–70%) could we discern the influence of vegetation on MoM content. This highlights the importance of chemical protection mechanisms for long-term C sequestration.
Highlights
Efforts to optimize C sequestration in forest ecosystems have mainly focused on enhancing stand biomass productivity and density by adapting rotation length, thinning intensity and tree species composition
While differences in soil organic carbon (SOC) storage across the aspen-conifer gradient were not always clearcut, potentially due to the high variability in abiotic factors, our results suggest that aspen stores more SOC in association with silt and clay, increasing the pool of longer residence time SOC
In conifer-dominated stands, on the other hand, SOC is more susceptible to losses through microbial decomposition
Summary
Efforts to optimize C sequestration in forest ecosystems have mainly focused on enhancing stand biomass productivity and density by adapting rotation length, thinning intensity and tree species composition. The persistence of SOC is further enhanced by the mineral matrix through additional protection mechanisms, such as the isolation of organic matter inside aggregates (i.e., physical protection) and surface interactions between organic compounds and mineral particles, mainly from the silt and clay fraction (i.e., chemical protection) [12]. The interaction of these protection mechanisms and soil microclimate creates a continuum of SOC pools with different chemical composition and residence time [13,14] that differs among forest species [7,15]
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