Storage and Stability of Soil Organic Carbon in Aspen and Conifer Forest Soils of Northern Utah

Abstract

This study compares the amount, distribution, and stability of soil organic carbon (SOC) in six paired quaking aspen (Populus tremuloides Michx) and conifer plots at three locations in northern Utah, to assess the influence of vegetation cover and other biotic and abiotic drivers on SOC storage capacity in seasonally dry environments. Aspen soils accumulated significantly more SOC in the mineral soil (0-60 cm) (92.2 ± 26.7 Mg C ha"^ vs. 66.9 ± 18.6 Mg C ha'^ under conifers), and despite thicker O horizons under conifers that contained higher amounts of SOC (11.6 ± 8.8 Mg C ha"^ under conifers vs. 1.65 ± 0.38 Mg C ha"^ in aspen), across all sites SOC storage was 25% higher under aspen. Shallow soil cores (0-15 cm) did not indicate significant differences in SOC with vegetation type. The SOC under aspen was also more stable, indicated by well-developed mollic epipedon (A horizon 38-53-cm thick vs. 5.5-34 cm under conifers), slower turnover of surficial SOC deduced from long-term laboratory incubations (67.7 ± 15.7 g CO2-C per kg C for aspen vs. 130.9 ± 41.3 g CO2-C per kg C for conifer soils), and a greater preponderance of mineralassociated SOC (55±13% in aspen vs. 41 ±13% in conifer). Aspen soils were generally wetter and we hypothesize that rapid litter turnover coupled with greater water supply may have caused greater downward redistribution and adsorption of dissolved organic carbon (DOC) in aspen soils.

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

“…SOC content did not follow any pattern nor did it change abruptly at a critical LBA threshold across the aspen-conifer gradient, but it varied across sampling sites ( Figure 2 and Table 3). Our values for SOC content are comparable to those found by Woldeselassie et al [26], who similarly did not find significant differences in SOC content in the top 15 cm of mineral soil between aspen (49.5 ± 7.9 Mg C ha −1 ) and conifer stands (54.9 ± 20.3 Mg C ha −1 ) in montane forests from northern Utah. Although we did not find differences in SOC storage along the vegetation gradient, differences in distribution among labile and stable SOC fractions may be more relevant for C sequestration.…”

“…Pure conifer clusters were absent in one site at CM (CM1), while samples from one transect at another site (CM57) were excluded from the inventory, due to discrepancies in sampling and storage protocol. We did not describe soil pedons in our sites, but the characteristics of the topsoil were in agreement with those described in previous studies [26,37]. We generally observed a thin O horizon (i.e., superficial horizon dominated by organic matter, > 20% by weight of SOC) and a relatively deep A horizon (i.e., mineral topsoil with accumulation of organic matter, < 20% by weight of SOC) under aspen, whereas conifer soils had a thicker O horizon and a shallower and lighter A horizon.…”

“…C recovery was on average 98.0% ± 11.3%, with five samples somewhat outside this range. SOC decomposability (i.e., biologically available SOC) was determined with long-term (10 months) aerobic laboratory incubations of fresh soil samples (0-5 cm transect, 0-15 cm plot) following the protocol of Paul [45], as modified by Woldeselassie et al [26]. Cumulative CO 2 -C respired was expressed on a dry soil weight basis (mg CO 2 -C g soil −1 ) and normalized to C content (mg CO 2 -C g C −1 ) as an indicator of qualitative differences in SOC.…”

“…Aspen and conifer stands from semi-arid montane and subalpine forests differ considerably in soil microclimate [24][25][26][27], hydrology [24], litter quality [27,28], soil chemistry [27,29], soil microbial community structure [27] and SOC content and dynamics [25][26][27]. Woldeselassie et al [26] found that montane aspen stands in northern Utah had higher SOC stock (96.2 ± 26.7 Mg C ha −1 ) than adjacent conifer stands (66.9 ± 18.6 Mg C ha −1 ) in the top 60 cm of mineral soil.…”

“…Woldeselassie et al [26] found that montane aspen stands in northern Utah had higher SOC stock (96.2 ± 26.7 Mg C ha −1 ) than adjacent conifer stands (66.9 ± 18.6 Mg C ha −1 ) in the top 60 cm of mineral soil. SOC under aspen was also more persistent than SOC from conifer soils and had a higher proportion of mineral-associated SOC (55% ± 13% in aspen vs. 41% ± 13% in conifers) [25,26]. 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).…”