Soil organic matter (SOM) is involved in many important soil processes such as carbon sequestration and the solubility of plant nutrients and metals. Ultrahigh resolution mass spectrometry was used to determine the influence of forest vegetation type and soil depth on the molecular composition of the water-extractable organic matter (WEOM) fraction. Contrasting the upper 0-5 cm with the 25-50 cm B horizon depth increment, the relative abundance of lipids and carbohydrates significantly increased, whereas condensed aromatics and tannins significantly decreased for the deciduous stand WEOM. No significant abundance changes were found for the coniferous stand DOM. Kendrick mass defect analysis showed that the WEOM of the 25-50 cm B horizon was depleted in oxygen-rich and higher mass components as compared to the 0-5 cm B horizon WEOM, suggesting that higher mass WEOM components with oxygen-containing functionality show greater reactivity in abiotic and/or biotic reactions. Furthermore, using an inoculated 14-day laboratory incubation study and multivariate ordination methods, we identified the WEOM components with H:C > 1.2 and O:C > 0.5 as being correlated most strongly with biodegradability. Our findings highlight the importance of understanding soil depth differences for various forest types in the chemical composition of SOM and the processes governing SOM production and transformations to fully understand the ecological implications of changes in forest composition and function in a changing climate.

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.

A meta‐analysis using 77 studies from 28 countries was performed to assess the effect of hardwood vs. conifer overstory on soil organic C (SOC) storage in forest floor (FF), mineral soil, and whole soil (FF + mineral soil). Overall, FF stocks were 38% higher under conifers, mineral SOC stocks were similar, and whole soil SOC was 14% higher under conifers. An analysis with six of the seven most reported tree genera reaffirmed higher FF and whole soil C stocks under conifer stands. Analysis with all seven of the genera showed more pronounced variability in mineral SOC results compared with the overall results. Eucalyptus was the only hardwood that stored significantly (17%) more SOC in the mineral soil than adjacent conifers. Picea was the only conifer that stored significantly (7%) more SOC in the mineral soil than hardwoods. Differences in FF SOC stocks had a limited predictive power in explaining the variability of mineral SOC stock differences, suggesting that they are not very closely linked with regard to SOC storage. Only when comparing FF SOC stocks among genera did precipitation, age difference, soil texture, and previous land use moderate SOC storage differences between conifers and hardwoods. In other cases, neither climate nor soil variables could explain differences between SOC stocks. Our findings suggest that using plant‐trait‐driven vegetation categories may be a more descriptive way of detecting vegetation effects on SOC.

An important issue, in times of climate change and more extreme weather events, is the investigation of forest ecosystem reactions to these events. Longer drought periods stress the vitality of trees and promote mass insect outbreaks, which strongly affect ecosystem processes and services. Cavity-enhanced Raman gas spectrometry was applied for online multi-gas analysis of the gas exchange rates of O2 and CO2 and the labeling of Fagus sylvatica L. (European beech) seedlings with (13)CO2. The rapid monitoring of all these gases simultaneously allowed for the separation of photosynthetic uptake of CO2 by the beech seedlings and a constant (12)CO2 efflux via respiration and thus for a correction of the measured (12)CO2 concentrations in course of the labeling experiment. The effects of aphid infestation with the woolly beech aphid (Phyllaphis fagi L.) as well as the effect of a drought period on the respirational gas exchange were investigated. A slightly decreased respirational activity of drought-stressed seedlings in comparison to normally watered seedlings was found already for a low drought intensity. Cavity-enhanced Raman gas monitoring of O2, (12)CO2, and (13)CO2 was proven to be a powerful new tool for studying the effect of drought stress and aphid infestation on the respirational activity of European beech seedlings as an example of important forest species in Central Europe.