Forests constitute important ecosystems in the global carbon cycle. However, how trees and environmental conditions interact to determine the amount of organic carbon stored in forest soils is a hotly debated subject. In particular, how tree species influence soil organic carbon (SOC) remains unclear. Based on a global compilation of data, we show that functional traits of trees and forest standing biomass explain half of the local variability in forest SOC. The effects of functional traits on SOC depended on the climatic and soil conditions with the strongest effect observed under boreal climate and on acidic, poor, coarse-textured soils. Mixing tree species in forests also favours the storage of SOC, provided that a biomass over-yielding occurs in mixed forests. We propose that the forest carbon sink can be optimised by (i) increasing standing biomass, (ii) increasing forest species richness, and (iii) choosing forest composition based on tree functional traits according to the local conditions.

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.

Abstract. The analysis of soil phosphorus (P) in fractions of different plant availability is a common approach to characterize the P status of forest soils. However, quantification of organic and inorganic P fractions in different extracts is labor intensive and therefore rarely applied for large sample numbers. Therefore, we examined whether different P fractions can be predicted using near-infrared spectroscopy (NIRS). We used the Hedley sequential extraction method (modified by Tiessen and Moir, 2008) with increasingly strong extractants to determine P in fractions of different plant availability and measured near-infrared (NIR) spectra for soil samples from sites of the German forest soil inventory and from a nature reserve in southeastern China. The R2 of NIRS calibrations to predict P in individual Hedley fractions ranged between 0.08 and 0.85. When these fractions were combined into labile, moderately labile and stable P pools, R2 of calibration models was between 0.38 and 0.88 (all significant). Model prediction quality was higher for organic than for inorganic P fractions and increased with the homogeneity of soil properties in soil sample sets. Useable models were obtained for samples originating from one soil type in subtropical China, whereas prediction models for sample sets from a range of soil types in Germany were only moderately useable or not useable. Our results indicate that prediction of Hedley P fractions with NIRS can be a promising approach to replace conventional analysis, if models are developed for sets of soil samples with similar physical and chemical properties, e.g., from the same soil type or study site.

Climate-and management-induced changes in tree species distributions are raising questions regarding tree species-specific effects on soil organic carbon (SOC) storage and stability. Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, but fire exclusion often promotes the succession to conifer dominated forests. Aspen in the Western US have been found to store more SOC in the mineral soil than nearby conifers, but we do not yet fully understand the source of this differential SOC accumulation. We measured total SOC storage (0-50 cm), characterized stable and labile SOC pools, and quantified above-and belowground litter inputs and dissolved organic carbon (DOC) fluxes during snowmelt in plots located in N and S Utah, to elucidate the role of foliage vs. root detritus in SOC storage and stabilization in both ecosystems. While leaf litterfall was twice as high under aspen as under conifers, input of litter-derived DOC with snowmelt water was consistently higher under conifers. Fine root (<2 mm) biomass, estimated root detritus input, and root-derived DOC fluxes were also higher under conifers. A strong positive relationship between root and light fraction C content suggests that root detritus mostly fueled the labile fraction of SOC. Overall, neither differences in above-and belowground detritus C inputs nor in detritus-derived DOC fluxes could explain the higher and more stable SOC pools under aspen. We hypothesize that root-microbe-soil interactions in the rhizosphere are more likely to drive these SOC pool differences.

Trembling aspen covers a large geographic range in North America, and previous studies reported that a better understanding of its singular influence on soil properties and processes is of high relevance for global change questions. Here we investigate the potential impact of a shift in aspen abundance on soil carbon sequestration and soil carbon stability at the continental scale by conducting a systematic literature review using 23 published studies. Our review shows that aspen's effect on soil carbon is relatively consistent throughout the species range. Aspen stores less C in the forest floor but similar amounts in the mineral soil relative to conifers. However, a robust set of indicators of soil C stability, for example, degree of organo-mineral associations, proportion of readily-available or labile C estimated during long-term soil incubations or using hot-water extraction, pattern of soil C distribution, and temperature sensitivity of soil heterotrophic respiration, reveals that the soil organic carbon (SOC) stock under aspen is more stable, rendering it more protected against environmental changes and soil disturbances. Therefore, our continental-scale analysis highlights that an increase in the abundance of trembling aspen in North American forests may increase the resistance and resilience of soil C stocks against global changes.