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Water-extractable organic matter (WEOM) is the most dynamic and bioavailable fraction of the soil organic matter pool. Although the litter floor is considered the main source of WEOM, roots also release a great amount of labile organic compounds through rhizodeposition processes. This makes the rhizosphere, the small soil volume in proximity to the roots, a soil compartment relatively enriched in WEOM. Since both the rhizosphere and the labile organic C pool are highly sensitive to the environmental conditions we evaluated the characteristics of WEOM from rhizosphere and bulk soil collected from the A horizons of European beech ( Fagus sylvatica L.) forest soils of Apennines mountains (central Italy) at two altitudes (800 and 1000 m), using elevation as a proxy for temperature change. Specifically, we tested if i ) the rhizosphere contains higher amounts of WEOM with a greater diversity of compounds with respect to the bulk soil, and ii ) this effect is more pronounced at higher altitude. At both 800 m and 1000 m above sea level, the main distinction between WEOM from rhizosphere and bulk soil was the larger amounts of sugars in the soil close to the roots. Further, our results indicated an influence of altitude on rhizospheric processes as suggested by the larger concentrations of organic C and soluble phenols, and richness of tannins in the rhizosphere WEOM than in the bulk soil at 1000 m. We attributed this influence to environmental constraints which enhanced the release of labile organics and secondary metabolites by rhizodeposition and humification processes in the rhizosphere. As a whole, our data draw a picture where the roots are able to affect the characteristics of WEOM and environmental constraints enhance the differentiation between rhizosphere and bulk soil. This view confirms the influence of the rhizosphere on the soil C cycle, and the importance of the rhizospheric processes when environmental conditions become harsher. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an important nutrient for plant growth but its availability in soil is limited. Although plants are able to respond to the P shortage, climatic factors might modify the soil-plant-microorganisms system and reduce P availability. In this study we evaluated the rhizosphere effect of beech ( Fagus sylvatica L.) in forest soils of Apennines mountains (central Italy) at two altitudes (800 and 1000 m) and along 1° of latitudinal gradient, using latitude and altitude as proxies for temperature change. Specifically, we tested if 1) soil organic C, total N, and organic and available P decrease with increasing latitude and altitude, and 2) the rhizosphere effect on P availability becomes more pronounced when potential nutrient limitations are more severe, as it happens with increasing latitude and altitude. The results showed that the small latitudinal gradient has no effect on soil properties. Conversely, significant changes occurred between 800 and 1000 m above sea level, as the soils at higher altitude showed greater total organic C (TOC) content, organic and available P contents, and alkaline mono-phosphatases activity than the soils at lower altitude. Further, at the higher altitude, a marked rhizosphere effect was detected, as indicated by greater concentration of TOC, water extractable organic C, and available P, and its fulfillment was mainly attributed to the release of labile organics through rhizodeposition. The availability of easy degradable compounds in the rhizosphere should foster the mineralization of the organic matter with a consequent increase of available P. Hence, we speculate that at high altitude the energy supplied by the plants through rhizodeposition to the rhizosphere heterotrophic microbial community is key for fuelling the rhizospheric processes and, in particular, P cycling. [ABSTRACT FROM AUTHOR]

The accumulation of eolian dust plays an important role in the development of alpine soils on the northern Tibetan Plateau. However, little is known either about the nature of soil-forming processes in the region, or about the magnitude of soil organic and inorganic carbon (SOC and SIC) storage. Here we report the results of an investigation of a typical profile, consisting of a series of buried soils developed within eolian deposits, situated at an altitude of 4951 m in the Amdo zone of the northern Tibetan Plateau. Bulk density, pH, SOC and SIC content, grain-size distribution, magnetic susceptibility and mineralogical composition were measured at high resolution, and AMS 14 C dating was used to provide a chronology. Based on all of the analytical data we conclude that this alpine accretionary soil profile contains three buried soils beneath the present surface soil layer. The deepest and oldest soil is the basal paleo-weathering crust (Ferralsol) which underwent active soil formation from 5540 to 7615 yr. BP, or even earlier. The other soils, which are developed within the overlying eolian dust deposits, are Luvisols and they document the occurrence of two intervals of more humid conditions than the present day: from ~ 3455 to 5540 yr. cal BP and from ~ 2000–2500 yr. cal BP. Microscopic analysis of coarse particles from the profile reveals that high values of magnetic susceptibility at the base of the profile reflect the presence of coarse magnetite grains present as inclusions within quartz grains derived from the weathering of granitic gneiss. In addition, the magnetic susceptibility profile reflects the effects of winnowing of the eolian fraction by wind activity, and not the production of fine magnetic grains during pedogenesis. SOC is the dominant form of carbon in this alpine soil and the SOC density and average accumulation rate values within the entire interval of eolian deposits are 19.67 kg C m − 2 and 3.55 g C m − 2 yr. − 1 , respectively. These remarkably high values indicate that this alpine accretionary soil is characterized by highly efficient SOC burial, and therefore that such soils are an important terrestrial CO 2 sink. Likewise, the buried soil plays an important role in SOC storage. Finally, the characteristics of the buried soils indicate a long-term climatic trend towards aridity in the study region. This natural trend has promoted the processes of desertification and grassland degradation on the northern Tibetan Plateau, and these processes may have been exacerbated by human activity. [ABSTRACT FROM AUTHOR]

Climate change and a related increase in temperature, particularly in alpine areas, force both flora and fauna to adapt to the new conditions. These changes should in turn affect soil formation processes. The aim of this study was to identify possible consequences for soils in a dry-alpine region with respect to weathering of primary minerals and leaching of elements under expected vegetation and climate changes. To achieve this, a field empirical approach investigating an altitudinal sequence was used in combination with laboratory weathering experiments simulating several scenarios. The study sites are located in Sinks Canyon and Stough Basin of the Wind River Range, Wyoming, USA. The following sites (from moist to dry with increasing temperature along the sequence) were investigated: 10 soil profiles (Typic Haplocryoll) in a tundra ecotone, 10 soil profiles (Ustic Haplocryoll) in a pine-fir forest and 20 soil profiles (Ustic Argicryoll) in sagebrush. All soils developed on granitoid moraines. Soil mineralogy was analysed using cathodoluminescence and X-ray diffraction. This revealed that biotite and plagioclase were both weathered to smectite while plagioclase also weathered to kaolinite. Cooler, wetter, altitude-dependent conditions promoted weathering of primary minerals. Furthermore, the soils of the tundra and forest zone exhibited a higher acidity and more organic carbon. In a series of wet laboratory batch experiments, materials from topsoils (A horizons) and subsoils (B horizons) in each ecotone were examined alone or in combination with other samples. In a first step, aqueous extracts of the topsoil samples were generated in batch reactors and analysed for the main ions. In a second and a third step the topsoil extracts were reacted with the subsoil samples of the same ecotone, and with the subsoil samples of the ecotones at higher altitude. The total duration of these batch experiments was 1800 h, and the solutes were measured using ICP-OES and ion chromatography. Dissolved Ca, Mg and K were mainly controlled by the chemical weathering of oligoclase, K-feldspar and biotite. With increasing altitude the total concentrations of Ca, Mg and K in the aqueous extracts decreased, the relative ionic contribution from K decreased, while the ionic contribution from Ca increased. Climate change (warming, changed precipitation) potentially will reduce weathering intensity, soil acidity and the content of organic carbon. An altitudinal shift in vegetation due to climate change seems to affect the ionic composition of the soil solution. In the case of a shift from forest to sagebrush and tundra to forest or sagebrush, the relative contribution from K would increase at the expense of Ca. We hypothesise that K will play an important role in future biogeochemical cycles under the assumptions of climate warming and subsequent vegetation shifts to higher altitudes. [ABSTRACT FROM AUTHOR]

Abstract: Forest soils are an important component of the global C cycle as they store large amounts of organic carbon (OC). Particularly in mountain forest ecosystems, soil organic matter is of crucial importance for site productivity and ecosystem services, but probably sensitive to climate change. Robust information about the OC stocks of mountain soils is rare due to their limited accessibility and large spatial heterogeneity. Our study covered the entire German Alps in a large-scale sampling campaign in 2011 and 2012, and provides soil organic carbon (SOC) stock data obtained from 150 forest soil profiles with different site conditions (elevation, aspect, air temperature, precipitation, parent material, soil type) and different intensities of historical forest utilization. The mean SOC stock of the investigated soils is 10.9kgm−2. The median value is 9.6kgm−2, indicating a skewed distribution of SOC stocks in forest soils of the German Alps. On average, 30% of the SOC stock is bound in the organic surface (O) layer, and 70% in the mineral soil. SOC stocks show a considerable dependency on site conditions (elevation, air temperature, precipitation, parent material). Soils in the German Limestone Alps show a significant OC stock gradient from W (Werdenfels) to E (Berchtesgaden region), which probably has been caused by more intense historical forest utilization in the latter compared to the former region. Soils at high-elevation sites with low air temperature and high precipitation have particularly large OC stocks. However, the elevation and climate effect is statistically significant only for precipitation due to the large variation of other factors with relevance for SOC stocks (e.g. parent material, soil type) in a given elevation/climate stratum. Histosols on consolidated calcareous bedrock and Histic Rendzic Leptosols have significantly larger SOC stocks than Rendzic Leptosols, Rendzic Cambisols, or soils on easily-weatherable parent material (marl, clayey sandstone, moraine). The fact that SOC stocks in forest soils of the German Alps are by trend larger at high-elevation sites with low air temperature and high precipitation suggests a sensitivity to the ongoing climate change and a risk of SOC losses for the predicted climate scenarios. [Copyright &y& Elsevier]

• A modified CENTURY model examines changes in soil organic carbon in Northern Tibet. • Climate change retards soil organic carbon sequestration in the permafrost region. • Acceleration in freeze-thaw circle enlarges decreasing trend in soil organic carbon. Climate warming can enhance the decomposition of soil organic matter (SOM), thereby increasing the rate of carbon release from soils. In permafrost regions, climate warming alters the nature of freeze–thaw cycles by affecting sub-surface hydrology and soil temperature, thereby impacting the decomposition of SOM. However, this process is rarely considered in projections of the long-term dynamics of soil organic carbon (SOC) on the Tibetan Plateau (TP) in response to climate change. Here, we employ the CENTURY-FTC model, which implements a freeze–thaw module in the CENTURY model, to simulate the response of top soil organic carbon density (SOCD) at 0–20 cm depth to climate change in the permafrost region of Northern Tibet. Our findings suggest that (i) the average SOCD was 2.123 kg C m−2 in 2015; (ii) the SOCD decreased at an average rate of 0.7 × 10−3 kg C m−2 year−1 for the period 1961–2015; and (iii) SOCD decreases spatially from south to north across Northern Tibet. Under various climate scenarios, the SOCD is projected to decrease significantly throughout Northern Tibet from 2016 to 2050, with the decline being most pronounced under the representative concentration-pathway (RCP) 8.5 scenario, which projects an increase in global mean surface temperature of 4.5 °C by 2100. Superimposed on this pattern, regional differences might be driven by vegetation type, with the largest decrements occurring in southern, alpine-meadow-dominated areas and the smallest in the northern alpine desert. We propose that this declining trend will be enhanced as climate warming continues and might be amplified by the increase in freeze–thaw processes. The addition of a freeze–thaw module to the CENTURY model changes projections of SOCD change due to climate. Whereas the freeze–thaw cycle has had little impact during the baseline period, its influence is likely to increase as climate warming continues. By 2050, freeze–thaw processes are projected to contribute 3% to SOCD decline under the RCP2.6 scenario, which projects a rise in global mean surface temperature of 1.5 °C by 2100, and as much as 10% under the RCP8.5 scenario. In general, future warming is likely to result in declining SOCD throughout Northern Tibet and a reduction in the capacity of alpine soils to sequester carbon. [ABSTRACT FROM AUTHOR]

• Enzyme activity and stoichiometry were studied along a climate and a vegetation type gradient in subalpine forests. • Climate has little impact on the enzyme activity and stoichiometry. • Vegetation type regulated enzyme activities and stoichiometry by affecting plant-associated microbial communities. Soil extracellular enzymes catalyze the rate limiting steps of organic matter decomposition, and enzyme stoichiometry has been used to reflect microbial resource acquisition strategies. However, the patterns and key driving factors of soil enzyme activities and stoichiometry in subalpine forests, which are areas sensitive to global changes, remain unclear. In this study, rhizosphere and bulk soils along two environmental gradients in the subalpine forests of Gongga Mountain, in the eastern Tibetan Plateau, including (1) a horizontal chronosequence with different vegetation types but a similar climate and (2) a vertical elevation gradient with the same vegetation type but different climates, were sampled during the growing and nongrowing seasons. The activities and stoichiometry of soil enzymes related to the microbial acquisition of carbon (C) (β-1,4-glucosidase, BG), nitrogen (N) (β-1,4-N-acetylglucosaminidase, NAG) and phosphorus (P) (acid phosphomonoesterase, AP) were analyzed to reveal their responses to environmental gradients. The results showed that enzyme activities and stoichiometry varied significantly among vegetation types, but were less affected by climate and root proximity. BG activity and ratios of BG to nutrient-acquiring enzymes were significantly higher in broadleaf forests than in coniferous forests. The differences in enzyme stoichiometry between vegetation types were weakly related to the microbial nutrient status (represented by the ratio of soil to microbial element stoichiometry) but might be explained by the higher proportion of ectomycorrhizal fungi in coniferous forests compared to broadleaf forests. The results of this study indicate that vegetation type was a major factor regulating soil enzyme activities and stoichiometry in the subalpine forests, possibly via its influences on plant-associated microbial communities. [ABSTRACT FROM AUTHOR]