Soil organic matter (SOM) is related to vegetation, soil bacteria, and soil properties; however, not many studies link all these parameters simultaneously, particularly in tundra ecosystems vulnerable to climate change. Our aim was to describe the relationships between vegetation, bacteria, soil properties, and SOM composition in moist acidic tundra by integrating physical, chemical, and molecular methods. A total of 70 soil samples were collected at two different depths from 36 spots systematically arranged over an area of about 300 m x 50 m. Pyrolysis-gas chromatography/mass spectrometry and pyrosequencing of the 16S rRNA gene were used to identify the molecular compositions of the SOM and bacterial community, respectively. Vegetation and soil physicochemical properties were also measured. The sampling sites were grouped into three, based on their SOM compositions: Sphagnum moss-derived SOM, lipid-rich materials, and aromatic-rich materials. Our results show that SOM composition is spatially structured and linked to microtopography; however, the vegetation, soil properties, and bacterial community composition did not show overall spatial structuring. Simultaneously, soil properties and bacterial community composition were the main factors explaining SOM compositional variation, while vegetation had a residual effect. Verrucomicrobia and Acidobacteria were related to polysaccharides, and Chloroflexi was linked to aromatic compounds. These relationships were consistent across different hierarchical levels. Our results suggest that SOM composition at a local scale is closely linked with soil factors and the bacterial community. Comprehensive observation of ecosystem components is recommended to understand the in-situ function of bacteria and the fate of SOM in the moist acidic tundra. (C) 2021 Elsevier B.V. All rights reserved.

Black carbon (BC) aerosols in the atmosphere strongly affect direct radiative forcing and climate, not only while suspended in the atmosphere but also after deposition onto high albedo surfaces. Snow surfaces are especially sensitive to BC deposition, because of their high surface albedo and additional positive feedbacks further enhance faster snowpack melting caused by BC deposition, resulting in modifications in water resources and recession of glaciers. For the analysis of BC deposition on snow, a precise quantification of BC mass is needed. Instead, optical methods have the potential of quantifying only BC, based on its characteristic spectral absorption. Commercial optical transmissometers commonly use quartz filters to filter BC and measure its optical attenuation. They are calibrated for the determination of BC mass concentrations in air, but not adapted or calibrated for their determination in water or snowmelt samples. Additionally, they are generally calibrated using BC-simulating materials that are not representative of ambient BC particles. Here, a new analytical method is demonstrated for the quantitative determination of BC mass concentration in snow samples that considers filtering of melted snow with polycarbonate filters in a new device, and optical filter attenuation BC mass concentration measurement (880 nm). The attenuation can be obtained with any optical equipment that can measure the 880-nm attenuation of filters impacted with BC/snow impurities. This method has been calibrated using real diesel vehicle exhaust soot with well-known optical properties as reference material, yielding a multipoint calibration curve for common BC concentration levels in snow. The limits of detection (0.011 mg of BC), quantification (0.036 mg of BC) and reproducibility (96.39%) of this new analytical method have been determined. Real surface snow samples collected at different locations in Los Andes mountains of Chile were measured with this method given a BC concentrations ranged from 151 to 5987 mu g kg(-1). (C) 2019 Elsevier B.V. All rights reserved.

Vast amounts of soil organic matter (SOM) have been preserved in arctic soils over millennia time scales due to the limiting effects of cold and wet environments on decomposer activity. With the increase in high latitude warming due to climate change, the potential decomposability of this SOM needs to be assessed. In this study, we investigated the capability of mid infrared (MIR) spectroscopy to quickly predict soil carbon and nitrogen concentrations and carbon (C) mineralized during short-term incubations of tundra soils. Active layer and upper permafrost soils collected from four tundra sites on the North Slope of Alaska were incubated at 1, 4, 8 and 16 C for 60 days. All incubated soils were scanned to obtain the MIR spectra and analyzed for total organic carbon (TOC) and total nitrogen (TN) concentrations, and salt-extractable organic matter carbon (SEOM). Partial least square regression (PLSR) models, constructed using the MIR spectral data for all soils, were excellent predictors of soil TOC and TN concentrations and good predictors of mineralized C for these tundra soils. We explored whether we could improve the prediction of mineralized C by splitting the soils into the groups defined by the influential factors and thresholds identified in a principal components analysis: (1) TOC > 10%, (2) TOC 0.6%, (5) acidic tundra, and (6) non-acidic tundra. The best PLSR mineralization models were found for soils with TOC < 10% and TN < 0.6%. Analysis of the PLSR loadings and beta coefficients from these models indicated a small number of influential spectral bands. These bands were associated with clay content, phenolics, aliphatics, silicates, carboxylic acids, and amides. Our results suggest that MIR could serve as a useful tool for quickly and reasonably estimating the initial decomposability of tundra soils, particularly for mineral soils and the mixed organic-mineral horizons of cryoturbated soils.

Carbon (C) release from thawing permafrost is potentially the largest climate feedback from terrestrial ecosystems. However, the magnitude of this feedback remains highly uncertain, partly due to the limited understanding of how abrupt permafrost thaw (e.g. permafrost collapse) alters soil organic matter (SOM) quality. Here we employed elemental analysis, stable isotope analysis, biomarker and nuclear magnetic resonance techniques to explore changes in soil C concentration and stock as well as SOM quality following permafrost collapse on the Tibetan Plateau. Our results showed that permafrost collapse resulted in a 21% decrease in soil C concentration and a 32% reduction in C stock of the top 15 cm of soil over 16 years. Moreover, permafrost collapse led to a significant decline in SOM quality: the relative abundance of labile SOM fractions (e.g. carbohydrates) decreased, whereas recalcitrant SOM fractions (e.g. suberin-derived compounds) increased 16 years after collapse. By contrast, the relative abundances of labile and recalcitrant compounds showed no significant differences in the control plots along the thaw sequence. These results demonstrate that permafrost collapse and consequent changes in soil environmental conditions could trigger substantial C release on decadal timescales, implying that abrupt thaw maybe a dominant mechanism exposing soil C to mineralization.

Pleistocene yedoma sediments store large amounts of soil organic matter (SOM) and are vulnerable to permafrost degradation. Here we contribute to our understanding of yedoma SOM dynamics and potential response to thaw, by molecular characterization of samples from a 5.7 m yedoma exposure, as well as upper permafrost samples that were previously incubated, using Thermally assisted Hydrolysis and Methylation (THM-GC-MS). In general, the SOM is derived from aliphatic material (including cutin and suberin), phenols (lignin, sphagnum acid), polysaccharides and N-containing components (largely microbial SOM). Soil organic carbon (SOC) content and molecular SOM composition follow a sawtooth pattern where local maxima in SOC coincide with lignin and aliphatic material that experienced only slight degradation, and minima with degraded plant-derived SOM and microbial tissue, representing a stratified cryopedolith. The SOC-depleted top 0.9 m (active layer and transition zone) is enriched in microbial SOM probably due to recent thawing. Comparison with CO2 respiration rates indicates that SOM of microbial origin (low C/N) is more labile than aliphatic SOM from well-preserved plant tissue (high C/N). However, we argue that the more stable aliphatic SOM in SOC-rich layers might also be vulnerable to decay, which could, due to its abundance in SOC-rich layers, dominate overall Yedoma C losses due to thermal erosion.

Labile soil organic matter (SOM) plays a crucial role in nutrient and carbon cycling, particularly in permafrost ecosystems. Understanding its variation is therefore very important. In the present study, we evaluated the seasonal patterns of labile SOM from April 2013 to March 2014 under alpine swamp meadow (ASM), meadow (AM), steppe (AS) and desert (AD) vegetation in permafrost regions of the China's Qinghai-Tibet Plateau. The fractions (0 to 10 cm depth) included dissolved organic carbon (DOC), light-fraction carbon (LFC) and nitrogen (LFN), and microbial biomass carbon (MBC) and nitrogen (MBN). These fractions showed dramatic seasonal patterns in ASM and AM soils, but were relatively stable in AD soil. Soil DOC concentrations in the ASM, AM, and AD soils increased from April to May 2013, then increased again from July to August 2013 and from February to March 2014. The LFC and LFN concentrations in all four vegetation types were higher from June to August 2013. The highest MBC and MBN concentrations in the ASM, AM, and AS soils all occurred in the summer and the ASM soil showed a second peak in October or November 2013. Seasonal changes in climatic factors, vegetation types, and permafrost features were great causes of labile SOM variations in this study. Throughout the entire sampling period, the ASM soil generally had the highest labile SOM, followed by the AM, AS, and AD soils; thus, the ASM soil is the best system conserving soil nutrient (especially labile fractions) and microbial activity. Correlation analysis indicated that these fractions were not related to soil moisture and temperature in AS or AD soils, but soil temperature and moisture were significantly related to MBC and MBN in AM soil and DOC in ASM soil. Thus, the response of the labile SOM fractions in this high-altitude permafrost soils to climate change depended strongly on vegetation types. (C) 2015 Elsevier B.V. All rights reserved.

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 degrees N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO2 concentration stimulate vegetation productivity, taking up CO2. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 +/- 0.3 Pg C yr-1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 +/- 20 Pg C over 1901 levels, of which 17 +/- 8 Pg C comes from vegetation (43%) and 21 +/- 16 Pg C from the soil (8%). Both CO2 fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO2 fertilization effect, permafrost, and the role of soil moisture. Unlike the CO2 fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid-latitudes. These effects are further enhanced as a result of positive carbon cycle-climate feedbacks due to additional CO2 and warming.