Largest Organic Carbon Pool Research Articles

Reduced CO2 Release from Arctic Soils Due to CO2 Binding to Calcium Forming Aragonite.

Arctic soils are the largest pool of organic carbon compared with other soils globally and serve as a main source for greenhouse gases, especially in the course of the predicted future temperature increase. With increasing temperatures, substantial thawing of the permafrost layer of soils is expected, altering the availability of calcium in those soils, with an increase by ∼5 mg Ca g-1 DW predicted for Alaska. Here we show for two representative soils in Alaska (initially Ca-poor or Ca-rich) that this increase in Ca availability will lead to decreases in CO2 release by 50% and 57%. It is already well-known that the cation bridging of Ca ions to organic carbon renders this carbon unavailable for microbial respiration and that Ca is altering the transformation of Corg by microbes. Here we show that the decrease of the soil CO2 release may be also due to enhanced aragonite formation (by 300% for Ca-poor and 90-200% for Ca-rich soils), as revealed by synchrotron-based scanning transmission X-ray microscopy. We therefore call upon field experiments for validation of this process and inclusion of this process in global and local carbon budget models.

Dynamics of chromophoric dissolved organic matter in a highly productive Amundsen Sea polynya

Dissolved organic carbon (DOC) is the largest organic carbon pool in the ocean, and is the most active component in respect to the ocean carbon cycling. However, its study in Antarctica has been limited due to challenges associated with sample collection. In this study, we conduct an investigation on the chromophoric dissolved organic matter (CDOM) in a highly productive Amundsen Sea Polynya (ASP), where phytoplankton blooms occur annually during the austral summer, serving as the primary source of DOC and CDOM. The relative abundances of CDOM, as indicated by the absorption coefficient at 254 nm (a254), exhibit significant variability, reaching up to 6.34 m−1. Four fluorescent components, including two humic-like components (C1 and C4) and two protein-like components (C2 and C3), are identified by excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Our findings suggest that heterotrophic metabolism primarily contributes to the formation of humic-like fluorescent components and DOC removal. Water mass, solar radiation, primary productivity as well as microbial degradation are identified as the main factors influencing CDOM dynamics in ASP. This study bears significant implications for advancing our understandings of the CDOM and DOC dynamics in the coastal polynyas of Antarctica, thus facilitating improved evaluation of carbon cycle in the Antarctica.

Soil carbon stocks can be negatively affected by land use and climate change in natural ecosystems of semi-arid environment of Iran

Soil is the largest organic carbon pool in the terrestrial biosphere, and a minor variation in the soil organic carbon (SOC) stock can substantially affect the global carbon cycle and climate. This study examined SOC storage considering land use and land use change across a semi-arid environment in northern Iran with a double aim: studying the effects of available soil parameters using a regression based modeling, and simulating the effect of the temporal variations of SOC under future climate change projections. In this regard, the following land covers were considered: forest cover, rangeland cover, conversion of forest to cropland, and conversion of rangeland to cropland. A stepwise regression analysis method was used to derive the relevant models of soil properties at of 0–10 and 10–20 cm depths. The results of regression modeling indicate that fungal abundance (for 0–10 cm soil depth) and the stoichiometry of microbial biomass nitrogen to total nitrogen (for 10–20 cm soil depth) are the most important factors for predicting SOC under the studied land covers. The effect of climate change on SOC stock differed among land uses. Simulated SOC stock decrease was in the range 5.14–11.95 Mg C ha−1 in forest, 4.48–10.37 Mg C ha−1 in rangeland, 9.34–15.38 Mg C ha−1 in cropland converted from forest and 8.00–13.72 Mg C ha−1 in cropland converted from rangeland. Therefore, conversion from natural ecosystems to cropland requires targeted intervention for a proper management of soils through a better residue management, the inclusion of a cover crop to be used as green manure in rotation with wheat, minimum tillage intensity and addition of both mineral and organic fertilizers.

Incorporating permafrost into climate mitigation and adaptation policy

Permafrost thaw is drastically altering Arctic lands and creating hazardous conditions for its residents, who are being forced to make difficult and urgent decisions about where and how to live to protect themselves and their lifeways from the impacts of climate change. Permafrost thaw also poses a risk to global climate due to the large pool of organic carbon in permafrost, which, when thawed, can release greenhouse gasses to the atmosphere, exacerbating an already rapidly warming climate. Permafrost thaw has significant implications for adaptation and mitigation policy worldwide. However, it remains almost entirely excluded from policy dialogues at the regional, national, and international levels. Here we discuss current gaps and recommendations for increasing the integration of permafrost science into policy, focusing on three core components: reducing scientific uncertainty; targeting scientific outputs to address climate policy needs; and co-developing just and equitable climate adaptation plans to respond to the hazards of permafrost thaw.

Grazing exclusion alters carbon flux of alpine meadow in the Tibetan Plateau

• Carbon flux in grazing and enclosure grasslands is highly inconsistent. • Fencing in alpine grassland is a promising measure to carbon sequestration. • Compared to FG, Q 10 of respiration raised in GE5 but declined in GE9. • C, N, and P content of plant and soil provides a new insight for carbon flux. Grasslands contain a large global pool of soil organic carbon (C), but the carbon flux in response to grazing exclusion and the affecting factors are still under debate. In this study, a manipulative experiment was conducted in alpine meadow of the eastern Tibetan Plateau. The sites were: 9 years grazing exclusion, GE9; 5 years grazing exclusion, GE5; and free grazing, FG in degraded alpine meadow. Results showed that there had been a significant increase in soil respiration (SR) and net ecosystem exchange (NEE) with fencing duration. However, no significant difference of ecosystem respiration (ER) was found between grazing and fencing sites. Though, carbon sink occurred in all three sites during the growing season, only the site for 9 years fencing increased gross ecosystem productivity (GEP). Remarkably, temperature sensitivity (Q 10 ) of both soil and ecosystem respiration raised in GE5 but declined in GE9. These findings indicated that grazing exclusion could be a promising measure to CO 2 sequestration in alpine meadow. Of the governing factors, exponential relationships among SR, ER and soil temperature and significant relationships among NEE, GEP and plant biomass were found. Besides, our results showed that plant nitrogen (N), phosphorus (P) content exhibited significant effects on SR, NEE, respectively. While soil C and N were significant explanatory factors for ER. These results indicated that C, N, and P content of plant and soil could provide a new insight for understanding carbon flux in alpine meadow. Together, the findings are not only vital to improve understanding of C cycle in grasslands, but also to provide scientific data for assessing the ecological effect and carbon budget of grazing exclusion.

Highly Productive Tropical Seagrass Beds Support Diverse Consumers and a Large Organic Carbon Pool in the Sediments

Tropical seagrass beds are productive coastal ecosystems that are important blue carbon sinks and crucial habitats and feeding grounds for consumers at high trophic levels. To understand how energy sustains the ecosystem from seagrass production, we constructed an Ecopath trophic model to reveal the possible pathways of energy flow in the tropical seagrass beds around Dongsha Island, South China Sea. The model indicates that Dongsha seagrass beds were developing but well-structured ecosystems. The productive seagrasses were rarely directly consumed by herbivores and, ultimately, flowed into detrital pools. Detritus was the main food source used to support diverse consumers in the food web. Nevertheless, the low cycling rate (2.74%) suggests that most detritus was not reused or exported and was stored as a large organic carbon pool in the sediments. The detritus-feeding invertebrates are keystone groups in the Dongsha seagrass beds, as they recycle energy from detritus and transfer to top predators such as sharks. The predation of top predators affected the biomasses of other compartments, leading to strong top-down control via a trophic cascade effect.

When Forests Take Over After Land Abandonment: Dissolved Organic Matter Response in Headwater Mountain Streams

Dissolved organic matter (DOM) represents the largest pool of organic carbon in fluvial ecosystems. The majority of DOM in rivers is of terrigenous origin—making DOM composition highly dependent on vegetation cover and soil properties. While deforestation is still a worldwide anthropogenic phenomenon, current land cover change in temperate regions is often characterized by secondary succession processes following the abandonment of agricultural activities including grazing on pasturelands. This results in (secondary) forest expansion with a consequent, time-lagged transformation of soil properties. Predicting the time scale and spatial scale (i.e., location in the catchment: riparian vs. upslope areas) at which such land cover changes affect the terrestrial-aquatic carbon linkage and concomitantly alter properties of fluvial DOM as drivers of carbon cycling in freshwater ecosystems represents a new scientific challenge. In an attempt to identify potential legacy effects of land cover, i.e., reaction delays of fluvial DOM to changes in land cover, we here investigate the influence of specific current and historic (2 decade-old) land cover types on molecularly resolved fluvial DOM composition in headwater mountain streams. Our analysis is based on a scale-sensitive approach weighing in the distance of land cover (changes) to the stream and ultrahigh-resolution mass spectrometric analyses. Results identified the dominance of terrigenous DOM, with phenolic and polyphenolic sum formulae commonly associated to lignins and tannins, in all the studied streams. DOM properties mostly reflected present-day gradients of forest cover in the riparian area. In more forested catchments, DOM had on average higher molecular weight and a greater abundance of O-rich phenols and polyphenols but less aliphatics. Besides the modulation of the DOM source, our results also point to an important influence of photodegradation associated to variation in light exposition with riparian land cover in defining fluvial DOM properties. Despite expectations, we were unable to detect an effect of historic land cover on present-day DOM composition, at least at the investigated baseflow conditions, probably because of an overriding effect of current riparian vegetation.

Responses of soil organic carbon to climate change in the Qilian Mountains and its future projection

Soil is the largest organic carbon pool in the terrestrial biosphere, and a small variation of soil organic carbon (SOC) can substantially affect the global carbon cycle and climate. Climate change is a major driver affecting the dynamics of SOC; however, our understanding about the responses of SOC in alpine ecosystems to climate change is quite limited. In particular, the differences of SOC dynamics at different depths were rarely reported. In this study, we investigated the impacts of precipitation variations and warming on SOC dynamics at both top and deep soils in the Qilian Mountains in Northwestern China using a machine learning approach and climate sensitivity experiments. Our simulation revealed the temporal inconsistency between topsoil SOC (in the top 20 cm, denoted as SOC20) and deeper soil SOC dynamics—SOC20 showed a decreasing trend since 2009 which is earlier than that (2012) in the top 100 cm soil (SOC100). We also found that SOC100 may be more sensitive to warming due to the strengthened microbial decomposition rate and additional carbon source through deepened active layer. On the contrary, SOC20 presented more intense responses to precipitation than SOC100, which was mainly attributed to the different responses of upland and lowland SOC to precipitation variations. Our projection indicated that SOC20 may not substantially change under future climate trajectories because the enriched SOC induced by increased precipitation may offset the carbon loss via warming. However, SOC100 was projected to decrease significantly due to the enhanced carbon emissions via warming induced strengthened decomposition rate, additional carbon source from the deepened active layer, and exposed soil carbon to the atmosphere caused by ground subsidence and disrupted soil horizons resulting from thawed frozen soil. In brief, this study deepened our understanding of the mechanism of climate effect on SOC dynamics and can be helpful for regional soil ecological security assessment and risk projection.

帽儿山3种森林生态系统土壤动物与土壤呼吸及其相互关系研究

PDF HTML阅读 XML下载 导出引用 引用提醒 帽儿山3种森林生态系统土壤动物与土壤呼吸及其相互关系研究 DOI: 10.5846/stxb201911252555 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31670619，41101048）；黑龙江省留学归国人员科学基金（LC2018011）；哈尔滨师范大学硕士研究生创新项目（HSDSSCX2019-01） Analysis on soil animals, soil respiration and the correlation in three forest ecosystems in Maoershan Author: Affiliation: Fund Project: Project of National Natural Science Foundation of China (31670619,41101048); Science Foundation of returned students of Heilongjiang Province (LC2018011); Innovation Project of postgraduates of Harbin normal University (HSDSSCX2019-01) 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤呼吸是森林生态系统碳循环的关键过程，土壤动物可通过自身代谢及影响微生物活动调控土壤呼吸，因此研究土壤动物与土壤呼吸的相互关系对进一步揭示生态系统碳循环的规律和机理具有重要意义。通过野外定点，以帽儿山3种森林生态系统的土壤呼吸及土壤动物为研究对象，探讨不同森林生态系统的土壤呼吸、土壤动物个体密度和生物量的时间变化规律及二者相互关系。结果表明：（1）3种森林生态系统土壤总呼吸速率与土壤异养呼吸速率均呈现先增强后减弱的时间动态变化（P<0.05），且不同森林生态系统土壤异养呼吸速率差异显著（P<0.05），表现为硬阔叶林最高，红松人工林最低；（2）3种森林生态系统土壤动物生物量也具有显著的时间动态变化（P<0.05），均在9月份达到最大，且不同森林生态系统土壤动物个体密度显著不同（P<0.05），蒙古栎林土壤动物个体密度显著小于红松人工林与硬阔叶林；（3）通过回归分析可得，土壤动物数量及生物量的增加抑制了土壤呼吸速率，尤其在生长季初期、末期。研究表明土壤动物可通过抑制微生物生命活动和降低根系呼吸从而对土壤总呼吸及异养呼吸产生负反馈作用，三者是不可分割的整体，与土壤温度、水分等环境因子共同调控着土壤呼吸。 Abstract:Forest ecosystem is the largest organic carbon pool in terrestrial ecosystem, and its slight changes may cause significant variations in global climate. Soil respiration is the key ecological process of carbon cycle in forest ecosystem, which has a great influence on global climate change. Soil animals, as the important consumers of material cycle in the ecosystem, can regulate soil respiration the help of their own metabolism and affecting microbial activities. Therefore, study of the correlation of soil animals and soil respiration is of great significance to further reveal the law and mechanism of carbon cycle in ecosystem. Based on field experiment and indoor analysis, we chose soil respiration and soil animals in three forest ecosystems of Korean pine plantation forest (HS), Mongolian oak forest (MGL), and Hard-wood forest (YK) in Maoershan as the research object, and discuss the temporal variation and correlation between soil respiration as well as individual density or biomass of soil animals in different forest ecosystems. The results showed that: (1) the total soil respiration rate of the three forest ecosystems increased at first and then decreased (P<0.05), with the lowest value appearing in October. The soil heterotrophic respiration rate also showed significantly temporal dynamic change (P<0.05). The difference of soil heterotrophic respiration rate among different forest ecosystems was significant (P<0.05), which was the highest in Hard-wood forest and the lowest in Korean pine plantation forest. (2) The soil animal biomass and individual density of the three forest ecosystems also had notable temporal dynamic changes (P<0.05). The change trends of soil animal biomass and individual density were different, but they all had the maximum values in September. The individual density of soil animals in Mongolian oak forest was much lower than that in Korean pine plantation forest and Hard-wood forest. (3) Through regression analysis, the increase of the number and biomass of soil animals inhibited the soil respiration rate, especially at the beginning and end of the growing season. And the correlation between soil respiration and soil animals in Korean pine plantation forest was different from that in Mongolian oak forest and Hard-wood forest. It could be concluded that soil animals could produce negative feedback on total soil respiration and heterotrophic respiration by inhibiting microbial life activities and reducing root respiration. Soil animals, soil microorganisms and plant roots are an inseparable whole, which together with soil temperature, water and other environmental factors regulate soil respiration. 参考文献 相似文献 引证文献

Soil organic carbon potential of Bozhurishte municipality: A case study from western Bulgaria

The current study focuses on the soil organic carbon contents in topsoil (0-30 cm) in Bozhurishte Municipality. The municipality is a part of Sofia Province and it represents a major section of the “Sofia-Bozhurishte Economic Zone”. The aim of the investigation is to reveal how soil organic carbon storages may change in the forthcoming 20 years during a medium and a high scenario. The results of the study display several major zones of a large soil organic carbon pool, located to the northeast and the central parts of the municipality. The section, containing the largest soil organic carbon pool, may experience an increase of 376 400 tons/ha for 20 years in the medium scenario and 499 100 tons/ha in the high scenario. The author concludes that promising results may be used as a basis for an expansion of the study in the neighboring municipalities.

Stocks of organic carbon in German agricultural soils—Key results of the first comprehensive inventory

AbstractBackground: There is considerable uncertainty about the actual size of the global soil organic carbon (SOC) pool and its spatial distribution due to insufficient and heterogeneous data coverage.Aims: We aimed to assess the size of the German agricultural SOC stock and develop a stratification approach that could be used in national greenhouse gas reporting.Methods: Soils from a total of 3104 sites, comprising 2234 croplands, 820 permanent grasslands and 50 sites with permanent crops (vineyards, orchards) were sampled in a grid of 8 × 8 km to a depth of 100 cm in fixed depth increments. In addition, a decade of management data was recorded in a questionnaire completed by farmers. Two different approaches were used to stratify cropland and grassland mineral soils and derive homogeneous groups: stratification via soil type (pedogenesis) and via SOC‐relevant soil properties.Results: A total of 146 soils were identified as organic soils, which stored by far the highest average SOC stock of 528 ± 201 Mg ha−1 in 0–100 cm depth. Of the mineral soils, croplands and permanent crops stored on average 61 ± 25 and 62 ± 25 Mg ha−1 in 0–30 cm (topsoil) and 35 ± 30 and 44 ± 28 Mg ha−1 in 30–100 cm (subsoil), while permanent grasslands stored significantly more SOC (88 ± 32 and 47 ± 50 Mg ha−1 in topsoil and subsoil). Overall, topsoils stored 67 ± 14% and subsoils 33 ± 14% of total SOC stocks. Soil C:N ratio, clay content and groundwater level were major factors that explained the spatial variability of SOC stocks in mineral soils. Accordingly, Podzols, Gleysols and Vertisols were found to have the highest SOC stocks.Conclusions: Stratification via soil properties yielded the most comparable cropland and grassland strata and is thus preferable for estimating land‐use change effects, e.g., for greenhouse gas inventories.In total, 2.5 Pg C are stored in the upper 100 cm of German agricultural soils, making them the largest organic carbon pool in terrestrial ecosystems of Germany. This bares a large responsibility for the agricultural sector and society as a whole to maintain and, if possible, enhance this pool.

Effects of Drying-Rewetting Frequency on Vertical and Lateral Loss of Soil Organic Carbon in a Tidal Salt Marsh

Tidal salt marshes, as “blue carbon” ecosystems, play a critical role in mitigation of global climate change since their large soil organic carbon (SOC) pool. Drying-rewetting cycles induced by periodic tides have profound influence on soil carbon cycling in tidal salt marshes. However, the magnitude and mechaanism of the effects of drying-rewetting frequency on SOC loss in tidal salt marshes is still uncertain. Here, we conducted a mesocosm experiment to identify how drying-rewetting frequency changes alter the vertical (CO2 and CH4) and lateral (dissolved organic carbon) carbon losses of soils in a tidal salt marsh in the Yellow River Delta (YRD). We found that increasing soil moisture inhibited CO2 emission but stimulated CH4 emission in a tidal salt marsh. Soil dissolved organic carbon (DOC) was produced in the drying phase and rewetting lead to the loss of DOC. Soil moisture and salinity change induced by drying-rewetting cycles were the critical factors controlling vertical organic carbon loss in a tidal salt marsh. DOC had significant effects on CO2 emissions. Changes of tidal action and drying-rewetting cycle induced by global change can affect the pathway of carbon loss in a tidal salt marsh.

Better estimates of soil carbon from geographical data: a revised global approach

Soils hold the largest pool of organic carbon (C) on Earth; yet, soil organic carbon (SOC) reservoirs are not well represented in climate change mitigation strategies because our database for ecosystems where human impacts are minimal is still fragmentary. Here, we provide a tool for generating a global baseline of SOC stocks. We used partial least square (PLS) regression and available geographic datasets that describe SOC, climate, organisms, relief, parent material and time. The accuracy of the model was determined by the root mean square deviation (RMSD) of predicted SOC against 100 independent measurements. The best predictors were related to primary productivity, climate, topography, biome classification, and soil type. The largest C stocks for the top 1 m were found in boreal forests (254 ± 14.3 t ha−1) and tundra (310 ± 15.3 t ha−1). Deserts had the lowest C stocks (53.2 ± 6.3 t ha−1) and statistically similar C stocks were found for temperate and Mediterranean forests (142 - 221 t ha−1), tropical and subtropical forests (94 - 143 t ha−1) and grasslands (99-104 t ha−1). Solar radiation, evapotranspiration, and annual mean temperature were negatively correlated with SOC, whereas soil water content was positively correlated with SOC. Our model explained 49% of SOC variability, with RMSD (0.68) representing approximately 14% of observed C stock variance, overestimating extremely low and underestimating extremely high stocks, respectively. Our baseline PLS predictions of SOC stocks can be used for estimating the maximum amount of C that may be sequestered in soils across biomes.

Organic carbon storage change in China's urban landfills from 1978–2014

China has produced increasingly large quantities of waste associated with its accelerated urbanization and economic development and deposited these wastes into landfills, potentially sequestering carbon. However, the magnitude of the carbon storage in China’s urban landfills and its spatial and temporal change remain unclear. Here, we estimate the total amount of organic carbon (OC) stored in China's urban landfills between 1978 and 2014 using a first order organic matter decomposition model and data compiled from literature review and statistical yearbooks. Our results show that total OC stored in China’s urban landfills increased nearly 68-fold from the 1970s to the 2010s, and reached 225.2–264.5 Tg C (95% confidence interval, hereafter) in 2014. Construction waste was the largest OC pool (128.4–157.5 Tg C) in 2014, followed by household waste (67.7–83.8 Tg C), and sewage sludge was the least (19.7–34.1 Tg C). Carbon stored in urban landfills accounts for more than 10% of the country’s carbon stocks in urban ecosystems. The annual increase (i.e. sequestration rate) of OC in urban landfills in the 2010s (25.1 ± 4.3 Tg C yr−1, mean ± 2SD, hereafter) is equivalent to 1% of China's carbon emissions from fossil fuel combustion and cement production during the same period, but represents about 9% of the total terrestrial carbon sequestration in the country. Our study clearly indicates that OC dynamics in landfills should not be neglected in regional to national carbon cycle studies as landfills not only account for a substantial part of the carbon stored in urban ecosystems but also have a respectable contribution to national carbon sequestration.

Constraining seasonal active layer dynamics and chemical weathering reactions occurring in North Slope Alaskan watersheds with major ion and isotope (δ34SSO4, δ13CDIC, 87Sr/86Sr, δ44/40Ca, and δ44/42Ca) measurements

Rising air temperatures in the Arctic may destabilize a large pool of organic carbon stored in permafrost, thereby causing a positive feedback to global climate warming. Permafrost thaw could also deepen hydrologic flow paths and expose previously frozen rock and mineral fragments to chemical weathering. Future shifts in the inorganic solute geochemistry of Arctic rivers may signal changes in soil processes that also affect organic carbon storage. Tracing permafrost thaw with dissolved riverine loads requires understanding the spatial and seasonal variation of chemical weathering reactions and other biogeochemical phenomena that affect elemental mass-transport. To help identify connections between mineral weathering and active layer processes, we studied the major ion and isotope (δ34SSO4, δ13CDIC, 87Sr/86Sr, δ44/40Ca, and δ44/42Ca) geochemistry of five streams draining the North Slope of Alaska. Continuous permafrost underlies all streams, but the Atigun River, Roche Moutonnée Creek, and Trevor Creek primarily drain bare bedrock outcrops in the Brooks Range, while the Upper Kuparuk River and Imnavait Creek primarily drain tundra. In total, we collected 546 water samples spanning the spring freshet through fall freeze-up in 2009 and 2010. We also analyzed snow, rock, sediment, soil, and plant samples.Major ion ratios and δ13CDIC values point to the overall dominance of carbonate weathering by carbonic and sulfuric acids, with additional influences from atmospheric deposition, plant decay, sulfate salt dissolution, and silicate weathering by carbonic acid. δ13CDIC values may also reflect partial equilibration with soil and atmospheric CO2. All streams display large seasonal variations in major ion ratios and δ13CDIC values that are consistent with progressive deepening of the seasonally thawed zone over the summer. In the mountain watersheds, carbonate weathering dominates during the spring and summer, while sulfate salt (primarily CaSO4 and MgSO4) dissolution dominates during the fall. Riverine δ34SSO4 values reveal that the sulfate salts are secondary precipitates. We propose a conceptual model where cryoconcentration in soils during the late fall and winter causes secondary mineral formation at depth and re-exposure during subsequent thaw seasons produces the observed geochemical signals in rivers. The tundra streams lack definitive evidence for sulfate salt dissolution, presumably because thick peat soils limit the exposure and weathering of underlying glacial sediment where the salts are expected to form and dissolve. Appearance of a sulfate salt dissolution signal in tundra streams may correlate with future permafrost degradation. Carbonate weathering dominates riverine 87Sr/86Sr ratios, but the compositional heterogeneity of bedrock limits interpretation of the data. All rivers have higher δ44/40Ca values compared to bedrock, likely due to plant uptake of lighter Ca isotopes. In the tundra watersheds, freshet δ44/40Ca values were 0.10–0.20‰ lower than summer and fall values. These trends likely reflect contributions from plant decay, as comparison between δ44/40Ca and δ44/42Ca values suggests that all isotopic variation is mass-dependent with minimal radiogenic 40Ca inputs from the weathering of old silicate minerals with high K/Ca ratios.

Effects of warming and nitrogen fertilization on GHG flux in an alpine swamp meadow of a permafrost region

Uncertainties in the seasonal changes of greenhouse gases (GHG) fluxes in wetlands limit our accurate understanding of the responses of permafrost ecosystems to future warming and increased nitrogen (N) deposition. Therefore, in an alpine swamp meadow in the hinterland of the Qinghai-Tibet Plateau, a simulated warming with N fertilization experiment was conducted to investigate the key GHG fluxes (ecosystem respiration [Re], CH4 and N2O) in the early (EG), mid (MG) and late (LG) growing seasons. Results showed that warming (6.2 °C) increased the average seasonal Re by 30.9% and transformed the alpine swamp meadow from a N2O sink to a source, whereas CH4 flux was not significantly affected. N fertilization (4 g N m−2 a−1) alone had no significant effect on the fluxes of GHGs. The interaction of warming and N fertilization increased CH4 uptake by 69.6% and N2O emissions by 26.2% compared with warming, whereas the Re was not significantly affected. During the EG, although the soil temperature sensitivity of the Re was the highest, the effect of warming on the Re was the weakest. The primary driving factor for Re was soil surface temperature, whereas soil moisture controlled CH4 flux, and the N2O flux was primarily affected by rain events. The results indicated: (i) increasing N deposition has both positive and negative feedbacks on GHG fluxes in response to climate warming; (ii) during soil thawing process at active layer, low temperature of deep frozen soils have a negative contribution to Re in alpine ecosystems; and (iii) although these alpine wetland ecosystems are buffers against increased temperature, their feedbacks on climate change cannot be ignored because of the large soil organic carbon pool and high temperature sensitivity of the Re.

Enrichment Characteristics of Heavy Metals in Particulate Organic Matter of Purple Paddy Soil

Twenty purple paddy soil samples were collected from the Prefectures of Hechuan and Tongliang, Chongqing, and sieved into particulate organic matter (0.053-2 mm, POM), sand fraction (0.053-2 mm) and fine soil fraction (<0.053 mm). By comparing the concentration and distribution of soil organic carbon (SOC) and heavy metals among different components, the enrichment characteristics of POM for heavy metals (Cu, Zn, Pb and Zn) and its relationship with soil properties such as pH, CEC, clay content (<0.002 mm) and SOC were investigated. The result showed that the contents of POM fraction in purple paddy soils ranged from 7.31 to 44.76 g·kg-1, with an average level of 19.20 g·kg-1, while the carbon contents ranged from 96.61 to 263.17 g·kg-1, which were significantly higher than those of their original soils and other two fractions. The contribution rates of organic carbon by POM to the total organic carbon of soils ranged from 8.63% to 48.62%, which accounted for a large pool of organic carbon. The average enrichment factors of POM fraction to Cu, Zn, Pb and Cd were 3.35, 1.14, 2.88 and 2.14, respectively. Compared with sand fraction and fine soil fraction, POM fraction showed a significantly higher contribution rate to heavy metals, which were 6.02%, 2.22%, 5.24% and 3.21%. In addition, the enrichment ability and contribution rate of POM to Cu and Pb were obviously greater than those of Zn and Cd. There was a significant positive correlation between content of POM fraction and soil properties such as SOC, pH, CEC and clay content, in which SOC was the key factor. The POM-C content and total amount were significantly correlated with SOC and clay content. The pH value, SOC and POM-C amount had a significant or extremely significant correlation with the amount of heavy metals in POM, in which SOC was the most critical one. Comparing with other heavy metals, the amount of zinc in POM had the highest regression coefficient and the closest relationship with soil properties.

Soil organic carbon distribution in roadside soils of Singapore

Soil is the largest pool of organic carbon in terrestrial systems and plays a key role in carbon cycle. Global population living in urban areas are increasing substantially; however, the effects of urbanization on soil carbon storage and distribution are largely unknown. Here, we characterized the soil organic carbon (SOC) in roadside soils across the city-state of Singapore. We tested three hypotheses that SOC contents (concentration and density) in Singapore would be positively related to aboveground tree biomass, soil microbial biomass and land-use patterns. Overall mean SOC concentrations and densities (0–100 cm) of Singapore's roadside soils were 29 g kg−1 (4–106 g kg−1) and 11 kg m−2 (1.1–42.5 kg m−2) with median values of 26 g kg−1 and 10 kg m−2, respectively. There was significantly higher concentration of organic carbon (10.3 g kg−1) in the top 0–30 cm soil depth compared to the deeper (30–50 cm, and 50–100 cm) soil depths. Singapore's roadside soils represent 4% of Singapore's land, but store 2.9 million Mg C (estimated range of 0.3–11 million Mg C). This amount of SOC is equivalent to 25% of annual anthropogenic C emissions in Singapore. Soil organic C contents in Singapore's soils were not related to aboveground vegetation or soil microbial biomass, whereas land-use patterns to best explain variance in SOC in Singapore's roadside soils. We found SOC in Singapore's roadside soils to be inversely related to urbanization. We conclude that high SOC in Singapore roadside soils are probably due to management, such as specifications of high quality top-soil, high use of irrigation and fertilization and also due to an optimal climate promoting rapid growth and biological activity.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced.

Biodegradability of dissolved organic carbon in permafrost soils and aquatic systems: a meta-analysis

Abstract. As Arctic regions warm and frozen soils thaw, the large organic carbon pool stored in permafrost becomes increasingly vulnerable to decomposition or transport. The transfer of newly mobilized carbon to the atmosphere and its potential influence upon climate change will largely depend on the degradability of carbon delivered to aquatic ecosystems. Dissolved organic carbon (DOC) is a key regulator of aquatic metabolism, yet knowledge of the mechanistic controls on DOC biodegradability is currently poor due to a scarcity of long-term data sets, limited spatial coverage of available data, and methodological diversity. Here, we performed parallel biodegradable DOC (BDOC) experiments at six Arctic sites (16 experiments) using a standardized incubation protocol to examine the effect of methodological differences commonly used in the literature. We also synthesized results from 14 aquatic and soil leachate BDOC studies from across the circum-arctic permafrost region to examine pan-arctic trends in BDOC. An increasing extent of permafrost across the landscape resulted in higher DOC losses in both soil and aquatic systems. We hypothesize that the unique composition of (yedoma) permafrost-derived DOC combined with limited prior microbial processing due to low soil temperature and relatively short flow path lengths and transport times, contributed to a higher overall terrestrial and freshwater DOC loss. Additionally, we found that the fraction of BDOC decreased moving down the fluvial network in continuous permafrost regions, i.e. from streams to large rivers, suggesting that highly biodegradable DOC is lost in headwater streams. We also observed a seasonal (January–December) decrease in BDOC in large streams and rivers, but saw no apparent change in smaller streams or soil leachates. We attribute this seasonal change to a combination of factors including shifts in carbon source, changing DOC residence time related to increasing thaw-depth, increasing water temperatures later in the summer, as well as decreasing hydrologic connectivity between soils and surface water as the thaw season progresses. Our results suggest that future climate warming-induced shifts of continuous permafrost into discontinuous permafrost regions could affect the degradation potential of thaw-released DOC, the amount of BDOC, as well as its variability throughout the Arctic summer. We lastly recommend a standardized BDOC protocol to facilitate the comparison of future work and improve our knowledge of processing and transport of DOC in a changing Arctic.