The Experts below are selected from a list of 3333 Experts worldwide ranked by ideXlab platform
Joshua P Schimel - One of the best experts on this subject based on the ideXlab platform.
-
limited effects of early snowmelt on plants decomposers and soil nutrients in arctic Tundra Soils
Ecology and Evolution, 2019Co-Authors: Joshua P Schimel, Anthony Darrouzetnardi, Heidi Steltzer, Patrick F Sullivan, A D Segal, Amanda M Koltz, C Livensperger, Michael N. WeintraubAbstract:In addition to warming temperatures, Arctic ecosystems are responding to climate change with earlier snowmelt and soil thaw. Earlier snowmelt has been examined infrequently in field experiments, and we lack a comprehensive look at belowground responses of the soil biogeochemical system that includes plant roots, decomposers, and soil nutrients. We experimentally advanced the timing of snowmelt in factorial combination with an open-top chamber warming treatment over a 3-year period and evaluated the responses of decomposers and nutrient cycling processes. We tested two alternative hypotheses: (a) Early snowmelt and warming advance the timing of root growth and nutrient uptake, altering the timing of microbial and invertebrate activity and key nutrient cycling events; and (b) loss of insulating snow cover damages plants, leading to reductions in root growth and altered biological activity. During the 3 years of our study (2010-2012), we advanced snowmelt by 4, 15, and 10 days, respectively. Despite advancing aboveground plant phenology, particularly in the year with the warmest early-season temperatures (2012), belowground effects were primarily seen only on the first sampling date of the season or restricted to particular years or soil type. Overall, consistent and substantial responses to early snowmelt were not observed, counter to both of our hypotheses. The data on soil physical conditions, as well interannual comparisons of our results, suggest that this limited response was because of the earlier date of snowmelt that did not coincide with substantially warmer air and soil temperatures as they might in response to a natural climate event. We conclude that the interaction of snowmelt timing with soil temperatures is important to how the ecosystem will respond, but that 1- to 2-week changes in timing of snowmelt alone are not enough to drive season-long changes in soil microbial and nutrient cycling processes.
-
shifting patterns of microbial n metabolism across seasons in upland alaskan Tundra Soils
Soil Biology & Biochemistry, 2017Co-Authors: Shawna K Mcmahon, Joshua P SchimelAbstract:Abstract In the arctic Tundra, N controls the productivity and composition of Tundra plant communities. In the Alaskan Tundra, microbes only ever appear to be N-limited in Eriophorum vaginatum -dominated tussock Tundra, and that only during the summer growing season; during winter microbes continue to respire but show no evidence of N-limitation. What drives the shifts in microbial N-limitation and how are they developed by the metabolic pathways involved in processing available soil organic compounds? To answer these questions, we collected Soils from tussock and shrub Tundra during four seasons and incubated them with 13 C-labeled glucose, glutamate, and protein; then we measured 13 C in CO 2 , microbial biomass, and specific phospholipid fatty acids (PLFAs). By analyzing 13 C-PLFAs, we were able to assess whether different groups of microbes processed the substrates differently, and to assess changes in N-use brought on by the Arctic winter. Monomers were metabolized during both seasons. In tussock Tundra, glutamate-C was assimilated into PLFAs more extensively in winter than summer, suggesting glutamate was used as a C-source during winter but as a N-source during summer. In shrub Soils, the flow of C from glutamate to PLFAs tracked with glucose year-round suggesting that the communities were primarily using glutamate as a C source. These results parallel biogeochemical evidence showing shifts in N availability and limitation. Protein metabolism was negligible in winter in Soils other than tussocks. In summer, protein was broken down and all communities incorporated C; however, fungi did not assimilate protein-C at all while Gram-positive bacteria appeared to be proteolytic specialists. The different patterns of metabolism of C vs. N containing compounds across microbial groups regulate the dynamics of both soil communities and of soil carbon and nitrogen in Tundra Soils. As such, the flow of 13 C into different biomarker PLFAs provided a lens to evaluate the shifting dynamics of microbial communities and of the resource environment in which they find themselves.
-
seasonal variation in enzyme activities and temperature sensitivities in arctic Tundra Soils
Global Change Biology, 2009Co-Authors: Matthew D. Wallenstein, Shawna K Mcmahon, Joshua P SchimelAbstract:Arctic Soils contain large amounts of organic matter due to very slow rates of detritus decomposition. The first step in decomposition results from the activity of extracellular enzymes produced by soil microbes. We hypothesized that potential enzyme activities are low relative to the large stocks of organic matter in Arctic Tundra Soils, and that enzyme activity is low at in situ temperatures. We measured the potential activity of six hydrolytic enzymes at 4 and 20 °C on four sampling dates in tussock, intertussock, shrub organic, and shrub mineral Soils at Toolik Lake, Alaska. Potential activities of N-acetyl glucosaminidase, β-glucosidase, and peptidase tended to be greatest at the end of winter, suggesting that microbes produced enzymes while Soils were frozen. In general, enzyme activities did not increase during the Arctic summer, suggesting that enzyme production is N-limited during the period when temperatures would otherwise drive higher enzyme activity in situ. We also detected seasonal variations in the temperature sensitivity (Q10) of soil enzymes. In general, soil enzyme pools were more sensitive to temperature at the end of the winter than during the summer. We modeled potential in situβ-glucosidase activities for tussock and shrub organic Soils based on measured enzyme activities, temperature sensitivities, and daily soil temperature data. Modeled in situ enzyme activity in tussock Soils increased briefly during the spring, then declined through the summer. In shrub Soils, modeled enzyme activities increased through the spring thaw into early August, and then declined through the late summer and into winter. Overall, temperature is the strongest factor driving low in situ enzyme activities in the Arctic. However, enzyme activity was low during the summer, possibly due to N-limitation of enzyme production, which would constrain enzyme activity during the brief period when temperatures would otherwise drive higher rates of decomposition.
-
cold season production of co2 in arctic Soils can laboratory and field estimates be reconciled through a simple modeling approach
Arctic Antarctic and Alpine Research, 2006Co-Authors: Joshua P Schimel, Carl Mikan, Chien-lu Ping, Jace T Fahnestock, G J Michaelson, V E Romanovsky, Jeffrey M WelkerAbstract:Abstract Microbial activity in arctic Tundra Soils has been evaluated through both lab incubations and field flux measurements. To determine whether these different measurement approaches can be directly linked to each other, we developed a simple model of soil microbial CO2 production during the cold season in tussock Tundra, moss Tundra, and wet meadow Tundra in the Alaskan Arctic. The model incorporated laboratory-based estimates of microbial temperature responses at sub-zero temperatures with field measurements of C stocks through the soil profile and daily temperature measurements at the sites. Estimates of total CO2 production overestimated in situ cold season CO2 fluxes for the studied sites by as much as two- to threefold, suggesting that either CO2 produced in situ does not efflux during the cold season or that microbial respiration potentials are constrained by some other factor in situ. Average estimated winter CO2 production was near 120 g C m−2 in moist Tundra and 60 g C m−2 in wet meadow tun...
-
changing microbial substrate use in arctic Tundra Soils through a freeze thaw cycle
Soil Biology & Biochemistry, 2005Co-Authors: Joshua P Schimel, Carl MikanAbstract:Abstract Recent research has established that microbial processes in the arctic continue even when Soils are frozen, and that cold-season processes can be important in the overall annual carbon and nitrogen cycles. Despite the importance of wintertime soil microbial processes, our understanding of their controls remains extremely poor. We particularly have a poor understanding of how microbial substrate use patterns change as Soils freeze: do microbes use the same substrates as during the growing season, only slower, or do they switch to using different substrates? We used a 14 C isotope equilibration technique to partition respiration between the actively turning over microbial biomass and products pool and the plant detritus pool in a range of Arctic Tundra Soils. Microbes showed a step-function shift in their metabolism as Soils cool from +2 to +0.5 °C, roughly doubling the contribution of recycling of microbial C to total soil respiration. There was no additional shift in substrate use as Soils underwent bulk soil freezing. The above-0 °C substrate shift is important because Tundra Soils spend a long time at or just below 0 °C as they are freezing in the early winter. The change in substrate use represents a shift from processing N-poor detritus to N-rich microbial products, causing N available for either plant uptake or leaching to be greatest when Soils are near 0 °C. This may explain the observed patterns of growing season N immobilization vs. cold-season mineralization that appear common in Arctic Tundra ecosystems.
Elizabeth M Herndon - One of the best experts on this subject based on the ideXlab platform.
-
iron and iron bound phosphate accumulate in surface Soils of ice wedge polygons in arctic Tundra
Environmental Science: Processes & Impacts, 2020Co-Authors: Elizabeth M Herndon, Lauren E Kinsmancostello, Nicolle Di Domenico, Kiersten Duroe, Maximilian R Barczok, Chelsea E Smith, Stan D WullschlegerAbstract:Phosphorus (P) is a limiting or co-limiting nutrient to plants and microorganisms in diverse ecosystems that include the arctic Tundra. Certain soil minerals can adsorb or co-precipitate with phosphate, and this mineral-bound P provides a potentially large P reservoir in Soils. Iron (Fe) oxyhydroxides have a high capacity to adsorb phosphate; however, the ability of Fe oxyhydroxides to adsorb phosphate and limit P bioavailability in organic Tundra Soils is not known. Here, we examined the depth distribution of soil Fe and P species in the active layer ( 4× greater than water-soluble Pi. These results demonstrate that Fe-bound Pi is a large and ecologically important reservoir of phosphate. We contend that Fe oxyhydroxides and other minerals may regulate Pi solubility under fluctuating redox conditions in organic surface Soils on the arctic Tundra.
-
Geochemical drivers of organic matter decomposition in arctic Tundra Soils
Biogeochemistry, 2015Co-Authors: Elizabeth M Herndon, Ziming Yang, Stan D Wullschleger, David E Graham, John Bargar, Noemie Janot, Tom Z. Regier, Liyuan LiangAbstract:Climate change is warming Tundra ecosystems in the Arctic, resulting in the decomposition of previously-frozen soil organic matter (SOM) and release of carbon (C) to the atmosphere; however, the processes that control SOM decomposition and C emissions remain highly uncertain. In this study, we evaluate geochemical factors that influence microbial production of carbon dioxide (CO2) and methane (CH4) in the seasonally-thawed active layer of interstitial polygonal Tundra near Barrow, Alaska. We report spatial and seasonal patterns of dissolved gases in relation to the geochemical properties of Fe and organic C in soil and soil solution, as determined using spectroscopic and chromatographic techniques. The chemical composition of soil water collected during the annual thaw season varied significantly with depth. Soil water in the middle of the active layer contained abundant Fe(III), and aromatic-C and low-molecular-weight organic acids derived from SOM decomposition. At these depths, CH4 was positively correlated with the ratio of Fe(III) to total Fe in waterlogged transitional and low-centered polygons but negatively correlated in the drier flat- and high-centered polygons. These observations contradict the expectation that CH4 would be uniformly low where Fe(III) was high due to inhibition of methanogenesis by Fe(III)-reduction reactions. Our results suggest that vertically-stratified Fe redox reactions influence respiration/fermentation of SOM and production of substrates (e.g., low-molecular-weight organic acids) for methanogenesis, but that these effects vary with soil moisture. We infer that geochemical differences induced by water saturation dictate microbial products of SOM decomposition, and Fe geochemistry is an important factor regulating methanogenesis in anoxic Tundra Soils.
-
pathways of anaerobic organic matter decomposition in Tundra Soils from barrow alaska
Journal of Geophysical Research, 2015Co-Authors: Elizabeth M Herndon, Benjamin F Mann, Taniya Roy Chowdhury, Ziming Yang, Stan D Wullschleger, David E Graham, Liyuan LiangAbstract:Arctic Tundra Soils store a large quantity of organic carbon that is susceptible to decomposition and release to the atmosphere as methane (CH4) and carbon dioxide (CO2) under a warming climate. Anaerobic processes that generate CH4 and CO2 remain unclear because previous studies have focused on aerobic decomposition pathways. To predict releases of CO2 and CH4 from Tundra Soils, it is necessary to identify pathways of soil organic matter decomposition under the anoxic conditions that are prevalent in Arctic ecosystems. Here molecular and spectroscopic techniques were used to monitor biological degradation of water-extractable organic carbon (WEOC) during anoxic incubation of Tundra Soils from a region of continuous permafrost in northern Alaska. Organic and mineral Soils from the Tundra active layer were incubated at –2, +4, or +8°C for up to 60 days to mimic the short-term thaw season. Results suggest that, under anoxic conditions, fermentation converted complex organic molecules into simple organic acids that were used in concomitant Fe-reduction and acetoclastic methanogenesis reactions. Nonaromatic compounds increased over time as WEOC increased. Organic acid metabolites initially accumulated in Soils but were mostly depleted by day 60 because organic acids were consumed to produce Fe(II), CO2, and CH4. We conclude that fermentationmore » of nonprotected organic matter facilitates methanogenesis and Fe reduction reactions, and that the proportion of organic acids consumed by methanogenesis increases relative to Fe reduction with increasing temperature. As a result, the decomposition pathways observed in this study are important to consider in numerical modeling of greenhouse gas production in the Arctic.« less
Stan D Wullschleger - One of the best experts on this subject based on the ideXlab platform.
-
iron and iron bound phosphate accumulate in surface Soils of ice wedge polygons in arctic Tundra
Environmental Science: Processes & Impacts, 2020Co-Authors: Elizabeth M Herndon, Lauren E Kinsmancostello, Nicolle Di Domenico, Kiersten Duroe, Maximilian R Barczok, Chelsea E Smith, Stan D WullschlegerAbstract:Phosphorus (P) is a limiting or co-limiting nutrient to plants and microorganisms in diverse ecosystems that include the arctic Tundra. Certain soil minerals can adsorb or co-precipitate with phosphate, and this mineral-bound P provides a potentially large P reservoir in Soils. Iron (Fe) oxyhydroxides have a high capacity to adsorb phosphate; however, the ability of Fe oxyhydroxides to adsorb phosphate and limit P bioavailability in organic Tundra Soils is not known. Here, we examined the depth distribution of soil Fe and P species in the active layer ( 4× greater than water-soluble Pi. These results demonstrate that Fe-bound Pi is a large and ecologically important reservoir of phosphate. We contend that Fe oxyhydroxides and other minerals may regulate Pi solubility under fluctuating redox conditions in organic surface Soils on the arctic Tundra.
-
stimulation of anaerobic organic matter decomposition by subsurface organic n addition in Tundra Soils
Soil Biology & Biochemistry, 2019Co-Authors: Michael Philben, Ziming Yang, Stan D Wullschleger, Jianqiu Zheng, Markus Bill, Jeffrey M Heikoop, George Perkins, David E GrahamAbstract:Abstract Increasing nitrogen (N) availability in Arctic Soils could stimulate the growth of both plants and microorganisms by relieving the constraints of nutrient limitation. It was hypothesized that organic N addition to anoxic Tundra soil would increase CH4 production by stimulating the fermentation of labile substrates, which is considered the rate-limiting step in anaerobic C mineralization. We tested this hypothesis through both field and lab-based experiments. In the field experiment, we injected a solution of 13C- and 15N-labeled glutamate 35 cm belowground at a site near Nome on the Seward Peninsula, Alaska, and observed the resulting changes in porewater geochemistry and dissolved greenhouse gas concentrations. The concentration of free glutamate declined rapidly within hours of injection, and the 15N label was recovered almost exclusively as dissolved organic N within 62 h. These results indicate rapid microbial assimilation of the added N and transformation into novel organic compounds. We observed increasing concentrations of dissolved CH4 and Fe(II), indicating rapid stimulation of methanogenesis and Fe(III) reduction. Low molecular weight organic acids such as acetate and propionate accumulated despite increasing consumption through anaerobic C mineralization. A laboratory soil column flow experiment using active layer soil collected from the same site further supported these findings. Glutamate recovery was low compared to a conservative bromide tracer, but concentrations of NO3− and NH4+ remained low, consistent with microbial uptake of the added N. Similar to the field experiment, we observed both increasing Fe(II) and organic acid concentrations. Together, these results support our hypothesis of increased fermentation in response to organic N addition and suggest that increasing N availability could accelerate CH4 production in Tundra Soils.
-
microbial community and functional gene changes in arctic Tundra Soils in a microcosm warming experiment
Frontiers in Microbiology, 2017Co-Authors: Ziming Yang, Sihang Yang, Joy D Van Nostrand, Jizhong Zhou, Wei Fang, Yurong Liu, Stan D WullschlegerAbstract:Microbial decomposition of soil organic carbon (SOC) in thawing Arctic permafrost is important in determining greenhouse gas feedbacks of Tundra ecosystems to climate. However, the changes in microbial community structure during SOC decomposition are poorly known. Here we examine these changes using frozen Soils from Barrow, Alaska, USA, in anoxic microcosm incubation at -2 and 8°C for 122 days. The functional gene array GeoChip was used to determine microbial community structure and the functional genes associated with SOC degradation, methanogenesis, and Fe(III) reduction. Results show that soil incubation after 122 days at 8°C significantly decreased functional gene abundance (P < 0.05) associated with SOC degradation, fermentation, methanogenesis, and iron cycling, particularly in organic-rich soil. These observations correspond well with decreases in labile SOC content (e.g., reducing sugar and ethanol), methane and CO2 production, and Fe(III) reduction. In contrast, the community functional structure was largely unchanged in the -2°C incubation. Soil type (i.e., organic vs. mineral) and the availability of labile SOC were among the most significant factors impacting microbial community structure. These results demonstrate the important roles of microbial community in SOC degradation and support previous findings that SOC in organic-rich Arctic Tundra is highly vulnerable to microbial degradation under warming.
-
Geochemical drivers of organic matter decomposition in arctic Tundra Soils
Biogeochemistry, 2015Co-Authors: Elizabeth M Herndon, Ziming Yang, Stan D Wullschleger, David E Graham, John Bargar, Noemie Janot, Tom Z. Regier, Liyuan LiangAbstract:Climate change is warming Tundra ecosystems in the Arctic, resulting in the decomposition of previously-frozen soil organic matter (SOM) and release of carbon (C) to the atmosphere; however, the processes that control SOM decomposition and C emissions remain highly uncertain. In this study, we evaluate geochemical factors that influence microbial production of carbon dioxide (CO2) and methane (CH4) in the seasonally-thawed active layer of interstitial polygonal Tundra near Barrow, Alaska. We report spatial and seasonal patterns of dissolved gases in relation to the geochemical properties of Fe and organic C in soil and soil solution, as determined using spectroscopic and chromatographic techniques. The chemical composition of soil water collected during the annual thaw season varied significantly with depth. Soil water in the middle of the active layer contained abundant Fe(III), and aromatic-C and low-molecular-weight organic acids derived from SOM decomposition. At these depths, CH4 was positively correlated with the ratio of Fe(III) to total Fe in waterlogged transitional and low-centered polygons but negatively correlated in the drier flat- and high-centered polygons. These observations contradict the expectation that CH4 would be uniformly low where Fe(III) was high due to inhibition of methanogenesis by Fe(III)-reduction reactions. Our results suggest that vertically-stratified Fe redox reactions influence respiration/fermentation of SOM and production of substrates (e.g., low-molecular-weight organic acids) for methanogenesis, but that these effects vary with soil moisture. We infer that geochemical differences induced by water saturation dictate microbial products of SOM decomposition, and Fe geochemistry is an important factor regulating methanogenesis in anoxic Tundra Soils.
-
pathways of anaerobic organic matter decomposition in Tundra Soils from barrow alaska
Journal of Geophysical Research, 2015Co-Authors: Elizabeth M Herndon, Benjamin F Mann, Taniya Roy Chowdhury, Ziming Yang, Stan D Wullschleger, David E Graham, Liyuan LiangAbstract:Arctic Tundra Soils store a large quantity of organic carbon that is susceptible to decomposition and release to the atmosphere as methane (CH4) and carbon dioxide (CO2) under a warming climate. Anaerobic processes that generate CH4 and CO2 remain unclear because previous studies have focused on aerobic decomposition pathways. To predict releases of CO2 and CH4 from Tundra Soils, it is necessary to identify pathways of soil organic matter decomposition under the anoxic conditions that are prevalent in Arctic ecosystems. Here molecular and spectroscopic techniques were used to monitor biological degradation of water-extractable organic carbon (WEOC) during anoxic incubation of Tundra Soils from a region of continuous permafrost in northern Alaska. Organic and mineral Soils from the Tundra active layer were incubated at –2, +4, or +8°C for up to 60 days to mimic the short-term thaw season. Results suggest that, under anoxic conditions, fermentation converted complex organic molecules into simple organic acids that were used in concomitant Fe-reduction and acetoclastic methanogenesis reactions. Nonaromatic compounds increased over time as WEOC increased. Organic acid metabolites initially accumulated in Soils but were mostly depleted by day 60 because organic acids were consumed to produce Fe(II), CO2, and CH4. We conclude that fermentationmore » of nonprotected organic matter facilitates methanogenesis and Fe reduction reactions, and that the proportion of organic acids consumed by methanogenesis increases relative to Fe reduction with increasing temperature. As a result, the decomposition pathways observed in this study are important to consider in numerical modeling of greenhouse gas production in the Arctic.« less
Ziming Yang - One of the best experts on this subject based on the ideXlab platform.
-
stimulation of anaerobic organic matter decomposition by subsurface organic n addition in Tundra Soils
Soil Biology & Biochemistry, 2019Co-Authors: Michael Philben, Ziming Yang, Stan D Wullschleger, Jianqiu Zheng, Markus Bill, Jeffrey M Heikoop, George Perkins, David E GrahamAbstract:Abstract Increasing nitrogen (N) availability in Arctic Soils could stimulate the growth of both plants and microorganisms by relieving the constraints of nutrient limitation. It was hypothesized that organic N addition to anoxic Tundra soil would increase CH4 production by stimulating the fermentation of labile substrates, which is considered the rate-limiting step in anaerobic C mineralization. We tested this hypothesis through both field and lab-based experiments. In the field experiment, we injected a solution of 13C- and 15N-labeled glutamate 35 cm belowground at a site near Nome on the Seward Peninsula, Alaska, and observed the resulting changes in porewater geochemistry and dissolved greenhouse gas concentrations. The concentration of free glutamate declined rapidly within hours of injection, and the 15N label was recovered almost exclusively as dissolved organic N within 62 h. These results indicate rapid microbial assimilation of the added N and transformation into novel organic compounds. We observed increasing concentrations of dissolved CH4 and Fe(II), indicating rapid stimulation of methanogenesis and Fe(III) reduction. Low molecular weight organic acids such as acetate and propionate accumulated despite increasing consumption through anaerobic C mineralization. A laboratory soil column flow experiment using active layer soil collected from the same site further supported these findings. Glutamate recovery was low compared to a conservative bromide tracer, but concentrations of NO3− and NH4+ remained low, consistent with microbial uptake of the added N. Similar to the field experiment, we observed both increasing Fe(II) and organic acid concentrations. Together, these results support our hypothesis of increased fermentation in response to organic N addition and suggest that increasing N availability could accelerate CH4 production in Tundra Soils.
-
microbial community and functional gene changes in arctic Tundra Soils in a microcosm warming experiment
Frontiers in Microbiology, 2017Co-Authors: Ziming Yang, Sihang Yang, Joy D Van Nostrand, Jizhong Zhou, Wei Fang, Yurong Liu, Stan D WullschlegerAbstract:Microbial decomposition of soil organic carbon (SOC) in thawing Arctic permafrost is important in determining greenhouse gas feedbacks of Tundra ecosystems to climate. However, the changes in microbial community structure during SOC decomposition are poorly known. Here we examine these changes using frozen Soils from Barrow, Alaska, USA, in anoxic microcosm incubation at -2 and 8°C for 122 days. The functional gene array GeoChip was used to determine microbial community structure and the functional genes associated with SOC degradation, methanogenesis, and Fe(III) reduction. Results show that soil incubation after 122 days at 8°C significantly decreased functional gene abundance (P < 0.05) associated with SOC degradation, fermentation, methanogenesis, and iron cycling, particularly in organic-rich soil. These observations correspond well with decreases in labile SOC content (e.g., reducing sugar and ethanol), methane and CO2 production, and Fe(III) reduction. In contrast, the community functional structure was largely unchanged in the -2°C incubation. Soil type (i.e., organic vs. mineral) and the availability of labile SOC were among the most significant factors impacting microbial community structure. These results demonstrate the important roles of microbial community in SOC degradation and support previous findings that SOC in organic-rich Arctic Tundra is highly vulnerable to microbial degradation under warming.
-
Geochemical drivers of organic matter decomposition in arctic Tundra Soils
Biogeochemistry, 2015Co-Authors: Elizabeth M Herndon, Ziming Yang, Stan D Wullschleger, David E Graham, John Bargar, Noemie Janot, Tom Z. Regier, Liyuan LiangAbstract:Climate change is warming Tundra ecosystems in the Arctic, resulting in the decomposition of previously-frozen soil organic matter (SOM) and release of carbon (C) to the atmosphere; however, the processes that control SOM decomposition and C emissions remain highly uncertain. In this study, we evaluate geochemical factors that influence microbial production of carbon dioxide (CO2) and methane (CH4) in the seasonally-thawed active layer of interstitial polygonal Tundra near Barrow, Alaska. We report spatial and seasonal patterns of dissolved gases in relation to the geochemical properties of Fe and organic C in soil and soil solution, as determined using spectroscopic and chromatographic techniques. The chemical composition of soil water collected during the annual thaw season varied significantly with depth. Soil water in the middle of the active layer contained abundant Fe(III), and aromatic-C and low-molecular-weight organic acids derived from SOM decomposition. At these depths, CH4 was positively correlated with the ratio of Fe(III) to total Fe in waterlogged transitional and low-centered polygons but negatively correlated in the drier flat- and high-centered polygons. These observations contradict the expectation that CH4 would be uniformly low where Fe(III) was high due to inhibition of methanogenesis by Fe(III)-reduction reactions. Our results suggest that vertically-stratified Fe redox reactions influence respiration/fermentation of SOM and production of substrates (e.g., low-molecular-weight organic acids) for methanogenesis, but that these effects vary with soil moisture. We infer that geochemical differences induced by water saturation dictate microbial products of SOM decomposition, and Fe geochemistry is an important factor regulating methanogenesis in anoxic Tundra Soils.
-
pathways of anaerobic organic matter decomposition in Tundra Soils from barrow alaska
Journal of Geophysical Research, 2015Co-Authors: Elizabeth M Herndon, Benjamin F Mann, Taniya Roy Chowdhury, Ziming Yang, Stan D Wullschleger, David E Graham, Liyuan LiangAbstract:Arctic Tundra Soils store a large quantity of organic carbon that is susceptible to decomposition and release to the atmosphere as methane (CH4) and carbon dioxide (CO2) under a warming climate. Anaerobic processes that generate CH4 and CO2 remain unclear because previous studies have focused on aerobic decomposition pathways. To predict releases of CO2 and CH4 from Tundra Soils, it is necessary to identify pathways of soil organic matter decomposition under the anoxic conditions that are prevalent in Arctic ecosystems. Here molecular and spectroscopic techniques were used to monitor biological degradation of water-extractable organic carbon (WEOC) during anoxic incubation of Tundra Soils from a region of continuous permafrost in northern Alaska. Organic and mineral Soils from the Tundra active layer were incubated at –2, +4, or +8°C for up to 60 days to mimic the short-term thaw season. Results suggest that, under anoxic conditions, fermentation converted complex organic molecules into simple organic acids that were used in concomitant Fe-reduction and acetoclastic methanogenesis reactions. Nonaromatic compounds increased over time as WEOC increased. Organic acid metabolites initially accumulated in Soils but were mostly depleted by day 60 because organic acids were consumed to produce Fe(II), CO2, and CH4. We conclude that fermentationmore » of nonprotected organic matter facilitates methanogenesis and Fe reduction reactions, and that the proportion of organic acids consumed by methanogenesis increases relative to Fe reduction with increasing temperature. As a result, the decomposition pathways observed in this study are important to consider in numerical modeling of greenhouse gas production in the Arctic.« less
Michael N. Weintraub - One of the best experts on this subject based on the ideXlab platform.
-
limited effects of early snowmelt on plants decomposers and soil nutrients in arctic Tundra Soils
Ecology and Evolution, 2019Co-Authors: Joshua P Schimel, Anthony Darrouzetnardi, Heidi Steltzer, Patrick F Sullivan, A D Segal, Amanda M Koltz, C Livensperger, Michael N. WeintraubAbstract:In addition to warming temperatures, Arctic ecosystems are responding to climate change with earlier snowmelt and soil thaw. Earlier snowmelt has been examined infrequently in field experiments, and we lack a comprehensive look at belowground responses of the soil biogeochemical system that includes plant roots, decomposers, and soil nutrients. We experimentally advanced the timing of snowmelt in factorial combination with an open-top chamber warming treatment over a 3-year period and evaluated the responses of decomposers and nutrient cycling processes. We tested two alternative hypotheses: (a) Early snowmelt and warming advance the timing of root growth and nutrient uptake, altering the timing of microbial and invertebrate activity and key nutrient cycling events; and (b) loss of insulating snow cover damages plants, leading to reductions in root growth and altered biological activity. During the 3 years of our study (2010-2012), we advanced snowmelt by 4, 15, and 10 days, respectively. Despite advancing aboveground plant phenology, particularly in the year with the warmest early-season temperatures (2012), belowground effects were primarily seen only on the first sampling date of the season or restricted to particular years or soil type. Overall, consistent and substantial responses to early snowmelt were not observed, counter to both of our hypotheses. The data on soil physical conditions, as well interannual comparisons of our results, suggest that this limited response was because of the earlier date of snowmelt that did not coincide with substantially warmer air and soil temperatures as they might in response to a natural climate event. We conclude that the interaction of snowmelt timing with soil temperatures is important to how the ecosystem will respond, but that 1- to 2-week changes in timing of snowmelt alone are not enough to drive season-long changes in soil microbial and nutrient cycling processes.
-
Microbial activity is not always limited by nitrogen in Arctic Tundra Soils
Soil Biology and Biochemistry, 2015Co-Authors: Caroline Melle, Matthew D. Wallenstein, Anthony Darrouzet-nardi, Michael N. WeintraubAbstract:Abstract Both primary productivity and decomposition appear to be limited by low soil nitrogen (N) availability throughout much of the Arctic Tundra active growing season, making these ecosystems among the most N-limited in the world. Climate warming may potentially stimulate microbial activities such as N mineralization, which could have cascading long-term effects on Arctic Tundra ecosystems. Despite previous evidence of N limitation to microbial decomposition in Arctic Tundra, N may not limit microbial activity throughout the entire active season. Labile carbon (C) may be co-limiting for portions of the active season when there is relatively high inorganic N and/or low labile C availability. To assess seasonal variation in the controls on microbial activity, we conducted a series of laboratory incubations with Soils collected at the beginning and peak of the active season in two years to examine intra-seasonal and annual variability of soil microbial N limitation in an Arctic moist acidic Tundra (MAT) soil. The soil incubations were set-up in a factorial design with treatments of added N or DI water as a control and incubation temperatures of 5 °C and 15 °C. We measured chloroform-labile C and N (as a proxy for microbial biomass), extractable nutrients, C-mineralization and potential enzyme activities. In contrast to previous studies, we found that these metrics of microbial activity were not consistently stimulated by N additions; rather, added N was primarily immobilized by microbes resulting in decreased chloroform-labile C:N ratios. Stimulation of C mineralization by N addition was short-lived and variable between our two sampling dates within a single active season. Additionally, there were differences in temperature sensitivities of C mineralization and contrasting effects of N amendment on enzyme activities between the two study years. These findings suggest that, at times, other factors co-limit microbial activities in MAT Soils. The current dogma of universal N-limitation to microbial activity may need to be refined in light of these results, to more accurately predict the fate of the large amounts of C currently sequestered in Arctic Tundra Soils.
-
seasonal protein dynamics in alaskan arctic Tundra Soils
Soil Biology & Biochemistry, 2005Co-Authors: Michael N. Weintraub, Joshua P SchimelAbstract:Abstract In the arctic Tundra of Alaska, plant growth is limited by N supply, especially in tussock Tundra. Because proteins are the predominant form of soil organic N, proteolysis is considered to be the rate-limiting step in both the release of amino acids and in N mineralization. To help understand the controls on N availability in Tundra Soils, and to determine whether proteolysis is controlled by enzyme activity or by substrate availability, we measured soil protein concentrations, and proteolysis rates with and without added protein, every 1–2 weeks through the summer of 2000 and twice in the summer of 2001. Protease activity with added protein (‘potential’) was higher than without added protein (‘actual’). However, differences between the two tended to be driven by relatively brief peaks in potential protease activity. In fact, actual and potential rates were usually similar, suggesting that much of the time proteolysis was not limited by substrate availability, but rather by enzyme activity. Our data suggest that protease activity was actually only substrate limited at the times when it was highest. Potential rates peaked at the same times that soluble proteins were also high. These increases in protease activity and soluble protein concentrations occurred when soil amino acid and NH 4 + concentrations were at their lowest, drawn down by the seasonal peaks in root growth. The fact that the peaks in protease activity coincided with the peak in soil amino acid and NH 4 + demand strongly suggests that proteolysis was stimulated by high soil amino acid demand, and resulted in increases in soluble protein concentrations caused by the solubilization of larger proteins.
-
The seasonal dynamics of amino acids and other nutrients in Alaskan Arctic Tundra Soils
Biogeochemistry, 2005Co-Authors: Michael N. Weintraub, Joshua P SchimelAbstract:Past research strongly indicates the importance of amino acids in the N economy of the Arctic Tundra, but little is known about the seasonal dynamics of amino acids in Tundra Soils. We repeatedly sampled Soils from tussock, shrub, and wet sedge Tundra communities in the summers of 2000 and 2001 and extracted them with water (H_2O) and potassium sulfate (K_2SO_4) to determine the seasonal dynamics of soil amino acids, ammonium (NH_4^+), nitrate (NO_3^−), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and phosphate (PO_4^2−). In the H_2O extractions mean concentrations of total free amino acids (TFAA) were higher than NH_4^+ in all Soils but shrub. TFAA and NH_4^+ were highest in wet sedge and tussock Soils and lowest in shrub soil. The most predominant amino acids were alanine, arginine, glycine, serine, and threonine. None of the highest amino acids were significantly different than NH_4^+ in any soil but shrub, in which NH_4^+ was significantly higher than all of the highest individual amino acids. Mean NO_3^− concentrations were not significantly different from mean TFAA and NH_4^+ concentrations in any soil but tussock, where NO_3^− was significantly higher than NH_4^+. In all Soils amino acid and NH_4^+ concentrations dropped to barely detectable levels in the middle of July, suggesting intense competition for N at the height of the growing season. In all Soils but tussock, amino acid and NH_4^+ concentrations rebounded in August as the end of the Arctic growing season approached and plant N demand decreased. This pattern suggests that low N concentrations in Tundra Soils at the height of the growing season are likely the result of an increase in soil N uptake associated with the peak in plant growth, either directly by roots or indirectly by microbes fueled by increased root C inputs in mid-July. As N availability decreased in July, PO_4^2− concentrations in the K_2SO_4 extractions increased dramatically in all Soils but shrub, where there was a comparable increase in PO_4^2− later in the growing season. Previous research suggests that these increases in PO_4^2− concentrations are due to the mineralization of organic phosphorus by phosphatase enzymes associated with soil microbes and plant roots, and that they may have been caused by an increase in organic P availability.
-
interactions between carbon and nitrogen mineralization and soil organic matter chemistry in arctic Tundra Soils
Ecosystems, 2003Co-Authors: Michael N. Weintraub, Joshua P SchimelAbstract:We used long-term laboratory incubations and chemical fractionation to characterize the mineralization dynamics of organic Soils from tussock, shrub, and wet meadow Tundra communities, to determine the relationship between soil organic matter (SOM) decomposition and chemistry, and to quantify the relative proportions of carbon (C) and nitrogen (N) in Tundra SOM that are biologically available for decomposition. In all Soils but shrub, we found little decline in respiration rates over 1 year, although Soils respired approximately a tenth to a third of total soil C. The lack of decline in respiration rates despite large C losses indicates that the quantity of organic matter available was not controlling respiration and thus suggests that something else was limiting microbial activity. To determine the nature of the respired C, we analyzed soil chemistry before and after the incubation using a peat fractionation scheme. Despite the large losses of soil C, SOM chemistry was relatively unchanged after the incubation. The decomposition dynamics we observed suggest that Tundra SOM, which is largely plant detritus, fits within existing concepts of the litter decay continuum. The lack of changes in organic matter chemistry indicates that this material had already decomposed to the point where the breakdown of labile constituents was tied to lignin decomposition. N mineralization was correlated with C mineralization in our study, but shrub soil mineralized more and tussock soil less N than would have been predicted by this correlation. Our results suggest that a large proportion of Tundra SOM is potentially mineralizable, despite the fact that decomposition was dependent on lignin breakdown, and that the historical accumulation of organic matter in Tundra Soils is the result of field conditions unfavorable to decomposition and not the result of fundamental chemical limitations to decomposition. Our study also suggests that the anticipated increases in shrub dominance may substantially alter the dynamics of SOM decomposition in the Tundra.
