Permafrost collapse alters soil carbon stocks,respiration, CH4, and N2O in upland tundra

Release of greenhouse gases from thawing permafrost is potentially the largest terrestrial feedback to climate change and one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Some of this uncertainty stems from abrupt thaw processes known as thermokarst (permafrost collapse due to ground ice melt), which alter controls on carbon and nitrogen cycling and expose organic matter from meters below the surface. Thermokarst may affect 20–50% of tundra uplands by the end of the century; however, little is known about the effect of different thermokarst morphologies on carbon and nitrogen release. We measured soil organic matter displacement, ecosystem respiration, and soil gas concentrations at 26 upland thermokarst features on the North Slope of Alaska. Features included the three most common upland thermokarst morphologies: active‐layer detachment slides, thermo‐erosion gullies, and retrogressive thaw slumps. We found that thermokarst morphology interacted with landscape parameters to determine both the initial displacement of organic matter and subsequent carbon and nitrogen cycling. The large proportion of ecosystem carbon exported off‐site by slumps and slides resulted in decreased ecosystem respiration postfailure, while gullies removed a smaller portion of ecosystem carbon but strongly increased respiration and N₂O concentration. Elevated N₂O in gully soils persisted through most of the growing season, indicating sustained nitrification and denitrification in disturbed soils, representing a potential noncarbon permafrost climate feedback. While upland thermokarst formation did not substantially alter redox conditions within features, it redistributed organic matter into both oxic and anoxic environments. Across morphologies, residual organic matter cover, and predisturbance respiration explained 83% of the variation in respiration response. Consistent differences between upland thermokarst types may contribute to the incorporation of this nonlinear process into projections of carbon and nitrogen release from degrading permafrost.     

Headwater Mires Constitute a Major Source of Nitrogen (N) to Surface Waters in the Boreal Landscape

Nutrient exports from soils have important implications for long-term patterns of nutrient limitation on land and resource delivery to aquatic environments. While plant–soil systems are notably efficient at retaining limiting nutrients, spatial and temporal mismatches in resource supply and demand may create opportunities for hydrologic losses to occur. Spatial mismatches may be particularly important in peat-forming landscapes, where the development of a two-layer vertical structure can isolate plant communities on the surface from resource pools that accumulate at depth. Our objectives were to test this idea in northern Sweden, where nitrogen (N) limitation of terrestrial plants is widespread, and where peat-forming, mire ecosystems are dominant features of the landscape. We quantified vertical patterns of N chemistry in a minerogenic mire, estimated the seasonal and annual hydrologic export of organic and inorganic N from this system, and evaluated the broader influence of mire cover on N chemistry across a stream network. Relatively high concentrations of ammonium (up to 2 mg l^?1) were observed in groundwater several meters below the peat surface, and N was routed to the outlet stream along deep, preferential flowpaths. Areal estimates of inorganic N export from the mire were several times greater than from an adjacent, forested catchment, with markedly higher loss rates during the growing season, when plant N demand is ostensibly greatest. At broader scales, mire cover was positively correlated with long-term concentrations of inorganic and organic N in streams across the drainage network. This study provides an example of how mire formation and peat accumulation can create broad-scale heterogeneity in nutrient supply and demand across boreal landscapes. This mismatch allows for hydrologic losses of reactive N that are independent of annual plant demand and potentially important to receiving lakes and streams.     

Regulation of Microbial Community Composition and Activity by Soil Nutrient Availability, Soil pH, and Herbivory in the Tundra

Soil nitrogen (N) availability and pH constitute major abiotic controls over microbial community composition and activity in tundra ecosystems. On the other hand, mammalian grazers form an important biotic factor influencing resource coupling between plants and soil microorganisms. To investigate individual effects and interactions among soil nutrients, pH, and grazing on tundra soils, we performed factorial treatments of fertilization, liming, and grazer exclusion in the field for 3 years at 2 contrasting tundra habitats, acidic (N-poor) and non-acidic (N-rich) tundra heaths. The effects of all treatments were small in the non-acidic tundra heaths. In the acidic tundra heaths, fertilization decreased the fungal:bacterial ratio as analyzed by soil PLFAs, but there were no effects of liming. Fertilization increased soil N concentrations more drastically in ungrazed than grazed plots, and in parallel, fertilization decreased the fungal:bacterial ratio to a greater extent in the ungrazed plots. Liming, on the other hand, partly negated the effects of fertilization on both soil N concentrations and PLFAs. Fertilization drastically increased the activity of phenol oxidase, a microbial enzyme synthesized for degradation of soil phenols, in grazed plots, but had no effect in ungrazed plots. Taken together, our results demonstrate that grazers have the potential to regulate the fungal:bacterial ratio in soils through influencing N availability for the soil microorganisms.     

Thaw depth determines reaction and transport of inorganic nitrogen in valley bottom permafrost soils

Nitrate (NO₃^–) export coupled with high inorganic nitrogen (N) concentrations in Alaskan streams suggests that N cycles of permafrost‐influenced ecosystems are more open than expected for N‐limited ecosystems. We tested the hypothesis that soil thaw depth governs inorganic N retention and removal in soils due to vertical patterns in the dominant N transformation pathways. Using an in situ, push–pull method, we estimated rates of inorganic N uptake and denitrification during snow melt, summer, and autumn, as depth of soil–stream flowpaths increased in the valley bottom of an arctic and a boreal catchment. Net NO₃^– uptake declined sharply from snow melt to summer and decreased as a nonlinear function of thaw depth. Peak denitrification rate occurred during snow melt at the arctic site, in summer at the boreal site, and declined as a nonlinear function of thaw depth across both sites. Seasonal patterns in ammonium (NH₄⁺) uptake were not significant, but low rates during the peak growing season suggest uptake that is balanced by mineralization. Despite rapid rates of hydrologic transport during snow melt runoff, rates of uptake and removal of inorganic N tended to exceed water residence time during snow melt, indicating potential for retention of N in valley bottom soils when flowpaths are shallow. Decreased reaction rates relative to water residence time in subsequent seasons suggest greater export of inorganic N as the soil–stream flowpath deepens due to thawing soils. Using seasonal thaw as a proxy for longer term deepening of the thaw layer caused by climate warming and permafrost degradation, these results suggest increasing potential for export of inorganic N from permafrost‐influenced soils to streams.     

Standing biomass and production in water drainages of the foothills of the Philip Smith Mountains, Alaska

In the foothills of the Philip Smith Mountains, Brooks Range, Alaska, tussock tundra is the most widely distributed vegetation, and it occurs on rolling hills and in valleys that were shaped by a sequence of Pleistocene glaciations. In this study, aboveground standing biomass and production were compared in "intertrack tundra" areas that were relatively homogenous with respect to downslope drainage and adjacent "water tracks" that acted to channel water flow to the valley bottom stream. Comparisons of biomass, leaf area index, and specific leaf weight were also made between upper and lower slope positions. Similarities and differences of vegetation structure are examined with respect to graminoid, deciduous shrub, evergreen shrub, herbaceous, and bryophyte components.
Water tracks were found to have 1.5–1.7 times the biomass of intertrack tundra, and production (excluding secondary growth) in water tracks was 40% greater than in intertrack tundra. The aboveground biomass for all areas studied and the annual production values were similar to those found in other studies of tussock tundra. While only slight differences in depth of thaw occurred in water tracks and intertrack tundra during June and early July, water tracks thawed more deeply with the onset of summer rains. Warmer temperatures at 40 cm depth in July and August may have increased nutrient availability, whereas greater rooting depth and movement of water may have increased nutrient capture in water tracks as compared with the intertrack areas. Greater biomass and a deeper thaw depth occurred at upper slope locations.     

Elaeagnus angustifolia Elevates Soil Inorganic Nitrogen Pools in Riparian Ecosystems

Elaeagnus angustifolia L., a nonnative N₂-fixer, has established within riparian corridors of the interior western United States and is now the fourth most frequently occurring woody riparian plant in this region. We examined whether E. angustifolia alters pools and fluxes of soil inorganic N at eight sites dominated by Populus deltoides ssp. wislizeni along the Rio Grande in New Mexico over 2 years. E. angustifolia contributed a small fraction of total leaf fall (<5% across sites) but accounted for a disproportionately high amount of N (19%) that entered the system from P. deltoides and E. angustifolia leaf fall, due to the high N content (>2%) of E. angustifolia senesced leaves. Soil inorganic N concentrations and potential rates of nitrification and net N mineralization varied across sites. E. angustifolia leaf fall explained 59% of the variation in soil inorganic N concentrations across years. This relationship suggests that inputs of N-rich leaf litter from E. angustifolia may increase N availability in riparian soils. We detected no relationship between E. angustifolia leaf fall and fluxes of soil inorganic N, whereas others have measured both stimulation and inhibition of soil N cycling by E. angustifolia. Greater abundance of N₂-fixing species in riparian forests may augment growth of neighboring plants or increase N export to rivers. Given these possibilities, ecosystem studies and restoration projects should further examine the potential for E. angustifolia to affect N pools and fluxes along western North American rivers.     

高等植物对有机氮吸收与利用研究进展

主要综述植物氨基酸营养生理生化和分子生物学研究的最新进展。长期以来,人们一直认为植物只能吸收无机态N,有机N必须矿化为无机N后才能被植物吸收利用,而近年来越来越多实验证明植物能吸收有机N,特别是氨基酸,其吸收能力因植物种类而异,生长在有机N丰富的北极,高山和亚高山生态环境中的植物甚至嗜好氨基酸,因此,不应过分夸大有机N矿化的重要性,迄今一些植物细胞质膜上的氨基酸转运子基因已被描述并加以克隆。     

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The inverse texture hypothesis for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.     

Landscape patterns of free amino acids in arctic tundra soils

Concentrations of free amino acids were measured in soils from four major ecosystem types in arctic Alaska. Total free amino acid concentrations were several-fold higher than ammonium (the major form of inorganic nitrogen) in water extracts of soils. The dominant free amino acids in these soils were glycine, aspartic acid, glutamic acid, serine, and arginine. Concentrations of total amino acids ranged 5-fold across communities, being highest in tussock tundra and lowest in wet meadows. Incubation experiments indicate that the turnover of amino acids is rapid, which suggests high rates of gross nitrogen mineralization in these soils. The high concentrations and dynamic nature of soil free amino acids suggest that this nitrogen pool is a significant component of nitrogen cycling in these tundra ecosystems.     

Inorganic nitrogen and microbial biomass dynamics before and during spring snowmelt

Recent work in seasonally snow covered ecosystems has identifiedthawed soil and high levels of heterotrophic activity throughout the winterunder consistent snow cover. We performed measurements during the winter of1994 to determine how the depth and timing of seasonal snow cover affectsoil microbial populations, surface water NO loss during snowmelt, and plant Navailability early in the growing season. Soil under early accumulating,consistent snow cover remained thawed during most of the winter and bothmicrobial biomass and soil inorganic N pools gradually increased under thesnowpack. At the initiation of snowmelt, microbial biomass N pools increasedfrom 3.0 to 5.9 g n m^-2,concurrent with a decrease in soil inorganic N pools. During the latterstages of snowmelt, microbial biomass N pools decreased sharply without aconcurrent increase in inorganic N pools or significant leaching losses. Incontrast, soil under inconsistent snow cover remained frozen during most ofthe winter. During snowmelt, microbial biomass initially increased from 1.7to 3.1 g N m^-2 and thendecreased as sites became snow-free. In contrast to smaller pool sizes,NO export during snowmeltfrom the inconsistent snow cover sites of 1.14 (±0.511) g N m^-2 was significantly greater (p< 0.001) than the 0.27 (±0.16) g N m^-2 exported from sites with consistent snowcover. These data suggest that microbial biomass in consistentlysnow-covered soil provides a significant buffer limiting the export ofinorganic N to surface water during snowmelt. However, this buffer is verysensitive to changes in snowpack regime. Therefore, interannual variabilityin the timing and depth of snowpack accumulation may explain the year toyear variability in inorganic N concentrations in surface water theseecosystems.     

Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi–plant interaction

Uptake of nitrogen (N) via root-mycorrhizal associations accounts for a significant portion of total N supply to many vascular plants. Using stable isotope ratios (δ¹⁵N) and the mass balance among N pools of plants, fungal tissues, and soils, a number of efforts have been made in recent years to quantify the flux of N from mycorrhizal fungi to host plants. Current estimates of this flux for arctic tundra ecosystems rely on the untested assumption that the δ¹⁵N of labile organic N taken up by the fungi is approximately the same as the δ¹⁵N of bulk soil. We report here hydrolysable amino acids are more depleted in ¹⁵N relative to hydrolysable ammonium and amino sugars in arctic tundra soils near Toolik Lake, Alaska, USA. We demonstrate, using a case study, that recognizing the depletion in ¹⁵N for hydrolysable amino acids (δ¹⁵N = ?5.6‰ on average) would alter recent estimates of N flux between mycorrhizal fungi and host plants in an arctic tundra ecosystem.     

Soil microbial responses to elevated phosphorus and pH in acidic temperate deciduous forests

Although northern temperate forests are generally not considered phosphorus (P) limited, ecosystem P limitation may occur on highly weathered or strongly acidic soils where bioavailable inorganic P is low. In such environments, soil organisms may compensate by increasing the utilization of organic P via the production of extracellular enzymes to prevent limitation. In this study, we experimentally increased available P and/or pH in several acidic eastern deciduous forests underlain by glaciated and unglaciated soils in eastern Ohio, USA. We hypothesized that where inorganic P is low; soil microbes are able to access organic P by increasing production of phosphatase enzymes, thereby overcoming biogeochemical P limitations. We measured surface soil for: available P pools, N mineralization and nitrification rates, total C and N, enzymes responsible for C, N, and P hydrolysis, and microbial community composition (PLFA). Increasing surface soil pH a whole unit had little effect on microbial community composition, but increased N cycling rates in unglaciated soils. Phosphorus additions suppressed phosphatase activities over 60% in the unglaciated soils but were unchanged in the glaciated soils. All treatments had minimal influence on microbial biomass, but available pools of P strongly correlated with microbial composition. Microbes may be dependent on sources of organic P in some forest ecosystems and from a microbial perspective soil pH might be less important overall than P availability. Although our sampling was conducted less than 1 year after treatment initiation, microbial community composition was strongly influenced by available P pools and these effects may be greater than short-term increases in soil pH.     

Neutron radiography as a tool for revealing root development in soil: capabilities and limitations

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and ¹³C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched ¹³C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.     

Immobilization, stabilization and remobilization of nitrogen in forest soils at elevated CO2: a 15N and 13C tracer study

The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO₂ and N deposition. In a dual isotope tracer experiment (¹³C, ¹⁵N) within a 4‐year CO₂ enrichment (+200 ppm_v) study with forest model ecosystems, we (i) quantified the effects of elevated CO₂ on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their ¹³C signals; and (iii) estimated the remobilization and thus, the bio‐availability of newly sequestered C and N. (1) CO₂ enrichment significantly decreased NO₃^? concentrations in soil waters and export from 1.5 m deep lysimeters by 30–80%. Consequently, elevated CO₂ increased the overall retention of N in the model ecosystems. (2) About 60–80% of added ¹⁵NH₄¹⁵NO₃ were retained in soils. The clay fraction was the greatest sink for the immobilized ¹⁵N sequestering 50–60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (<13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added ¹⁵N was mainly immobilized in stable mineral‐bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO₂. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO₂ can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.     

Effects of fertilisation and irrigation on ‘foliar afterlife’ in alpine tundra

Question: How do increases in soil nutrient and water availability alter the nutrient fluxes through the resorption and litter decomposition pathways and how do they affect litter nutrient pools in a low‐productive alpine tundra ecosystem? Location: An alpine lichen‐rich tundra on Mt. Malaya Khati‐para in the NW Caucasus, Russia (43°27’ N, 41°42’ E; altitude 2800 m a.s.l.). Methods: We conducted a 4‐year fertilisation (N, P, N+P, lime) and irrigation experiment, and analysed the responses of nutrient resorption from senescing leaves, leaf litter quality and decomposability of six pre‐dominant vascular plant species, total plant community litter production and litter (nutrient) accumulation. Results: Vascular plant litter [N] and [P] increased 1.5 and 10 fold in response to N and P additions, due to increased concentrations of the nutrients in fresh leaves and unchanged or reduced resorption efficiency. Litter decomposability was not affected by nutrient amendments. Fertilisation enhanced litter production (180%; N+P treatment) and litter accumulation (80%; N+P), owing to tremendously increased production and low decomposability of graminoids. Together with increased litter [N] and [P] this led to great increases in total litter nutrient pools. Conclusions: Due to increased production of graminoids, nutrients added to the alpine tundra soil were mostly immobilised in recalcitrant, nutrient‐rich litter. This suggests that changing species composition in low productive ecosystems may act as an internal buffer mechanism, which under increased soil nutrient availability prevents the community from rapidly acquiring features typical of a high productive ecosystem such as high decomposability and high nutrient availability.     

Environmental and physical controls on northern terrestrial methane emissions across permafrost zones

Methane (CH₄) emissions from the northern high‐latitude region represent potentially significant biogeochemical feedbacks to the climate system. We compiled a database of growing‐season CH₄ emissions from terrestrial ecosystems located across permafrost zones, including 303 sites described in 65 studies. Data on environmental and physical variables, including permafrost conditions, were used to assess controls on CH₄ emissions. Water table position, soil temperature, and vegetation composition strongly influenced emissions and had interacting effects. Sites with a dense sedge cover had higher emissions than other sites at comparable water table positions, and this was an effect that was more pronounced at low soil temperatures. Sensitivity analysis suggested that CH₄ emissions from ecosystems where the water table on average is at or above the soil surface (wet tundra, fen underlain by permafrost, and littoral ecosystems) are more sensitive to variability in soil temperature than drier ecosystems (palsa dry tundra, bog, and fen), whereas the latter ecosystems conversely are relatively more sensitive to changes of the water table position. Sites with near‐surface permafrost had lower CH₄ fluxes than sites without permafrost at comparable water table positions, a difference that was explained by lower soil temperatures. Neither the active layer depth nor the organic soil layer depth was related to CH₄ emissions. Permafrost thaw in lowland regions is often associated with increased soil moisture, higher soil temperatures, and increased sedge cover. In our database, lowland thermokarst sites generally had higher emissions than adjacent sites with intact permafrost, but emissions from thermokarst sites were not statistically higher than emissions from permafrost‐free sites with comparable environmental conditions. Overall, these results suggest that future changes to terrestrial high‐latitude CH₄ emissions will be more proximately related to changes in moisture, soil temperature, and vegetation composition than to increased availability of organic matter following permafrost thaw.     

Soil–plant N processes in a High Arctic ecosystem,NW Greenland are altered by long‐term experimental warming and higher rainfall

Rapid temperature and precipitation changes in High Arctic tundra ecosystems are altering the biogeochemical cycles of carbon (C) and nitrogen (N), but in ways that are difficult to predict. The challenge grows from the uncertainty of N cycle responses and the extent to which shifts in soil N are coupled with the C cycle and productivity of tundra systems. We used a long‐term (since 2003) experiment of summer warming and supplemental summer water additions to a High Arctic ecosystem in NW Greenland, and applied a combination of discrete sampling and in situ soil core incubations to measure C and N pools and seasonal microbial processes that might control plant‐available N. We hypothesized that elevated temperature and increased precipitation would stimulate microbial activity and net inorganic N mineralization, thereby increasing plant N‐availability through the growing season. While we did find increased N mineralization rates under both global change scenarios, water addition also significantly increased net nitrification rates, loss of NO₃^?‐N via leaching, and lowered rates of labile organic N production. We also expected the chronic warming and watering would lead to long‐term changes in soil N‐cycling that would be reflected in soil δ¹⁵N values. We found that soil δ¹⁵N decreased under the different climate change scenarios. Our results suggest that temperature accelerates biological processes and existing C and N transformations, but moisture increases soil hydraulic connectivity and so alters the pathways, and changes the fate of the products of C and N transformations. In addition, our findings indicate that warmer, wetter High Arctic tundra will be cycling N and C in ways that may transform these landscapes in part leading to greater C sequestration, but simultaneously, N losses from the upper soil profile that may be transported to depth dissolved in water and or transported off site in lateral flow.     

Spatial Heterogeneity of Denitrification in Semi-Arid Floodplains

Riparian ecosystems are recognized as sinks for inorganic nitrogen (N). Denitrification, a heterotrophic microbial process, often accounts for a significant fraction of the N removed. Characteristics of both riparian soils and hydrologic vectors may constrain the locations where denitrification can occur within riparian ecosystems by influencing the distribution of substrates, water, and suitable redox conditions. We employed spatially explicit methods to quantify heterogeneity of soil characteristics and potential rate of denitrification in semi-arid riparian ecosystems. These results allow us to evaluate the relative contributions of hydrologic vectors (water courses that convey materials) and soil resources (materials required by biota) to spatial heterogeneity of denitrification. During dry and monsoon seasons we contrasted a mesic site, characterized by shallow groundwater and annual inundation by floods, with a xeric site that is inundated less often and has a deeper water table. Potential denitrification was detected throughout the mesic floodplain and the average rate of denitrification was greater at the mesic site than at the xeric site, indicating the influence of water availability on denitrification. At the xeric reach, sharp declines in pools of soil resources and rate of denitrification occurred away from the stream, demonstrating the importance of the stream in determining spatial patterns. Using geographically weighted regression analysis, we determined that soil organic matter and soil nitrate were significant predictors of denitrification at the xeric site, but that factors influencing denitrification varied spatially. Spatial heterogeneity of carbon (C) and N substrates in soils likely influenced spatial patterns of denitrification, but distribution of C and N substrates was ultimately organized by hydrologic vectors. Droughts will increase the abundance of reaches with hydrogeomorphic templates similar to the xeric reach studied here. Consequences of such a transition may include a reduced rate of denitrification and patchy distribution of denitrification in floodplain soils, which will decrease the contribution of riparian ecosystems to N removal. TKH designed and completed the study and wrote the paper; EAW contributed methods and edited the paper; NBG designed the study and edited the paper.     

Production and export of dissolved C in arctic tundra mesocosms: the roles of vegetation and water flow

To better understand carbon (C) cycling in arctic tundra we measureddissolved C production and export rates in mesocosms of three tundra vegetationtypes: tussock, inter-tussock and wet sedge. Three flushing frequencies wereused to simulate storm events and determine potential mass export of dissolved Cunder increased soil water flow scenarios. Dissolved C production and exportrates differed between vegetation types (inter-tussock < tussock < wetsedge). In the absence of flushing, dissolved organic C (DOC) dominatedproduction in tussock and inter-tussock soils but was consumed in wet sedgesoils (8.3, 32.7, and –0.4 g C g soil^–1day^–1). Soil water dissolved C concentrations declined over time when flushedat high and medium frequencies but were variable at low flushing frequency.Total yield of dissolved C and DOC increased with increased flushing frequency.The ratio of DOC to dissolved inorganic C exported dropped with increasedflushing under tussock but not inter-tussock or wet sedge vegetation. Massexport per liter of water added declined as flushing frequency increased intussock and inter-tussock mesocosms. Export and production of dissolved C werestrongly correlated with above ground biomass, but not with photosynthetic ratesor below ground biomass. DOC quality was examined by measuring production ofToolik Lake bacteria fed mesocosm soil water. When normalized for DOCconcentration, wet sedge soil water supported significantly higher bacterialproduction. Our results indicate that arctic tundra soils have high potentialsfor dissolved C export, that water flow and vegetation type mainly controldissolved C export, and that responses of aquatic microbes to terrestrial inputsdepend on the vegetation type in the watershed.     

Responses of terrestrial nitrogen pools and dynamics to different patterns of freeze‐thaw cycle: A meta‐analysis

Altered freeze‐thaw cycle (FTC) patterns due to global climate change may affect nitrogen (N) cycling in terrestrial ecosystems. However, the general responses of soil N pools and fluxes to different FTC patterns are still poorly understood. Here, we compiled data of 1519 observations from 63 studies and conducted a meta‐analysis of the responses of 17 variables involved in terrestrial N pools and fluxes to FTC. Results showed that under FTC treatment, soil NH₄⁺, NO₃^?, NO₃^? leaching, and N₂O emission significantly increased by 18.5%, 18.3%, 66.9%, and 144.9%, respectively; and soil total N (TN) and microbial biomass N (MBN) significantly decreased by 26.2% and 4.7%, respectively; while net N mineralization or nitrification rates did not change. Temperate and cropland ecosystems with relatively high soil nutrient contents were more responsive to FTC than alpine and arctic tundra ecosystems with rapid microbial acclimation. Therefore, altered FTC patterns (such as increased duration of FTC, temperature of freeze, amplitude of freeze, and frequency of FTC) due to global climate warming would enhance the release of inorganic N and the losses of N via leaching and N₂O emissions. Results of this meta‐analysis help better understand the responses of N cycling to FTC and the relationships between FTC patterns and N pools and N fluxes.