Will CO<Subscript>2</Subscript> Emissions from Drained Tropical Peatlands Decline Over Time? Links Between Soil Organic Matter Quality,Nutrients, and C Mineralization Rates

Conversion, drainage, and cultivation of tropical peatlands can change soil conditions, shifting the C balance of these systems, which is important for the global C cycle. We examined the effect of soil organic matter (SOM) quality and nutrients on CO₂ production from peat decomposition using laboratory incubations of Indonesian peat soils from undrained forest in Kalimantan and drained oil palm plantations in Kalimantan and Sumatra. We found that oil palm soils had higher C/N and lower SOM quality than forest soils. Higher substrate quality and nutrient availability, particularly lower ratios of aromatic/aliphatic carbon and C/N, rather than total SOM or carbon, explained the higher rate of CO₂ production by forest soils (10.80 ± 0.23 µg CO₂–C g C h^?1) compared to oil palm soils (5.34 ± 0.26 µg CO₂–C g C h^?1) from Kalimantan. These factors also explained lower rates in Sumatran oil palm (3.90 ± 0.25 µg CO₂–C g C h^?1). We amended peat with nitrogen (N), phosphorus (P), and glucose to further investigate observed substrate and nutrient constraints across the range of observed peat quality. Available N limited CO₂ production, in unamended and amended soils. P addition raised CO₂ production when substrate quality was high and initial P state was low. Glucose addition raised CO₂ production in the presence of added N and P. Our results suggest that decline in SOM quality and nutrients associated with conversion may decrease substrate-driven rates of CO₂ production from peat decomposition over time.     

The Temperature Response of CO<Subscript>2</Subscript> Production from Bulk Soils and Soil Fractions is Related to Soil Organic Matter Quality

The projected increase in global mean temperature could accelerate the turnover of soil organic matter (SOM). Enhanced soil CO₂ emissions could feedback on the climate system, depending on the balance between the sensitivity to temperature of net carbon fixation by vegetation and SOM decomposition. Most of the SOM is stabilised by several physico-chemical mechanisms within the soil architecture, but the response of this quantitatively important fraction to increasing temperature is largely unknown. The aim of this study was to relate the temperature sensitivity of decomposition of physical and chemical soil fractions (size fractions, hydrolysis residues), and of bulk soil, to their quality and turnover time. Soil samples were taken from arable and grassland soils from the Swiss Central Plateau, and CO₂ production was measured under strictly controlled conditions at 5, 15, 25, and 35 °C by using sequential incubation. Physico-chemical properties of the samples were characterised by measuring elemental composition, surface area, ¹⁴C age, and by using DRIFT spectroscopy. CO₂ production rates per unit (g) organic carbon (OC) strongly varied between samples, in relation to the difference in the biochemical quality of the substrates. The temperature response of all samples was exponential up to 25 °C, with the largest variability at lower temperatures. Q₁₀ values were negatively related to CO₂ production over the whole temperature range, indicating higher temperature sensitivity of SOM of lower quality. In particular, hydrolysis residues, representing a more stabilised SOM pool containing older C, produced less CO₂ g⁻¹ OC than non-hydrolysed fractions or bulk samples at lower temperatures, but similar rates at ≥25 °C, leading to higher Q₁₀ values than in other samples. Based on these results and provided that they apply also to other soils it is suggested that because of the higher sensitivity of passive SOM the overall response of SOM to increasing temperatures might be higher than previously expected from SOM models. Finally, surface area measurements revealed that micro-aggregation rather than organo-mineral association mainly contributes to the longer turnover time of SOM isolated by acid hydrolysis.     

Vulnerability of wetland soil carbon stocks to climate warming in the perhumid coastal temperate rainforest

The perhumid coastal temperate rainforest (PCTR) of southeast Alaska has some of the densest soil organic carbon (SOC) stocks in the world (>300 Mg C ha^?1) but the fate of this SOC with continued warming remains largely unknown. We quantified dissolved organic carbon (DOC) and carbon dioxide (CO₂) yields from four different wetland types (rich fen, poor fen, forested wetland and cedar wetland) using controlled laboratory incubations of surface (10 cm) and subsurface (25 cm) soils incubated at 8 and 15 °C for 37 weeks. Furthermore, we used fluorescence characterization of DOC and laboratory bioassays to assess how climate-induced soil warming may impact the quality and bioavailability of DOC delivered to fluvial systems. Soil temperature was the strongest control on SOC turnover, with wetland type and soil depth less important in controlling CO₂ flux and extractable DOC. The high temperature incubation increased average CO₂ yield by ~40 and ~25% for DOC suggesting PCTR soils contain a sizeable pool of readily biodegradable SOC that can be mineralized to DOC and CO₂ with future climate warming. Fluxes of CO₂ were positively correlated to both extractable DOC and percent bioavailable DOC during the last few months of the incubation suggesting mineralization of SOC to DOC is a strong control of soil respiration rates. Whether the net result is increased export of either carbon form will depend on the balance between the land to water transport of DOC and the ability of soil microbial communities to mineralize DOC to CO₂.     

Temperature sensitivity of greenhouse gas production in wetland soils of different vegetation

Organic matter decomposition regulates rates of carbon loss (CO₂ and CH₄) in wetlands and has implications for carbon sequestration in the context of changing global temperature. Here we determined the influence of temperature and vegetation type on both aerobic and anaerobic decomposition of organic matter in subtropical wetland soils. As in many other studies, increased temperature resulted in higher rates of respiration and methanogenesis under both aerobic and anaerobic conditions, and the positive effect of temperature depended on vegetation (source of carbon substrate to soil). Under anaerobic incubations, the proportion of gaseous C (CO₂ and CH₄) lost as CH₄ increased with temperature indicating a greater sensitivity of methanogenesis to temperature. This was further supported by a wider range of Q₁₀ values (1.4–3.6) for methane production as compared with anaerobic CO₂ (1.3–2.5) or aerobic CO₂ (1.4–2.1) production. The increasing strength of positive linear correlation between CO₂:CH₄ ratio and the soil organic matter ligno-cellulose index at higher temperature indicated that the temperature sensitivity of methanogenesis was likely the result of increased C availability at higher temperature. This information adds to our basic understanding of decomposition in warmer subtropical and tropical wetland systems and has implications for C models in wetlands with different vegetation types.     

Experimental evidence for the attenuating effect of SOM protection on temperature sensitivity of SOM decomposition

The ability to predict C cycle responses to temperature changes depends on the accurate representation of temperature sensitivity (Q₁₀) of soil organic matter (SOM) decomposition in C models for different C pools and soil depths. Theoretically, Q₁₀ of SOM decomposition is determined by SOM quality and availability (referred to here as SOM protection). Here, we focus on the role of SOM protection in attenuating the intrinsic, SOM quality dependent Q₁₀. To assess the separate effects of SOM quality and protection, we incubated topsoil and subsoil samples characterized by differences in SOM protection under optimum moisture conditions at 25 °C and 35 °C. Although lower SOM quality in the subsoil should lead to a higher Q₁₀ according to kinetic theory, we observed a much lower overall temperature response in subsoil compared with the topsoil. Q₁₀ values determined for respired SOM fractions of decreasing lability within the topsoil increased from 1.9 for the most labile to 3.8 for the least labile respired SOM, whereas corresponding Q₁₀ values for the subsoil did not show this trend (Q₁₀ between 1.4 and 0.9). These results indicate the existence of a limiting factor that attenuates the intrinsic effect of SOM quality on Q₁₀ in the subsoil. A parallel incubation experiment of ¹³C‐labeled plant material added to top‐ and subsoil showed that decomposition of an unprotected C substrate of equal quality responds similarly to temperature changes in top‐ and subsoil. This further confirms that the attenuating effect on Q₁₀ in the subsoil originates from SOM protection rather than from microbial properties or other nutrient limitations. In conclusion, we found experimental evidence that SOM protection can attenuate the intrinsic Q₁₀ of SOM decomposition.     

Short-term responses of ecosystem carbon fluxes to experimental soil warming at the Swiss alpine treeline

Climatic warming will probably have particularly large impacts on carbon fluxes in high altitude and latitude ecosystems due to their great stocks of labile soil C and high temperature sensitivity. At the alpine treeline, we experimentally warmed undisturbed soils by 4 K for one growing season with heating cables at the soil surface and measured the response of net C uptake by plants, of soil respiration, and of leaching of dissolved organic carbon (DOC). Soil warming increased soil CO₂ effluxes instantaneously and throughout the whole vegetation period (+45%; +120 g C m y^?1). In contrast, DOC leaching showed a negligible response of a 5% increase (NS). Annual C uptake of new shoots was not significantly affected by elevated soil temperatures, with a 17, 12, and 14% increase for larch, pine, and dwarf shrubs, respectively, resulting in an overall increase in net C uptake by plants of 20–40 g C m^?2y^?1. The Q ₁₀ of 3.0 measured for soil respiration did not change compared to a 3-year period before the warming treatment started, suggesting little impact of warming-induced lower soil moisture (?15% relative decrease) or increased soil C losses. The fraction of recent plant-derived C in soil respired CO₂ from warmed soils was smaller than that from control soils (25 vs. 40% of total C respired), which implies that the warming-induced increase in soil CO₂ efflux resulted mainly from mineralization of older SOM rather than from stimulated root respiration. In summary, one season of 4 K soil warming, representative of hot years, led to C losses from the studied alpine treeline ecosystem by increasing SOM decomposition more than C gains through plant growth.     

Rate of warming affects temperature sensitivity of anaerobic peat decomposition and greenhouse gas production

Temperature sensitivity of anaerobic carbon mineralization in wetlands remains poorly represented in most climate models and is especially unconstrained for warmer subtropical and tropical systems which account for a large proportion of global methane emissions. Several studies of experimental warming have documented thermal acclimation of soil respiration involving adjustments in microbial physiology or carbon use efficiency (CUE), with an initial decline in CUE with warming followed by a partial recovery in CUE at a later stage. The variable CUE implies that the rate of warming may impact microbial acclimation and the rate of carbon‐dioxide (CO₂) and methane (CH₄) production. Here, we assessed the effects of warming rate on the decomposition of subtropical peats, by applying either a large single‐step (10°C within a day) or a slow ramping (0.1°C/day for 100 days) temperature increase. The extent of thermal acclimation was tested by monitoring CO₂ and CH₄ production, CUE, and microbial biomass. Total gaseous C loss, CUE, and MBC were greater in the slow (ramp) warming treatment. However, greater values of CH₄–C:CO₂–C ratios lead to a greater global warming potential in the fast (step) warming treatment. The effect of gradual warming on decomposition was more pronounced in recalcitrant and nutrient‐limited soils. Stable carbon isotopes of CH₄ and CO₂ further indicated the possibility of different carbon processing pathways under the contrasting warming rates. Different responses in fast vs. slow warming treatment combined with different endpoints may indicate alternate pathways with long‐term consequences. Incorporations of experimental results into organic matter decomposition models suggest that parameter uncertainties in CUE and CH₄–C:CO₂–C ratios have a larger impact on long‐term soil organic carbon and global warming potential than uncertainty in model structure, and shows that particular rates of warming are central to understand the response of wetland soils to global climate change.     

Can composition and physical protection of soil organic matter explain soil respiration temperature sensitivity?

The importance of soil organic matter (SOM) in the global carbon (C) cycle has been highlighted by many studies, but the way in which SOM stabilization processes and chemical composition affect decomposition rates under natural climatic conditions is not yet well understood. To relate the temperature sensitivity of heterotrophic soil respiration to the decomposition potential of SOM, we compared temperature sensitivities of respiration rates from a 2-year long soil translocation experiment from four elevations along a ~3000 m tropical forest gradient. We determined SOM stabilization mechanisms and the molecular structure of soil C from different horizons collected before and after the translocation. Soil samples were analysed by physical fractionation procedures, ¹³C nuclear magnetic resonance (NMR) spectroscopy, and differential scanning calorimetry (DSC). The temperature sensitivity (Q ₁₀) of heterotrophic soil respiration at the four sites along the elevation transect did not correlate with either the available amount of SOM or its chemical structure. Only the relative distribution of C into physical soil fractions correlated with Q ₁₀ values. We therefore conclude that physical fractionation of soil samples is the most appropriate way to assess the temperature sensitivity of SOM.     

Source and age of dissolved and gaseous carbon in a peatland–riparian–stream continuum: a dual isotope (14C and δ13C) analysis

Radiocarbon isotopes are increasingly being used to investigate the age and source of carbon released from peatlands. Here we use combined ¹⁴C and δ¹³C measurements to determine the isotopic composition of soil and soil decomposition products [dissolved organic carbon (DOC), CO₂ and CH₄] in a peatland–riparian–stream transect, to establish the isotopic signature and potential connectivity between carbon pools. Sampling was conducted during two time periods in 2012 to investigate processes under different temperature, hydrological and flux conditions. Isotopic differences existed in the peatland and riparian zone soil organic matter as a result of the riparian depositional formation. The peatland had a mean radiocarbon age of 551 ± 133 years BP, with age increasing with depth, and δ¹³C values consistent with C3 plant material as the primary source. In contrast the riparian zone had a much older radiocarbon age of 1,055 ± 107 years BP and showed no age/depth relationship; δ¹³C in the riparian zone was also consistent with C3 plant material. With the exception of DOC in September, soil decomposition products were predominately >100 %modern with ¹⁴C values consistent with derivation from organic matter fixed in the previous 5 years. Emissions of CO₂ and CH₄ from the soil surface were also modern. In contrast, CO₂ and CH₄ evaded from the stream surface was older (CH₄: 310–537 years BP, CO₂: 36 years BP to modern) and contained a more complex mix of sources combining soil organic matter and geogenic carbon. The results suggest considerable vertical transport of modern carbon to depth within the soil profile. The importance of modern recently fixed carbon and the differences between riparian and stream isotopic signatures suggests that the peatland (not the riparian zone) is the most important source of carbon to stream water.     

Temperature and peat type control CO2 and CH4 production in Alaskan permafrost peats

Controls on the fate of ~277 Pg of soil organic carbon (C) stored in permafrost peatland soils remain poorly understood despite the potential for a significant positive feedback to climate change. Our objective was to quantify the temperature, moisture, organic matter, and microbial controls on soil organic carbon (SOC) losses following permafrost thaw in peat soils across Alaska. We compared the carbon dioxide (CO₂) and methane (CH₄) emissions from peat samples collected at active layer and permafrost depths when incubated aerobically and anaerobically at ?5, ?0.5, +4, and +20 °C. Temperature had a strong, positive effect on C emissions; global warming potential (GWP) was >3× larger at 20 °C than at 4 °C. Anaerobic conditions significantly reduced CO₂ emissions and GWP by 47% at 20 °C but did not have a significant effect at ?0.5 °C. Net anaerobic CH₄ production over 30 days was 7.1 ± 2.8 μg CH₄‐C gC^?1 at 20 °C. Cumulative CO₂ emissions were related to organic matter chemistry and best predicted by the relative abundance of polysaccharides and proteins (R² = 0.81) in SOC. Carbon emissions (CO₂‐C + CH₄‐C) from the active layer depth peat ranged from 77% larger to not significantly different than permafrost depths and varied depending on the peat type and peat decomposition stage rather than thermal state. Potential SOC losses with warming depend not only on the magnitude of temperature increase and hydrology but also organic matter quality, permafrost history, and vegetation dynamics, which will ultimately determine net radiative forcing due to permafrost thaw.     

Regulation of Decomposition and Methane Dynamics across Natural,Commercially Mined,and Restored Northern Peatlands

Abstract We examined aerobic and anaerobic microbial carbon dioxide (CO₂) and methane (CH₄) exchange in peat samples representing different profiles at natural, mined, mined-abandoned, and restored northern peatlands and characterized the nutrient and substrate chemistry and microbial biomass of these soils. Mining and abandonment led to reduced nutrient and substrate availability and occasionally drier conditions in surface peat resulting in a drastic reduction in CO₂ and CH₄ production, in agreement with previous studies. Owing mainly to wetter conditions, CH₄ production and oxidation were faster in restored block-cut than natural sites, whereas in one restored site, increased substrate and nutrient availability led to much more rapid rates of CO₂ production. Our work in restored block-cut sites compliments that in vacuum-harvested peatlands undergoing more recent active restoration attempts. The sites we examined covered a large range of soil C substrate quality, nutrient availability, microbial biomass, and microbial activities, allowing us to draw general conclusions about controls on microbial CO₂ and CH₄ dynamics using stepwise regression analysis among all sites and soil depths. Aerobic and anaerobic decomposition of peat was constrained by organic matter quality, particularly phosphorus (P) and carbon (C) chemistry, and closely linked to the size of the microbial biomass supported by these limiting resources. Methane production was more dominantly controlled by field moisture content (a proxy for anaerobism), even after 20 days of anaerobic laboratory incubation, and to a lesser extent by C substrate availability. As methanogens likely represented only a small proportion of the total microbial biomass, there were no links between total microbial biomass and CH₄ production. Methane oxidation was controlled by the same factors influencing CH₄ production, leading to the conclusion that CH₄ oxidation is primarily controlled by substrate (that is, CH₄) availability. Although restoring hydrology similar to natural sites may re-establish CH₄ dynamics, there is geographic or site-specific variability in the ability to restore peat decomposition dynamics.     

Recovery of Methanogenesis Following Summer Drought in Soils from Two Cool Temperate Peatlands,New York State,USA

Following a summer drought, intact cores of peat soil from two cool temperate peatlands (a rain-fed bog and a groundwater-fed swamp) were exposed experimentally to three different water table levels. The goal was to examine recovery of anaerobic methanogenesis and to evaluate peat soil decomposition to methane (CH₄), carbon dioxide (CO₂), and dissolved organic carbon (DOC) upon rewetting. Methane emission from soils to the atmosphere was greatest (mean = 80 μmol m^?2 s^?1) when the entire peat core was rewetted quickly; emission was negligible at low water level and when peat cores were rewetted gradually. Rates of CO₂ emission (mean = 1.0 μmol m^?2 s^?1) were relatively insensitive to water level. Concentrations of CH₄ in soil air spaces suggest that onset of methanogenesis induces, but later represses, aerobic oxidation of CH₄ above the water table. Concentrations of CO₂ suggest production at the soil surface of swamp peat versus at greater depths in bog peat. Portions of peat soil incubated in vitro without oxygen (O₂) exhibited a lag before the onset of methanogenesis, and the lag time was less in peat from the cores rewetted quickly. The inhibition of methanogenesis by the selective inhibitor 2-bromoethanesulfonic acid (BES) decreased CO₂ production by 20 to 30% but resulted in an increase in concentrations of DOC by 2 to 5 times. The results show that methanogens in peat soils tolerate moderate drought, and recovery varies among different peat types. In peat soils, the inhibition of methanogenesis might enhance DOC availability.     

Effects of elevated CO2 and temperature on the growth,elemental composition,and cell size of two marine diatoms: potential implications of global climate change

Two pennate diatoms, Amphora coffeaeformis and Nitzschia ovalis, were used to evaluate potential responses to the future CO₂ and temperature increases with respect to cell-specific growth rate, elemental composition, size, population growth rate, and carrying capacity. Diatoms were subjected to four different treatments over a 2 week period (approximately 4 generations): a control (28°C and present-day CO₂, ~400 ppm), high CO₂ (28°C with high CO₂, ~750 ppm), high temperature (31°C and present-day CO₂, ~400 ppm), and greenhouse-effect treatment (31°C with high CO₂, ~750 ppm). The results indicated that both the cell-specific growth rates and the carrying capacity of A. coffeaeformis decreased at the higher temperature treatment, whereas N. ovalis did not differ among all treatments. No significant difference was found in either species’ elemental cell composition, but higher C:N and C:P ratios were observed for A. coffeaeformis and N. ovalis, respectively, in high CO₂ and greenhouse-effect treatments. Smaller cell sizes were observed for both species under the greenhouse-effect treatment, a phenomenon that could alter benthic food webs in the future.     

Biogeochemical effects of simulated sea level rise on carbon loss in an Everglades mangrove peat soil

Saltwater intrusion and inundation can affect soil microbial activity, which regulates the carbon (C) balance in mangroves and helps to determine if these coastal forests can keep pace with sea level rise (SLR). This study evaluated the effects of increased salinity (+15 ppt), increased inundation (?8 cm), and their combination, on soil organic C loss from a mangrove peat soil (Everglades, Florida, USA) under simulated tides. Soil respiration (CO₂ flux), methane (CH₄) flux, dissolved organic carbon (DOC) production, and porewater nutrient concentrations were quantified. Soil respiration was the major pathway of soil organic C loss (94–98%) and was approximately 90% higher in the control water level than the inundated treatment under elevated salinity. Respiration rate increased with water temperature, but depended upon salinity and tidal range. CH₄ flux was minimal, while porewater DOC increased with a concomitant, significant decline in soil bulk density under increased inundation. Porewater ammonium increased (73%) with inundation and soluble reactive phosphorus increased (32%) with salinity. Overall, the decline in soil organic C mineralization from combined saltwater intrusion and prolonged inundation was not significant, but results suggest SLR could increase this soil’s susceptibility to peat collapse and accelerate nutrient and DOC export to adjacent Florida Bay.     

No temperature acclimation of soil extracellular enzymes to experimental warming in an alpine grassland ecosystem on the Tibetan Plateau

Alpine grassland soils store large amounts of soil organic carbon (SOC) and are susceptible to rising air temperature. Soil extracellular enzymes catalyze the rate-limiting step in SOC decomposition and their catalysis, production and degradation rates are regulated by temperature. Therefore, the responses of these enzymes to warming could have a profound impact on carbon cycling in the alpine grassland ecosystems. This study was conducted to measure the responses of soil extracellular enzyme activity and temperature sensitivity (Q₁₀) to experimental warming in samples from an alpine grassland ecosystem on the Tibetan Plateau. A free air-temperature enhancement system was set up in May 2006. We measured soil microbial biomass, nutrient availability and the activity of five extracellular enzymes in 2009 and 2010. The Q₁₀ of each enzyme was calculated using a simple first-order exponential equation. We found that warming had no significant effects on soil microbial biomass C, the labile C or N content, or nutrient availability. Significant differences in the activity of most extracellular enzymes among sampling dates were found, with typically higher enzyme activity during the warm period of the year. The effects of warming on the activity of the five extracellular enzymes at 20 °C were not significant. Enzyme activity in vitro strongly increased with temperature up to 27 °C or over 30 °C (optimum temperature; T_opt). Seasonal variations in the Q₁₀ were found, but the effects of warming on Q₁₀ were not significant. We conclude that soil extracellular enzymes adapted to seasonal temperature variations, but did not acclimate to the field experimental warming.     

Vegetation and climate controls on potential CO2, DOC and DON production in northern latitude soils

Climatic change may influence decomposition dynamics in arctic and boreal ecosystems, affecting both atmospheric CO₂ levels, and the flux of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) to aquatic systems. In this study, we investigated landscape‐scale controls on potential production of these compounds using a one‐year laboratory incubation at two temperatures (10° and 30 °C). We measured the release of CO₂, DOC and DON from tundra soils collected from a variety of vegetation types and climatic regimes: tussock tundra at four sites along a latitudinal gradient from the interior to the north slope of Alaska, and soils from additional vegetation types at two of those sites (upland spruce at Fairbanks, and wet sedge and shrub tundra at Toolik Lake in northern Alaska). Vegetation type strongly influenced carbon fluxes. The highest CO₂ and DOC release at the high incubation temperature occurred in the soils of shrub tundra communities. Tussock tundra soils exhibited the next highest DOC fluxes followed by spruce and wet sedge tundra soils, respectively. Of the fluxes, CO₂ showed the greatest sensitivity to incubation temperatures and vegetation type, followed by DOC. DON fluxes were less variable. Total CO₂ and total DOC release were positively correlated, with DOC fluxes approximately 10% of total CO₂ fluxes. The ratio of CO₂ production to DOC release varied significantly across vegetation types with Tussock soils producing an average of four times as much CO₂ per unit DOC released compared to Spruce soils from the Fairbanks site. Sites in this study released 80–370 mg CO₂‐C g soil C^?1 and 5–46 mg DOC g soil C^?1 at high temperatures. The magnitude of these fluxes indicates that arctic carbon pools contain a large proportion of labile carbon that could be easily decomposed given optimal conditions. The size of this labile pool ranged between 9 and 41% of soil carbon on a g soil C basis, with most variation related to vegetation type rather than climate.     

Release of aquatic carbon from two peatland catchments in E. Finland during the spring snowmelt period

Spring snowmelt in the arctic and boreal regions represents the most significant event in the hydrological year. We measured concentrations and fluxes of different carbon species in 2 small contrasting (control v drained) forested peatland catchments in E. Finland between April and June 2008 and compared these to long-term annual fluxes. Measurements were made using a combination of continuous sensors (CO₂, temperature, pH, discharge) and routine spot sampling (DOC, POC, DIC, CO₂, CH₄, N₂O). The highest concentrations of CO₂ and CH₄ in streamwater were observed under low flow conditions before the spring flood event, reflecting accumulation and downstream release of gaseous C at the end of the winter period. Over the length of the study mean CH₄ concentrations were 10× higher in the drained site. The snowmelt event was associated with a dilution of DOC and CO₂, with the drained catchment showing a much flashier hydrological response compared to the control site, and post-event, a slower recovery in DOC and CO₂ concentrations. Fluxes of all carbon species during the snowmelt event were significant and represented 37?C45% of the annual flux. This highlights the challenge of quantifying aquatic C fluxes in areas with large temporal variability and suggests that inability to ??capture?? the spring snowmelt event may lead to under-estimation of C fluxes in northern regions.     

Rapid laboratory measurement of the temperature dependence of soil respiration and application to changes in three diverse soils through the year

Determining the temperature dependence of soil respiration is needed to test predictive models such as Arrhenius-like functions and macro-molecular rate theory (MMRT). We tested a method for rapid measurement of respiration using a temperature gradient block, cooled at one end (~2 °C) and heated at the other (~50 °C) that accommodated 44 tubes containing soil incubated at roughly 1 °C increments. Gas samples were taken after 5 h incubation and analysed for CO₂. The temperature gradient block allowed rapid assessment of temperature dependence of soil respiration with the precision needed to test models and explore existing theories of how temperature and moisture interact to control biochemical processes. Temperature response curves were well fitted by MMRT and allowed calculation of the temperature at which absolute temperature sensitivity was maximal (T_inf). We measured temperature response of three soils at seven moisture contents and showed that the absolute rate and sensitivity of respiration was partly dependent on adjusted moisture content. This result implied that comparisons between soils need to be made at a common moisture content. We also measured potential changes in the temperature dependence (and sensitivity) of respiration for three different soils collected at one site throughout a year. T_inf ranged from 43 to 51 °C for the three soils. T_inf and temperature sensitivity were not dependent on soil type collected but was partly dependent on time of year of collection. Temporal changes in temperature response suggested that the microbial communities may tune their metabolisms in response to changes in soil temperatures.     

Age and source of different forms of carbon released from boreal peatland streams during spring snowmelt in E. Finland

Isotopic data are increasingly being used to quantify and understand the processes that control the release of carbon (C) from northern peatlands. We used δ¹³C and ¹⁴C measurements to investigate the source and age of different forms of aquatic C (DOC, POC and evasion CO₂) released from 2 contrasting (undrained v drained) forested peatland catchments at the end of the winter snowmelt period in boreal E Finland. The δ¹³C_VPDB values of DOC (range ?28.3 to ?28.8 ‰) were generally more ¹³C depleted than evasion CO₂ (range ?22.7 to ?31.5 ‰) and showed no clear differences between the pre-flood, flood and post-flood periods. Both forms of C had evidence of bomb-¹⁴C (i.e. >100%modern), indicating that they contained substantial quantities of C fixed since the mid AD 1950s. However, DOC was ¹⁴C enriched compared to evasion CO₂, with ¹⁴C concentrations suggesting that, on average, DOC-C was ~5–6 years younger than evasion CO₂–C, with the most recently fixed C being released when temperatures were highest. POC was significantly depleted in ¹⁴C with conventional (uncalibrated) radiocarbon ages of 805–1135 BP. In contrast to other studies, the isotopic compositions of DOC and evasion CO₂ were very similar, suggesting a predominantly single and consistent C source (decomposition of soil organic matter; SOM) during the snowmelt period. Whilst we found no evidence to suggest that old (pre-bomb) C was being released at the end of the winter period, the drained site was associated with more ¹⁴C depleted and ¹³C enriched evasion CO₂, suggesting a closer link to the atmospheric CO₂ pool. Our isotopic data suggest that the various forms of C released to the aquatic system from these forested Finnish peatlands are closely related, largely unaffected by drainage and (at least in the case of evasion CO₂ and DOC) indicate strong connectivity between C cycling in the soil–plant–water system.