Beavers affect carbon biogeochemistry: both short‐term and long‐term processes are involved

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Circumpolar assessment of permafrost C quality and its vulnerability over time using long‐term incubation data

High‐latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long‐term (>1 year) aerobic soil incubations from 121 individual samples from 23 high‐latitude ecosystems located across the northern circumpolar permafrost zone. Using a three‐pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO₂ within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes.     

Potential carbon release from permafrost soils of Northeastern Siberia

Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO₂ is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO₂ with radiocarbon (¹⁴C) ages ranging between～21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q₁₀ value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of Δ¹⁴CO₂ values in respiration CO₂ and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO₂ in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of～40 Pg C to the atmosphere over a period of four decades.     

A High Arctic soil ecosystem resists long‐term environmental manipulations

We evaluated above‐ and belowground ecosystem changes in a 16 year, combined fertilization and warming experiment in a High Arctic tundra deciduous shrub heath (Alexandra Fiord, Ellesmere Island, NU, Canada). Soil emissions of the three key greenhouse gases (GHGs) (carbon dioxide, methane, and nitrous oxide) were measured in mid‐July 2009 using soil respiration chambers attached to a FTIR system. Soil chemical and biochemical properties including Q₁₀ values for CO₂, CH₄, and N₂O, Bacteria and Archaea assemblage composition, and the diversity and prevalence of key nitrogen cycling genes including bacterial amoA, crenarchaeal amoA, and nosZ were measured. Warming and fertilization caused strong increases in plant community cover and height but had limited effects on GHG fluxes and no substantial effect on soil chemistry or biochemistry. Similarly, there was a surprising lack of directional shifts in the soil microbial community as a whole or any change at all in microbial functional groups associated with CH₄ consumption or N₂O cycling in any treatment. Thus, it appears that while warming and increased nutrient availability have strongly affected the plant community over the last 16 years, the belowground ecosystem has not yet responded. This resistance of the soil ecosystem has resulted in limited changes in GHG fluxes in response to the experimental treatments.     

Carbon accumulation in a permafrost polygon peatland: steady long‐term rates in spite of shifts between dry and wet conditions

Ice‐wedge polygon peatlands contain a substantial part of the carbon stored in permafrost soils. However, little is known about their long‐term carbon accumulation rates (CAR) in relation to shifts in vegetation and climate. We collected four peat profiles from one single polygon in NE Yakutia and cut them into contiguous 0.5 cm slices. Pollen density interpolation between AMS ¹⁴C dated levels provided the time span contained in each of the sample slices, which – in combination with the volumetric carbon content – allowed for the reconstruction of CAR over decadal and centennial timescales. Vegetation representing dry palaeo‐ridges and wet depressions was reconstructed with detailed micro‐ and macrofossil analysis. We found repeated shifts between wet and dry conditions during the past millennium. Dry ridges with associated permafrost growth originated during phases of (relatively) warm summer temperature and collapsed during relatively cold phases, illustrating the important role of vegetation and peat as intermediaries between ambient air temperature and the permafrost. The average long‐term CAR across the four profiles was 10.6 ± 5.5 g C m^?2 yr^?1. Time‐weighted mean CAR did not differ significantly between wet depression and dry ridge/hummock phases (10.6 ± 5.2 g C m^?2 yr^?1 and 10.3 ± 5.7 g C m^?2 yr^?1, respectively). Although we observed increased CAR in relation to warm shifts, we also found changes in the opposite direction and the highest CAR actually occurred during the Little Ice Age. In fact, CAR rather seems to be governed by strong internal feedback mechanisms and has roughly remained stable on centennial time scales. The absence of significant differences in CAR between dry ridge and wet depression phases suggests that recent warming and associated expansion of shrubs will not affect long‐term rates of carbon burial in ice‐wedge polygon peatlands.     

Potential for long‐term transfer of dissolved organic carbon from riparian zones to streams in boreal catchments

Boreal regions store most of the global terrestrial carbon, which can be transferred as dissolved organic carbon (DOC) to inland waters with implications for both aquatic ecology and carbon budgets. Headwater riparian zones (RZ) are important sources of DOC, and often just a narrow ‘dominant source layer’ (DSL) within the riparian profile is responsible for most of the DOC export. Two important questions arise: how long boreal RZ could sustain lateral DOC fluxes as the sole source of exported carbon and how its hydromorphological variability influences this role. We estimate theoretical turnover times by comparing carbon pools and lateral exports in the DSL of 13 riparian profiles distributed over a 69 km² catchment in northern Sweden. The thickness of the DSL was 36 ± 18 (average ± SD) cm. Thus, only about one‐third of the 1‐m‐deep riparian profile contributed 90% of the lateral DOC flux. The 13 RZ exported 8.7 ± 6.5 g C m^?2 year^?1, covering the whole range of boreal stream DOC exports. The variation could be explained by local hydromorphological characteristics including RZ width (R² = 0.90). The estimated theoretical turnover times were hundreds to a few thousands of years, that is there is a potential long‐lasting supply of DOC. Estimates of net ecosystem production in the RZ suggest that lateral fluxes, including both organic and inorganic C, could be maintained without drawing down the riparian pools. This was supported by measurements of stream DO¹⁴C that indicated modern carbon as the predominant fraction exported, including streams disturbed by ditching. The transfer of DOC into boreal inland waters from new and old carbon sources has a major influence on surface water quality and global carbon balances. This study highlights the importance of local variations in RZ hydromorphology and DSL extent for future DOC fluxes under a changing climate.     

Soil organic matter quality influences mineralization and GHG emissions in cryosols: a field‐based study of sub‐ to high Arctic

Arctic soils store large amounts of labile soil organic matter (SOM) and several studies have suggested that SOM characteristics may explain variations in SOM cycling rates across Arctic landscapes and Arctic ecosystems. The objective of this study was to investigate the influence of routinely measured soil properties and SOM characteristics on soil gross N mineralization and soil GHG emissions at the landscape scale. This study was carried out in three Canadian Arctic ecosystems: Sub‐Arctic (Churchill, MB), Low‐Arctic (Daring Lake, NWT), and High‐Arctic (Truelove Lowlands, NU). The landscapes were divided into five landform units: (1) upper slope, (2) back slope, (3) lower slope, (4) hummock, and (5) interhummock, which represented a great diversity of Static and Turbic Cryosolic soils including Brunisolic, Gleysolic, and Organic subgroups. Soil gross N mineralization was measured using the ¹⁵N dilution technique, whereas soil GHG emissions (N₂O, CH₄, and CO₂) were measured using a multicomponent Fourier transform infrared gas analyzer. Soil organic matter characteristics were determined by (1) water‐extractable organic matter, (2) density fractionation of SOM, and (3) solid‐state CPMAS ¹³C nuclear magnetic resonance (NMR) spectroscopy. Results showed that gross N mineralization, N₂O, and CO₂ emissions were affected by SOM quantity and SOM characteristics. Soil moisture, soil organic carbon (SOC), light fraction (LF) of SOM, and O‐Alkyl‐C to Aromatic‐C ratio positively influenced gross N mineralization, N₂O and CO₂ emissions, whereas the relative proportion of Aromatic‐C negatively influenced those N and C cycling processes. Relationships between SOM characteristics and CH₄ emissions were not significant throughout all Arctic ecosystems. Furthermore, results showed that lower slope and interhummock areas store relatively more labile C than upper and back slope locations. These results are particularly important because they can be used to produce better models that evaluate SOM stocks and dynamics under several climate scenarios and across Arctic landscapes and ecosystems.     

Future active layer dynamics and carbon dioxide production from thawing permafrost layers in Northeast Greenland

Thawing permafrost and the resulting mineralization of previously frozen organic carbon (C) is considered an important future feedback from terrestrial ecosystems to the atmosphere. Here, we use a dynamic process oriented permafrost model, the CoupModel, to link surface and subsurface temperatures from a moist permafrost soil in High‐Arctic Greenland with observed heat production and carbon dioxide (CO₂) release rates from decomposition of previously frozen organic matter. Observations show that the maximum thickness of the active layer at the end of the summer has increased 1 cm yr^?1 since 1996. The model is successfully adjusted and applied for the study area and shown to be able to simulate active layer dynamics. Subsequently, the model is used to predict the active layer thickness under future warming scenarios. The model predicts an increase of maximum active layer thickness from today 70 to 80–105 cm as a result of a 2–6 °C warming. An additional increase in the maximum active layer thickness of a few centimetres may be expected due to heat production from decomposition of organic matter. Simulated future soil temperatures and water contents are subsequently used with measured basal soil respiration rates in a respiration model to predict the corresponding depth‐integrated CO₂ production from permafrost layers between 0.7 and 2 m below the surface. Results show an increase from present values of <40 g C m^?2 yr^?1 to between 120 and 213 g C m^?2 yr^?1 depending on the magnitude of predicted warming. These rates are more than 50% of the present soil CO₂ efflux measured at the soil surface. Future modelling accounting for snow, vegetation and internal biological heat feedbacks are of interest in order to test the robustness of the above predictions and to describe the entire ecosystem response.     

Net annual global warming potential and greenhouse gas intensity in Chinese double rice‐cropping systems: a 3‐year field measurement in long‐term fertilizer experiments

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long‐term fertilizer experiment in Chinese double rice‐cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH₄) and nitrous oxide (N₂O) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0–20 cm) organic carbon (SOC) density over the 10‐year period 1999–2009. Long‐term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha^?1 yr^?1 for the control and 1.01–1.43 t C ha^?1 yr^?1 for the fertilizer plots. Long‐term inorganic fertilizer application tended to increase CH₄ emissions during the flooded rice season and significantly increased N₂O emissions from drained soils during the nonrice season. Annual mean CH₄ emissions ranged from 621 kg CH₄ ha^?1 for the control to 1175 kg CH₄ ha^?1 for the FOM plots, 63–83% of which derived from the late‐rice season. Annual N₂O emission averaged 1.15–4.11 kg N₂O–N ha^?1 in the double rice‐cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application.     

典型温带雨养泥炭沼泽湿地地下部二氧化碳和甲烷浓度变化规律及其影响因素

为研究北方泥炭沼泽湿地二氧化碳(CO₂)和甲烷(CH₄)浓度随深度的变化规律及其影响因素,选取欧洲北部典型雨养泥炭地贝尔山湿地(BBM)和舒特兹山湿地(SBM)两个采样点,通过原位采集泥炭剖面温室气体、孔隙水以及土壤样品,结合傅里叶变换红外光谱(FTIR)技术、碳氮同位素技术,探讨泥炭土壤的分解程度及温室气体浓度变化的关系。研究结果表明:(1)BBM采样点地下部的CO₂浓度变化规律总体呈现随深度波动减少趋势,值多在3000μmol/L附近波动,最大值为4210.74μmol/L(120 cm),SBM采样点的CO₂浓度随深度先增后减,60 cm以上在1800μmol/L附近波动,60 cm以下在3000μmol/L附近波动,最大值为4191.94μmol/L(90 cm);BBM和SBM地下部CH₄浓度都随深度增大,并且在60cm以下浓度增加较快,BBM最大值为735.90μmol/L(260 cm),SBM最大值为543.51μmol/L(170 cm)。(2)BBM和SBM...     

Storage and mobility of black carbon in permafrost soils of the forest tundra ecotone in Northern Siberia

Boreal permafrost soils store large amounts of organic carbon (OC). Parts of this carbon (C) might be black carbon (BC) generated during vegetation fires. Rising temperature and permafrost degradation is expected to have different consequences for OC and BC, because BC is considered to be a refractory subfraction of soil organic matter. To get some insight into stocks, variability, and characteristics of BC in permafrost soils, we estimated the benzene polycarboxylic acid (BPCA) method‐specific composition and storage of BC, i.e. BPCA‐BC, in a 0.44 km²‐sized catchment at the forest tundra ecotone in northern Siberia. Furthermore, we assessed the BPCA‐BC export with the stream draining the catchment. The catchment is composed of various landscape units with south‐southwest (SSW) exposed mineral soils characterized by thick active layer or lacking permafrost, north‐northeast (NNE) faced mineral soils with thin active layer, and permafrost‐affected raised bogs in plateau positions showing in part thermokarst formation. There were indications of vegetation fires at all landscape units. BC was ubiquitous in the catchment soils and BPCA‐BC amounted to 0.6–3.0% of OC. This corresponded to a BC storage of 22–3440 g m^?2. The relative contribution of BPCA‐BC to OC, as well as the absolute stocks of BPCA‐BC were largest in the intact bogs with a shallow active layer followed by mineral soils of the NNE aspects. In both landscape units, a large proportion of BPCA‐BC was stored within the permafrost. In contrast, mineral soils with thick active layer or lacking permafrost and organic soils subjected to thermokarst formation stored less BPCA‐BC. Permafrost is, hence, not only a crucial factor in the storage of OC but also of BC. In the stream water BPCA‐BC amounted on an average to 3.9% of OC, and a yearly export of 0.10 g BPCA‐BC m^?2 was calculated, most of it occurring during the period of snow melt with dominance of surface flow. This suggests that BC mobility in dissolved and colloidal phase is an important pathway of BC export from the catchment. Such a transport mechanism may explain the high BC concentrations found in sediments of the Arctic Ocean.     

Nitrogen addition alters mineralization dynamics of 13C‐depleted leaf and twig litter and reduces leaching of older DOC from mineral soil

Recent reviews indicate that N deposition increases soil organic matter (SOM) storage in forests but the undelying processes are poorly understood. Our aim was to quantify the impacts of increased N inputs on soil C fluxes such as C mineralization and leaching of dissolved organic carbon (DOC) from different litter materials and native SOM. We added 5.5 g N m^?2 yr^?1 as NH₄NO₃ over 1 year to two beech forest stands on calcareous soils in the Swiss Jura. We replaced the native litter layer with ¹³C‐depleted twigs and leaves (δ¹³C: ?38.4 and ?40.8‰) in late fall and measured N effects on litter‐ and SOM‐derived C fluxes. Nitrogen addition did not significantly affect annual C losses through mineralization, but altered the temporal dynamics in litter mineralization: increased N inputs stimulated initial mineralization during winter (leaves: +25%; twigs: +22%), but suppressed rates in the subsequent summer. The switch from a positive to a negative response occurred earlier and more strongly for leaves than for twigs (?21% vs. 0%). Nitrogen addition did not influence microbial respiration from the nonlabeled calcareous mineral soil below the litter which contrasts with recent meta‐analysis primarily based on acidic soils. Leaching of DOC from the litter layer was not affected by NH₄NO₃ additions, but DOC fluxes from the mineral soils at 5 and 10 cm depth were significantly reduced by 17%. The ¹³C tracking indicated that litter‐derived C contributed less than 15% of the DOC flux from the mineral soil, with N additions not affecting this fraction. Hence, the suppressed DOC fluxes from the mineral soil at higher N inputs can be attributed to reduced mobilization of nonlitter derived ‘older’ DOC. We relate this decline to an altered solute chemistry by NH₄NO₃ additions, an increased ionic strength and acidification resulting from nitrification, rather than to a change in microbial decomposition.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Emission of CO2 from biochar‐amended soils and implications for soil organic carbon

Soil amendment with pyrogenic organic matter (PyOM), also named biochar, is claimed to sequester carbon (C). However, possible interactions between PyOM and native soil organic carbon (SOC) may accelerate the loss of SOC, thus reducing PyOM's C sequestration potential. We combined the results of 46 studies in a meta‐analysis to investigate changes in CO₂ emission of PyOM‐amended soils and to identify the causes of these changes and the possible factors involved. Our results showed a statistically significant increase of 28% in CO₂ emission from PyOM‐amended soils. When grouped by PyOM C (PyC):SOC ratios, the group of studies with a ratio >2 showed a significant increase in CO₂ emissions, but those with a ratio <2 showed no significant effect of PyOM application on CO₂ emission. Our data are consistent with the hypothesis that increased CO₂ emission after PyOM addition is additive and mainly derived from PyOM's labile C fractions. The PyC:SOC ratio provided the best predictor of increases in CO₂ production after PyOM addition to soil. This meta‐analysis highlights the importance of taking into account the amount of applied PyC in relation to SOC for designing future decomposition experiments.     

Predicting ecosystem carbon balance in a warming Arctic: the importance of long‐term thermal acclimation potential and inhibitory effects of light on respiration

The carbon balance of Arctic ecosystems is particularly sensitive to global environmental change. Leaf respiration (R), a temperature‐dependent key process in determining the carbon balance, is not well‐understood in Arctic plants. The potential for plants to acclimate to warmer conditions could strongly impact future global carbon balance. Two key unanswered questions are (1) whether short‐term temperature responses can predict long‐term respiratory responses to growth in elevated temperatures and (2) to what extent the constant daylight conditions of the Arctic growing season inhibit leaf respiration. In two dominant Arctic species E riophorum vaginatum (tussock grass) and B etula nana (woody shrub), we assessed the extent of respiratory inhibition in the light (R _L/R _D), respiratory response to short‐term temperature change, and respiratory acclimation to long‐term warming treatments. We found that R of both species is strongly inhibited by light (averaging 35% across all measurement temperatures). In E . vaginatum both R _L and R _D acclimated to the long‐term warming treatment, reducing the magnitude of respiratory response relative to the short‐term response to temperature increase. In B . nana, both R _L and R _D responded to short‐term temperature increase but showed no acclimation to the long‐term warming. The ability to predict plant respiratory response to global warming with short‐term temperature responses will depend on species‐specific acclimation potential and the differential response of R _L and R _D to temperature. With projected woody shrub encroachment in Arctic tundra and continued warming, changing species dominance between these two functional groups, may impact ecosystem respiratory response and carbon balance.     

Land‐atmosphere exchange of methane from soil thawing to soil freezing in a high‐Arctic wet tundra ecosystem

The land‐atmosphere exchange of methane (CH₄) and carbon dioxide (CO₂) in a high‐Arctic wet tundra ecosystem (Rylekærene) in Zackenberg, north‐eastern Greenland, was studied over the full growing season and until early winter in 2008 and from before snow melt until early winter in 2009. The eddy covariance technique was used to estimate CO₂ fluxes and a combination of the gradient and eddy covariance methods was used to estimate CH₄ fluxes. Small CH₄ bursts were observed during spring thawing 2009, but these existed during short periods and would not have any significant effect on the annual budget. Growing season CH₄ fluxes were well correlated with soil temperature, gross primary production, and active layer thickness. The CH₄ fluxes remained low during the entire autumn, and until early winter. No increase in CH₄ fluxes were seen as the soil started to freeze. However, in autumn 2008 there were two CH₄ burst events that were highly correlated with atmospheric turbulence. They were likely associated with the release of stored CH₄ from soil and vegetation cavities. Over the measurement period, 7.6 and 6.5 g C m^?2 was emitted as CH₄ in 2008 and in 2009, respectively. Rylekærene acted as a C source during the warmer and wetter measurement period 2008, whereas it was a C sink for the colder and drier period of 2009. Wet tundra ecosystems, such as Rylekærene may thus play a more significant role for the climate in the future, as temperature and precipitation are predicted to increase in the high‐Arctic.     

Decoupled trophic responses to long‐term recovery from acidification and associated browning in lakes

Increases in the concentration of dissolved organic matter (DOM) have been documented in many inland waters in recent decades, a process known as “browning”. Previous studies have often used space‐for‐time substitution to examine the direct consequences of increased DOM on lake ecosystems. However, browning often occurs concomitant with other ecologically important water chemistry changes that may interact with or overwhelm any potential ecological response to browning itself. Here we examine a long‐term (~20 year) dataset of 28 lakes in the Adirondack Park, New York, USA, that have undergone strong browning in response to recovery from acidification. With these data, we explored how primary producer and zooplankton consumer populations changed during this time and what physical and chemical changes best predicted these long‐term ecosystem changes. Our results indicate that changes in primary producers are likely driven by reduced water clarity due to browning, independent of changes in nutrients, counter to previously hypothesized primary producer response to browning. In contrast, declines in calcium concomitant with browning play an important role in driving long‐term declines in zooplankton biomass. Our results indicate that responses to browning at different trophic levels are decoupled from one another. Concomitant chemical changes have important implications for our understanding of the response of aquatic ecosystems to browning.     

Enhanced decomposition of stable soil organic carbon and microbial catabolic potentials by long‐term field warming

Quantifying soil organic carbon (SOC) decomposition under warming is critical to predict carbon–climate feedbacks. According to the substrate regulating principle, SOC decomposition would decrease as labile SOC declines under field warming, but observations of SOC decomposition under warming do not always support this prediction. This discrepancy could result from varying changes in SOC components and soil microbial communities under warming. This study aimed to determine the decomposition of SOC components with different turnover times after subjected to long‐term field warming and/or root exclusion to limit C input, and to test whether SOC decomposition is driven by substrate lability under warming. Taking advantage of a 12‐year field warming experiment in a prairie, we assessed the decomposition of SOC components by incubating soils from control and warmed plots, with and without root exclusion for 3 years. We assayed SOC decomposition from these incubations by combining inverse modeling and microbial functional genes during decomposition with a metagenomic technique (GeoChip). The decomposition of SOC components with turnover times of years and decades, which contributed to 95% of total cumulative CO₂ respiration, was greater in soils from warmed plots. But the decomposition of labile SOC was similar in warmed plots compared to the control. The diversity of C‐degradation microbial genes generally declined with time during the incubation in all treatments, suggesting shifts of microbial functional groups as substrate composition was changing. Compared to the control, soils from warmed plots showed significant increase in the signal intensities of microbial genes involved in degrading complex organic compounds, implying enhanced potential abilities of microbial catabolism. These are likely responsible for accelerated decomposition of SOC components with slow turnover rates. Overall, the shifted microbial community induced by long‐term warming accelerates the decomposition of SOC components with slow turnover rates and thus amplify the positive feedback to climate change.     

Evaluating litter decomposition in earth system models with long‐term litterbag experiments: an example using the Community Land Model version 4 (CLM4)

Decomposition is a large term in the global carbon budget, but models of the earth system that simulate carbon cycle‐climate feedbacks are largely untested with respect to litter decomposition. We tested the litter decomposition parameterization of the community land model version 4 (CLM4), the terrestrial component of the community earth system model, with data from the long‐term intersite decomposition experiment team (LIDET). The LIDET dataset is a 10‐year study of litter decomposition at multiple sites across North America and Central America. We performed 10‐year litter decomposition simulations comparable with LIDET for 9 litter types and 20 sites in tundra, grassland, and boreal, conifer, deciduous, and tropical forest biomes using the LIDET‐provided climatic decomposition index to constrain temperature and moisture effects on decomposition. We performed additional simulations with DAYCENT, a version of the CENTURY model, to ask how well an established ecosystem model matches the observations. The results show large discrepancy between the laboratory microcosm studies used to parameterize the CLM4 litter decomposition and the LIDET field study. Simulated carbon loss is more rapid than the observations across all sites, and nitrogen immobilization is biased high. Closer agreement with the observations requires much lower decomposition rates, obtained with the assumption that soil mineral nitrogen severely limits decomposition. DAYCENT better replicates the observations, for both carbon mass remaining and nitrogen, independent of nitrogen limitation. CLM4 has low soil carbon in global earth system simulations. These results suggest that this bias arises, in part, from too rapid litter decomposition. More broadly, the terrestrial biogeochemistry of earth system models must be critically tested with observations, and the consequences of particular model choices must be documented. Long‐term litter decomposition experiments such as LIDET provide a real‐world process‐oriented benchmark to evaluate models.