CO2 Flux Rates Research Articles

Disentangling thermal acclimation and substrate limitation effects on C and N cycling in peatlands

Temperature and substrate availability are among the key factors controlling microbial metabolism. The relative importance of these two drivers on soil organic matter turnover is, however, hotly debated. In this study, we investigated the effect of temperature changes on the potential enzyme activities involved in C (phenol-oxidase) and N (protease and amidase) cycling by incubating peat soils collected in winter and summer at the two typical temperatures recorded in the field during these two distinct periods (4 and 19 °C, respectively). In addition, to evaluate the effect of substrate limitations, we also compared the respiration rates of the thermally adapted soils with and without plant litter additions. Results showed that both collection season and incubation temperature had a significant effect on the two enzymes involved in N-cycling, with summer and increasing temperatures having detrimental effects on the potential activities of protease and amidase, whereas none of these factors affected phenol-oxidase activity. Furthermore, while adding readily decomposable substrate accelerated decomposition rates, CO2 flux rates were similar for all temperature conditions. Interestingly, the greatest contribution of litter to CO2 emissions occurred in the summer samples incubated at the lower temperature, whereas for the winter samples the stimulating effect on soil respiration was observed under warmer conditions. These findings suggest that the responses of soil microbial communities to temperature and substrate availability seem to strongly depend on the long-term temperature conditions and its interaction with substrate availability.

Diurnal to annual changes in latent, sensible heat, and CO2 fluxes over a Laurentian Great Lake: A case study in Western Lake Erie

AbstractTo understand the carbon and energy exchange between the lake surface and the atmosphere, direct measurements of latent, sensible heat, and CO2 fluxes were taken using the eddy covariance (EC) technique in Western Lake Erie during October 2011 to September 2013. We found that the latent heat flux (LE) had a marked one‐peak seasonal change in both years that differed from the diurnal course and lacked a sinusoidal dynamic common in terrestrial ecosystems. Daily mean LE was 4.8 ± 0.1 and 4.3 ± 0.2 MJ m−2 d−1 in Year 1 and Year 2, respectively. The sensible heat flux (H) remained much lower than the LE, with a daily mean of 0.9 ± 0.1 and 1.1 ± 0.1 MJ m−2 d−1 in Year 1 and Year 2, respectively. As a result, the Bowen ratio was <1 during most of the 2 year period, with the lowest summer value at 0.14. The vapor pressure deficit explained 35% of the variation in half hourly LE, while the temperature difference between the water surface and air explained 65% of the variation in half hourly H. Western Lake Erie acted as a small carbon sink holding −19.0 ± 5.4 and −40.2 ± 13.3 g C m−2 in the first and second summers (May–September) but as an annual source of 77.7 ± 18.6 and 49.5 ± 17.9 g C m−2 yr−1 in Year 1 and Year 2, respectively. The CO2 flux ( ) rate varied from −0.45 g C m−2 d−1 to 0.98 g C m−2 d−1. Similar to LE, had noticeable diurnal changes during the months that had high chlorophyll a months but not during other months. A significantly negative correlation (P < 0.05) was found between and chlorophyll a on monthly fluxes. Three gap‐filling methods, including marginal distribution sampling, mean diurnal variation, and monthly mean, were quantitatively assessed, yielding an uncertainty of 4%, 6%, and 10% in LE, H, and , respectively.

Temporal trends and sources of variation in carbon flux from coarse woody debris in experimental forest canopy openings.

Pulses of respiration from coarse woody debris (CWD) have been observed immediately following canopy disturbances, but it is unclear how long these pulses are sustained. Several factors are known to influence carbon flux rates from CWD, but few studies have evaluated more than temperature and moisture. We experimentally manipulated forest structure in a second-growth northern hardwood forest and measured CO2 flux periodically for seven growing seasons following gap creation. We present an analysis of which factors, including the composition of the wood-decay fungal community influence CO2 flux. CO2 flux from CWD was strongly and positively related to wood temperature and varied significantly between substrate types (logs vs. stumps). For five growing seasons after treatment, the CO2 flux of stumps reached rates up to seven times higher than that of logs. CO2 flux of logs did not differ significantly between canopy-gap and closed-canopy conditions in the fourth through seventh post-treatment growing seasons. By the seventh season, the seasonal carbon flux of both logs and stumps had decreased significantly from prior years. Linear mixed models indicated the variation in the wood inhabiting fungal community composition explained a significant portion of variability in the CO2 flux along with measures of substrate conditions. CO2 flux rates were inversely related to fungal diversity, with logs hosting more species but emitting less CO2 than stumps. Overall, our results suggest that the current treatment of CWD in dynamic forest carbon models may be oversimplified, thereby hampering our ability to predict realistic carbon fluxes associated with wood decomposition.

Penguin activity influences soil biogeochemistry and soil respiration in rookeries on Ross Island, Antarctica

Ross Island, Antarctica, is home to the southernmost breeding colonies of Adelie penguins, with associated ornithogenic nutrient enrichment to soil. Most of our knowledge about ornithogenic soils in Antarctica comes from studies of the Peninsula region, with much less known about soils in the Ross Sea region of continental Antarctica where climate is more extreme and biological communities are less diverse. We measured soil properties (pH, salinity, total and organic C and N, extractable mineral N and P), microbial biomass, and CO2 flux at three penguin rookeries on Ross Island differing in colony size and along gradients of penguin activity extending from the colony to soils lacking visible penguin influence. We determined both in situ and potential rates of CO2 flux, as well as the nutrient limitations on C cycling by fertilizing soils with N and P, in controlled laboratory incubations. Larger pools of soil C, N, and P along with elevated soil CO2 flux were found in soils with high penguin activity as compared to areas of low or no apparent activity. In laboratory incubations, CO2 flux was stimulated by addition of C with N or P, but not with N or P alone, suggesting that even with the deposition of large amounts of penguin guano, soil biota are not released from C limitation. The influence of penguin activity was not directly related to colony size but rather to the intensity of activity within the rookery (e.g., frequency of penguin presence). Our results show that penguin-influenced soils are biogeochemical hot spots where altered nutrient pools are associated with increased CO2 flux.

Component-specific dynamics of riverine mangrove CO2 efflux in the Florida coastal Everglades

Carbon cycling in mangrove forests represents a significant portion of the coastal wetland carbon (C) budget across the latitudes of the tropics and subtropics. Previous research suggests fluctuations in tidal inundation, temperature and salinity can influence forest metabolism and C cycling. Carbon dioxide (CO2) from respiration that occurs from below the canopy is contributed from different components. In this study, we investigated variation in CO2 flux among different below-canopy components (soil, leaf litter, course woody debris, soil including pneumatophores, prop roots, and surface water) in a riverine mangrove forest of Shark River Slough estuary, Everglades National Park (Florida, USA). The range in CO2 flux from different components exceeded that measured among sites along the oligohaline-saline gradient. Black mangrove (Avicennia germinans) pneumatophores contributed the largest average CO2 flux. Over a narrow range of estuarine salinity (25–35 practical salinity units (PSU)), increased salinity resulted in lower CO2 flux to the atmosphere. Tidal inundation reduced soil CO2 flux overall but increased the partial pressure of CO2 (pCO2) observed in the overlying surface water upon flooding. Higher pCO2 in surface water is then subject to tidally driven export, largely as HCO3. Integration and scaling of CO2 flux rates to forest scale allowed for improved understanding of the relative contribution of different below-canopy components to mangrove forest ecosystem respiration (ER). Summing component CO2 fluxes suggests a more significant contribution of below-canopy respiration to ER than previously considered. An understanding of below-canopy CO2 component fluxes and their contributions to ER can help to elucidate how C cycling will change with discrete disturbance events (e.g., hurricanes) and long-term change, including sea-level rise, and potential impact mangrove forests. As such, key controls on below-canopy ER must be taken into consideration when developing and modeling mangrove forest C budgets.

Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline.

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space-for-time’ approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg) is significantly lower at shrub (2.98 ± 0.48 kg m−2) and forest (2.04 ± 0.25 kg m−2) plots than at heath plots (7.03 ± 0.79 kg m−2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.

Scaling Analysis of the Convective Mixing in Porous Media for Geological Storage of CO2: An Experimental Approach

Prediction of the behavior of convective mixing and the effectiveness of this mechanism is essential for permanent sequestration of CO2 in deep saline aquifers. Simulation of the diffusion–convection mechanism at a large scale is very expensive and time-consuming; therefore, scaling relationships can be used to find suitable candidates for storage sites. In this study, scaling analysis is performed for the convective mixing of CO2 in saline aquifers based on experimental results. The scaling relationships are presented for the prediction of convective dissolution behavior. In the presented scaling analysis, different systems with a wide range of Rayleigh numbers were used. All experiments were conducted in a dissolution cell with different ranges of grain sizes. The pressure decay data are used to determine the dissolution rate of CO2, Sherwood number, and convective flux. In addition, the fraction of ultimate dissolution is calculated for each experiment to investigate the mixing regimes (convective mixing and diffusive mixing). The results indicate that the mixing of CO2 in water can be approximated by a scaling relationship for the Sherwood number and convective flux. These relations can be used in the implementation of large-scale CO2 storage in deep saline aquifers.

Multiyear greenhouse gas flux measurements on a temperate fen soil used for cropland or grassland

AbstractTo date there is still a lack of reliable data on greenhouse gas emissions from drained fens needed to determine the climatic relevance of land use and land use change on peatlands and to supply the National Inventory Report for the German Greenhouse Gas Inventory. In this study we present the results of monthly‐based multiyear measurements of CO2, N2O and CH4 flux rates in two drained agriculturally used fen ecosystems in NW Germany (cropland and grassland) over a period of 4.5 y using transparent and opaque closed chambers. CO2 exchange was modelled at high resolution with temperature and photosynthetic active radiation. The measured and modelled values fit very well (R2 ≥ 0.93). Annual GHG and Global Warming Potential (GWP) balances were determined. Net CO2 emissions at the cropland and grassland sites were similarly high, taking into account changes in management; net ecosystem C balance amounted to about 4.0 to 5.0 Mg C ha−1 y−1. Emissions of N2O and CH4 were low at both sites. The mean GWP balance for a time frame of 100 y (GWP100) amounted to about 17.0 to 19.0 Mg CO2‐eq. ha−1 y−1. The unexpectedly low greenhouse gas emissions from the cropland site are attributed to the high water table and a change in crop management. The change from corn for silage to corn‐cob mix lead transiently to rather small greenhouse gas emissions. The study confirms the need for multiyear measurements taking climatic and management variation into account.

Ecosystem CO2 flux rates in relation to vegetation type and age of Leymus arenarius dunes on Surtsey

The primary succession on the 50 year old volcanic island of Surtsey, Iceland, has been intensively studied. Initial soil development and other belowground processes are important drivers of primary succession but frequently overseen. A Leymus arenarius and Honckenya peploides dominated plant community has formed a relatively stable successional sere on the island, where external inputs of nutrients remain low. These plants have had a stable <10% aboveground surface cover during the past 20 years, but less is known about their belowground development. We investigated the organic matter (carbon) output and input processes (soil respiration, ecosystem respiration and photosynthesis) of the community and how they were affected by soil temperature, soil water content, vegetation and age of L. arenarius dunes. We found that both soil respiration and root stocks have increased substantially from 1987, when an earlier study was conducted. The same pattern was found when different aged L. arenarius dunes were studied. L. arenarius had a stronger effect on the soil respiration fluxes than its surface cover might indicate, through its much higher photosynthesis rates than H. peploides. The study furthermore illustrated how water stress may temporally limit belowground processes in this coastal community.

Soil Carbon Accumulation and CO2Flux in Experimental Restoration Plots, Southern Iceland: Comparing Soil Treatment Strategies

Experimental plots were established on severely eroded land surfaces in Iceland in 1999 to study the rates and limits of soil carbon sequestration during restoration and succession. The carbon content in the upper 10 cm of soils increased substantially during the initial eight years in all plots for which the treatments included both fertilizer and seeding with grasses, concomitant with the increase in vegetative cover. In the following five years, however, the soil carbon accumulation rates declined to negligible for most treatments and the carbon content in soils mainly remained relatively constant. We suggest that burial of vegetated surfaces by aeolian drift and nutrient limitation inhibited productivity and carbon sequestration in most plots. Only plots seeded with lupine demonstrated continued long-term soil carbon accumulation and soil CO2flux rates significantly higher than background levels. This demonstrates that lupine was the sole treatment that resulted in vegetation capable of sustained growth independent of nutrient availability and resistant to disruption by aeolian processes.

Automated modeling of ecosystem CO2 fluxes based on periodic closed chamber measurements: A standardized conceptual and practical approach

Closed chamber measurements are widely used for determining the CO2 exchange of different ecosystems. Among the chamber design and operational handling, the data processing procedure is a considerable source of uncertainty of obtained results. We developed a standardized automatic data processing algorithm, based on the language and statistical computing environment R© to (i) calculate measured CO2 flux rates, (ii) parameterize ecosystem respiration (Reco) and gross primary production (GPP) models, (iii) optionally compute an adaptive temperature model, (iv) model Reco, GPP and net ecosystem exchange (NEE), and (v) evaluate model uncertainty. The algorithm was tested in a case study performed at a cultivated fen situated in the northeast of Germany. Our study shows that even minor changes within the modeling approach may result in considerable differences of calculated flux rates, derived photosynthetic active radiation and temperature dependencies. Subsequently modeled Reco, GPP and NEE balance can therefore vary by up to 25%. Thus, automated and standardized data processing procedures, based on clearly defined criteria, such as statistical parameters and thresholds, are a prerequisite and highly desirable to guarantee the reproducibility and traceability of modeling results. Moreover, a standardized and automated data processing procedure also encourage a better comparability between closed chamber-based CO2 measurements.

Gopher mounds decrease nutrient cycling rates and increase adjacent vegetation in volcanic primary succession

Fossorial mammals may affect nutrient dynamics and vegetation in recently initiated primary successional ecosystems differently than in more developed systems because of strong C and N limitation to primary productivity and microbial communities. We investigated northern pocket gopher (Thomomys talpoides) effects on soil nutrient dynamics, soil physical properties, and plant communities on surfaces created by Mount St. Helens' 1980 eruption. For comparison to later successional systems, we summarized published studies on gopher effects on soil C and N and plant communities. In 2010, 18 years after gopher colonization, we found that gophers were active in ~2.5% of the study area and formed ~328 mounds ha(-1). Mounds exhibited decreased species density compared to undisturbed areas, while plant abundance on mound margins increased 77%. Plant burial increased total soil carbon (TC) by 13% and nitrogen (TN) by 11%, compared to undisturbed soils. Mound crusts decreased water infiltration, likely explaining the lack of detectable increases in rates of NO3-N, NH4-N or PO4-P leaching out of the rooting zone or in CO2 flux rates. We concluded that plant burial and reduced infiltration on gopher mounds may accelerate soil carbon accumulation, facilitate vegetation development at mound edges through resource concentration and competitive release, and increase small-scale heterogeneity of soils and communities across substantial sections of the primary successional landscape. Our review indicated that increases in TC, TN and plant density at mound margins contrasted with later successional systems, likely due to differences in physical effects and microbial resources between primary successional and older systems.

Possibility of effective magma degassing into groundwater flow systems beneath Unzen volcanic area, SW Japan, inferred from the evaluation of volcanic gas fluxes using electrical conductivity structures

The mass and heat budget of volcanic gases released from magma is critical to understanding a number of volcanic activities, including the ease with which magma can ascend. Due to its elevated temperature and salinity, the crustal electrical conductivity in a groundwater flow system increases through the addition of hydrothermal fluids which are produced by mixing of volcanic gases with meteoric-origin water. Therefore, the spatial extent of high electrical conductivity regions within groundwater flow systems may be used to evaluate the mass flux of volcanic gases to the systems. The present study attempts to estimate the mass flux of volcanic gases beneath the Unzen volcanic area in Southwest Japan, by developing a simple flow model of hydrothermal fluids and applying this model to the electrical conductivity structure of the area. The estimated mass flux of volcanic gases (104.8±0.3t/yr) yields results for CO2 flux (103.1±0.3t/yr) and magma input rate (100.1±0.3millionm3/yr) that are consistent with those estimated by geochemical and geodetic observations. This suggests that volcanic gases are steadily released from magma into the overlying groundwater flow system beneath the area, and that effective degassing may be one of the factors controlling the relatively effusive style of recent volcanism at Unzen volcano.

Spatial and temporal variability of pCO2 and CO2 efflux in seven Amazonian Rivers

Current estimates of CO2 outgassing from Amazonian rivers and streams have considerable uncertainty since they are based on limited-time surveys of pCO2 measurements along the Amazon mainstem and mouths of major tributaries, using conservative estimates of gas exchange velocities. In order to refine basin-scale CO2 efflux estimates from Amazonian rivers, we present a long time (5-year) dataset of direct measurements of CO2 fluxes, gas transfer velocities and pCO2 measurements in seven representative rivers of the lowland Amazon basin fluvial network, six non-tidal (Negro, Solimoes, Teles Pires, Cristalino, Araguaia and Javaes) and one tidal river (Caxiuana), with sizes ranging from 4th to 9th order. Surveys were conducted from January 2006 to December 2010, in a total of 389 campaigns covering all stages of their hydrographs. CO2 fluxes and gas transfer velocities (k) were measured using floating chambers and pCO2 was measured simultaneously by headspace extraction followed by gas chromatography analysis. Results show high CO2 flux rate variability among rivers and hydrograph stages, ranging from −0.8 to 15.3 μmol CO2 m−2 s−1, with unexpected negative fluxes in clear-water rivers during low waters. Non-tidal rivers showed marked seasonal CO2 flux patterns, with significantly higher exchange during high waters. Seasonality was modulated by pCO2, which was positive and strongly correlated with discharge. In these rivers k was well correlated with wind speed, which allowed the use of wind data to model k. We estimate a release of 360 ± 60 Tg C year−1 from Amazonian rivers and streams within a 1.47 million km2 quadrant in the central lowland Amazon. Extrapolating these values to the basin upstream of Obidos, results in an outgassing of 0.8 Pg C to the atmosphere each year. Our results are a step forward in achieving more accurate gas emission values for Amazonian rivers and their role in the annual carbon budget of the Amazon basin.

The effect of nutrient-rich effluents from shrimp farming on mangrove soil carbon storage and geochemistry under semi-arid climate conditions in northern Brazil

A semi-arid mangrove estuary system in the northeast Brazilian coast (Ceará state) was selected for this study to (i) evaluate the impact of shrimp farm nutrient-rich wastewater effluents on the soil geochemistry and organic carbon (OC) storage and (ii) estimate the total amount of OC stored in mangrove soils (0–40cm). Wastewater-affected mangrove forests were referred to as WAM and undisturbed areas as Non-WAM. Redox conditions and OC content were statistically correlated (P<0.05) with seasonality and type of land use (WAM vs. Non-WAM). Eh values were from anoxic to oxic conditions in the wet season (from −5 to 68mV in WAM and from <40 to >400mV in Non-WAM soils) and significantly higher (from 66 to 411mV) in the dry season (P<0.01). OC contents (0–40cm soil depth) were significantly higher (P<0.01) in the wet season than the dry season, and higher in Non-WAM soils than in WAM soils (values of 8.1 and 6.7kgm−2 in the wet and dry seasons, respectively, for Non-WAM, and values of 3.8 and 2.9kgm−2 in the wet and dry seasons, respectively, for WAM soils; P<0.01). Iron partitioning was significantly dependent (P<0.05) on type of land use, with a smaller degree of pyritization and lower Fe-pyrite presence in WAM soils compared to Non-WAM soils. Basal respiration of soil sediments was significantly influenced (P<0.01) by type of land use with highest CO2 flux rates measured in the WAM soils (mean values of 0.20mgCO2h−1–g−1 C vs. 0.04mgCO2h−1–g−1 C). The OC storage reduction in WAM soils was potentially caused (i) by an increase in microbial activity induced by loading of nutrient-rich effluents and (ii) by an increase of strong electron acceptors [e.g., NO3−] that promote a decrease in pyrite concentration and hence a reduction in soil OC burial. The current estimated OC stored in mangrove soils (0–40cm) in the state of Ceará is approximately 1milliont.

Effect of methodological consideration on soil carbon parameter estimates obtained via the acid hydrolysis-incubation method

Many techniques such as the acid hydrolysis – incubation (AHI) method have been developed with the aim of elucidating the inherent complexity of soil organic carbon (SOC). While the utility of the AHI method has been demonstrated, there is no standardized protocol developed for conducting the long-term incubation component of the method. In the current study we evaluated the effects of chamber venting and mechanical headspace mixing on soil CO2 flux rates and the resultant size and mean residence time of three operationally defined pools of SOC obtained via the AHI method. Continuous chamber venting resulted in an estimate of the readily mineralized carbon pool that was 2.3 times larger and turned over 2.9 times slower than the same pool estimated using periodically vented chambers. These differences were primarily attributed to the suppression of CO2 flux in periodically vented chambers as a result of high internal CO2 concentrations, and a concomitantly reduced diffusivity gradient. Prior to venting the periodically-vented chambers, CO2 flux rates averaged 2.3 μg C (g soil)−1 d−1, while CO2 flux rates following venting averaged 222.6 μg C (g soil)−1 d−1. We did not detect internal stratification of CO2 suggesting that mechanical headspace mixing is unnecessary in incubation chambers ranging from 1 to 2 L. A standardized protocol is called for that isolates SOC fractions that are useful in hypothesis testing, while simultaneously seeking to minimize laboratory artifacts.

Estimating soil subsidence and carbon loss in the Everglades Agricultural Area, Florida using geospatial techniques

Climate change due to elevated carbon dioxide levels in the atmosphere presents a long-term threat to the biosphere. The contribution of soil oxidation to global carbon dioxide levels is of growing concern. Until the past century, for over five millennia, the Everglades has been accreting peat soils and acting as a carbon sink. Anthropogenic drainage of the northernmost one-fourth of the Everglades, one of the largest deposits of organic soils in North America, began in the 1880s. Subsequently, the peat soils of that area began subsiding and releasing carbon dioxide (CO2) into the atmosphere. We quantified subsidence and CO2 evolution at the regional scale by calculating the changes in peat volume using surface maps reconstructed from historical and current data. The estimated peat volume was originally about 7×109m3 and is currently about 3×109m3. The average subsidence was about two meters and the CO2 emitted was 4.9×108metric tons. Assuming a constant CO2 emission rate during the century since drainage, then the regional scale CO2 flux rate from this study (0.2gCO2m−2yr−1) was similar to short-term, small-scale measurements made under controlled conditions (0.2–6gCO2m−2yr−1).

Tradeoffs in microbial carbon allocation may mediate soil carbon storage in future climates

Climate-induced changes in soil microbial physiology impact ecosystem carbon (C) storage and alter the rate of CO2 flux from soils to the atmosphere (Allison et al., 2010). The direction and magnitude of these microbial feedbacks depend on changes in saprotrophic bacterial and fungal C allocation in response to altered temperature, precipitation, and nutrient availability. Soil microbes may differentially allocate C in changing environments by altering processes such as enzyme production, C use efficiency (CUE), or biomass stoichiometry (Figure ​(Figure1).1). However, because these mechanisms may operate simultaneously and interact, microbial physiological feedbacks on soil C storage are difficult to predict. For example, initial increases in microbial CUE or biomass C:N may be counteracted by increases in enzyme production to acquire limiting organic nutrients. Figure 1 Three mechanisms through which microorganisms can shift C allocation: (A) extracellular enzyme activities, (B) carbon use efficiency, or (C) biomass stoichiometry. Each of these pathways can alter C storage in soils. Trend lines indicate expected responses ... Few studies have standardized microbial process rates, such as extracellular enzyme production or respiration, to the size of the microbial biomass. Examining process rates alone may obscure the microbial physiological mechanisms that underlie climate-induced changes in soil C cycling, leading to contradictory patterns among different studies. For instance, in a large-scale survey of soil protease activities from climate manipulations, drier and warmer conditions resulted in lower extracellular enzyme activities (EEA) compared to ambient conditions (Brzostek et al., 2012). In contrast, drier soils have also been found to stabilize extracellular enzymes in water films, reducing enzyme turnover rates and increasing potential activities (Lawrence et al., 2009; German et al., 2012).

Changes in Soil Composition and Floral Coverage on a Glacial Foreland Chronosequence in Southern Iceland

The land surface in front of the Skaftafellsj?kull in southern Iceland, exposed by ice recession commencing about the start of the twentieth century, constitutes a foreland with a maximum age of about 100 years and a more distal outwash plain. The ages of different surfaces within this sequence are constrained by moraines of known or estimated ages. Across this chronosequence, we measured at various sites the extent of floral coverage of the surface, the soil carbon and nitrogen contents of the substrate and the soil CO2 flux rate. All measured parameters exhibit values increasing with distance from the ice front, which correlates approximately with age. The strongest correlations are seen between distance and the carbon and nitrogen concentrations of the soil. Marked horizonation of the soil is observed only on the oldest surfaces (100+ years).

Correction to “Contribution of winter soil respiration to annual soil CO2 emission in a Mollisol under different tillage practices in northeast China”

[1] Winter soil CO2 emission is a very important component of the annual carbon budgets, however, almost no information on winter CO2 emission is available from the cropland soil in northeast China. In this study, soil CO2flux was measured for a 2-year period from an ongoing tillage trial on Black soil in northeast China to quantify seasonal patterns in soil CO2 flux rate and wintertime contribution to annual soil respiration. Average soil CO2 flux rates in the winter (November to March) were between 0.64 to 1.22 g CO2 m−2 d−1, in the non-growing season (October and April) were 2.09–3.56 g CO2 m−2 d−1, whereas in the growing season (May to September) they were between 10.9 to 12.7 g CO2 m−2 d−1, with no significant differences among tillage treatments. Total winter, non-growing and growing season soil CO2 emissions were 0.28–0.45 Mg C ha−1, 0.36–0.53 Mg C ha−1, and 4.52–5.55 Mg C ha−1, respectively, among tillage treatments. The contributions of winter soil respiration to annual soil CO2emission ranged from 5.1 to 7.1%, and the non-growing season emission ranged from 11.4 to 15.2% among tillage treatments. Our results indicate that in northeast China, cropland Black soil continuously emits CO2throughout the non-growing season, and the wintertime soil respiration plays a significant role in annual soil carbon budgets. Hence winter soil CO2 emission must be taken into consideration when the role of the soil ecosystem is assessed as either a sink or source of CO2 to the atmosphere.