Carbon dynamics and budget in a Zoysia japonica grassland, central Japan

The ecosystem carbon budget was estimated in a Japanese Zoysia japonica grassland. The green biomass started to grow in May and peaked from mid-July to September. Seasonal variations in soil CO₂ flux and root respiration were mediated by changes in soil temperature. Annual soil CO₂ flux was 1,121.4 and 1,213.6 g C m⁻² and root respiration was 471.0 and 544.3 g C m⁻² in 2007 and 2008, respectively. The root respiration contribution to soil CO₂ flux ranged from 33% to 71%. During the growing season, net primary production (NPP) was 747.5 and 770.1 g C m⁻² in 2007 and 2008, respectively. The biomass removed by livestock grazing (GL) was 122.1 and 102.7 g C m⁻², and the livestock returned 28.2 and 25.6 g C m⁻² as fecal input (FI) in 2007 and 2008, respectively. The decomposition of FI (DL, the dry weight loss due to decomposition) was very low, 1.5 and 1.4 g C m⁻², in 2007 and 2008. Based on the values of annual NPP, soil CO₂ flux, root respiration, GL, FI, and DL, the estimated carbon budget of the grassland was 1.7 and 22.3 g C m⁻² in 2007 and 2008, respectively. Thus, the carbon budget of this Z. japonica grassland ecosystem remained in equilibrium with the atmosphere under current grazing conditions over the 2 years of the study.     

Spatial Patterns of Soil Surface C Flux in Experimental Canopy Gaps

To explore within-gap spatial patterns of soil surface CO₂ flux, we measured instantaneous soil surface CO₂ flux, soil surface temperature, and soil moisture in north–south transects across canopy gaps and in adjacent contiguous forest from April to November 2010 in a second-growth northern hardwood forest in Wisconsin, USA. Throughout the growing season, soil surface CO₂ flux was higher in the northern 1/3 and northern edge of gaps compared to the central and southern portions. These patterns were driven primarily by within-gap variation in soil temperature, which was itself driven by within-gap patterns of insolation. Most locations in the northern 1/3 and northern edge of gaps had significantly higher modeled total growing season C flux (mean 725 g C m⁻²) compared to the contiguous forest (mean 706 g C m⁻²), whereas C flux in the central and southern portions of gaps (mean 555 g C m⁻²) was significantly lower than both the contiguous forest and the northern portions of gaps.     

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m⁻² year⁻¹). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m⁻² year⁻¹ in the north to +9 g C m⁻² year⁻¹ in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m⁻² year⁻¹. An erratum to this article can be found at     

Different patterns of ecosystem carbon accumulation between a young and an old-growth subtropical forest in Southern China

Using long-term (22 years) measurements from a young and an old-growth subtropical forest in southern China, we found that both forests accumulated carbon from 1982 to 2004, with the mean carbon accumulation rate at 227 ± 59 g C m⁻² year⁻¹ for young forest and 115 ± 89 g C m⁻² year⁻¹ for the old-growth forest. Allocation of the accumulated carbon was quite different between these two forests: the young forest accumulated a significant amount of carbon in plant live biomass, whereas the old-growth forest accumulated a significant amount of carbon in the soil. From 1982 to 2004, net primary productivity (NPP) increased for the young forest, and did not change significantly for the old-growth forest. The increase in NPP of the young forest resulted from recruitment of some dominant tree species characteristic of the subtropical mature forest in the region and an increase in tree density; decline of NPP of the old-growth forest was caused by increased mortality of the dominant trees.     

Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog

Anthropogenic nitrogen (N) loading has the potential to affect plant community structure and function, and the carbon dioxide (CO₂) sink of peatlands. Our aim is to study how vegetation changes, induced by nutrient input, affect the CO₂ exchange of a nutrient-limited bog. We conducted 9- and 4-year fertilization experiments at Mer Bleue bog, where we applied N addition levels of 1.6, 3.2, and 6.4 g N m⁻² a⁻¹, upon a background deposition of about 0.8 g N m⁻² a⁻¹, with or without phosphorus and potassium (PK). Only the treatments 3.2 and 6.4 g N m⁻² a⁻¹ with PK significantly affected CO₂ fluxes. These treatments shifted the Sphagnum moss and dwarf shrub community to taller dwarf shrub thickets without moss, and the CO₂ responses depended on the phase of vegetation transition. Overall, compared to the large observed changes in the vegetation, the changes in CO₂ fluxes were small. Following Sphagnum loss after 5 years, maximum ecosystem photosynthesis (Pg_max) and net CO₂ exchange (NEE_max) were lowered (−19 and −46%, respectively) in the highest NPK treatment. In the following years, while shrub height increased, the vascular foliar biomass did not fully compensate for the loss of moss biomass; yet, by year 8 there were no significant differences in Pg_max and NEE_max between the nutrient and the control treatments. At the same time, an increase (24–32%) in ecosystem respiration (ER) became evident. Trends in the N-only experiment resembled those in the older NPK experiment by the fourth year. The increasing ER with increasing vascular plant and decreasing Sphagnum moss biomass across the experimental plots suggest that high N deposition may lessen the CO₂ sink of a bog.     

Plant and soil carbon accumulation following fire in Mediterranean woodlands in Spain

We measured plant and soil carbon (C) storage following canopy-replacing wildfires in woodlands of northeastern Spain that include an understory of shrubs dominated by Quercus coccifera and an overstory of Pinus halepensis trees. Established plant succession models predict rapid shrub recovery in these ecosystems, and we build on this model by contrasting shrub succession with long-term C storage in soils, trees, and the whole ecosystem. We used chronosequence and repeated sampling approaches to detect change over time. Aboveground plant C increased from <100 to ~3,000 g C m⁻² over 30 years following fire, which is substantially less than the 5,942 ± 487 g C m⁻² (mean ±1 standard error) in unburned sites. As expected, shrubs accumulated C rapidly, but the capacity for C storage in shrubs was <600 g C m⁻². Pines were the largest plant C pool in sites >20 years post fire, and accounted for all of the difference in plant C between older burned sites and unburned sites. In contrast, soil C was initially higher in burned sites (~4,500 g C m⁻²) than in unburned sites (3,264 ± 261 g C m⁻²) but burned site C declined to unburned levels within 10 years after fire. Combining these results with prior research suggests two states for C storage. When pine regeneration is successful, ~9,200 g C m⁻² accumulate in woodlands but when tree regeneration fails (due to microclimatic stress or short fire return intervals), ecosystem C storage of ~4,000 g C m⁻² will occur in the resulting shrublands.     

Microphytobenthos of Arctic Kongsfjorden (Svalbard, Norway): biomass and potential primary production along the shore line

During summer 2007, Arctic microphytobenthic potential primary production was measured at several stations around the coastline of Kongsfjorden (Svalbard, Norway) at ≤5 m water depth and at two stations at five different water depths (5, 10, 15, 20, 30 m). Oxygen planar optode sensor spots were used ex situ to determine oxygen exchange in the overlying water of intact sediment cores under controlled light (ca. 100 μmol photons m⁻² s⁻¹) and temperature (2–4°C) conditions. Patches of microalgae (mainly diatoms) covering sandy sediments at water depths down to 30 m showed high biomass of up to 317 mg chl a m⁻². In spite of increasing water depth, no significant trend in “photoautotrophic active biomass” (chl a, ratio living/dead cells, cell sizes) and, thus, in primary production was measured at both stations. All sites from ≤5 to 30 m water depth exhibited variable rates of net production from −19 to +40 mg O₂ m⁻² h⁻¹ (−168 to +360 mg C m⁻² day⁻¹) and gross production of about 2–62 mg O₂ m⁻² h⁻¹ (17–554 mg C m⁻² day⁻¹), which is comparable to other polar as well as temperate regions. No relation between photoautotrophic biomass and gross/net production values was found. Microphytobenthos demonstrated significant rates of primary production that is comparable to pelagic production of Kongsfjorden and, hence, emphasised the importance as C source for the zoobenthos.     

Six-year Stable Annual Uptake of Carbon Dioxide in Intensively Managed Humid Temperate Grassland

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO₂) exchange of grassland ecosystems. We used 6 years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO₂ exchange (NEE), gross ecosystem production (GEP), and ecosystem respiration (ER) of an intensively managed grassland in the humid temperate climate of southern Ireland. In all the years of the study period, considerable uptake of atmospheric CO₂ occurred in this grassland with a narrow range in the annual NEE from −245 to −284 g C m⁻² y⁻¹, with the exception of 2008 in which the NEE reached −352 g C m⁻² y⁻¹. None of the measured environmental variables (air temperature (Ta), soil moisture, photosynthetically active radiation, vapor pressure deficit (VPD), precipitation (PPT), and so on) correlated with NEE on a seasonal or annual scale because of the equal responses from the component fluxes GEP and ER to variances in these variables. Pronounced reduction of summer PPT in two out of the six studied years correlated with decreases in both GEP and ER, but not with NEE. Thus, the stable annual NEE was primarily achieved through a strong coupling of ER and GEP on seasonal and annual scales. Limited inter-annual variations in Ta (±0.5°C) and generally sufficient soil moisture availability may have further favored a stable annual NEE. Monthly ecosystem carbon use efficiency (CUE; as the ratio of NEE:GEP) during the main growing season (April 1–September 30) was negatively correlated with temperature and VPD, but positively correlated with soil moisture, whereas the annual CUE correlated negatively with annual NEE. Thus, although drier and warmer summers may mildly reduce the uptake potential, the annual uptake of atmospheric CO₂, in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.     

Holocene Carbon Stocks and Carbon Accumulation Rates Altered in Soils Undergoing Permafrost Thaw

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆¹⁴C, C:N, δ¹³C, and δ¹⁵N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m⁻² in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ¹³C, and δ¹⁵N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m⁻² y⁻¹ and the rate for the deep soil accumulation was 2.3 g C m⁻² y⁻¹, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m⁻² y⁻¹. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.     

Measurement of net ecosystem production and ecosystem respiration in a Zoysia japonica grassland,central Japan,by the chamber method

Measuring light, temperature, soil moisture, and growth provides a better understanding of net ecosystem production (NEP), ecosystem respiration (R _eco), and their response functions. Here, we studied the variations in NEP and R _eco in a grassland dominated by a perennial warm-season C₄ grass, Zoysia japonica. We used the chamber method to measure NEP and R _eco from August to September 2007. Biomass and leaf area index (LAI) were also measured to observe their effects on NEP and R _eco. Diurnal variations in NEP and R _eco were predicted well by light intensity (PPFD) and by soil temperature, respectively. Maximum NEP (NEP_max) values on days of year 221, 233, 247, and 262, were 2.44, 2.55, 3.90, and 4.17 μmol m⁻² s⁻¹, respectively. Throughout the growing period, the apparent quantum yield (α) increased with increasing NEP_max that ranged from 0.0154 to 0.0515, and NEP responded to the soil temperature changes by 44% and R _eco changes by 48%, and R _eco responded from 88 to 94% with the soil temperature diurnally. NEP’s light response and R _eco’s temperature response were affected by soil water content; more than 27% of the variation in NEP and 67% of the variation in R _eco could be explained by this parameter. NEP was strongly correlated with biomass and LAI, but R _eco was not, because environmental variables affected R _eco more strongly than growth parameters. Using the light response of NEP, the temperature response of R _eco, and meteorological data, daily NEP and R _eco were estimated at 0.67, 0.81, 1.17, and 1.56 g C m⁻², and at 2.88, 2.50, 3.51, and 3.04 g C m⁻², respectively, on days of year 221, 233, 247, and 262. The corresponding daily gross primary production (NEP + R _eco) was 3.5, 3.3, 4.6, and 4.6 g C m⁻².     

Attenuation of the vertical flux of copepod fecal pellets under Arctic sea ice: evidence for an active detrital food web in winter

A variable fraction of fecal pellets produced in the epipelagic layer is intercepted and retained before reaching the bottom. We assessed fecal pellet retention in the ice-covered Beaufort Sea in early February by comparing the shape and size-frequency distribution of pellets collected by a sediment trap moored at 210 m to that produced in vitro. Appendicularian ellipsoidal and copepod cylindrical pellets made up 75 and 24% of the flux (165 μg C m⁻² day⁻¹). In contrast, production (135 μg C m⁻² day⁻¹) was dominated by cylindrical pellets (93%). The vertical flux of cylindrical pellets at 210 m was attenuated by 70%. Pellets >120 μm in width, represented 42% of the production, but were not detected in the trap. Retention most likely resulted from coprorhexic feeding by copepods such as Metridia longa. Our observations suggest that the detritivore food web prevailing under the ice of the Arctic Ocean in winter is dominated by appendicularians feeding on pellets fragmented by copepods.     

Winter-time ecology in the Bothnian Bay, Baltic Sea: nutrients and algae in fast ice

Landfast ice algal communities were studied in the strongly riverine-influenced northernmost part of the Baltic Sea, the Bothnian Bay, during the winter-spring transition of 2004. The under-ice river plume, detected by its low salinity and elevated nutrient concentrations, was observed only at the station closest to the river mouth. The bottommost ice layer at this station was formed from the plume water (brine volume 0.71%). This was reflected by the low flagellate-dominated (93%) algal biomass in the bottom layer, which was one-fifth of the diatom-dominated (74%) surface-layer biomass of 88 μg C l⁻¹. Our results indicate that habitable space plays a controlling role for ice algae in the Bothnian Bay fast ice. Similarly to the water column in the Bothnian Bay, average dissolved inorganic N:P-ratios in the ice were high, varying between 12 and 265. The integrated chlorophyll a (0.1–2.2 mg m⁻²) and algal biomass in the ice (1–31 mg C m⁻²) correlated significantly (Spearman ρ = 0.79), with the highest values being measured close to the river mouth in March and during the melt season in April. Flagellates <20 μm generally dominated in both the ice and water columns in February–March. In April the main ice-algal biomass was composed of Melosira arctica and unidentified pennate diatoms, while in the water column Achnanthes taeniata, Scrippsiella hangoei and flagellates dominated. The photosynthetic efficiency (0.003–0.013 (μg C [μg chl a ⁻¹] h⁻¹)(μE m⁻²s⁻¹)⁻¹) and maximum capacity (0.18–1.11 μg C [μg chl a ⁻¹] h⁻¹) could not always be linked to the algal composition, but in the case of a clear diatom dominance, pennate species showed to be more dark-adapted than centric diatoms.     

Net Primary Production and Carbon Stocks for Subarctic Mesic–Dry Tundras with Contrasting Microtopography, Altitude, and Dominant Species

Mesic–dry tundras are widespread in the Arctic but detailed assessments of net primary production (NPP) and ecosystem carbon (C) stocks are lacking. We addressed this lack of knowledge by determining the seasonal dynamics of aboveground vascular NPP, annual NPP, and whole-ecosystem C stocks in five mesic–dry tundras in Northern Sweden with contrasting microtopography, altitude, and dominant species. Those measurements were paralleled by the stock assessments of nitrogen (N), the limiting nutrient. The vascular production was determined by harvest or in situ growing units, whereas the nonvascular production was obtained from average species growth rates, previously assessed at the sites. Results showed that aboveground vascular NPP (15–270 g m⁻²), annual NPP (214–282 g m⁻² or 102–137 g C m⁻²) and vegetation biomass (330–2450 g m⁻²) varied greatly among communities. Vegetation dominated by Empetrum hermaphroditum is more productive than Cassiope tetragona vegetation. Although the large majority of the apical NPP occurred in early-mid season (85%), production of stems and evergreen leaves proceeded until about 2 weeks before senescence. Most of the vascular vegetation was belowground (80%), whereas most of the vegetation production occurred aboveground (85%). Ecosystem C and N stocks were 2100–8200 g C m⁻² and 80–330 g N m⁻², respectively, stored mainly in the soil turf and in the fine organic soil. Such stocks are comparable to the C and N stocks of moister tundra types, such as tussock tundra. Author Contributions  Matteo Campioli, Anders Michelsen, Roeland Samson, Raoul Lemeur—conceived and designed study, Matteo Campioli, Anders Michelsen, Andreas Demey, Annemie Vermeulen—performed research, Matteo Campioli—analyzed data, and Matteo Campioli—wrote the paper.     

Increasing fish production from wetlands at Lake Victoria, Uganda using organically manured seasonal wetland fish ponds

The processes driving primary productivity and its impacts on fish production were investigated in field trials in eight seasonal earthen wetland ponds ‘Fingerponds’ (192 m²) in Uganda between 2003 and 2005. The ponds were stocked by the seasonal flood with predominantly Oreochromis spp. at densities ranging from 0.1 to 0.5 fish m⁻². Chicken manure (521, 833 or 1,563 kg ha⁻¹) was applied fortnightly. Results showed that primary productivity was enhanced with maximum average net primary productivity (±Standard Error) of 11.7 (±2.5) g O₂ m⁻² day⁻¹ at the Gaba site and 8.3 (±1.5) g O₂ m⁻² day⁻¹ at the Walukuba site. Net fish yields were higher in manured ponds with up to 2,670 kg ha⁻¹ yield for a 310 day growth period compared to less than 700 kg ha⁻¹ in unmanured ponds. Fish production was limited mainly by high recruitment, falling water levels, light limitation from high suspended solids and turbidity, and low zooplankton biomass. It was concluded that Fingerponds have a high potential for sustainable fish production and can contribute to the alleviation of protein shortages amongst the riparian communities around Lake Victoria. Production can be enhanced further with improved stock management.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Annual sediment respiration in estuarine sandy intertidal flats in the Seto Inland Sea, Japan

To quantify organic matter mineralization at estuarine intertidal flats, we measured in situ sediment respiration rates using an infrared gas analyzer in estuarine sandy intertidal flats located in the northwestern Seto Inland Sea, Japan. In situ sediment respiration rates showed spatial and seasonal variations, and the mean of the rates is 38.8 mg CO₂-C m⁻² h⁻¹ in summer. In situ sediment respiration rates changed significantly with sediment temperature at the study sites (r ² = 0.70, p < 0.05), although we did not detect any significant correlations between the rates and sediment characteristics. We prepared a model for estimating the annual sediment respiration based on the in situ sediment respiration rates and their temperature coefficient (Q ₁₀ = 1.8). The annual sediment respiration was estimated to be 92 g CO₂-C m⁻² year⁻¹. The total amount of organic carbon mineralization for the entire estuarine intertidal flats through sediment respiration (43 t C year⁻¹) is equivalent to approximately 25% of the annual organic carbon load supplied from the river basin of the estuary.     

Process-level controls on CO<Subscript>2</Subscript> fluxes from a seasonally snow-covered subalpine meadow soil,Niwot Ridge,Colorado

Fluxes of CO₂ during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO₂ fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO₂ fluxes of 0.71 μmol m⁻² s⁻¹ in 2006 and 0.86 μmol m⁻² s⁻¹ in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m⁻² emitted over the winter, ~30% of the annual soil CO₂ efflux at this site. In general, the CO₂ flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO₂ flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO₂ flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO₂ flux in each zone changes from periodic pulses of CO₂ during thaw events (zone I), to CO₂ fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO₂ fluxes increasing gradually as soil moisture increases (zone III), to CO₂ fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO₂ flux from seasonally snow-covered soils.     

Effects of nitrogen source and empty bed residence time on the removal of styrene gaseous emissions by biotrickling filtration

The removal of styrene-polluted air emissions by biotrickling filtration was performed to evaluate the influence of using nitrate and urea as a nitrogen source in the nutrient solution supplied to two bioreactors run in parallel under the same operational conditions for 3 months. The use of urea resulted in less biomass content along the packed bed and better performance of the process, with a maximum elimination capacity (EC) of 57.6 g C m⁻³ h⁻¹ (removal efficiency (RE) of 88.3% and empty bed residence time (EBRT) of 60 s), which was around 54% higher than when using nitrate. EBRTs of 60, 30 and 15 s were evaluated with a urea-based nutrient supply. By decreasing the EBRT from 60 to 30 s the styrene concentration that could be treated with REs above 80% was almost the half, from 1,100 to 600 mg C m⁻³, resulting in ECs of 52.8 g C m⁻³ h⁻¹. Working at 15 s was not possible to obtain REs higher than 40% with a maximum EC of 28.5 g C m⁻³ h⁻¹.     

Winter and summer nitrous oxide and nitrogen oxides fluxes from a seasonally snow-covered subalpine meadow at Niwot Ridge,Colorado

The soil emission rates (fluxes) of nitrous oxide (N₂O) and nitrogen oxides (NO + NO₂ = NO _x ) through a seasonal snowpack were determined by a flux gradient method from near-continuous 2-year measurements using an automated system for sampling interstitial air at various heights within the snowpack from a subalpine site at Niwot Ridge, Colorado. The winter seasonal-averaged N₂O fluxes of 0.047–0.069 nmol m⁻² s⁻¹ were ~15 times higher than observed NO _x fluxes of 0.0030–0.0067 nmol m⁻² s⁻¹. During spring N₂O emissions first peaked and then dropped sharply as the soil water content increased from the release of snowpack meltwater, while other gases, including NO _x and CO₂ did not show this behavior. To compare and contrast the winter fluxes with snow-free conditions, N₂O fluxes were also measured at the same site in the summers of 2006 and 2007 using a closed soil chamber method. Summer N₂O fluxes followed a decreasing trend during the dry-out period after snowmelt, interrupted by higher values related to precipitation events. These peaks were up to 2–3 times higher than the background summer levels. The integrated N₂O-N loss over the summer period was calculated to be 1.1–2.4 kg N ha⁻¹, compared to ~0.24–0.34 kg N ha⁻¹ for the winter season. These wintertime N₂O fluxes from subniveal soil are generally higher than the few previously published data. These results are of the same order of magnitude as data from more productive ecosystems such as fertilized grasslands and high-N-cycling forests, most likely because of a combination of the relatively well-developed soils and the fact that subnivean biogeochemical processes are promoted by the deep, insulating snowpack. Hence, microbially mediated oxidized nitrogen emissions occurring during the winter can be a significant part of the N-cycle in seasonally snow-covered subalpine ecosystems.     

Evidence for large carbon sink and long residence time in semiarid forests based on 15 year flux and inventory records

The rate of change in atmospheric CO₂ is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001–2016) in a 55 year old 30 km² pine forest growing at the semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145–160 g C m^?2 year^?1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^?2 year^?1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m² and 372 g N/m². Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.