Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO₂) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils (<2 to >50 mg l⁻¹). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g⁻¹ soil C day⁻¹, falling to averages of 0.01 mg g⁻¹ soil C day⁻¹; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g⁻¹ soil C day⁻¹ (0.5–47.6 mg g⁻¹ soil C), with an average of 0.021 mg g⁻¹ soil C day⁻¹ (8.2 mg g⁻¹ soil C). DOC production rate was weakly related to temperature, equivalent to Q₁₀ values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO₂–C and DOC showed a quotient (CO₂–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO₂–C:DOC production (log₁₀ transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO₂ is produced. The CO₂–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO₂–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO₂–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.     

Carbon acquisition by diatoms

Diatoms are responsible for up to 40% of primary productivity in the ocean, and complete genome sequences are available for two species. However, there are very significant gaps in our understanding of how diatoms take up and assimilate inorganic C. Diatom plastids originate from secondary endosymbiosis with a red alga and their Form ID Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) from horizontal gene transfer, which means that embryophyte paradigms can only give general guidance as to their C acquisition mechanisms. Although diatom Rubiscos have relatively high CO₂ affinity and CO₂/O₂ selectivity, the low diffusion coefficient for CO₂ in water has the potential to restrict the rate of photosynthesis. Diatoms growing in their natural aquatic habitats operate inorganic C concentrating mechanisms (CCMs), which provide a steady-state CO₂ concentration around Rubisco higher than that in the medium. How these CCMs work is still a matter of debate. However, it is known that both CO₂ and HCO₃⁻ are taken up, and an obvious but as yet unproven possibility is that active transport of these species across the plasmalemma and/or the four-membrane plastid envelope is the basis of the CCM. In one marine diatom there is evidence of C₄-like biochemistry which could act as, or be part of, a CCM. Alternative mechanisms which have not been eliminated include the production of CO₂ from HCO₃⁻ at low pH maintained by a H⁺ pump, in a compartment close to that containing Rubisco.     

Acquisition of Ca2+ and HCO3 −/CO3 2− for shell formation in embryos of the common pond snail Lymnaea stagnalis

Embryos of the freshwater common pond snail Lymnaea stagnalis develop to hatch within 10 days under control conditions (22°C, Miami-Dade tap water) and this development is impaired by removal of ambient calcium. In contrast, embryos did not exhibit dependence upon an ambient HCO₃ ⁻/CO₃ ²⁻ source, developing and hatching in HCO₃ ⁻/CO₃ ²⁻-free water at rates comparable to controls. Post-metamorphic, shell-laying embryos exhibited a significant saturation-type calcium uptake as a function of increasing ambient calcium concentration. However, changes in ambient bicarbonate concentration did not influence calcium or apparent titratable alkalinity uptake. There was a distinct shift from no significant flux in pre-metamorphic embryos to net uptake of calcium in post-metamorphic stages as indicated by an increased uptake from the micro-environment surrounding the egg mass and increased net uptake in 24-h, whole egg mass flux measurements. Furthermore, HCO₃ ⁻/CO₃ ²⁻ acquisition as measured by titratable alkalinity flux is at least partially attributable to an endogenous carbonate source that is associated with acid extrusion. Thus, calcium requirements for embryonic shell formation are met via uptake but HCO₃ ⁻/CO₃ ²⁻, which is also necessary for shell formation is acquired in part from endogenous sources with no detectable correlation to ambient HCO₃ ⁻/CO₃ ²⁻ availability.     

Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole Catchment Soil Respiration

The concentration of CO₂ in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca²⁺, and Mg²⁺ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry model, we investigated how CO₂ concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO₂ (pCO₂) did not vary seasonally (P > 0.05). Over the 5 years, pCO₂ was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric equilibrium), and CO₂ evasion from the stream to the atmosphere ranged from 146 to 353 mmol m⁻² d⁻¹. Whereas pCO₂ in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO₂ were revealed by the catchment CO₂ model. Seasonally, soil pCO₂ increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels, evasion of CO₂ from soils to the atmosphere ranged from 83 mmol m⁻² d⁻¹ in winter to 287 mmol m⁻² d⁻¹ in summer. The seasonal variation in soil CO₂ tracked soil temperature (r ²= 0.46, P < 0.001) and model-derived estimates of CO₂ evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient = 0.62, P < 0.05) supporting the model assumptions. Although rates of CO₂ evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752 mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess whole catchment CO₂ dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil ecosystem respiration. Received 24 July 1997; accepted 14 November 1997.     

Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China

Responses of soil respiration (CO₂ emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting) and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October 2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments (Control, no N addition; Low-N, 5 g N m⁻² year⁻¹; Medium-N, 10 g N m⁻² year⁻¹), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited significant positive exponential relationship with soil temperature and significant positive linear relationship with soil moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO₂-C m⁻² h⁻¹) and 10% lower in medium-N plots (80 ± 4 mg CO₂-C m⁻² h⁻¹) than in the control plots (89 ± 5 mg CO₂-C m⁻² h⁻¹), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest, annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO₂-C m⁻² h⁻¹) and 8% lower in medium-N plots (61 ± 3 mg CO₂-C m⁻² h⁻¹) than in the control plots (66 ± 4 mg CO₂-C m⁻² h⁻¹), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in the reforested tropical forests may vary depending on the status of human disturbance. Responsible Editor: Hans Lambers.     

Soil CO<Subscript>2</Subscript> flux affected by <Emphasis Type="Italic">Aporrectodea caliginosa</Emphasis> earthworms

The effects of Aporrectodea caliginosa earthworms on both carbon dioxide (CO₂) accumulation in and emissions from soil, as well as the simultaneous impact of earthworms on soil microbiological properties were investigated in a microcosm experiment carried out over 5.5 months. Concentration of CO₂ in soil air was greater at a depth of 15 cm when compared with a depth of 5 cm, but varied during the season both in control and earthworm-inhabited chambers. Peaks of CO₂ concentrations at both depths occurred in both treatments during August, approximately 80 days after the experiment started. Generally, the presence of earthworms increased the CO₂ concentration at 15-cm depth. Larger CO₂ emissions were consistently recorded in conjunction with higher amounts of CO₂ in soil air when chambers were inhabited by earthworms. The total CO₂ emissions during the experimental period covering 161 days were estimated at 118 g CO₂-C m⁻² and 99 g CO₂-C m⁻² from chambers with and without earthworms respectively. Moreover, the presence of earthworms increased microbial biomass in the centre and at the bottom of chambers, and enhanced both dehydrogenase activity and nitrifying enzyme activity in the soils. We suggest that the effect of earthworms on both the enhanced soil accumulation of CO₂ as well as emissions of CO₂ was mostly indirect, due to the impacts of earthworms on soil microbial community.     

The impact of seawater saturation state and bicarbonate ion concentration on calcification by new recruits of two Atlantic corals

Rising concentrations of atmospheric CO₂ are changing the carbonate chemistry of the oceans, a process known as ocean acidification (OA). Absorption of this CO₂ by the surface oceans is increasing the amount of total dissolved inorganic carbon (DIC) and bicarbonate ion (HCO₃ ⁻) available for marine calcification yet is simultaneously lowering the seawater pH and carbonate ion concentration ([CO₃ ²⁻]), and thus the saturation state of seawater with respect to aragonite (Ω_ar). We investigated the relative importance of [HCO₃ ⁻] versus [CO₃ ²⁻] for early calcification by new recruits (primary polyps settled from zooxanthellate larvae) of two tropical coral species, Favia fragum and Porites astreoides. The polyps were reared over a range of Ω_ar values, which were manipulated by both acid-addition at constant pCO₂ (decreased total [HCO₃ ⁻] and [CO₃ ²⁻]) and by pCO₂ elevation at constant alkalinity (increased [HCO₃ ⁻], decreased [CO₃ ²⁻]). Calcification after 2 weeks was quantified by weighing the complete skeleton (corallite) accreted by each polyp over the course of the experiment. Both species exhibited the same negative response to decreasing [CO₃ ²⁻] whether Ω_ar was lowered by acid-addition or by pCO₂ elevation—calcification did not follow total DIC or [HCO₃ ⁻]. Nevertheless, the calcification response to decreasing [CO₃ ²⁻] was nonlinear. A statistically significant decrease in calcification was only detected between Ω_ar = <2.5 and Ω_ar = 1.1–1.5, where calcification of new recruits was reduced by 22–37% per 1.0 decrease in Ω_ar. Our results differ from many previous studies that report a linear coral calcification response to OA, and from those showing that calcification increases with increasing [HCO₃ ⁻]. Clearly, the coral calcification response to OA is variable and complex. A deeper understanding of the biomineralization mechanisms and environmental conditions underlying these variable responses is needed to support informed predictions about future OA impacts on corals and coral reefs.     

Photosynthetic CO2 affinity of the aquatic carnivorous plant Utricularia australis (Lentibulariaceae) and its investment in carnivory

Aquatic carnivorous plants usually grow in shallow dystrophic waters poor in inorganic N and P. Utricularia australis was chosen as a model plant for its prolific distribution and great ecological plasticity. The photosynthetic CO₂ compensation point and factors associated with investment in carnivory and capture of prey were measured in 17 U. australis micropopulations in Třeboň basin, Czech Republic, together with water chemistry factors at these sites differing greatly in their trophic level, water hardness, and prey availability. Apical shoot growth rate was estimated at some oligotrophic sites. The micropopulations differed greatly in the proportion of traps with animal prey (2.7–70%, mean 26%), trap proportion to total biomass (1.4–42%, mean 26%), mean trap biomass (0.7–63 μg trap⁻¹, mean 19 μg), and maximum trap size (1–3 mm, mean 2.0 mm). CO₂ compensation points ranged from 0.7 to 6.1 μM (mean 2.6 μM). A weak HCO₃ ⁻ use (compensation point 0.51 mM) was found in plants growing in alkaline water. Trap biomass proportion did not correlate significantly with prey capture and CO₂ compensation points with ambient [CO₂]. A very rapid apical growth (2.5–4.2 new nodes day⁻¹) occurred in sand pits. Thus, HCO₃ ⁻ use in U. australis can be induced by growing at very high pH. CO₂ compensation points resembled those known in other aquatic non-carnivorous plants. They did not reflect carnivory. In spite of very rapid apical shoot growth, the relative growth rate of U. australis can be zero in oligotrophic habitats without prey.     

Effect of Increased Carbon Dioxide and Temperature on Runoff Chemistry at a Forested Catchment in Southern Norway (CLIMEX Project)

CLIMEX (Climate Change Experiment) is an integrated, whole-ecosystem research project that focuses on the response of forest ecosystems at the catchment scale to increased CO₂ and temperature. KIM catchment (860 m²) is completely enclosed by a transparent greenhouse, receives deacidifed “clean” rain, and has elevated CO₂ (560 ppmv) and elevated air temperature (3°–5°C above ambient). The uppermost 20% of the catchment is partitioned off, is not subject to changed CO₂ or temperature, and serves as an untreated control. Fluxes of nitrate and ammonium in runoff from KIM catchment increased from 2 mmol m^{− 2} y^{− 1} each in the 3 years before treatment to 6 and 3 mmol m^{− 2} y^{− 1}, respectively, in the 3 years after treatment (May 1994–April 1997), despite a 15 mmol m^{− 2} y^{− 1} decrease in N dry deposition due to the sealing of the walls to the enclosure. N flux in runoff from three reference catchments and the control section did not change. The net loss of inorganic N was thus about 20 mmol m^{− 2} treated soil y^{− 1}. There were no changes in organic N or total organic carbon in runoff. The ecosystem switched from a net sink to a net source of inorganic nitrogen (N). The increased loss of N may be due to accelerated decomposition of soil organic matter induced by higher temperature. Due to many decades of N deposition from long-range transported pollutants, the ecosystem prior to treatment was N saturated. If global change induces persistent losses of inorganic N on a regional scale, the result may be a significant increase in nitrate concentrations in fresh waters and N loading to coastal marine ecosystems. In regions with acid sensitive waters, such as southern Norway, the increased nitrate release caused by global change may offset improvements achieved by reduced sulfur and N deposition. Received 15 October 1997; accepted 18 November 1997.     

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with ¹³C and ¹⁵N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the ¹³C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter ¹³C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m⁻² y⁻¹ of SOM accumulation. Recovery of litter ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of ¹⁵N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha⁻¹ y⁻¹) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.     

Carbon and water cycles in tropical papyrus wetlands

Highly productive papyrus (Cyperus papyrus L.) wetlands dominate many permanently flooded areas of tropical East Africa; however, the cycling of carbon and water within these ecosystems is poorly understood. The objective of this study was to utilise Eddy Covariance (EC) techniques to measure the fluxes of carbon dioxide and water vapour between papyrus vegetation and the atmosphere in a wetland located near Jinja, Uganda on the Northern shore of Lake Victoria. Peak, midday rates of photosynthetic CO₂ net assimilation were approximately 40 μmol CO₂ m⁻² s⁻¹, while night time losses through respiration ranged between 10 and 20 μmol CO₂ m⁻² s⁻¹. Numerical integration of the flux data suggests that papyrus wetlands have the potential to sequester approximately 0.48 kg C m⁻² y⁻¹. The average daily water vapour flux from the papyrus vegetation through canopy evapotranspiration was approximately 4.75 kg H₂O m⁻² d⁻¹, which is approximately 25% higher than water loss through evaporation from open water.     

Acid–base regulation in the plainfin midshipman (Porichthys notatus): an aglomerular marine teleost

The plainfin midshipman (Porichthys notatus) possesses an aglomerular kidney and like other marine teleosts, secretes base into the intestine to aid water absorption. Each of these features could potentially influence acid–base regulation during respiratory acidosis either by facilitating or constraining HCO₃ ⁻ accumulation, respectively. Thus, in the present study, we evaluated the capacity of P. notatus to regulate blood acid–base status during exposure to increasing levels of hypercapnia (nominally 1–5% CO₂). Fish exhibited a well-developed ability to increase plasma HCO₃ ⁻ levels with values of 39.8 ± 2.8 mmol l⁻¹ being achieved at the most severe stage of hypercapnic exposure (arterial blood PCO₂ = 21.9 ± 1.7 mmHg). Consequently, blood pH, while lowered by 0.15 units (pH = 7.63 ± 0.06) during the final step of hypercapnia, was regulated far above values predicted by chemical buffering (predicted pH = 7.0). The accumulation of plasma HCO₃ ⁻ during hypercapnia was associated with marked increases in branchial net acid excretion (J _NETH⁺) owing exclusively to increases in the titratable alkalinity component; total ammonia excretion was actually reduced during hypercapnia. The increase in J _NETH⁺ was accompanied by increases in branchial carbonic anhydrase (CA) enzymatic activity (2.8×) and CA protein levels (1.6×); branchial Na⁺/K⁺-ATPase activity was unaffected. Rectal fluids sampled from control fish contained on average HCO₃ ⁻ concentrations of 92.2 ± 4.8 mmol l⁻¹. At the highest level of hypercapnia, rectal fluid HCO₃ ⁻ levels were increased significantly to 141.8 ± 7.4 mmol l⁻¹ but returned to control levels during post-hypercapnia recovery (96.0 ± 13.2 mmol l⁻¹). Thus, the impressive accumulation of plasma HCO₃ ⁻ to compensate for hypercapnic acidosis occurred against a backdrop of increasing intestinal HCO₃ ⁻ excretion. Based on in vitro measurements of intestinal base secretion in Ussing chambers, it would appear that P. notatus did not respond by minimizing base loss during hypercapnia; the increases in base flux across the intestinal epithelium in response to alterations in serosal HCO₃ ⁻ concentration were similar in preparations obtained from control or hypercapnic fish. Fish returned to normocapnia developed profound metabolic alkalosis owing to unusually slow clearance of the accumulated plasma HCO₃ ⁻. The apparent inability of P. notatus to effectively excrete HCO₃ ⁻ following hypercapnia may reflect its aglomerular (i.e., non-filtering) kidney coupled with the normally low rates of urine production in marine teleosts.     

Primary Productivity and Water Balance of Grassland Vegetation on Three Soils in a Continuous CO2 Gradient: Initial Results from the Lysimeter CO2 Gradient Experiment

Field studies of atmospheric CO₂ effects on ecosystems usually include few levels of CO₂ and a single soil type, making it difficult to ascertain the shape of responses to increasing CO₂ or to generalize across soil types. The Lysimeter CO₂ Gradient (LYCOG) chambers were constructed to maintain a linear gradient of atmospheric CO₂ (~250 to 500 μl l⁻¹) on grassland vegetation established on intact soil monoliths from three soil series. The chambers maintained a linear daytime CO₂ gradient from 263 μl l⁻¹ at the subambient end of the gradient to 502 μl l⁻¹ at the superambient end, as well as a linear nighttime CO₂ gradient. Temperature variation within the chambers affected aboveground biomass and evapotranspiration, but the effects of temperature were small compared to the expected effects of CO₂. Aboveground biomass on Austin soils was 40% less than on Bastrop and Houston soils. Biomass differences between soils resulted from variation in biomass of Sorghastrum nutans, Bouteloua curtipendula, Schizachyrium scoparium (C₄ grasses), and Solidago canadensis (C₃ forb), suggesting the CO₂ sensitivity of these species may differ among soils. Evapotranspiration did not differ among the soils, but the CO₂ sensitivity of leaf-level photosynthesis and water use efficiency in S. canadensis was greater on Houston and Bastrop than on Austin soils, whereas the CO₂ sensitivity of soil CO₂ efflux was greater on Bastrop soils than on Austin or Houston soils. The effects of soil type on CO₂ sensitivity may be smaller for some processes that are tightly coupled to microclimate. LYCOG is useful for discerning the effects of soil type on the CO₂ sensitivity of ecosystem function in grasslands. Author Contributions: PF conceived study, analyzed data, and wrote the paper. AK, AP analyzed data. DH, VJ, RJ, HJ, and WP conceived study, and conducted research.     

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N₂O, CO₂ and CH₄ soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N₂O emission in these regions on a landscape level. The mean N₂O emission at site PF was 6.0 ± 0.1 SE μg N m⁻² h⁻¹. Fluxes of N₂O increased from <5 μg N m⁻² h⁻¹ during dry season conditions to up to 24.5 μg N m⁻² h⁻¹ with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N₂O were measured in the SF and RP sites, where mean N₂O emissions were 7.3 ± 0.7 SE μg N m⁻² h⁻¹ and 4.1 ± 0.5 SE μg N m⁻² h⁻¹, respectively. The dependency of N₂O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N₂O emission peaks, not the total amount of N₂O emitted. For all sites, significant positive correlations existed between N₂O emissions and both soil moisture and soil temperature. Mean CH₄ uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m⁻² h⁻¹), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m⁻² h⁻¹) and lowest for the RP site (−5.7 ± 0.5 SE μg C m⁻² h⁻¹). At all sites, CH₄ uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N₂O emissions, CH₄ uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO₂ emissions at the PF site increased significantly from 50 mg C m⁻² h⁻¹ up to 100 mg C m⁻² h⁻¹ (mean value 68.8 ± 0.8 SE mg C m⁻² h⁻¹), whereas CO₂ emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m⁻² h⁻¹ (SF) and 34.9 ± 1.1 SE mg C m⁻² h⁻¹ (RP). A dependency of soil CO₂ emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO₂ emissions as compared to control chambers. Correlation of CO₂ emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.     

Elevated [CO2], temperature increase and N supply effects on the accumulation of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l⁻¹ [CO₂]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha⁻¹ year⁻¹) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO₂ concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO₂] reduced it. At the last sampling date, elevated [CO₂] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO₂2] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha⁻¹, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha⁻¹. There was no significant effect of a 3 °C temperature increase under elevated [CO₂] on C accumulation in the OM fractions above 50 μm. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO₃⁻) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO₃⁻ leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m², 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO₃⁻ inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO₃⁻ was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO₃⁻ was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO₃⁻ leaching, undisturbed forest plots in these late-successional forests are not losing NO₃⁻ (net annual gain of 2.8 kg ha⁻¹) and are likely not N-saturated. Net annual NO₃⁻ losses were observed in gaps (1.3 kg ha⁻¹) and gap-edges (0.2 kg ha⁻¹), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.     

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.     

Inorganic Carbon Uptake during Photosynthesis : II. Uptake by Isolated Asparagus Mesophyll Cells during Isotopic Disequilibrium

The species of inorganic carbon (CO₂ or HCO₃⁻) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO₂ or HCO₃⁻ transport, during steady state photosynthesis, is achieved by monitoring the changes (by ¹⁴C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO₂ or HCO₃⁻ transport) and experimental time-courses of ¹⁴C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO₂, rather than HCO₃⁻, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO₂ is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO₃⁻ transport, as the incorporation of ¹⁴C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO₂ uptake alone. The contribution of HCO₃⁻ to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO₃⁻ concentration. The evidence for direct HCO₃⁻ transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of ¹⁴C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO₂, which is partially alleviated by a high extracellular concentration of HCO₃⁻.     

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO₂) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO₂ efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO₂ efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO₂ efflux varied widely in space; the range of variation averaged 17.4 μmol m⁻² s⁻¹ in total. While most CO₂ flux rates were under 10 μmol m⁻² s⁻¹, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO₂ efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO₂ efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO₂ efflux, estimated from the relationship between CO₂ efflux and SMC at 30 cm depth, was 165 mol m⁻² year⁻¹ (1,986 g C m⁻² year⁻¹). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO₂ efflux, leading an annual variation in soil C balance.     

Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l^-1 CO₂) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha^-1 y^-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 μl l^-1 [CO₂] at outdoor air temperature) were pulse labelled by the addition of ¹³CO₂. The elevated [CO₂] treatments were continuously labelled by the addition of fossil-fuel derived CO₂ (^{13 C} of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO₂] treatments, respectively. During the experiment, the ¹³C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO₂] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply, the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO₂], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of ‘new’ C, the soil C pools did not turnover faster in elevated [CO₂], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO₂] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO₂]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.