Long-term effects of successive Ca(OH)2 and CaCO3 treatments on the water quality of two eutrophic hardwater lakes

1. Whole-lake experiments were conducted in two hardwater lakes (Halfmoon and Figure Eight) in Alberta, Canada, to investigate the effectiveness of repeated lime (slaked lime: Ca(OH)₂ and/or calcite: CaCO₃) treatments (5–78 mg L^–1) for up to 7 years.
2. Randomized intervention analysis of intersystem differences between the experimental and three reference lakes demonstrated a decline in euphotic total phosphorus and chlorophyll a concentrations in the experimental lakes after repeated lime treatments.
3. After the second lime application to Halfmoon Lake, mean winter total phosphorus release rates (TPRR) decreased to < 1 mg m^–2 day^–1 compared with 3.6 mg m^–2 day^–1 during the winter after initial treatment. In the final year of lime application, mean summer TPRR decreased to 4.5 mg m^–2 day^–1 compared with 7.6 mg m^–2 day^–1 in the pre-treatment year.
4. Mean macrophyte biomass declined and species composition was altered at 1 and 2 m depths in Figure Eight Lake during lime application. Over the first 6 years of treatment, macrophyte biomass at 2 m declined by 95% compared with concentrations recorded during the initial treatment year. In the last year of the study, macrophyte biomass at 2 m reached initial treatment concentrations, which coincided with the greatest water transparency. Over the treatment period, macrophyte species shifted from floating to rooted plants.
5. Multiple lime applications can improve water quality in eutrophic hardwater lakes for periods of up to 7 years.     

Response of plankton communities to whole-lake Ca(OH)2 and CaCO3 additions in eutrophic hardwater lakes

1. The impact of whole-lake lime (slaked lime, Ca(OH)₂, and/or calcite, CaCO₃) addition on plankton communities was evaluated in eutrophic hardwater lakes on the North American Boreal Plain.
2. Two lakes received a single treatment of lime (Ca(OH)₂ at 74 or 107 mg L^–1), two lakes received multiple treatments with Ca(OH)₂ and/or CaCO₃ (5–78 mg L^–1), and four lakes were untreated and served as reference systems.
3. Over the long-term (> 1 year), phytoplankton biomass was reduced in multiple-dose lakes, but not in single-dose lakes. Cyanobacteria typically dominated the algal community in the years before, during and after lime treatment in both single- and multiple-dose lakes.
4. In the single-dose lakes, randomized intervention analysis showed no significant change in the biomass of zooplankton after lime addition.     

Ecosystem CO2 exchange and plant biomass in the littoral zone of a boreal eutrophic lake

• 1 In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO₂) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998–1999). We reconstructed the seasonal net CO₂ exchange and identified the key factors controlling CO₂ dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass.
• 2 In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO₂ fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO₂ uptake of 1.8–6.2 mol m^?2 per open water period, but the upper flooded zone with the highest photosynthetic capacity and above‐ground plant biomass, had a net CO₂ loss of 1.1–7.1 mol m^?2 per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment.
• 3 Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.

     

Sediment respiration and lake trophic state are important predictors of large CO2 evasion from small boreal lakes

We show that sediment respiration is one of the key factors contributing to the high CO₂ supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km²) and 32 lakes with the highest total phosphorus (P_tot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO₂ concentrations plotted against depth and time demonstrate strong CO₂ accumulation in hypolimnetic waters during the stratification periods. The relationship between O₂ departure from the saturation and CO₂ departure from the saturation was strong in the entire data set (r²=0.79, n=2 740, P<0.0001). CO₂ concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO₂ concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake‐area‐weighted annual CO₂ evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km² ( Rantakari & Kortelainen, 2005 ) was 42 g C m^?2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO₂ evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.     

Influence of NaCl, Cd(NO3)2 and air humidity on transpiration of Tamarix aphylla

Transpiration rates of young Tamarix aphylla (L.) Karst, plants grown in hydroponics were measured under NaCl- and Cd(NO₃)₂-stress. Transpiration rates were negatively correlated with the relative humidity of the ambient air at all NaCl concentrations investigated. Low and intermediate concentrations of Cd²⁺ (45 and 90 μ M , respectively) in the medium caused an increase in transpiration rates. This was particularly pronounced at low levels of relative humidity. At 180 μ M Cd²⁺, transpiration rates dropped, probably as a result of root damage due to Cd²⁺ toxicity. Since the transpiration rates differed by a factor of ca 3 between day and night, it is concluded that the stomata did not lose their ability to regulate transpiration under the influence of NaCl or of Cd(NO₃)₂. The transpiration behaviour of T. aphylla indicates that the effect of water vapour pressure (presented as relative humidity) on the degree of stomatal opening is small. Under conditions of ample water supply transpiration follows the evaporative demand of the ambient air and is influenced by the water uptake capacity of the root system as well as by other environmental factors, e.g. light.     

Interaction between atmospheric CO2 concentration and water deficit on gas exchange and crop growth: testing of ecosys with data from the Free Air CO2 Enrichment (FACE) experiment

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO₂ concentrations (C_a), but the extent of this influence is uncertain. To study the interaction of water deficits and C_a on crop growth, the ecosystem simulation model ecosys was tested with data for diurnal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at C_a= 370 and 550 μmol mol^?1 in the Free Air CO₂ Enrichment (FACE) experiment near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy conductance (g_c) at C_a= 550 μmol mol^?1 was reduced to about 0.75 that at C_a= 370 μmol mol^?1, but that under low irrigation, g_c was reduced less. Consequently when C_a was increased from 370 to 550 μmol mol^?1, canopy transpiration was reduced less, and net CO₂ fixation was increased more, under low irrigation than under high irrigation. The simulated effects of C_a and irrigation on diurnal gas exchange were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was reduced by 6% vs. 4% at C_a= 550 vs. 370 μmol mol^?1 but that under low irrigation was increased by 3% vs. 5%. Simulated vs. measured grain yield of wheat under high irrigation was increased by 16% vs. 8%, but that under low irrigation was increased by 38% vs. 21%. In ecosys, the interaction between C_a and irrigation on diurnal gas exchange, and hence on seasonal crop growth and water use, was attributed to a convergence of simulated g_c towards common values under both C_a as canopy turgor declined. This convergence caused transpiration to decrease comparatively less, but CO₂ fixation to increase comparatively more, under high vs. low C_a. Convergence of g_c was in turn attributed to improved turgor maintenance under elevated C_a caused by greater storage C concentrations in the leaves, and by greater rooting density in the soil.     

Seasonal fine-root biomass development of sour orange trees grown in atmospheres of ambient and elevated CO2 concentration

Sour orange trees have been grown from the seedling stage out-of-doors at Phoenix, Arizona, USA, in open-top enclosures with clear plastic walls for 3.5 years. For the last 3 years of this period, half of the trees have been continuously exposed to air enriched with CO₂ to 300 μmol mol^?1 above the ambient concentration. At 2-month intervals over the last 12 months, we have determined the fine-root biomass in the top 0.4 m of the soil profile beneath the trees. Results from both treatments define a single relationship between fine-root biomass and trunk cross-sectional area. The data also show the CO₂-enriched trees to have approximately 2.3 times more fine-root biomass in this soil layer than the trees grown in ambient air.     

Effects of raised CO2 on potential CH4 production and oxidation in, and CH4 emission from, a boreal mire

1 In a glasshouse experiment we studied the effect of raised CO₂ concentration (720 p.p.m.) on CH₄ emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH₄ production and oxidation was measured at the end of the emission experiment.
2 The vascular cores ('Sedge') consisted of a moss layer with sedges, and the moss cores (' Sphagnum ') of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH ₄ from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures.
3 In both Sedge and Sphagnum samples, CO₂ treatment doubled the potential CH₄ production but had no effect on the potential CH₄ oxidation. A raised concentration of CO₂ increased CH₄ efflux weakly and only at the highest peat temperatures (17–20 °C).
4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO₂ concentrations, and would thus retard any consequent increase in CH₄ emission.     

Interactions between atmospheric CO2 concentration and phosphorus nutrition on the formation of proteoid roots in white lupin (Lupinus albus L.)

Atmospheric [CO₂] affects photosynthesis and therefore should affect the supply of carbon to roots. To evaluate interactions between carbon supply and nutrient acquisition, the [CO₂] effects on root growth, proteoid root formation and phosphorus (P) uptake capacity were studied in white lupin (Lupinus albus L.) grown hydroponically at 200, 410 and 750 µmol mol^?1 CO₂, under sufficient (0·25 mm P) and deficient (0·69 µm P) phosphorus. Plant size increased with increasing [CO₂] only at high P. Both P deficiency and increasing [CO₂] increased the production of proteoid clusters; the increase in response to increased [CO₂] was proportionally greater from low to ambient [CO₂] than from ambient to high. The activity of phosphoenol pyruvate carboxylase in the proteoid root, the exudation of organic acids from the roots, and the specific uptake of P increased with P deficiency, but were unaffected by [CO₂]. Increasing [CO₂] from Pleistocene levels to those predicted for the next century increased plant size and allocation to proteoid roots, but did not change the specific P uptake capacity per unit root mass. Hence, rising [CO₂] should promote nutrient uptake by allowing lupins to mine greater volumes of soil.     

Seasonal patterns of soil CO2 efflux and soil air CO2 concentration in a Scots pine forest: comparison of two chamber techniques

A non‐vented non‐steady state flow‐through chamber and a non‐vented non‐steady state non‐flow‐through chamber technique were used to measure CO₂ efflux of a young Scots pine forest on a fertile till soil in southern Finland. Soil temperature, soil moisture and soil CO₂ concentration were measured concurrently with CO₂ efflux for two and a half successive years. The CO₂ efflux showed a seasonal pattern, effluxes ranging from low 0.0–0.1 g CO₂ m ^{? 2} h ^{? 1} in winter to peak values of 2.3 g CO₂ m ^{? 2} h ^{? 1} occurring in late June and in July. The daily average effluxes in July measured by flow through chambers were 1.23 and 0.98 g CO₂ m ^{? 2} h ^{? 1} in 1998 and 1999, respectively. The annual accumulated CO₂ efflux was 3117 and 3326 g CO₂ m ^{? 2} in 1998 and 1999, respectively. The spatial variation in CO₂ efflux was high (CV 0.18–0.45) and increased with increasing efflux. Soil air CO₂ concentration showed similar seasonal pattern the peak concentrations occurring in July–August. The CO₂ concentrations ranged from 580 to 780 µ mol mol ^{? 1} in the humus layer to 13 620–14 470 µ mol mol ^{? 1} in the C‐horizon. In winter the soil air CO₂ concentrations were lower, especially in deeper soil layers. Drought decreased CO₂ efflux and soil air CO₂ concentration. The in situ comparison on forest soil between the chamber methods showed the non‐flow‐through chamber to give ～～50% lower efflux values than that of the flow‐through chamber. When calibrated against known CO₂ efflux ranging from 0.4 to 0.8 g CO₂ m ^{? 2} h ^{? 1} generated with a diffusion box method developed by Widén and Lindroth [Acta Universitatis Agriculturae Suecia Silvestria, 2001], the flow‐through chamber gave equal effluxes at the lower end of the calibration range, but overestimated high effluxes by 20%. Non‐flow‐through chamber underestimated the CO₂ efflux by 30%.     

Geographic and microgeographic genetic differentiation in two aphid species over southern England using the multilocus (GATA)4 probe

Samples of the grain aphid Sitobion avenue (F.) and the rose-grain aphid Metopolophium dirhodum (Walker) were collected in late March from wheat fields and adjacent road-side grasses at a number of locations in southern England. Unparasitized aphids were DNA fingerprinted using the multilocus (GATA)₄ probe. Over all locations, the fingerprints of individual S. avenue caught in wheat had lower overall average distances of band migration (ADBM) and shared a higher proportion of bands, than fingerprints of individuals caught in adjacent road-side grasses. The ADBM of fingerprints of S. avenue collected on road-side grasses altered significantly with geographical location, while the ADBM of fingerprints of S. avenue caught on wheat did not. A comparison of the fingerprints of individual M. dirhodum caught in wheat and neighbouring road-side grasses did not reveal any genetic differentiation. Fingerprints of M. dirhodum that were caught in the same host type did however, show significant variation in ADBM between different locations. With both S. avenue and M. dirhodum, spatial autocorrelation revealed that locations that were close together were no more likely to have individuals with similar ADBM than locations mat were far apart Our results suggest that (i) particular clones of S. avenue prefer to colonize wheat; and/or that (ii) particular clones of S. avenae perform better on wheat man other clones. It is unclear why M. dirhodum did not show any genetic structuring according to host type, but this species appears to engage in sexual reproduction much more frequently than S. avenae in southern England. M. dirhodum is likely to have displayed genetic heterogeneity between locations either because of founder effects, or because of genetic drift.     

Differential Effects of Presynaptic Phospholipase A2 Neurotoxins on Torpedo Synaptosomes

The effects of several phospholipase A2 neurotoxins from snake venoms were examined on purely cholinergic synaptosomes from Torpedo electric organ. The noncatalytic component A of crotoxin had no effect, whereas its phospholipase component B, used alone or complexed to component A, elicited a rapid and dose-dependent acetylcholine (ACh) release and a depolarization of the preparation. Subsequent ACh release evoked by high K+ levels or calcium ionophore was identical to the control after the action of component A but reduced after the action of crotoxin or of component B. These effects were not observed when the phospholipase A2 activity of the toxin was blocked either by replacing Ca2+ by Ba2+ (respectively, activator and inhibitor of phospholipase A2) or by alkylation of component B with p-bromophenacyl bromide. beta-Bungarotoxin, another very potent phospholipase A2 neurotoxin, induced release of little ACh, did not affect ionophore-evoked ACh release, but significantly reduced depolarization-induced ACh release. The single-chain phospholipase A2 neurotoxin agkistrodotoxin behaved like crotoxin component B. A nonneurotoxic phospholipase A2 from mammalian pancrease induced release of an amount of ACh similar to that released by crotoxin but did not affect the evoked responses. The obvious differences in effect of the various neurotoxins suggest that they exert their specific actions on the excitation-secretion coupling process at different sites or by different mechanisms.     

Effects of cold on CO2 exchange in winter rape leaves

A viable wheat hybrid intermediate of the same height as the parents was obtained by crossing the female parent of tall variety NP4 with the male parent of the dwarf variety HD2160. Seeds of the hybrid and its parents were germinated and their growth pattern as well as the activities of peroxidase, indolyl-3-acetic acid oxidase and amylase in extracts made from them were studied at the early seedling stages i.e. up to 96 h.
A positive correlation existed between the length of the axis at the early seedling stage and at mature plant height as far as the parental varieties are concerned but no such correlation was observed with the hybrid. Growth of the hybrid seedlings was less than of its parents. Light appeared to stimulate the longitudinal growth of the axis to different extents in the parents and hybrid. Higher activities of peroxidase, indolyl-3-acetic acid oxidase and amylase were observed in the hybrid as compared to both of its parents. Lethal wheat hybrid also exhibits increased activities of amylase, indolyl-3-acetic acid oxidase and peroxidase. Therefore, it appears that seedling growth and enzyme activities at the seedling stage are not always correlated with hybrid vigour.     

Effect of nitrogen and phosphorus availability on the growth response of Eucalyptus grandis to high CO2

The response of Eucalyptus grandis seedlings to elevated atmospheric CO₂ concentrations was examined by growing seedlings at either 340 or 660 n mol CO₂ mol^-1 for 6 weeks. Graded increments of phosphorus and nitrogen fertilizers were added to a soil deficient in these nutrients to establish if the growth response to increasing nutrient availability was affected by CO₂ concentration. At 660 μmol CO₂ mol^-1, seedling dry weight was up to five times greater than at 340 μmol CO₂ mol^-1. The absolute response was largest when both nitrogen and phosphorus availability was high but the relative increase in dry weight was greatest at low phosphorus availability. At 340 μmol CO₂ mol^-1 and high nitrogen availability, growth was stimulated by addition of phosphorus up to 76 mg kg 1 soil. Further additions of phosphorus had little effect. However, at 660 μmol CO₂ mol^-1, growth only began to plateau at a phosphorus addition rate of 920mg kg^-1 soil. At 340 μmol CO₂ mol^-1 and high phosphorus availability, increasing nitrogen from 40 to 160mg kg^-1 soil had little effect on plant growth. At high CO₂, growth reached a maximum at between 80 and 160mg nitrogen kg^-1 soil. Total uptake of phosphorus was greater at high CO₂ concentration at all fertilizer addition rates, but nitrogen uptake was either lower or unchanged at high CO₂ concentration except at the highest nitrogen fertilizer rate. The shoot to root ratio was increased by CO₂ enrichment, primarily because the specific leaf weight was greater. The nitrogen and phosphorus concentration in the foliage was lower at elevated CO₂ concentration partly because of the higher specific leaf weight. These results indicate that critical foliar concentrations currently used to define nutritional status and fertilizer management may need to be reassessed as the atmospheric CO₂ concentration rises.     

Effects of elevated CO2 on grain yield and quality of wheat: results from a 3-year free-air CO2 enrichment experiment

Spring wheat ( Triticum aestivum L. cv. TRISO) was grown for three consecutive seasons in a free-air carbon dioxide (CO₂) enrichment (FACE) field experiment in order to examine the effects on crop yield and grain quality. CO₂ enrichment promoted aboveground biomass (+11.8%) and grain yield (+10.4%). However, adverse effects were predominantly observed on wholegrain quality characteristics. Although the thousand-grain weight remained unchanged, size distribution was significantly shifted towards smaller grains, which may directly relate to lower market value. Total grain protein concentration decreased significantly by 7.4% under elevated CO₂, and protein and amino acid composition were altered. Corresponding to the decline in grain protein concentration, CO₂ enrichment resulted in an overall decrease in amino acid concentrations, with greater reductions in non-essential than essential amino acids. Minerals such as potassium, molybdenum and lead increased, while manganese, iron, cadmium and silicon decreased, suggesting that adjustments of agricultural practices may be required to retain current grain quality standards. The concentration of fructose and fructan, as well as amounts per area of total and individual non-structural carbohydrates, except for starch, significantly increased in the grain. The same holds true for the amount of lipids. With regard to mixing and rheological properties of the flour, a significant increase in gluten resistance under elevated CO₂ was observed. CO₂ enrichment obviously affected grain quality characteristics that are important for consumer nutrition and health, and for industrial processing and marketing, which have to date received little attention.     

Time dependency of 1,25(OH)2D3 induction of calbindin mRNA and calbindin expression in chick enterocytes during their differentiation along the crypt-villus axis

Quantitative methods of in situ hybridization and immunocytochemistry have been used to measure 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) induction of calbindin mRNA and calbindin protein expressed in jejunal enterocytes at all points along the crypt-villus axis over a 24 h period. Small amounts of calbindin mRNA detected in vitamin D3 deficient (D-deficient) chick intestine increased rapidly to maximal values 8 h after hormone injection. The magnitude of this response was inversely related to age of enterocyte measured separately by injecting tritiated thymidine into D-deficient and 1,25(OH)2D3-injected birds. Enterocytes of all ages expressed small amounts of calbindin 3 h after hormone injection. This amount of calbindin then increased up to 24 h after hormone injection. Maximal calbindin expression took place in basal villus enterocytes. Later decrease in the ability of upper villus enterocytes to express calbindin was associated with a similar fall in calbindin mRNA expression. Previously it was suggested that inefficient translation to calbindin mRNA might take place in basal villus enterocytes 48 h after vitamin D injection. Present work using 1,25 (OH)2D3 shows that calbindin expression takes place at a constant rate during this early stage of enterocyte development. Secondary events limiting higher rates of calbindin synthesis in upper crypt and basal villus enterocytes remain to be identified.     

Effects of Antidepressant Drug Treatments on α2-Adrenoceptor Control of [3H]Noradrenaline Release from Hypothalamic Synaptosomes

KCl (16 mM) stimulated the release of [3H]noradrenaline ([3H]NA) from rat hypothalamic synaptosomes in a Ca2+-dependent manner; this release was attenuated by clonidine (0.01-100 microM). Changes in the release of [3H]NA and the functional status of alpha 2-adrenoceptors in the medial hypothalamus of rats treated acutely and chronically with clorgyline (1 mg/kg/day) or desipramine (DMI, 10 mg/kg/day) were assessed using superfused synaptosomes in which the attenuating effects of clonidine (1 microM) or the potentiating effects of yohimbine (1 microM) on K+-evoked release of [3H]NA were measured. After acute administration of DMI, significantly less [3H]NA was accumulated into synaptosomes. Although total (spontaneous + K+-evoked) [3H]NA release from these synaptosomes was unchanged, a significant reduction was apparent in the K+-evoked release from the DMI-treated tissue. Attenuation of K+-evoked release by clonidine was abolished in both these acute treatment groups. Following the chronic antidepressant drug regimens, [3H]NA uptake into DMI-treated tissue remained significantly reduced although total percent and K+-evoked [3H]NA release were unchanged. The K+-evoked release of [3H]NA in S1 was significantly enhanced (by 22%) in the clorgyline treatment group. Attenuation of K+-evoked [3H]NA release by clonidine in both chronic antidepressant-treated tissues was not significantly changed. It is concluded that the functional sensitivity of alpha 2-adrenoceptors on nerve endings in the medial hypothalamus is unchanged by these chronic antidepressant drug regimens. In synaptosomes from untreated tissue, yohimbine significantly potentiated K+-evoked release of [3H]NA; this effect was unchanged after acute regimens and reduced after chronic administration of both the antidepressants.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland

Microbial responses to three years of CO₂ enrichment (600 μL L^–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO₂. Microbial C:N ratios significantly decreased under elevated CO₂ (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO₂ (qCO₂ = basal respiration/microbial C) did not exhibit any significant differences between CO₂ treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO₂ and phosphorus (P) fertilizer (1 g P m^–2 y^–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO₂. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO₂ on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO₂ enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO₂ must be studied in natural, undisturbed systems.