Nitrate influx kinetic parameters of five potato cultivars during vegetative growth

This study examines the effect of elevated CO₂ on short-term partitioning of inorganic N between a grass and soil micro-organisms. ¹⁵N-labelled NH₄⁺ was injected in the soil of mesocosms of Holcus lanatus (L.) that had been grown for more than 15 months at ambient or elevated CO₂ in reconstituted grassland soil. After 48 h, the percentage recovery of added ¹⁵N was increased in soil microbial biomass N at elevated CO₂, was unchanged in total plant N and was decreased in soil extractable N. However, plant N content and microbial biomass N were not significantly affected by elevated CO₂. These results and literature data from plant–microbial ¹⁵N partitioning experiments at elevated CO₂ suggest that the mechanisms controlling the effects of CO₂ on short- vs. long-term N uptake and turnover differ. In particular, short-term immobilisation of added N by soil micro-organisms at elevated CO₂ does not appear to lead to long-term increases in N in soil microbial biomass. In addition, the increased soil microbial C:N ratios that we observed at elevated CO₂ suggest that long-term exposure to CO₂ alters either the functioning or structure of these microbial communities.     

Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3

Increases in atmospheric CO₂ and tropospheric O₃ may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free‐air CO₂–O₃ enrichment experiment, plant growth has been stimulated by elevated CO₂ resulting in greater substrate input to soil; elevated O₃ has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO₂, and that CO₂ effects would be dampened by O₃. We found that elevated CO₂ did not alter gross N transformation rates. Elevated O₃ significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO₂ and O₃: (i) gross N mineralization was greater under elevated CO₂ (1.0 mg N kg^?1 day^?1) than in the presence of both CO₂ and O₃ (0.5 mg N kg^?1 day^?1) and (ii) gross NH₄⁺ immobilization was also greater under elevated CO₂ (0.8 mg N kg^?1 day^?1) than under CO₂ plus O₃ (0.4 mg N kg^?1 day^?1). We used a laboratory ¹⁵N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO₂ led to greater recovery of NH₄⁺‐¹⁵N in microbial biomass and corresponding lower recovery in the extractable NO₃^? pool. Elevated CO₂ resulted in a substantial increase in NO₃^?‐¹⁵N recovery in soil organic matter. We observed no O₃ main effect and no CO₂ by O₃ interaction effect on ¹⁵N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO₂ has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre‐existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity.     

Intracellular Zn2+ accumulation enhances suppression of synaptic activity following spreading depolarization

Spreading depolarization (SD) is a feed‐forward wave that propagates slowly throughout brain tissue and recovery from SD involves substantial metabolic demand. Presynaptic Zn²⁺ release and intracellular accumulation occurs with SD, and elevated intracellular Zn²⁺ ([Zn²⁺]_i) can impair cellular metabolism through multiple pathways. We tested here whether increased [Zn²⁺]_i could exacerbate the metabolic challenge of SD, induced by KCl, and delay recovery in acute murine hippocampal slices. [Zn²⁺]_i loading prior to SD, by transient ZnCl₂ application with the Zn²⁺ ionophore pyrithione (Zn/Pyr), delayed recovery of field excitatory post‐synaptic potentials (fEPSPs) in a concentration‐dependent manner, prolonged DC shifts, and significantly increased extracellular adenosine accumulation. These effects could be due to metabolic inhibition, occurring downstream of pyruvate utilization. Prolonged [Zn²⁺]_i accumulation prior to SD was required for effects on fEPSP recovery and consistent with this, endogenous synaptic Zn²⁺ release during SD propagation did not delay recovery from SD. The effects of exogenous [Zn²⁺]_i loading were also lost in slices preconditioned with repetitive SDs, implying a rapid adaptation. Together, these results suggest that [Zn²⁺]_i loading prior to SD can provide significant additional challenge to brain tissue, and could contribute to deleterious effects of [Zn²⁺]_i accumulation in a range of brain injury models.     

Comparative Toxicities of Salts on Microbial Processes in Soil

Soil salinization is a growing threat to global agriculture and carbon sequestration, but to date it remains unclear how microbial processes will respond. We studied the acute response to salt exposure of a range of anabolic and catabolic microbial processes, including bacterial (leucine incorporation) and fungal (acetate incorporation into ergosterol) growth rates, respiration, and gross N mineralization and nitrification rates. To distinguish effects of specific ions from those of overall ionic strength, we compared the addition of four salts frequently associated with soil salinization (NaCl, KCl, Na₂SO₄, and K₂SO₄) to a nonsaline soil. To compare the tolerance of different microbial processes to salt and to interrelate the toxicity of different salts, concentration-response relationships were established. Growth-based measurements revealed that fungi were more resistant to salt exposure than bacteria. Effects by salt on C and N mineralization were indistinguishable, and in contrast to previous studies, nitrification was not found to be more sensitive to salt exposure than other microbial processes. The ion-specific toxicity of certain salts could be observed only for respiration, which was less inhibited by salts containing SO₄²⁻ than Cl⁻ salts, in contrast to the microbial growth assessments. This suggested that the inhibition of microbial growth was explained solely by total ionic strength, while ion-specific toxicity also should be considered for effects on microbial decomposition. This difference resulted in an apparent reduction of microbial growth efficiency in response to exposure to SO₄²⁻ salts but not to Cl⁻ salts; no evidence was found to distinguish K⁺ and Na⁺ salts.     

Rapid Conversion of Added Nitrate to Nitrous Oxide and Dinitrogen in Northern Forest Soil

The aims of this study were to simulate wet deposition of atmospheric nitrate (NO₃^?) onto forest soils and trace its fate via conversion spatially and temporally into gaseous products nitrous oxide (N₂O) and dinitrogen (N₂). The most likely mechanism is microbial denitrification, but an intermediate product nitrite (NO₂^?) can fuel N₂O production via a chemical pathway involving reactions with iron and/or organic matter referred to as chemodenitrification. During summer months, we applied tracer amounts of ¹⁵N-labeled NO₃^? onto forest soils (pH ～ 4) at three sites in the White Mountain Region of New Hampshire, USA. We recovered ¹⁵N as N₂O in 210 of 504 measurements (42%) versus ¹⁵N as N₂ in 51 of 504 measurements (10%), suggesting partial microbial denitrification and/or chemodenitrification. When recovery occurred, the mean percent recovery of added ¹⁵N was just 1.1% as N₂O, although N₂ recovery was 33%. A site with old-growth trees had a larger percentage recovery as N₂ (48%), whereas a site that had burned 100 years ago had a small percentage recovery as N₂O (0.24%). The ¹⁵N composition of N₂O in ambient air, collected before addition of the label, was markedly enriched in ¹⁵N. Since flux measurements were made 2 h after the addition, the results suggest that denitrification enzymes and conditions for chemodenitrification are present throughout the summer months but account for small amounts of NO₃^? conversion into N₂O and N₂.     

Sulfur-driven autotrophic denitrification of nitric oxide for efficient nitrous oxide recovery

Nitrous oxide (N₂O) was previously deemed as a potent greenhouse gas but is actually an untapped energy source, which can accumulate during the microbial denitrification of nitric oxide (NO). Compared with the organic electron donor required in heterotrophic denitrification, elemental sulfur (S⁰) is a promising electron donor alternative due to its cheap cost and low biomass yield in sulfur-driven autotrophic denitrification. However, no effort has been made to test N₂O recovery from sulfur-driven denitrification of NO so far. Therefore, in this study, batch and continuous experiments were carried out to investigate the NO removal performance and N₂O recovery potential via sulfur-driven NO-based denitrification under various Fe(II)EDTA-NO concentrations. Efficient energy recovery was achieved, as up to 35.5%–40.9% of NO was converted to N₂O under various NO concentrations. N₂O recovery from Fe(II)EDTA-NO could be enhanced by the low bioavailability of sulfur and the acid environment caused by sulfur oxidation. The NO reductase (NOR) and N₂O reductase (N₂OR) were inhibited distinctively at relatively low NO levels, leading to efficient N₂O accumulation, but were suppressed irreversibly at NO level beyond 15 mM in continuous experiments. Such results indicated that the regulation of NO at a relatively low level would benefit the system stability and NO removal capacity during long-term system operation. The continuous operation of the sulfur-driven Fe(II)EDTA-NO-based denitrification reduced the overall microbial diversity but enriched several key microbial community. Thauera, Thermomonas, and Arenimonas that are able to carry out sulfur-driven autotrophic denitrification became the dominant organisms with their relative abundance increased from 25.8% to 68.3%, collectively.     

Heterotrophic Fixation of CO2 in Soil

The occurrence of heterotrophic CO₂ fixation by soil microorganisms was tested in several mineral soils differing in pH and two artificial soils (a mixture of silica sand, alfalfa powder, and nutrient medium inoculated with a soil suspension). Soils were incubated at ambient (∼0.05 vol%) and elevated (∼5 vol%) CO₂ concentrations under aerobic conditions for up to 21 days. CO₂ fixation was detected using either a technique for determining the natural abundance of ¹³C or by measuring the distribution of labeled ¹⁴C-CO₂ in soil and bacteria. The effects of elevated CO₂ on microbial biomass (direct counts, chloroform fumigation extraction method), composition of microbial community (phospholipid fatty acids), microbial activity (respiration, dehydrogenase activity), and turnover rate were also measured. Heterotrophic CO₂ fixation was proven in all soils under study, being higher in neutral soils. The main portion of the fixed CO₂ (98–99%) was found in extracellular metabolites while only ∼1% CO₂ was incorporated into microbial cells. High CO₂ concentration always induced an increase in microbial activity, changes in the composition of the microbial community, and a decrease in microbial turnover. The results suggest that heterotrophic CO₂ fixation could be a widespread process in soils.     

Elevated CO2 increases nitrogen fixation and decreases soil nitrogen mineralization in Florida scrub oak

We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO₂ during the first 14 months of experimental treatment. Elevated CO₂ stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, Galactia elliottii Nuttall. During this period, elevated CO₂ reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO₂ did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH₄⁺ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO₂ best explains these changes. These results demonstrate that atmospheric CO₂ concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO₂. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.     

Protective Effects of Extracellular Acidosis and Blockade of Sodium/Hydrogen Ion Exchange During Recovery from Metabolic Inhibition in Neuronal Tissue Culture

Abstract: Acidosis is a universal response of tissue to ischemia. In the brain, severe acidosis has been linked to worsening of cerebral infarction. However, milder acidosis can have protective effects. As part of our investigations of the therapeutic window in our neuronal tissue culture model of ischemia, we investigated the effects of acidosis during recovery from brief simulated ischemia. Ischemic conditions were simulated in dissociated cortical cultures by metabolic inhibition with potassium cyanide to block oxidative metabolism and 2-deoxyglucose to block glycolysis. Lowering the extracellular pH (pH_e) to 6.2 during metabolic inhibition had no effect on injury, as measured by lactate dehydrogenase release from cultures after 24 h of recovery. Lowering the pH_e during the first hour of recovery, in contrast, had profound protective effects. When the duration of metabolic inhibition was lengthened to 30 min, most of the protective effects of the NMDA receptor antagonist MK-801 were lost. However, the protective effects of acidosis were unchanged. This suggested that the protective effects of extracellular acidosis could be due to more than blockade of NMDA receptors. Intracellular acidosis might be responsible. To test this, recovery of intracellular pH (pH_i) was slowed by incubation with blockers of Na⁺/H⁺ exchangers at normal pH_e. The two compounds tested, dimethylamiloride and harmaline, had protective effects when present during recovery from metabolic inhibition. Measurements of pH_i confirmed that the blockers slowed recovery from intracellular acidosis; more rapid pH_i recovery was correlated with injury. The protective effects of acidosis could be reversed by brief incubation with the protonophore monensin, which rapidly normalized pH_i. These results are the first demonstration of the protective effects of blocking Na⁺/H⁺ exchange in a model of cerebral ischemia. The protective effects of acidosis appear to arise either from suppressing pH-sensitive mechanisms of injury or from blocking sodium entry due to Na⁺/H⁺ exchange.     

Effects of slow-release urea fertilizers on urease activity,microbial biomass,and nematode communities in an aquic brown soil

A field experiment was carried out at the Shenyang Experimental Station of Ecology (CAS) in order to study the effects of slow-release urea fertilizers high polymer-coated urea (SRU1), SRU1 mixed with dicyandiamide DCD (SRU2), and SRU1 mixed with calcium carbide CaC₂ (SRU3) on urease activity, microbial biomass C and N, and nematode communities in an aquic brown soil during the maize growth period. The results demonstrated that the application of slow-release urea fertilizers inhibits soil urease activity and increases the soil NH₄ ⁺-N content. Soil available N increment could promote its immobilization by microorganisms. Determination of soil microbial biomass N indicated that a combined application of coated urea and nitrification inhibitors increased the soil active N pool. The population of predators/omnivores indicated that treatment with SRU2 could provide enough soil NH₄ ⁺-N to promote maize growth and increased the food resource for the soil fauna compared with the other treatments.     

Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils

Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO₂ on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community‐level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004–2005 growing season, an above‐average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO₂, and sampling date. Microbial C and N activities were lower in interspace soils compared with soils under Larrea, and responses to date and CO₂ treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO₂, but increased under elevated CO₂. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO₂ positively affected both. Elevated CO₂ also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO₂. Gross rates of NH₄⁺ transformations increased over the growing season, and late‐season NH₄⁺ fluxes were negatively affected by elevated CO₂. Gross NO₃⁻ fluxes decreased over time, with early season interspace soils positively affected by elevated CO₂. General increases in microbial activities under elevated CO₂ are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO₂, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO₂ shift the spatial distribution of Mojave Desert plant communities.     

Short-term nitrous oxide emissions from pasture soil as influenced by urea level and soil nitrate

Nitrogen excreted by cattle during grazing is a significant source of atmospheric nitrous oxide (N₂O). The regulation of N₂O emissions is not well understood, but may vary with urine composition and soil conditions. This laboratory study was undertaken to describe short-term effects on N₂O emissions and soil conditions, including microbial dynamics, of urea amendment at two different rates (22 and 43 g N m⁻²). The lower urea concentration was also combined with an elevated soil NO ₃ ⁻ concentration. Urea solutions labelled with 25 atom%¹⁵N were added to the surface of repacked pasture soil cores and incubated for 1, 3, 6 or 9 days under constant conditions (60% WFPS, 14 °C). Soil inorganic N (NH ₄ ⁺ , NO ₂ ⁻ and NO ₃ ⁻ ), pH, electrical conductivity and dissolved organic C were quantified. Microbial dynamics were followed by measurements of CO₂ evolution, by analyses of membrane lipid (PLFA) composition, and by measurement of potential ammonium oxidation and denitrifying enzyme activity. The total recovery of¹⁵N averaged 84%. Conversion of urea-N to NO ₃ ⁻ was evident, but nitrification was delayed at the highest urea concentration and was accompanied by an accumulation of NO ₂ ⁻ . Nitrous oxide emissions were also delayed at the highest urea amendment level, but accelerated towards the end of the study. The pH interacted with NH ₄ ⁺ to produce inhibitory concentrations of NH₃(aq) at the highest urea concentration, and there was evidence for transient negative effects of urea amendment on both nitrifying and denitrifying bacteria in this treatment. However, PLFA dynamics indicated that initial inhibitory effects were replaced by increased microbial activity and net growth. It is concluded that urea-N level has qualitative, as well as quantitative effects on soil N transformations in urine patches.     

Effects of slow-release urea fertilizers on urease activity,microbial biomass,and nematode communities in an aquic brown soil

A field experiment was carried out at the Shenyang Experimental Station of Ecology (CAS) in order to study the effects of slow-release urea fertilizers high polymer-coated urea (SRU1), SRU1 mixed with dicyandiamide DCD (SRU2), and SRU1 mixed with calcium carbide CaC₂ (SRU3) on urease activity, microbial biomass C and N, and nematode communities in an aquic brown soil during the maize growth period. The results demonstrated that the application of slow-release urea fertilizers inhibits soil urease activity and increases the soil NH₄ ⁺-N content. Soil available N increment could promote its immobilization by microorganisms. Determination of soil microbial biomass N indicated that a combined application of coated urea and nitrification inhibitors increased the soil active N pool. The population of predators/omnivores indicated that treatment with SRU2 could provide enough soil NH₄ ⁺-N to promote maize growth and increased the food resource for the soil fauna compared with the other treatments.

     

Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.     

C and N allocation in soil under ryegrass and alfalfa estimated by 13C and 15N labelling

Background and Aims

Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species.

Methods

A non-legume (Lolium perenne) and a legume (Medicago sativa) were labelled with ¹⁵N before the plants were clipped or shaded, and labelled twice with ¹³CO₂ thereafter. Ten days after clipping and shading, the ¹⁵N and ¹³C in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the ¹³C in soil CO₂ were analyzed.

Results

After clipping, about 50 % more ¹³C was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO₂ efflux under both species and the ¹³C recovery of soil CO₂ under L. perenne. The ¹⁵N recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated ¹³C was preferentially retained in the shoots of both species. This caused a decreased ¹³C recovery in the roots of M. sativa. Similarly, the total soil CO₂ efflux under M. sativa decreased more than 50 % after shading. The ¹⁵N recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the ¹⁵N recovery increased in the shoot of M. sativa.

Conclusions

The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading.     

南亚热带红椎和格木人工幼龄林土壤微生物群落结构特征

采用氯仿熏蒸浸提法和磷脂脂肪酸法(Phospholipids fatty acid,PLFA)研究了我国南亚热带地区非固氮树种红椎(Castanopsis hystrix)和固氮树种格木(Erythrophleum fordii)人工幼龄林土壤微生物生物量与微生物群落结构特征。结果表明,在旱季和雨季,红椎幼龄林土壤微生物总PLFAs量,细菌PLFAs量、放线菌PLFAs量及丛枝菌根真菌PLFAs量均大于格木幼龄林。红椎幼龄林土壤PLFA Shannon多样性指数(H_(PLFA))在旱季和雨季均大于格木幼龄林。主成分分析表明,土壤微生物群落结构组成受到林分类型和季节的双重影响。冗余分析表明,土壤硝态氮(NO_3~--N)含量、土壤含水量、p H及土壤微生物生物量氮(MBN)与特征磷脂脂肪酸之间呈显著相关关系。以上结果表明固氮树种格木与非固氮树种红椎人工幼龄林对土壤微生物生物量和群落结构的影响存在显著差异。     

Bioleaching of uranium from low grade black schists by <Emphasis Type="Italic">Acidithiobacillus ferrooxidans</Emphasis>

Summary The feasibility of bacterial recovery of uranium from the low grade black schists occurring in the Okcheon district, South Korea, was investigated. Following the introduction of Acidithiobacillus ferrooxidans, 80% of the uranium could be extracted from the schists, which contain 0.01% U₃O₈ by weight, within 60 h at a pulp density of 100 g-ore/l. Only 18% of the uranium was extracted without microbial activity. The uranium-leaching efficiency was not greatly affected by the addition of Fe²⁺ in the range of 5–9 g/l, and the leaching efficiency of uranium from the schists by A. ferrooxidanscould be efficiently maintained at high pulp densities (up to 500 g-ore/l).     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Gross Nitrogen Dynamics in the Mycorrhizosphere of an Organic Forest Soil

The rhizosphere is a hot-spot for biogeochemical cycles, including production of greenhouse gases, as microbial activity is stimulated by rhizodeposits released by roots and mycorrhizae. The biogeochemical cycle of nitrogen (N) in soil is complex, consisting of many simultaneously occurring processes. In situ studies investigating the effects of roots and mycorrhizae on gross N turnover rates are scarce. We conducted a ¹⁵N tracer study under field conditions in a spruce forest on organic soil, which was subjected to exclusion of roots and roots plus ectomycorrhizae (ECM) for 6 years by trenching. The forest soil had, over the 6-year period, an average emission of nitrous oxide (N₂O) of 5.9 ± 2.1 kg N₂O ha^?1 year^?1. Exclusion of roots + ECM nearly tripled N₂O emissions over all years, whereas root exclusion stimulated N₂O emission only in the latest years and to a smaller extent. Gross mineralization–ammonium (NH₄ ⁺) immobilization turnover was enhanced by the presence of roots, probably due to high inputs of labile carbon, stimulating microbial activity. We found contrasting effects of roots and ECM on N₂O emission and mineralization, as the former was decreased but the latter was stimulated by roots and ECM. The N₂O emission was positively related to the ratio of gross NH₄ ⁺ oxidation (that is, autotrophic nitrification) to NH₄ ⁺ immobilization. Ammonium oxidation was only stimulated by the presence of ECM, but not by the presence of roots. Overall, we conclude that plants and their mycorrhizal symbionts actively control soil N cycling, thereby also affecting N₂O emissions from forest soils. Consequently, adapted forest management with permanent tree cover avoiding clearcutting could be a means to reduce N₂O emissions and potential N leaching; despite higher mineralization in the presence of roots and ECM, N₂O emissions are decreased as the relative importance of NH₄ ⁺ oxidation is decreased, mainly due to a stimulated microbial NH₄ ⁺ immobilization in the mycorrhizosphere.     

Microbially mediated CH4 consumption and N2O emission is affected by elevated CO2, soil water content, and composition of semi-arid grassland species

Elevated CO₂ affects plant productivity, but also water availability and plant species composition in semi-arid grasslands, thereby potentially causing complex effects on CH₄ consumption and N₂O emission. We studied the effects of atmospheric CO₂ concentration (400 vs 780 μL L^?1), water content (15 vs 20% gravimetric soil moisture), and composition of semi-arid grassland species (perennial grasses Bouteloua gracilis, Hesperostipa comata, and Pascopyrum smithii; sub-shrub Artemisia frigida; invasive forb Linaria dalmatica grown in monoculture and all five species together) on CH₄ consumption and N₂O emission in a full factorial greenhouse experiment. We used a unique method where we measured microbial effects on CH₄ consumption and N₂O emission in isolation from effects of gas diffusivity. Microbially mediated CH₄ consumption was significantly higher under elevated CO₂ (by 20%), but was not affected by soil water content or plant species composition. Microbially mediated N₂O emission was not significantly affected by elevated CO₂, but was significantly higher with high water content (by 67%) and differed significantly among species. Treatment effects on CH₄ consumption and N₂O emission often could not be explained simply by differences in soil moisture, suggesting that treatment-induced changes in other soil and microbial properties played a role in causing these effects.