Plant nitrate use in deciduous woodland: the relationship between leaf N, N natural abundance of forbs and soil N mineralisation

Our aim was to study whether the in situ natural abundance ¹⁵N (δ¹⁵N)-values and N concentration of understory plants were correlated with the form and amount of mineral N available in the soil. Also to determine whether such differences were related to earlier demonstrations of differences in biomass increase in the same species exposed to nutrient solutions with both and or to alone. Several studies show that the δ¹⁵N of in soil solution generally is isotopically lighter than the δ¹⁵N of due to fractionation during nitrification. Hence, it is reasonable to assume that plant species benefiting from in ecosystems without significant leaching or denitrification have lower δ¹⁵N-values in their tissues than species growing equally well, or better, on We studied the δ¹⁵N of six understory species in oak woodlands in southern Sweden at 12 sites which varied fivefold in potential net N mineralisation rate The species decreased in benefit from in the following order: Geum urbanum, Aegopodium podagraria, Milium effusum, Convallaria majalis, Deschampsia flexuosa and Poa nemoralis. Four or five species demonstrated a negative correlation between and leaf δ¹⁵N and a positive correlation between and leaf N concentration. In wide contrast, only D. flexuosa, which grows on soils with little nitrification, showed a positive correlation between and the leaf N concentration and δ¹⁵N-value. Furthermore, δ¹⁵N of plants from the field and previously obtained indices of hydroponic growth on relative to were closely correlated at the species level. We conclude that δ¹⁵N may serve as a comparative index of uptake of among understory species, preferably in combination with other indices of N availability. The use of δ¹⁵N needs careful consideration of known restrictions of method, soils and plants.     

The impacts of inorganic nitrogen application on mineralization of C-labelled maize and glucose, and on priming effect in saline alkaline soil

Organic matter dynamics and nutrient availability in saline alkaline soil of the former lake Texcoco will determine the success of a planned reforestation program. Uniformly labelled ¹⁴C-maize (MAI-treatment) and glucose (GLU-treatment) with or without 200 mg  kg⁻¹ soil (MAI-N treatment and GLU-N treatment, respectively) were added to soils with electrolytic conductivity (EC) 56 dS m⁻¹ (soil A) and 12 dS m⁻¹ (soil B) to investigate the importance of N availability on decomposition of organic material. Production of CO₂ and and inorganic N dynamics were monitored. The amount of ¹⁴C-glucose mineralized increased 1.8-times in the soil A, but had no effect in the soil B when 200 mg  kg⁻¹ soil was added, while the amount of ¹⁴C-maize mineralized increased 1.7 and 1.3-times when 200  kg⁻¹ soil was added in the soils A and B, respectively. Application of increased priming effect 3.7-times in the MAI-treatment of the soil A and 3.4-times in the GLU-treatment, while in the soil B the increase of priming effect was 4.1-times in the MAI-treatment and 3.7-times in the GLU-treatment. Of the 200 mg  kg⁻¹ added to both soils less than 10 mg NH₃-N kg⁻¹ was volatilized within one day, while 22 and 44 mg  kg⁻¹ soil was fixed on the soil matrix in the soil A and the soil B, respectively. Therefore more than 100 mg −N kg⁻¹ was immobilized into the microbial biomass within the first day. Concentration of nitrite increased sharply in all the treatments of soil A at the onset of the incubation followed by a decrease. A similar pattern was observed in the GLU-N and MAI-N treatments of the soil B, but not in the other treatments. A decrease in concentration of was observed in both soils followed by an increase in the MAI-N and GLU-N treatments of the soil B. It was found that application of had a stimulating effect on the decomposition of maize and glucose, and on the priming effect, while assimilatory reduction of resulted in an increase of in the soil A, and nitrification in the soil B.     

Carbon and nitrogen mineralization of non-composted and composted municipal solid waste in sandy soils

A sterilized, but undecomposed, organic by-product of municipal waste processing was incubated in sandy soils to compare C and N mineralization with mature municipal waste compost. Waste products were added to two soils at rates of 17.9, 35.8, 71.6, and dry weight and incubated at for 90 d. Every 30 d, nitrate and ammonium concentrations were analyzed and C mineralization was measured as total CO₂-C evolved and added total organic C. Carbon mineralization of the undecomposed waste decreased over time, was directly related to application rate and soil nutrient status, and was significantly higher than C mineralization of the compost, in which C evolution was relatively unaffected across time, soils, and application rates. Carbon mineralization, measured as percentage C added by the wastes, also indicated no differences between composted waste treatments. However, mineralization as a percentage of C added in the undecomposed waste treatments was inversely related to application rate in the more productive soil, and no rate differences were observed in the highly degraded soil. Total inorganic N concentrations were much higher in the compost- and un-amended soils than in undecomposed waste treatments. Significant N immobilization occurred in all undecomposed waste treatments. Because C mineralization of the undecomposed waste was dependant on soil nutrient status and led to significant immobilization of N, this material appears to be best suited for highly degraded soils low in organic matter where restoration of vegetation adapted to nutrient poor soils is desired.     

Collembola effects on plant mass and nitrogen acquisition by ash seedlings (Fraxinus pennsylvanica)

We studied the effects of varied collembolan numbers on three compensatory mechanisms of nutrient uptake: fine root mass, endomycorrhizal development, and physiological uptake capacity. We grew ash (Fraxinus pennsylvanica) with or without the arbuscular mycorrhizal fungus Glomusintraradices, with 0, 10 or 50 initial Collembola (Folsomia candida). After 83 d root and uptake rates, endomycorrhizal development, and plant biomass were determined. Plant mass increased with Collembola number. Collembola interacted with mycorrhizae in their effects on N uptake and leaf N. Collembola in the absence of mycorrhizal roots were associated with lower N uptake and leaf N at 10 than at 0 or 50 initial Collembola. In contrast, Collembola in the presence of mycorrhizal roots were associated with the highest rate of N uptake and leaf N at 10 versus 0 or 50 initial Collembola. Hence as initial Collembola number increased, the relative importance of root system traits that determined N uptake changed from root physiological uptake capacity, presence of mycorrhizal roots, to fine root biomass.     

Rhizosphere activity, grass species and N availability effects on the soil C and N cycles

We examined whether grass species and soil nitrogen (N) availability could enhance Carbon (C) and N turnover during root litter decay in grassland. Three species with increasing competitiveness (Festuca ovina, Dactylis glomerata and Lolium perenne) were grown at two N fertiliser levels in an undisturbed grassland soil, in which soil organic fractions derived for the last 9 years from Lolium root litter which was ¹³C-depleted. During the subsequent experimental year, the C turnover was calculated using the respective δ¹³C values of the old and new C in the root phytomass, in two Particulate Organic Matter (POM) fractions above 200 μm and in the lightest part of the aggregated soil fraction between 50 and 200 μm. Soil N availability was monitored during the regrowth periods with ion exchange resins (IER). The C decay rates of each particle size fraction were calculated with a simple mechanistic model of C dynamics. The N mineralisation immobilisation turnover (MIT) was characterised by dilution of ¹⁵N-labelled fertiliser in the N harvestThe C:N ratio and the residence time of C in the fractions decreased with particle size. The presence of a grass rhizosphere increased the decay rate of old C. Accumulation of new C in particle size fractions increased with species competitiveness and with N supply. Species competitiveness increased C turnover in the aggregated fraction, as a result of greater accumulation of new C and faster decay of old C. Fertiliser N increased N turnover and C mineralisation in the SOM. Species competitiveness decreased soil -N exchanged with the IER and increased dissolved organic C (DOC) content. The nature of the current rhizosphere is thus an important factor driving C and N transformations of the old root litter, in relation with grass species strategy. Plant competitiveness may stimulate the C and N turnover in the more evolved SOM fractions in a similar way to the mineral N supply.     

Ammonium production during microbial nitrate removal in soil microcosms from a developing marsh estuary

Rapid development and urbanization in the South Carolina (SC) coastal plain may introduce significant nutrients to adjacent tidal creeks and salt marsh estuaries, and threaten estuarine water quality. Microbial denitrification in estuarine soils plays an important role in removing excessive nitrate in coastal waters. Relative contributions of denitrification and ammonium production during nitrate reduction via dissimilatory nitrate reduction to ammonium (DNRA) and soil mineralization determine whether N is lost from the system or retained as ammonium . The objectives of this study were to compare background, short-term and long-term potential denitrification ( and glucose added) rates, and production during microbial conversion in a developing marsh estuary, SC (USA). Denitrification rates were measured using the acetylene block technique in an undeveloped fresh water site (T1W), an undeveloped Spartina marsh (Grave's Dock, GD), and a Spartina marsh at a golf course resort (Chechessee marsh, C3M). Background denitrification with no added was primarily controlled by concentration in soils and surface water. Adding glucose did not enhance either short-term or long-term potential denitrification rates in GD marsh soils. production during microbial removal was significant at both marsh sites, and N-mass balance based on N₂O and production suggested a significant contribution of from sources other than DNRA. DNRA was estimated to account for approximately 16.3% and 0% of total added removal in GD surface (0-10 cm) and subsurface (30-40 cm) soils, and 1.9 and 23.2% in C3M surface and subsurface soils. Excessive generation from processes other than DNRA may be attributed to stimulated mineralization, and this stimulation was estimated to enhance soil ammonification by 0.5∼4 times compared to background generation with no added. Our results suggest that although the marsh soils displayed high potential of removal via denitrification, the produced via a combination of DNRA and enhanced mineralization may allow to accumulate and be transported to coastal waters.     

The use of camera traps for estimating tiger and leopard populations in the high altitude mountains of Bhutan

We used camera traps in combination with capture-recapture data analysis to provide the first reliable density estimates for tigers and leopards from the high altitude and rugged terrain in Bhutan’s Jigme Singye Wangchuck National Park. Fifty days of camera trapping in each of five study zones collapsed into two trapping blocks, resulted in a sampling effort of 4050 trap days. Camera trapping yielded 17 tiger photos (14 left flanked and 3 right flanked) and 48 leopard photos (25 left flanked and 23 right flanked). Using photos of these left flank, the closed heterogeneous Jackknife Model M_h was the best fit for the capture history data. A capture probability () of 0.04 was obtained for both tigers and leopards, thus generating population size (N) of 8 tigers (SE = 2.12) and 16 leopards (SE = 2.91) with densities of 0.52 tiger 100 km⁻² and 1.04 leopard 100 km⁻². Photographic evidence indicated that tigers and leopards did not overlap in their spatial use of space. Tigers preferred less disturbed areas located further away from settlements, while leopards appeared to be more resilient to disturbances in so far as they were found nearer to human settlements. Camera trapping using a capture-recapture framework was an effective tool for assessing population sizes for tiger and leopard in low density areas such as Bhutan.     

Impact of harvesting and logging slash on nitrogen and carbon dynamics in soils from upland spruce forests in northeastern Ontario

The potential impact of timber harvesting in the boreal forest on aquatic ecosystem water quality and productivity depends in part on the production of nutrients within the soil of the harvested catchment. Nitrogen supplied by organic matter decomposition is of particular interest because of the important role that N plays in biotic processes in surface waters, and in forest nutrition in general. Logging slash quality and input to the forest floor has the potential to influence N availability after harvest on clearcut sites. Net production of organic and inorganic-N and microbial biomass C and N concentrations were determined during a 90-day laboratory incubation at constant temperature and moisture. Incubated soils included F horizon and shallow mineral soil horizons (0-5 cm) from unharvested and full-tree harvested (2 and 12 growing seasons since harvest) boreal forest sites at the Esker Lakes Research Area (ELRA), in northeastern Ontario, Canada. In an ancillary experiment, black spruce foliage was added to unharvested forest floor material after 30 days during a 90-day laboratory incubation to simulate the influence of logging slash from full-tree harvesting on C and N dynamics. Twelve-year old clearcut F horizon material released on average 75 and 5 times more -N and 3 and 2 times as much inorganic-N than soil collected from unharvested and 2-year-old clearcuts, respectively. This increase in -N accumulation during the incubation was accompanied by decreases in both exchangeable -N and microbial biomass C and N levels. Net daily changes in microbial biomass N were significantly related to organic and inorganic-N accumulation or loss within the F horizon. Mineral soil release of inorganic-N was lower than release from the forest floor. Nitrate-nitrogen accumulation was lower, and -N accumulation was higher in mineral soil from unharvested sites when compared to 12-year-old clearcuts. Calculated harvest response ratios indicated that incubated mineral soil from the 12-year-old clearcut sites released significantly greater amounts of -N than 2-year-old clearcuts. Incorporation of black spruce needles into F horizon material reduced the production of organic and inorganic-N and increased microbial biomass N. Laboratory incubations of F horizon and shallow mineral soil from 12-year-old clearcuts suggested that these boreal soils have the capacity for increased inorganic-N production compared to uncut stands several years after harvesting. This has the potential to increase N availability to growing boreal forest plantations and increase N leaching due to greater -N levels in the forest soil.     

FT-IR study of the changes in carbohydrate chemistry of three New Jersey pine barrens leaf litters during simulated control burning

Low intensity control burns are a standard fuel reduction management tool used in pine barrens ecosystems. Periodic disturbances through fire can be an important influence on the cycling of nutrients within the ecosystem. Previous studies have shown that the inorganic chemistry of leaf litter residues differs with increasing temperature. Our study compared chemical changes in white oak (Quercus alba), pitch pine (Pinus rigida) and black huckleberry (Gaylussacia baccata), characteristic of the New Jersey pine barrens, during thermal decomposition using FT-IR spectroscopy. Three replicates of senescent leaf material were ground and separately heated for 2 h at: 100, 200, 300, 400 and 550 °C. These temperatures are representative of the range seen in fuel reducing prescribed burns in the pine barrens. Unburned litter of each species was used as a control. An optimization process using varying amounts of KBr and oak litter was performed to develop favorable FT-IR spectral conditions for a sample to KBr ratio of 0.75%. Chemometric analysis of the FT-IR spectra using principal component analysis (PCA) was used to analyze the changes in carbohydrate chemistry of each litter plant species (leaf litter species) at each temperature. In general, it appears that there is clear separation of leaf litter species at the different combustion temperatures. Infrared spectroscopy illustrated that all three species shared wavenumbers characteristic of the primary components of leaves such as cellulose, lignin and hemicellulose. Results from the PCA indicated separation of litter species and species by combustion temperature. PC axis 1 corresponds to the effects of temperature on leaf litter species and PC axis 2 separates the leaf litter species. At the low temperatures (control-200 °C), oak, pine and huckleberry litter species separated from each other. Wavenumbers that contributed to the separation of species at low temperatures belonged to functional group stretching frequencies of outer surface waxes, basic sugars, fatty acids and aldehydes. It appears that oak had more IR bands specific to suberin content. Convergence of these species occurs at 300 °C. Complexity of chemical composition decreases at this particular temperature as is shown by the decrease in wavenumber richness when compared to litters at low and high temperatures. Oak, pine and huckleberry had similar IR spectra showing bands belonging to outer surface wax content, pectin, lignin and hemicellulose. With increasing temperatures (400-550 °C), differences between litter species increased slightly. Plant material was reduced to similar composition due to thermal decomposition, which consisted of inorganic materials such as carbonate, phosphate and sulfate ions and possible fused aromatics.     

The role of plant residues in pH change of acid soils differing in initial pH

Reports on the effect of plant residues on soil pH have been contradictory. The conflicting accounts have been suggested to result from differences in compositions and types of plant residues and characteristics of soils. This incubation study examined the effect of plant residues differing in concentrations of N (3-49 g kg⁻¹) and of alkalinity (excess cations) (220-1560 mmol kg⁻¹) on pH change of three soils differing in initial pH (3.9-5.1 in 0.01 M CaCl₂). The addition of plant residues at a rate of 15 g kg⁻¹ soil weight increased the pH of all soils by up to 3.4 units and the pH reached the maximum at day 42 after incubation for Wodjil (initial pH 3.87) and Bodallin (pH 4.54) soils and day 14 for Lancelin soil (pH 5.1). The amount of pH buffering was decreased by residue addition in Wodjil soil, increased in Bodallin soil and remained unchanged in Lancelin soil, which closely related to changes of soil pH. Residue addition increased concentration and the increase in concentration generally correlated positively with the concentration of residue N. The concentration increased with time, reached the peak at Days 42-105 for Wodjil soil, Days 14-105 for Bodallin soil and Days 14-42 for Lancelin soil, and then decreased only in Lancelin soil. The concentration of was kept minimal in Wodjil and Bodallin soils. In Lancelin soil, concentrations increased with incubation time from days 14-28. Irrespective of plant residue and incubation time, the amounts of alkalinity produced due to residue addition correlated highly with the sum of the alkalinity added as plant residues (excess cations) and those resulting from mineralization of residue N, with the slope of regression lines decreasing with increase of the initial soil pH. Direct shaking of soil with the residues at the same rate of alkalinity (excess cations) under sterile conditions increased the pH of the Wodjil soil but decreased it in the Lancelin soil. It is suggested that the decarboxylation of organic anions (as indicated by excess cations) of added plant residues and ammonification of the residue N causes soil pH increase whereas nitrification of mineralised residue nitrogen causes soil pH decrease, and that the association/dissociation of organic compounds also plays a role in soil pH change, depending initial pH of the soil. The overall effect on soil pH after addition of plant residues would therefore depend on the extent of each of these processes under given conditions.     

Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates

Few studies have examined the kinetics of gross nitrogen (N) mineralization, immobilization, and nitrification rates in soil at temperatures above 15 °C. In this study, ¹⁵N isotopic pool dilution was used to evaluate the influence of retaining standing crop residues after harvest versus burning crop residues on short-term gross N transformation rates at constant temperatures of 5, 10, 15, 20, 30, and 40 °C. Gross N mineralization rates calculated per unit soil organic carbon were between 1 and 7 times lower in stubble burnt treatments than in stubble retained treatments. In addition, significant declines in soil microbial biomass (P=0.05) and CO₂-C evolution (P<0.001) were associated with stubble burning. Immobilization rates were of similar magnitude to gross N mineralization rates in stubble retained and burnt treatments incubated between 5 and 20 °C, but demonstrated significant divergence from gross N mineralization rates at temperatures between 20 and 40 °C. Separation in the mineralization immobilization turnover (MIT) in soil at high temperatures was not due to a lack of available C substrate, as glucose-C was added to one treatment to test this assumption. Nitrification increased linearly with temperature (P<0.001) and dominated over immobilization for available ammonium in soil incubated at 5 °C, and above 20 °C indicating that nitrification is often the principal process controlling consumption in a semi-arid soil. These findings illustrate that the MIT at soil temperatures above 20 °C is not tightly coupled, and consequently that the potential for loss of N (as nitrate) is considerably greater due to increased nitrification.     

Forest floor gross and net nitrogen mineralization in three forest types in Quebec, Canada

The effect of high nitrogen (N) depositions on forest ecosystems is an important concern in North America and may lead to N saturation of forest ecosystems and contribute to soils and surface water acidification. In this study, nitrogen dynamics in the FH layers of a sugar maple (SM), a balsam fir (BF) and a black spruce (BS) forest was characterized using a short term ¹⁵N isotopic pool dilutions approach and mid-term FH material incubation both in situ and in the laboratory. The short term dilutions approach indicated that the mean residence times of and in the FH material of the three sites were low (<1 d). The amount of inorganic nitrogen () recycled annually within the exchangeable forest floor reservoir was between one and two orders of magnitude larger than the annual atmospheric N deposition found at each of the sites. The BS site was clearly distinct than the two other forest types in that net N mineralization was negligible, even in absence of root uptake, suggesting that soil microorganisms were severely N limited. While net nitrification was not observed within the FH material of the BF site, did accumulate in the FH of the SM despite a low pH of 3.72 presumably because of heterotrophic nitrification or as a result of acid-tolerant autotrophic nitrification. The difference in N dynamics between the sites were most probably caused by dominant tree species. Transformation rates of inorganic N were higher in SM, followed by BF and BS stands. Given that the potential to mineralize inorganic N matches with a superimposed N atmospheric deposition gradient in Québec, the sugar maple forest is more likely to be affected by N saturation than coniferous forests.     

Nitrate uptake by Eriophorum vaginatum controls N2O production in a restored peatland

The clear dependence of N₂O production through denitrification on available nitrate in soil has been shown in many studies. Since N availability similarly limits the growth of plants, the resource competition with vegetation limits the activity of denitrifying microbes and may consequently moderate the N₂O emissions from peatlands. We used uptake by Eriophorum vaginatum L. as a vegetation competition factor for microbes. The species was selected for the experiment because it has high nutrient use efficiency in low-nutrient conditions and high nutrient uptake efficiency in luxuriant nutrient conditions. We measured gaseous N flux as N₂O (end product of denitrifier activity) in a restored peatland in central Finland with acetylene inhibition technique over a growing season from sample plots with varying addition levels and E. vaginatum cover. The resource competition effects were analysed with a model that used exponential decay dependence of N₂O flux on the leaf area of E. vaginatum, and saturating response of N₂O flux to addition level. The model explained the variation in N₂O fluxes well (R²=0.86). The model simulation showed that the increasing nutrient uptake of E. vaginatum decreased the N₂O fluxes exponentially. Simultaneously, denitrification appeared to saturate even in conditions with high availability of and low level of competition by vegetation. Thus, E. vaginatum is an effective competitor for in sedge-dominated peatlands that controls the availability of for denitrification, and consequently moderates the N₂O emissions from peatlands.     

Does the enhanced P acquisition by maize following legumes in a rotation result from improved soil P availability?

Field data have suggested that under P-deficient conditions, legumes supplied with phosphate rock (PR) increase P acquisition by a subsequent maize crop compared to direct application of PR to maize. This study assessed the mechanism of this positive rotational effect in terms of soil P availability using a greenhouse trial with large volume (74 l) containers. The rotation effect was analysed in relation to PR application, previous legume growth and incorporation of the legume residues. Velvet bean (Mucuna pruriens) and maize were grown in a representative Acrisol from the Nigerian Northern Guinea savannah (NGS). All soils were applied with sufficient urea to exclude N-effects in the rotations. In a first season, velvet bean and maize responded similarly to PR application, and P uptake by both crops increased by 45%. The soil total labile P quantity (E-value) and P concentration in soil solution after plant growth were increased by PR-application only in soils previously grown by velvet bean, suggesting enhanced PR solubilisation in the legume-grown soils. In the subsequent season, grain yields and P uptake of a maize crop following velvet bean were twice as large compared to maize following a first maize crop. This residual effect of velvet bean was even significant in treatments without PR-application, although both maize and velvet bean withdrew similar amounts of P during the first season and no differences in soil P availability were observed. Furthermore, legume residue incorporation in soils previously grown by maize did not affect yields or P uptake of the subsequent maize crop, while it significantly increased the E-value and during the first 7 weeks in the second season. As such, the positive rotational effects of velvet bean were larger than predicted by soil P availability measures. Maize yield significantly increased with increasing plant P concentration among all treatments. However, the rotational effect was unrelated to internal P concentration: significantly larger yields were obtained for maize following velvet bean than for maize following maize at identical internal P. This suggested the presence of another growth-limiting which is counteracted by the previous velvet bean growth. In conclusion, our results confirmed that the introduction of a legume supplied with PR into a maize-based cropping system increases yield and P-uptake by a subsequent maize crop, compared to maize following a first maize crop supplied with PR. These stimulations, however, went beyond improved P nutrition. Results strongly suggested that the legume in the rotation system has other positive, possibly soil-microbiological effects which enhance maize growth and production.     

Evaluation of nitrate leaching from mine tailings amended with biosolids under Mediterranean type climate conditions

Mine tailings are difficult to revegetate due to the lack of organic matter, severe nutrient limitations, and potential metal toxicity. Biosolids has been shown to be favorable for improving properties of mine tailings. The rates of biosolids required to reclaim mine tailings (up to ) may produce conditions where significant amounts of nitrates can leach into groundwater. Leaching column experiments were conducted to determine the influences of biosolids placement and plant cover on nitrate leaching from biosolids-amended mine tailings. PVC columns packed with 7.7 kg of tailings were treated with 168 g of biosolids (approximately 270 kg mineral ). Biosolids were either placed on the surface or mixed with the tailings and half of the columns were seeded with perennial ryegrass (Lolium perenne, L.). Columns were drip irrigated at a rate of 758 mm of water y^-1. This rate was twice the average precipitation for Central Chile. All leachates were collected weekly for up to 21 weeks and analyzed for nitrate, pH, electrical conductivity, and chemical oxygen demand. The electrical conductivity and nitrate concentration of percolates decreased with time, while the pH remained constant. In some cases the percolate had nitrate concentrations greater than the maximum amount allowed for human consumption (10 mg ). Vegetation cover and mixing the biosolids with tailings reduced NO₃-N concentrations in the percolate.     

A survey of symbiotic nitrogen fixation by white clover grown on metal contaminated soils

There is conflicting evidence about toxic effects of heavy metals in soil on symbiotic nitrogen fixation. This study was set-up to assess the general occurrence of such effects. Soils with metal concentration gradients were sampled from six established field trials, where sewage sludge or metal salts have been applied, or from a transect in a sludge treated soil. Additional contaminated soils were sampled near metal smelters, in floodplains, in sludge amended arable land and in a metalliferous area. Symbiotic nitrogen fixation was measured with ¹⁵N isotope dilution in white clover (Trifolium repens L.) grown in potted soil that was not re-inoculated, and using ryegrass (Lolium perenne L.) as reference crop. The fraction nitrogen in clover derived from fixation (N_dff) varied from 0 to 88% depending on soil. Pronounced metal toxicity on N_dff was only confirmed in a sludge treated soil where nitrogen fixation was halved from the control value at soil total metal concentration of 737 mg Zn kg⁻¹, 428 mg Cu kg⁻¹ and 10 mg Cd kg⁻¹. The N_dff was significantly reduced by increasing metal concentration in soils from two other sites where N_dff was low throughout and where these effects might be attributed to confounding factors. No significant effects of metals on N_dff were identified in all other gradients even up to elevated total metal concentration (e.g. 55 mg Cd kg⁻¹). The variation of N_dff among all soils (n=48), is mainly explained by the number of rhizobia in the soil (log MPN, log (cells g⁻¹ soil)), whereas correlations with total or soil solution metal concentrations were weak (R²<0.25). The is significantly affected by the presence or absence of the host plant at the sampling site. No effects of metals were identified at even at total Zn concentrations of about 2000 mg Zn kg⁻¹, whereas metal toxicity could be identified at lower most probable number (MPN) values. This survey shows that the metal toxicity on symbiotic nitrogen fixation cannot be generalized and that survival of a healthy population of the microsymbiont is probably the critical factor.     

Why does temperature affect relative uptake rates of nitrate, ammonium and glycine: A test with Eucalyptus pauciflora

Few studies have examined how temperature affects uptake of nitrate, ammonium and amino acids from soil. This study tests the hypothesis that cool temperatures favour uptake of the amino acid glycine while warm temperatures favour uptake of inorganic forms of N such as nitrate. We used glasshouse-grown ectomycorrhizal seedlings of the sub-alpine tree species Eucalyptus pauciflora Sieber ex Spreng. Seedlings were grown in soil (humic umbrosol, from species' habitat) that was dominated by amino acids and ammonium with only small amounts of nitrate. To examine if root physiology affects temperature responses of N uptake, we measured uptake from ¹⁵N-labelled hydrosolutions containing equimolar 100 μmol L⁻¹ mixtures of ammonium, nitrate and glycine at temperatures from 5 to 35 °C. We also examined if the effect of temperature on uptake of N forms was due to plant-microbe competition by following the fate of equimolar amounts of labelled ammonium, nitrate and glycine injected into the soil at temperatures of 5 °C and 25 °C. Hydrosolution experiments showed that uptake of glycine was favoured by warm temperatures and inorganic N by cool temperatures. In contrast, when ¹⁵N was injected into soil the uptake of glycine was favoured by low temperatures and nitrate by warm temperatures. At 25 °C, glycine was 17% of the N taken up from soil and nitrate was 51%; whereas at 5 °C glycine was 30% of the N taken up from soil and nitrate was 23%. Microbes were better competitors than seedlings for all forms of N, but temperature did not affect microbial preference for the different N forms. Hence, while microbes limit N available for plant uptake, they do not seem to be the cause of the greater plant uptake of glycine at cool temperatures and nitrate at warm temperatures. Intact uptake of glycine by plants was suggested by the positive relationship between uptake of ¹³C and ¹⁵N and detection by GC-MS of intact , ¹⁵N glycine molecules in roots. In conclusion, uptake of glycine is favoured by cool temperatures and nitrate by warm temperatures, but this is apparently not a function of root physiology or competition with soil microbes.     

Movements and activities of snow leopards in Southwestern Mongolia

Four adult (2M:2F) snow leopards (Uncia uncia) were radio-monitored (VHF; one also via satellite) year-round during 1994-1997 in the Altai Mountains of southwestern Mongolia where prey densities (i.e., ibex, Capra siberica) were relatively low (∼0.9/km²). Marked animals were more active at night (51%) than during the day (35%). Within the study area, marked leopards showed strong affinity for steep and rugged terrain, high use of areas rich in ungulate prey, and affinity for habitat edges. The satellite-monitored leopard moved more than 12 km on 14% of consecutive days monitored. Home ranges determined by standard telemetry techniques overlapped substantially and were at least 13-141 km²in size. However, the satellite-monitored individual apparently ranged over an area of at least 1590 km², and perhaps over as much as 4500 km². Since telemetry attempts from the ground were frequently unsuccessful , we suspect all marked animals likely had large home ranges. Relatively low prey abundance in the area also suggested that home ranges of >500 km²were not unreasonable to expect, though these are >10-fold larger than measured in any other part of snow leopard range. Home ranges of snow leopards may be larger than we suspect in many areas, and thus estimation of snow leopard conservation status must rigorously consider logistical constraints inherent in telemetry studies, and the relative abundance of prey.     

Influence of single trees on spatial and temporal patterns of belowground properties in native pine forest

Linkages between forest dynamics and ecosystem processes are poorly understood and this limits our ability to adequately estimate future changes in forest ecosystems due to human-induced global change. In particular at the single tree level, our understanding of temporal and spatial changes of belowground properties during forest succession is limited. Thus, our aim was to test whether we find a spatial and temporal gradient in nutrient availability and an associated shift in microbial community structure with increasing distance and age of single trees. We found that inorganic nitrogen was less available below the crown of single trees, while soluble organic carbon (DOC) was much more abundant, in particular in the inner zone of influence, i.e. close to the stem. The fungal:bacterial PLFA ratio was greater while microbial biomass carbon (MicC) was lower below the tree crown, indicating a strong influence of trees on spatial patterns of microbial biomass and community structure. Moreover, the positive correlation between MicC and total extractable N, and the negative correlation between fungal:bacterial biomass and δ¹⁵N, suggested that the microbial biomass was N limited below the tree crown and as a consequence nutrient cycling was presumably decelerated compared to open conditions. We also found a temporal pattern of increasing surface soil C and N content with increasing tree age (up to 250 years), underlining the significant role of single trees in creating spatial and temporal heterogeneity in forests.     

Nitrite production by Nitrosomonas europaea and Nitrosospira sp. AV in soils at different solution concentrations of ammonium

Although it remains unclear why NH₃-oxidizing bacteria (AOB) of the genus Nitrosospira dominate soil environments, and why Nitrosomonas spp. are less common, virtually no studies have compared their behavior in soil. In this study, the NH₃ oxidation rates of Nitrosomonas europaea (ATCC 19718) and Nitrosospira sp. AV were compared in three differently textured soils containing a range of extractable contents (2-11 μg soil). Soils were adjusted to pH 7.0-7.4 with CaCO₃ and sterilized with γ-radiation. Cell suspensions of each bacterium were inoculated into the soils to bring them to two-third of water-holding capacity and cell densities ∼2.5×10⁶ g⁻¹ soil. In virtually all cases, rates of production for both N. europaea and Nitrosospira sp. AV were linear over 48 h, and represented between 13 and 75%, respectively, of the maximum rates achieved in soil-free bacterial suspensions. Soil solution concentrations that supported these rates ranged between 0.2 and 1.5 mM. Addition of 21-36 μg soil raised soil solution levels to 1.8-2.5 mM and stimulated production to a greater extent in N. europaea (3.3-6.6-fold) than in Nitrosospira sp. AV (1-2.1-fold). Maximum rates of production were obtained by raising soil solution levels to 3-4 mM with a supplement of ∼80-90 μg soil. K_s values in soil for Nitrosospira sp. AV and N. europaea were estimated as 0.14 and 1.9 mM , respectively, and estimates of V_max were about 3.5-times higher for N. europaea (0.007 pmol h⁻¹ cell⁻¹) than for Nitrosospira sp. AV (0.002 pmol h⁻¹ cell⁻¹). The cell density of N. europaea increased in sterile Steiwer soil independent of supplemental . In the case of treatments receiving supplemental , growth yields of N. europaea calculated from either produced or consumed were similar to those reported in literature (3.5×10⁶-6×10⁶ cells μmol⁻¹). A higher growth yield was measured in the case of zero added (2.7×10⁷ cells μmol⁻¹), indicating that use of organic carbon compounds might have occurred and resulted in some energy sparing. Our results suggest that Nitrosospira spp. with a K_s similar to Nitrosospira sp. AV may have an advantage for survival in soil environments where soil solution levels are less than 1 mM. However, it is apparent that AOB like N. europaea are poised to take advantages of modest increases in extractable that raise soil solution levels to about 2.0-2.5 mM.