Differences between resource patches modify root herbivore effects on plants

While soil resource heterogeneity and root herbivory can have significant direct influences on plant growth, soil heterogeneity may also have indirect effects by influencing the foraging behavior of root herbivores. We used sand-filled greenhouse pots to assess root herbivore foraging behavior and potential interactions between patch quality, herbivore foraging, and plant biomass production (yield). Individual pots were divided into four quarters: one fertilized, and three unfertilized, two of which were planted with tree seedlings. Two treatments were used to create fertilized quarters: high-organic manure fertilizer, and slow-release mineral fertilizer. Seedlings of red maple (Acer rubrum L.) and Virginia pine (Pinus virginiana L.) were used to create two single-species and one mixed-species treatments. Root-feeding beetle larvae were added to the pots and allowed to forage freely for ∼8 weeks. At harvest, root herbivores in organic-fertilized pots were strongly attracted to fertilized quarters despite their relatively low-root biomass. Herbivore distribution was significantly different in mineral fertilized pots, where larvae were most abundant in planted quarters, which is also where most of the plant roots occurred. Whole pot plant yield was significantly reduced by larvae; this effect was stronger in the mineral fertilized pots than organic fertilized pots. While one of the plant species appeared more sensitive to herbivory, root herbivores had a greater influence on yield in mixed-species pots than in single-species pots. Overall, these results suggest that patch quality influences on herbivore foraging may indirectly alter yield and plant community composition. Responsible Editor: Angela Hodge.     

Long-term effects of mineral versus organic fertilizers on activity and structure of the methanotrophic community in agricultural soils

Agricultural practices, such as mineral nitrogen fertilization, have an impact on the soil's ability to oxidize methane, but little is known about the shifts in the methanotrophic community composition associated with these practices. Therefore, the long-term effect of both mineral (NH4NO3) and organic (manure and GFT-compost) fertilizer applications on the soil methanotrophic community activity and structure were investigated. Both high and low affinity methane oxidation rates were lower in the soil treated with mineral fertilizer compared to the other soils. An enhanced nitrate concentration was observed in the mineral fertilized soil but nitrate did not show a direct affect on the high affinity methane oxidation. In contrast, the low affinity methane oxidation was slowed down by increased nitrate concentrations, which suggests a direct effect of nitrate on low affinity methane oxidation. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA gene fragments specific for methanotrophs revealed a distinct community between the mineral and organic fertilized soils as extra Type I methanotrophic bands (phylotypes) became visible in the organic fertilized soils. These phylotypes were not visible in the patterns of the added organic fertilizers suggesting an indirect effect of the organic fertilizers on the methanotrophic community. Additionally, a molecular analysis was performed after the low affinity methane oxidation test. The enhanced methane concentrations used in the test enriched certain low affinity methanotrophs in the organic fertilized soils but not in the mineral fertilized soil. Supporting the molecular and functional observations, fatty acids characteristic for methanotrophs were less abundant in the soil treated with mineral fertilizer compared to the soil treated with compost. In conclusion, the function and molecular and chemical composition of the methanotrophic community are all altered in soil fertilized with mineral fertilizer.     

AMF对喀斯特土壤枯落物分解和对宿主植物的养分传递

为探索丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)在喀斯特土壤中养分利用机制,采用分室系统隔室装置(用20μm或0.45μm尼龙网双层隔离)对香樟(Cinnamomum camphora)幼苗进行幼套球囊霉(Glomus etunicatum)接种处理和施氮处理,并采用15N稳定同位素技术标记了黑麦草(Lolium perenne)枯落物作为土壤有机残体,幼苗生长15周后测定了隔室幼苗生长性状指标、氮、磷摄取量、植株和隔室土壤中的δ15N值、微生物量碳、微生物量氮以及菌丝体密度,结果表明:AMF具有腐生营养能力,促进了土壤枯落物的分解并吸收其释放的15N传递给宿主植物利用;香樟幼苗优先利用根际周围氮维持生长;在低氮状态下,香樟植株通过AMF菌丝体更多地利用了相邻隔室枯落物分解释放的15N;施加根际外源氮有利于AMF对隔室枯落物的分解,但降低了植株对枯落物氮的利用;根际高氮状态下植株的氮、磷摄取量较大;高养分状态下提高了相邻隔室微生物量碳、氮含量和菌丝体密度。     

Growth response of Pinus densiflora seedlings to different fertilizer compound ratios in a recently burned area in the eastern coast of Korea

This study was conducted to determine fertilizer compound ratios suitable for soil conditions and tree seedling growth in recently burned areas in Korean forests. Currently, the conventional fertilizer ratio applied to undisturbed forests in Korea is N:P:K 3:4:1. In this study, Japanese red pine (Pinus densiflora S. et Z.) seedlings planted in the burned area were fertilized over four growing seasons with the following NPK compound ratios: unfertilized (CON), 3:4:1, 6:4:1, 3:8:1, and 3:4:2. Fertilization generally increased current-year needle nutrient concentrations of the seedlings, and the chlorophyll a:b ratio in CON was significantly lower than in all fertilized plots. Fertilization significantly affected the growth of the pine seedlings, which had 71–87 % more height growth and 29–67 % increased root collar diameter (RCD) compared to CON. The increases in height and RCD were significantly higher with the 6:4:1 and 3:8:1 ratios than with the 3:4:1 ratio. The 3:4:2 and 3:4:1 fertilizer ratios had no effect on the RCD growth of seedlings. This suggests that the early growth of pine seedlings could be improved by providing high N and P supplies to areas affected by forest fires rather than the conventional fertilizer ratio of 3:4:1 in Korean forest soils. Therefore, application of the suitable fertilization ratio may be a very effective way to reduce reforestation cost as well as to shorten restoration period.     

Defence structure development inRhizopus nigricans during mycoparasitism byFusarium oxysporum F. sp.lycopersici

Summary Unfertilized and NPK-fertilized plots in the long term fertility trial at Los Baños, Philippines were used to observe the effect of nitrogen fertilizer on heterotrophic N₂-fixing activities in plow layer soil and in association with wetland rice. The activities were measured in the field byin situ acetylene reduction assays. Acetylene reduction activity of the plow layer soil in the paddy field was measured by soil cores. No appreciable difference was found-between fertilized and unfertilized plots.     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

Effects of growth medium, nutrients, water, and aeration on mycorrhization and biomass allocation of greenhouse-grown interior Douglas-fir seedlings

Commercial nursery practices usually fail to promote mycorrhization of interior Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco] seedlings in British Columbia, which may account for their poor performance following planting in the field. We tested the effects of four nursery cultivation factors (nitrogen fertilization, phosphorus fertilization, watering, and soil aeration) and field soil addition on mycorrhization, survival, growth, and biomass allocation of interior Douglas-fir seedlings in a series of greenhouse experiments. Where field soil was added to the growing medium, mycorrhization and root/shoot ratios were maximized at lower levels of mineral nutrient application and aeration. Where field soil was not added, mycorrhization was negligible across all fertilization and aeration treatments, but root/shoot ratio was maximized at lower levels of mineral nutrients and the highest level of aeration. Regardless of whether field soil was added, intermediate levels of soil water resulted in the best mycorrhizal colonization and root/shoot ratios. However, field soil addition reduced seedling mortality at the two lowest water levels. A cluster analysis placed ectomycorrhizal morphotypes into three groups (Mycelium radicis-atrovirens Melin, Wilcoxina, and mixed) based on their treatment response, with all but two morphotypes in the mixed group whose abundance was maximized under conditions common to advanced seedling establishment. For maximal mycorrhization and root development of interior Douglas-fir seedlings, nurseries should minimize addition of nitrogen and phosphorus nutrients, maximize aeration, provide water at moderate rates, and, where possible, add small amounts of field soil to the growing medium.     

On the synthesis of glutamine before and after fertilization of the sea urchin, Strongylocentrotus purpuratus

Unfertilized eggs of the sea urchin, Strongylocentrotus purpuratus, have a much lower capacity for glutamine synthesis than do fertilized eggs. This difference is not caused by an alteration of glutamine synthetase activity attendant upon fertilization. Neither the specific activity of glutamine synthetase nor its pattern of activation by divalent metal ions is affected by fertilization. The enzyme from both fertilized and unfertilized eggs is activated by α-ketoglutarate and inhibited by ultimate end products of glutamine metabolism. This type of regulation is similar to that seen with many other eucaryotic glutamine synthetases.Unfertilized eggs take up less glutamic acid than do fertilized eggs when the amino acid is presented at high concentrations (12.5 mM), whereas there is no difference in glutamic acid uptake at low concentrations (5 μM). Under conditions where glutamate uptake is identical, unfertilized eggs are dependent upon exogenous ammonia for glutamine synthesis in vivo; fertilized eggs are able to synthesize glutamine in the absence of added ammonia. Thus, our data suggest that the increased capacity for glutamine synthesis after fertilization is related to an increased availability of the substrate, ammonia.     

Nutrient addition retards decomposition and C immobilization in two wet grasslands

Eutrophication is one of the biggest environmental problems facing wetlands. However, its effect on soil functioning is not yet well understood. We tested the hypothesis that increased nutrient loading into wet grassland ecosystems accelerates soil C and N cycles and decreases microbial immobilization of C and N. Experimental sites were established on two wet grasslands, with either mineral or peaty soils, and fertilized by NPK fertilizer for 3 years. Soils were analyzed for soluble and microbial C and N contents and their transformations, profile of phospholipid fatty acids and number of nirK denitrifiers. Fertilization affected C more than N transformations. Opposite to what was predicted, decomposition was retarded, the soil C cycle was based more on labile C compounds, and the soil was more susceptible to C losses in fertilized versus unfertilized treatments in both soils. Fertilization resulted in lower microbial biomass C and microbial C immobilization and also decreased the activity of lignin-degrading enzymes. Shifts in the composition of the microbial communities led to decreased (1) decomposition of complex organic compounds and (2) immobilization of transformed C. Net nitrification and microbial N immobilization tended to increase in fertilized treatments indicating an acceleration of soil N cycling and losses, but only in the more vulnerable organic soil.     

Ecophysiological responses of Fagus sylvatica seedlings to changing light conditions. II. The interaction of light environment and soil fertility on seedling physiology

The survival and growth of natural beech regeneration after canopy removal is variable and little is known about ecophysiological mechanisms of these responses. Biomass, nonstructural carbohydrate levels and nitrogen concentrations were measured in an Italian population of European beech seedlings. Seedlings were container-grown in two types of soil, organic and mineral, collected at the study site. The seedlings were grown under three light treatments: under full beech canopy (understory), exposed to full sun only during midday (gap) and under full sun (clearing). Leaf gas exchange and chlorophyll a fluorescence parameters were measured and then foliar analyses were conducted for chlorophyll, phenolic and tannin levels. Biomass and allocation were significantly affected by light and soil treatments. The clearing seedlings and those in organic soil were larger than seedlings in the other light treatments or soil type. Total nonstructural carbohydrate concentrations were lower in the understory seedlings and significant differences between soil types were present in the gap and clearing seedlings. Nitrogen concentrations were higher in the understory seedlings and those growing in the organic soil compared to the other treatments. Gas exchange rates were highest in clearing and the organic soil seedlings. Gap seedlings exhibited photosynthetic acclimation that allowed them to utilize high light of midday and any sunflecks during the morning and afternoon. Relative fluorescence was significantly influenced by both light treatment and soil type, with the highest values observed in the gap seedlings. Light response curves showed decreasing apparent maximum quantum efficiency from the understory to clearing, while maximum photosynthetic rate was highest in the gap seedlings. Chlorophyll concentration was highest in understory seedlings and those growing in organic soil and higher in seedlings growing in organic than in mineral soil. Both foliar tannin and phenolic levels were highest in clearing seedlings, and only tannin concentrations were affected by soil type. Understory seedlings had the highest mortality and insect herbivory; the latter was found to be inversely related to tannin concentration. Overall, growth and photosynthesis in beech seedlings responded positively to high light associated with small canopy gaps. Organic soil increased seedling size, particularly in the gap and clearing environments. We conclude that forest gaps are favorable for photosynthesis and growth of European beech seedlings.     

Nutrient addition accelerates leaf breakdown in an alpine springbrook

This study assessed the effect of nutrient enrichment on organic matter breakdown in an alpine springbrook, using alder leaf packs to which phosphorus and nitrogen were added in the form of slow-release fertilizer briquettes. The breakdown of leaf packs with nutrients added (k=0.0284 day^–1) was significantly faster than that of unfertilized packs (k=0.0137 day^–1), resulting in a 30% higher mass loss after 42 days. Unfertilized leaves enclosed in fine-mesh bags broke down at an even slower rate (k=0.0062 day^–1). Phosphorus and nitrogen concentrations were initially higher in leaf packs with nutrients added, but this difference disappeared within 3 weeks. Fungal biomass developing in decomposing leaves was substantial (c. 55 mg dry mass per 1 g leaf dry mass) although similar between fertilized and unfertilized packs, as was the sporulation activity of aquatic hyphomycetes. There was a significantly greater number and higher biomass of macroinvertebrates (shredding nemourid stoneflies in particular) on the fertilized packs, suggesting that the increased leaf mass loss was brought about by shredder feeding. Received: 11 March 1999 / Accepted: 6 September 1999     

A bioassay of the availability of residual 15N fertilizer eight years after application to a forest soil in interior British Columbia

Although a high proportion of fertilizer N may be immobilized in organic forms in the soil, no studies have examined the long-term availability of residual fertilizer ¹⁵N in forestry situations. We investigated this by growing lodgepole pine (Pinus contorta) seedlings in surface (0–10 cm) soil sample eight years after application of ¹⁵N-urea, ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ to lodgepole pine in interior British Columbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the ¹⁵N and 4.6% of the native N per pot. Most of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. In contrast to the greenhouse study, there was no significantuptake of ¹⁵N by trees in the field study after the first growing season, although half of the soil organic ¹⁵N was lost between one and eight years after fertilization. This indicates the need to understand the mechanisms which limit the uptake of mineral N by trees in the field, and the possible mismatch of tree demand and mineral N availability.     

Availability of residual fertilizer 15 N from forest floor and mineral soil to Douglas-fir seedlings ten years after fertilization

Background and aims

As low initial uptake and essentially zero later uptake limit efficacy of N fertilization for temperate conifers, we investigated factors limiting long-term tree uptake of residual ¹⁵?N-labeled fertilizer.

Methods

We used a pot bioassay to assess availability of ¹⁵?N from soil sampled 10 years after fertilization of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand with ¹⁵?N-urea (200 kg N ha^?1). Douglas-fir seedlings were grown for 2 years in organic (designated LFH) and mineral soil (0–10 cm) layers reconstructed from control and fertilized plots; residual fertilizer N amounted to 10 % of LHF and 5 % of MIN N.

Results

Percentage recovery of residual ¹⁵?N in seedlings was not affected by the original season of fertilization (spring vs. fall), but differed by the source of ¹⁵?N excess. LFH was a better source of residual ¹⁵?N; 12.4 % of residual LFH ¹⁵?N was taken up by seedlings and 7.6 % transferred to soil, whereas mineral soil yielded only 8.3 % of residual ¹⁵?N to seedling uptake and 2.4 % to LFH. Extractable inorganic N was 2–3 orders of magnitude higher in fallow pots.

Conclusions

Ten-year residual fertilizer ¹⁵?N was clearly cycling between LFH and mineral soil and available to seedlings, indicating that other factors such as denitrification, leaching, and asynchrony of soil N mineralization and tree uptake limit long-term residual N fertilizer uptake in the field.     

The influence of living plants on mineralization and immobilization of nitrogen

Summary Changes in the pattern of distribution of the nitrogen of the soil and seedling grass plants have been investigated when the grass plants were grown in pots of sandy soil, from a pasture, at pH 5.7. Net mineralization of soil nitrogen was not observed during an experimental period of one month in the absence of added nitrogenous fertilizer (Table 2). Addition of labeled nitrogen (as ammonium sulphate) to the soil at the beginning of the experimental period resulted in a negative net mineralization during this period (Table 4b). When none of the fertilizer nitrogen remained in its original form in the soil it was found that approximately 12 per cent of the labeled nitrogen had been immobilized in soil organic compounds. Clipping of the grass at this date was followed by a decrease in the amount of labeled soil organic nitrogen, indicating that mineralization was not depressed by living plants. The application of unlabeled ammonium sulphate subsequent to the utilization of the labeled nitrogen did not decrease the amount of immobilized labeled nitrogen in the soil organic matter, as would be expected if the organic nitrogen compounds of the soil had been decomposed to ammonia. This was thought to be due to the fact that decomposition of organic nitrogen compounds in permanent grassland results in the production of peptides, amino acids etc. which are utilized by microorganisms without deamination taking place. In pots with ageing grass plants, labeled organic nitrogen compounds were found to be translocated from the grass shoots to the soil (Table 7). Net mineralization of soil organic nitrogen was positive in the contents of pots containing killed root systems (Table 3b). About 8 per cent of the labeled nitrogen added to the contents of such pots, in the form of ammonium sulphate, was found to be present in soil organic nitrogen compounds approximately 4 weeks after application, while a total of about twice this amount of soil organic nitrogen was mineralized during that period. From the results obtained in this investigation, it is concluded that the constant presence of living plants is responsible for the accumulation of nitrogen in organic compounds in permanent grassland. No evidence was obtained that the decomposition of such compounds in the soil is inhibited by living plants.     

Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull: the effect of fertilizer and defoliation on above- and below-ground performance

1 The role of nutrient supply and defoliation on the competitive interactions between pot-grown Calluna vulgaris and Nardus stricta plants was investigated.WP leading adjustment
2 Young plants were grown alone and together in pots under a combination of fertilizer and defoliation treatments. After 18 months, parameters reflecting both above- and below-ground performance were measured, namely: total above-ground biomass, shoot nitrogen and phosphorus content, root length and the extent of mycorrhizal infection of the roots.
3 In the pots that received fertilizer, the shoot nutrient content and above-ground biomass of Nardus plants increased to a greater extent than those of Calluna plants; this effect was more marked for Nardus plants growing with Calluna plants than for those growing with other Nardus plants. In contrast , Calluna plants growing in competition with Nardus failed to respond to the addition of nutrients. However, in unfertilized pots, Calluna gained more above-ground biomass during the experimental period than Nardus.
4 Calluna had greater root length than Nardus , but Nardus had a higher proportion of its root length infected by mycorrhizal fungi. In both plants, the addition of fertilizer reduced the mycorrhizal infection and increased the root length. Nardus root length was decreased when grown in competition with Calluna only in pots where no nutrients were added. Defoliation decreased the extent of mycorrhizal infection in Calluna roots but not in those of Nardus; defoliation decreased the shoot nutrient content in Calluna plants, but not in Nardus plants.
5 These results suggest that the competitive balance between Nardus and Calluna may be altered by the addition of nutrients, and by defoliation, which may have serious implications for the future dominance of Calluna in heathland ecosystems, particularly those where nutrient inputs are increasing significantly or where grazing pressures are high.     

Localized elongation of roots of cotton follows establishment of arbuscular mycorrhizas

In an experiment to determine the influence of mycorrhizal colonization on root development, fertilized seedlings of cotton were grown in long tubular pots. Inoculation of soil with an isolate of Glomus mosseae at 10, 25 or 40 cm below the soil surface resulted in spread of arbuscular mycorrhizas up and down the root system, and localized elongation of roots following colonization. Specific root length was not influenced by colonization. Increases in local root density at the point of inoculation were observed, though total root mass in relation to shoot mass declined following initiation of mycorrhizas.     

Long-term soil nutrient dynamics and lateral nutrient movement in fertilized and unfertilized red pine plantations

In this study, we use arepeated-measures analysis to test thehypothesis that soil fertility underpotassium-limited red pine (Pinusresinosa Ait.) stands at the Charles LathropPack Demonstration Forest in Warrensburg, NewYork is increasing toward a steady state thatwas artificially induced in fertilized standsby K-fertilization over 50 years ago. Wemeasured soil K by horizon and added new datato a 53-year database. We examine onemechanism that explains the higher rate of Kaccumulation in unfertilized stands comparedwith fertilized – lateral movement offertilizer K from treated plots to untreated –using the rubidium/potassium reverse tracermethod. Over the past five decades, soil Kconcentrations under both fertilized andunfertilized red pine have increasedsignificantly. The trends under fertilized andunfertilized plots demonstrate the gradualconvergence of soil K under unfertilized plotstoward concentrations in fertilized plots. Five decades after fertilization, treated soilsstill contain greater concentrations ofexchangeable K and lower bulk densities thanunfertilized plots. Analysis of Rb/K ratios inthe forest floor of fertilized and unfertilizedplots confirms the hypothesis that lateraltransport of surface broadcast fertilizer,applied over 50 years ago, extendsapproximately 11–16 m from the edges offertilized plots. The four unfertilized plotsclosest to fertilized plots have beensignificantly affected by inputs of fertilizerK, while the remaining five plots arerelatively unimpacted. Approximately 36% ofthe K in fertilized plots, and 23% of the K inunfertilized plots affected by fertilizermigration were derived directly from thefertilizer applied 5 decades ago, demonstratingthe highly conservative nature of mineralnutrient cycling in aggrading forests.     

Uptake of cadmium from an experimentally contaminated calcareous soil by arbuscular mycorrhizal maize (<Emphasis Type="Italic"> Zea mays </Emphasis>L.)

We investigated uptake of Cd by arbuscular mycorrhizal (AM) maize inoculated with Glomus mosseae from a low-P sandy calcareous soil in two glasshouse experiments. Plants grew in pots containing two compartments, one for root and hyphal growth and one for hyphal development only. Three levels of Cd (0, 25 and 100 mg kg^–1) and two of P (20 and 60 mg kg^–1) were applied separately to the two compartments to assess hyphal uptake of Cd. Neither Cd nor P addition inhibited root colonization by the AM fungus, but Cd depressed plant biomass. Mycorrhizal colonization, P addition and increasing added Cd level led to lower Cd partitioning to the shoots. Plant P uptake was enhanced by mycorrhizal colonization at all Cd levels studied. When Cd was added to the plant compartment and P to the hyphal compartment, plant biomass increased with AM colonization and the mycorrhizal effect was more pronounced with increasing Cd addition. When P was added to the plant compartment and Cd to the hyphal compartment, plant biomass was little affected by AM colonization, but shoot Cd uptake was increased by colonization at the low Cd addition rate (25 mg kg^–1) and lowered at the higher Cd rate (100 mg kg^–1) but with no difference in root Cd uptake. These effects may have been due to immobilization of Cd by the fungal mycelium or effects of the AM fungus on rhizosphere physicochemical conditions and are discussed in relation to possible phytostabilization of contaminated sites by AM plants.     

Effects of long-term temperature and nutrient manipulation on Norway spruce fine roots and mycelia production

Aims and methods

The effects of changing climate on ectomycorrhizal (EcM) fine roots were studied in northern Sweden by manipulating soil temperature for 14 years and/or by fertilizing for 22 years. Fine root biomass, necromass, EcM root tip biomass, morphology and number as well as mycelia production were determined from soil cores and mesh bags.

Results and conclusions

The fine root biomass and necromass were highest in the fertilized plots, following similar trends in the above-ground biomass, whereas the EcM root tip biomass per basal area decreased by 22 % in the fertilized plots compared to the control. Warming increased the fine root biomass, live/dead-ratio and the number of EcM root tips in the mineral soil and tended to increase the production of EcM mycelia. Greater fine root biomass meant more EcM root tips, although the tip frequency was not affected by fertilization or warming. Significantly higher specific root length of EcM root tips indicated an increased need for nutrients in warmed and in unfertilized plots. Better nutrient supply and warmer soil temperature provide a potential to increase the flow of carbon into the soil via increased fine root biomass, but the carbon balance also depends on root turnover.     

Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from32P-labelled organic matter during mineralization in soil

An experiment was set up to investigate the role of arbuscular mycorrhiza (AM) in utilization of P from organic matter during mineralization in soil. Cucumber (Cucumis sativus L.) inoculated with one of two AM fungi or left uninoculated were grown for 30 days in cross-shaped PVC pots. One of two horizontal compartments contained 100 g soil (quartz sand: clay loam, 1:1) with 0.5 g ground clover leaves labelled with³²P. The labelled soil received microbial inoculum without AM fungi to ensure mineralization of the added organic matter. The labelling compartment was separated from a central root compartment by either 37 m or 700 m nylon mesh giving only hyphae or both roots and hyphae, respectively, access to the labelled soil. The recovery of³²P from the hyphal compartment was 5.5 and 8.6% for plants colonized withGlomus sp. andG. caledonium, respectively, but only 0.6 % for the non-mycorrhizal controls. Interfungal differences were not related to root colonization or hyphal length densities, which were lowest forG. caledonium. Both fungi depleted the labelled soil of NaHCO₃-extractable P and³²P compared to controls. A 15–25% recovery of³²P by roots was not enhanced in the presence of mycorrhizas, probably due to high root densities in the labelled soil. The experiment confirms that AM fungi differ in P uptake characteristics, and that mycorrhizal hyphae can intercept some P immobilization by other microorganisms and P-sorbing clay minerals.