Atmospheric CO(2) and mycorrhiza effects on biomass allocation and nutrient uptake of nodulated pea (Pisum sativum L.) plants

The effect of ambient and elevated atmospheric CO(2) on biomass partitioning and nutrient uptake of mycorrhizal and non-mycorrhizal pea plants grown in pots in a controlled environment was studied. The hypothesis tested was that mycorrhizae would increase C assimilation by increasing photosynthetic rates and reduce below-ground biomass allocation by improving nutrient uptake. This effect was expected to be more pronounced at elevated CO(2) where plant C supply and nutrient demand would be increased. The results showed that mycorrhizae did not interact with atmospheric CO(2) concentration in the variables measured. Mycorrhizae did not affect photosynthetic rates, had no effect on root weight or root length density and almost no effect on nutrient uptake, but still significantly increased shoot weight and reduced root/shoot ratio at harvest. Elevated CO(2) increased photosynthetic rates with no evidence for down-regulation, increased shoot weight and nutrient uptake, had no effect on root weight, and actually reduced root/shoot ratio at harvest. Non-mycorrhizal plants growing at both CO(2) concentrations had lower shoot weight than mycorrhizal plants with similar nutritional status and photosynthetic rates. It is suggested that the positive effect of mycorrhizal inoculation was caused by an enhanced C supply and C use in mycorrhizal plants than in non-mycorrhizal plants. The results indicate that plant growth was not limited by mineral nutrients, but partially source and sink limited for carbon. Mycorrhizal inoculation and elevated CO(2) might have removed such limitations and their effects on above-ground biomass were independent, positive and additive.     

Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (38050 mol mol^-1) and elevated (600150 mol mol^-1) atmospheric CO2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO2 stimulated plant growth to the same extent in mycorrhizal and non0mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO2, and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, or plants to elevated CO2. ¹³C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the plants as dark-respiration. The need for detailed studies in this area is emphasized.     

Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment

Soil fungi couple plant and ecosystem resource demands to pools of soil resources. Research on these organisms is needed to predict how rising atmospheric CO₂ will influence forest ecosystem processes and soil carbon (C) sequestration potential. We examined the influence of free‐air‐CO₂‐enrichment (FACE) on mycorrhizal and extraradical rhizomorph dynamics over a 5‐year period in a loblolly pine forest using minirhizotrons. Standing crop of mycorrhizal root tips varied greatly spatially and through time. Summed across all years, CO₂ enrichment increased mycorrhizal root tip production by 194% in deep soil (15–30 cm) but did not influence mycorrhizal production in shallow soil (0–15 cm). Production and mortality of soil rhizomorph length was 27% and 25% greater in CO₂‐enriched plots compared with controls over a 5‐year period beginning in January of 2000 and running through autumn 2004. Effects of atmospheric CO₂ enrichment on longevity of mycorrhizal root tips and rhizomorphs varied with soil depth (mycorrhizae and rhizomorphs) and with diameter (rhizomorphs). For instance, survival of mycorrhizal tips was reduced in CO₂‐enriched plots in deep soil (15–30 cm depth) but was increased in shallower soil (0–15 cm). Rhizomorph turnover was accelerated in shallow soil but effects on survivorship in deep soil varied according to diameter. A drought in 2002 coupled with loss of leaf area to an ice storm late in 2002 were followed by reductions in rhizomorph and mycorrhizal production, increases in mortality, and decreases in standing crop during 2003 and 2004. These effects tended to be more severe in CO₂‐enriched plots. Positive effects of atmospheric CO₂ enrichment on mycorrhizal fungi, primarily observed in deeper soil, are probably contributing to the prolonged stimulation of NPP by CO₂ enrichment at the Duke FACE study site.     

Independent and contrasting effects of elevated CO2 and N-fertilization on root architecture in Pinus ponderosa

The effects of elevated CO₂ and N-fertilization on the architecture of Pinus ponderosa Dougl. ex P. Laws & C. Laws fine roots and their associated mycorrhizal symbionts were measured over a 4-year period using minirhizotron tubes. The study was conducted in open-top field-exposure chambers located near Placerville, Calif. A replicated (3 replicates), 3×3 factorial experimental design with three CO₂ concentrations [ambient air (354 mol mol^–1), 525 mol mol^–1, and 700 mol mol^–1] and three rates of N-fertilization (0, 100 and 200 kg ha^–1 year^–1) was used. Elevated CO₂ and N treatment had contrasting effects on the architecture of fine roots and their associated mycorrhizae. Elevated CO₂ increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae. In contrast, N-fertilization had no effect on fine root extensity or intensity but increased mycorrhizal extensity and intensity. To better understand and model the responses of systems to increasing CO₂ concentrations and N deposition/fertilization it is necessary to consider these contrasting root architectural responses.     

Effects of long-term free-air ozone fumigation on delta15N and total N in Fagus sylvatica and associated mycorrhizal fungi

Patterns of nitrogen (N) isotope composition (delta(15)N) and total N contents were determined in leaves, fine roots, root-associated ectomycorrhizal fungi (ECM) of adult beech trees (FAGUS SYLVATICA), and soil material under ambient (1 x O(3)) and double ambient (2 x O(3)) atmospheric ozone concentrations over a period of two years. From fine root to leaf material delta(15)N decreased consecutively. Under enhanced ozone concentrations total N was reduced in fine roots and delta(15)N showed a decrease in roots and leaves. In the soil and in most types of mycorrhizae, delta(15)N and total N were not altered due to ozone fumigation. The number of vital ectomycorrhizal root tips increased and the mycorrhizal community structure changed in 2 x O(3). Simultaneously, the specific rate of inorganic N-uptake by the roots was reduced under the double ozone regime. From these results it is assumed that 2 x O(3) changes N-nutrition of the trees at the level of N-acquisition, as indicated by enhanced mycorrhizal root tip density, altered mycorrhizal species composition, and reduced specific N-uptake rates.     

Influence of compaction from wheel traffic and tillage on arbuscular mycorrhizae infection and nutrient uptake by Zea mays

Interactive effects of seven years of compaction due to wheel traffic and tillage on root density, formation of arbuscular mycorrhizae, above-ground biomass, nutrient uptake and yield of corn (Zea mays L.) were measured on a coastal plain soil in eastern Alabama, USA. Tillage and soil compaction treatments initiated in 1987 were: 1) soil compaction from tractor traffic with conventional tillage (C,CT), 2) no soil compaction from tractor traffic with conventional tillage (NC,CT), 3) soil compaction from tractor traffic with no-tillage (C,NT), and, 4) no soil compaction from tractor traffic with no-tillage (NC,NT). The study was arranged as a split plot design with compaction from wheel traffic as main plots and tillage as subplots. The experiment had four replications. In May (49 days after planting) and June, (79 days after planting), root biomass and root biomass infected with arbuscular mycorrhizae was higher in treatments that received the NC,NT treatment than the other three treatments. In June and July (109 days after planting), corn plants that received C,CT treatment had less above-ground biomass, root biomass and root biomass infected with mycorrhizae than the other three treatments. Within compacted treatments, plants that received no-tillage had greater root biomass and root biomass infected with mycorrhizae in May and June than plants that received conventional tillage. Corn plants in no-tillage treatments had higher root biomass and root biomass infected with mycorrhizae than those in conventional tillage. After 7 years of treatment on a sandy Southeastern soil, the interactive effects of tillage and compaction from wheel traffic reduced root biomass and root biomass infected with mycorrhizae but did not affect plant nutrient concentration and yield. ei]J H Graham     

Availability and function of arbuscular mycorrhizal and ectomycorrhizal fungi during revegetation of dewatered reservoirs left after dam removal

Revegetation following dam removal projects may depend on recovery of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungal communities, which perform valuable ecosystem functions. This study assessed the availability and function of AM and EM fungi for plants colonizing dewatered reservoirs following a dam removal project on the Elwha River, Olympic Peninsula, Washington, United States. Availability was assessed via AM fungal spore density in soils and EM root tip colonization of Salix sitchensis (Sitka willow) in an observational field study. The effect of mycorrhizal fungi from 4 sources (reservoir soils, commercial inoculum, and 2 mature plant community soils) on growth and nutrient status of S. sitchensis was quantified in a greenhouse study. AM fungal spores and EM root tips were present in all field samples. In the greenhouse, plants receiving reservoir soil inoculum had only incipient mantle formation, while plants receiving inoculum from mature plant communities had fully formed EM root tips. EM formation corresponded with alleviation of phosphorus stress in plants (lower shoot nitrogen:phosphorus). Thus, revegetating plants have access to AM and EM fungi following dam removal, and EM formation may be especially important for plant P uptake in reservoir soils. However, availability of mycorrhizal fungi declines with distance from established plant communities. Furthermore, EM fungal communities in recently dewatered reservoirs may not be as effective at forming beneficial mycorrhizae as those from mature plant communities. Whole soil inoculum from mature plant communities may be important for the success of revegetating plants and recovery of mycorrhizal fungal communities.     

Mycorrhizal colonisation improves fruit yield and water use efficiency in watermelon (Citrullus lanatus Thunb.) grown under well-watered and water-stressed conditions

The effect of arbuscular mycorrhizal (AM) colonisation by Glomus clarum on fruit yield and water use efficiency (WUE) was evaluated in watermelon (Citrullus lanatus) cv. Crimson Sweet F1 under field conditions. Treatments were: (1) well-watered plants without mycorrhizae (WW-M), (2) well-watered plants with mycorrhizae (WW+M), (3) water- stressed plants without mycorrhizae (WS-M) and (4) water-stressed plants with mycorrhizae (WS+M). When soil water tension readings reached –20 and –50 kPa for well-watered (WW) and water-stressed (WS) treatments, respectively, irrigation was initiated to restore the top soil to near field capacity. Water stress reduced watermelon shoot and root dry matter, fruit yield, water use efficiency but not total soluble solids (TSS) in the fruit, compared with the non-stressed treatments. Mycorrhizal plants had significantly higher biomass and fruit yield compared to nonmycorrhizal plants, whether plants were water stressed or not. AM colonisation increased WUE in both WW and WS plants. Macro- (N, P, K, Ca and Mg) and micro- (Zn, Fe and Mn) nutrient concentrations in the leaves were significantly reduced by water stress. Mycorrhizal colonisation of WS plants restored leaf nutrient concentrations to levels in WW plants in most cases. This is the first report of the mitigation of the adverse effect of water stress on yield and quality of a fruit crop.     

Interactions between the root-knot nematode Meloidogyne incognita and Glomus mosseae in banana

The effects of the interaction between the arbuscular mycorrhizal fungus Glomus mosseae and the root-knot nematode Meloidogyne incognita on growth and nutrition of micropropagated ;Grand Naine banana (Musa AAA) cultivar was studied under greenhouse conditions. Inoculation with two G. mosseae isolates significantly increased growth of plants in relation to non-mycorrhizal plants. Response to mycorrhizae was as effective as with an optimum P fertilization in promoting plant development for most growth parameters. Meloidogyne incognita had no apparent effect on the percentage of root colonization in mycorrhizal plants. In contrast, G. mosseae suppressed root galling and nematode buildup in the roots. The percentage of mycorrhizal colonization was high (over 80%) in low P fertilized plants, but optimum P rates for bananas (four times higher than low P) significantly reduced mycorrhizal colonization. Most elements were within sufficiency levels for banana with exception of N which was low for all treatments. Mycorrhizal plants fertilized with a low P rate showed higher N, P, K, Ca, and Mg contents as compared to non-mycorrhizal plants low in P with or without the nematode. Inoculation with G. mosseae favours growth of banana plants by enhancing plant nutrition and by suppressing nematode reproduction and galling during the early stages of plant development.     

Zinc Extraction potential of two common crop plants,Nicotiana tabacum and Zea mays

White spruce [Picea glauca (Moench) Voss] seedlings were inoculated with Hebeloma crustuliniforme and treated with 25 mM NaCl to examine the effects of salinized soil and mycorrhizae on root hydraulic conductance and growth. Mycorrhizal seedlings had significantly greater shoot and root dry weights, number of lateral branches and chlorophyll content than non-mycorrhizal seedlings. Salt treatment reduced seedling growth in both non-mycorrhizal and mycorrhizal seedlings. However, needles of salt-treated mycorrhizal seedlings had several-fold higher needle chlorophyll content than that in non-mycorrhizal seedlings treated with salt. Mycorrhizae increased N and P concentrations in seedlings. Na levels in shoots and roots of salt-treated mycorrhizal seedlings were significantly lower and root hydraulic conductance was several-fold higher than in non-mycorrhizal seedlings. A reduction of about 50% in root hydraulic conductance of mycorrhizal seedlings was observed after removal of the fungal hyphal sheath. Transpiration and root respiration rates were reduced by salt treatments in both groups of seedlings compared with the controls, however, both transpiration and respiration rates of salt-treated mycorrhizal seedlings were as high as those in the non-mycorrhizal seedlings that had not been subjected to salt treatment. The reduction of shoot Na uptake while increasing N and P absorption and maintaining high transpiration rates and root hydraulic conductance may be important resistance mechanisms in ectomycorrhizal plants growing in salinized soil.     

Mycorrhizal inoculant alleviates salt stress in<Emphasis Type="Italic"> Sesbania aegyptiaca</Emphasis> and<Emphasis Type="Italic"> Sesbania grandiflora</Emphasis> under field conditions: evidence for reduced sodium and improved magnesium uptake

A field experiment was conducted to examine the effect of the arbuscular mycorrhizal fungus Glomus macrocarpum and salinity on growth of Sesbania aegyptiaca and S. grandiflora. In the salt-stressed soil, mycorrhizal root colonisation and sporulation was significantly higher in AM-inoculated than in uninoculated plants. Mycorrhizal seedlings had significantly higher root and shoot dry biomass production than non-mycorrhizal seedlings grown in saline soil. The content of chlorophyll was greater in the leaves of mycorrhiza-inoculated as compared to uninoculated seedlings. The number of nodules was significantly higher in mycorrhizal than non-mycorrhizal plants. Mycorrhizal seedling tissue had significantly increased concentrations of P, N and Mg but lower Na concentration than non-mycorrhizal seedlings. Under salinity stress conditions both Sesbania sp. showed a high degree of dependence on mycorrhizae, increasing with the age of the plants. The reduction in Na uptake together with a concomitant increase in P, N and Mg absorption and high chlorophyll content in mycorrhizal plants may be important salt-alleviating mechanisms for plants growing in saline soil.     

Long‐term dynamics of mycorrhizal root tips in a loblolly pine forest grown with free‐air CO2 enrichment and soil N fertilization for 6 years

Large‐scale, long‐term FACE (Free‐Air CO₂ enrichment) experiments indicate that increases in atmospheric CO₂ concentrations will influence forest C cycling in unpredictable ways. It has been recently suggested that responses of mycorrhizal fungi could determine whether forest net primary productivity (NPP) is increased by elevated CO₂ over long time periods and if forests soils will function as sources or sinks of C in the future. We studied the dynamic responses of ectomycorrhizae to N fertilization and atmospheric CO₂ enrichment at the Duke FACE experiment using minirhizotrons over a 6 year period (2005–2010). Stimulation of mycorrhizal production by elevated CO₂ was observed during only 1 (2007) of 6 years. This increased the standing crop of mycorrhizal tips during 2007 and 2008; during 2008, significantly higher mortality returned standing crop to ambient levels for the remainder of the experiment. It is therefore unlikely that increased production of mycorrhizal tips can explain the lack of progressive nitrogen limitations and associated increases in N uptake observed in CO₂‐enriched plots at this site. Fertilization generally decreased tree reliance on mycorrhizae as tip production declined with the addition of nitrogen as has been shown in many other studies. Annual NPP of mycorrhizal tips was greatest during years with warm January temperatures and during years with cool spring temperatures. A 2 °C increase in average late spring temperatures (May and June) decreased annual production of mycorrhizal root tip length by 50%. This has important implications for ecosystem function in a warmer world in addition to potential for forest soils to sequester atmospheric C.     

13C and 15N in microarthropods reveal little response of Douglas-fir ecosystems to climate change

Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO₂) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO₂ increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n= 4, r²= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO₂ increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.     

Mechanisms of Phosphorus Acquisition for Ponderosa Pine Seedlings under High CO2and Temperature

To test the hypothesis that elevated atmospheric CO₂and elevatedtemperature, simulating current and predicted future growingseason conditions, act antagonistically on phosphorus acquisitionof ponderosa pine, seedlings were grown in controlled-environmentchambers in a two temperature (25/10 °C and 30/15 °C)xtwoCO₂(350 and 700 µl^-1) experimental design. Mycorrhizalseedlings were watered daily with a nutrient solution with Padded in organic form as inositol hexaphosphate (64 ppm P).Thus seedlings were challenged to use active forms of P acquisition.Elevated CO₂increased the relative growth rate by approx. 5%which resulted in an approx. 33% increase in biomass after 4months. There was no main effect of temperature on growth. Increasedgrowth under elevated CO₂and temperature was supported by increasesin specific absorption rate and the specific utilization rateof P. The contribution of mycorrhizae to P uptake may have beengreater under simulated future conditions, as elevated CO₂increasedthe number of mycorrhizal roots. There was no main effect oftemperature on root phosphatase activity, but elevated CO₂causeda decrease in activity. The inverse pattern of root phosphataseactivity and mycorrhizal infection across treatments suggestsa physiological coordination between these avenues of P acquisition.The concentration of oxalate in the soil increased under elevatedCO₂and decreased under elevated temperature. This small molecularweight acid solubilizes inorganic P making it available foruptake. Increased mycorrhizal infection and exudation of oxalateincreased P uptake in ponderosa pine seedlings under elevatedCO₂, and there was no net negative effect of increased temperature.The increased carbon status of pine under elevated CO₂may facilitateuptake of limiting P in native ecosystems. Atmospheric CO₂; climate change; growth analysis; oxalate; Pinus ponderosa ; ponderosa pine; phosphorus uptake; rhizosphere; root phosphatase; temperature     

A comparison of phosphorus and nitrogen transfer between plants of different phosphorus status

Summary This study has two objections: (1) to compare transfers of phosphorus (³²P) with nitrogen (¹⁵N) from undefoliated and defoliated mycorrhizal P-rich plants to an adjacent mycorrhizal plant and (2) to determine whether the improved nutrient status of a plant growing with a nutrient-rich plant is due primarily to movement of nutrients from roots of its nutrient-rich neighbor (= nutrient transfer) or to reduced nutrient uptake by its nutrient-rich neighbor (=shift in competition). Two plants of Plantago lanceolata were grown in a three-pot unit in which each of their root systems were split, with part in the central shared pot and part by themselves in an outside pot. There were three treatments: (1) no added P; (2) P added in the outer pot to only plant, termed the donor plant, since it might provide P to the companion plant, acting as a receiver; and (3) as in the previous treatment but the P-fertilized donor plant was also clipped. To encourage the formation of hyphal links between roots of the different plants, transfers were determined when root length densities were high (90 to 130 cm cm^-3 soil) and when 56 to 85% of the root length was infected with vesicular-arbuscular mycorrhizae. Phosphorus fertilization enhanced P but not N movement within donor plants. Regardless of treatment, N transfer from donor to receiver plants was an order of magnitude greater than P transfer and in amounts that could potentially affect plant nutrition in very infertile soils. Phosphorus transfer was very small in any of the treatments. Although P fertilization and clipping improved P status of receiver plants, P transfer was not indicated as the main reason for the improved nutrition. A shift in competition between donor and receiver plants was likely the major factor in the shift in nutrition of the receiver plants.     

Growth and Arbuscular Mycorrhizal Fungal Colonization of Two Prairie Grasses Grown in Soil from Restorations of Three Ages

I compared growth and arbuscular mycorrhizal fungal (AMF) colonization of two prairie grasses (Wild rye [Elymus canadensis] and Little bluestem [Schizachyrium scoparium]), an early‐ and a late‐dominating species in prairie restorations, respectively, grown in soil from restored prairies of differing age, soil characteristics, and site history. There were no consistent patterns between restoration age and soil inorganic nutrients or organic matter. The oldest restoration site had higher soil mycorrhizal inoculum potential (MIP) than 2‐ and 12‐year‐old restorations. However, MIP did not translate into actual colonization for two species grown in soils from the three restorations, nor did MIP relate to phosphorus availability. There were significant differences in root mass and colonization among Wild rye plants but not among Little bluestem plants grown in soils from the three restorations. Wild rye grown in 2‐year‐old restoration soil had significantly higher AMF colonization than when it was grown in soils from the 12‐ and 17‐year‐old restorations. Wild rye grown in 2‐year‐old restoration soil also had higher colonization than Little bluestem grown in 2‐ and 12‐year‐old restoration soils. Little bluestem had no significant correlations between shoot biomass, root biomass or colonization, and concentrations of soil P, total N, or N:P. However, for Wild rye, total soil N was positively correlated with root mass and negatively correlated with colonization, suggesting that in this species, mycorrhizae may affect N availability. Collectively, these results suggest that soil properties unrelated to restoration age were important in determining differences in growth and AMF colonization of two species of prairie grasses.     

Distribution of lead in mycorrhizal and non-mycorrhizal Norway spruce seedlings

Non-mycorrhizal spruce seedlings (Picea abies Karst.) and spruce seedlings colonized with Lactarius rufus (Scop.) Fr. or two strains of Paxillits involutus (Batsch) Fr. were grown in an axenic silica sand culture system with frequently renewed nutrient solution. After successful mycorrhizal colonization, the seedlings were exposed to 1 μM PbCI₂ for 19 weeks. The degree of infection in all of the mycorrhizal treatments approached 100% during the experiment and was not affected by exposure to Pb. However, the number of root tips per root dry weight and the shoot: root ratio, both in the non-mycorrhizal and the mycorrhizal seedlings, had decreased after the 19 week treatment with PbCl₂ Using X-ray microanalysis, the distribution and concentration of Pb in the tissues of mycorrhizal and non-mycorrhizal root tips were compared. In the mycorrhizae of seedlings exposed to Pb no significant accumulation of Pb in the hyphal mantle or in fungal cell walls of the Hartig net were detected. Lead accumulated primarily in the cortex cell walls both of non-mycorrhizal and mycorrhizal root tips. No significant difference of Pb concentrations in root cortex cell walls of non-mycorrhizal and mycorrhizal seedlings was found; except for seedlings colonized with Paxillus involutus strain 537. However, at the endodermis no effect of mycorrhizal fungal colonization on the Pb tissue concentration was detected. The presence of the fungal sheath did not prevent Pb from reaching the root cortex. The endodermis acted as a barrier to Pb radial transport in both mycorrhizal and non-mycorrhizal seedling roots.     

接种丛枝菌根真菌对模拟大气氮沉降下灌木铁线莲根系形态及养分承载的影响

为了解菌根化处理的灌木铁线莲(Clematis fruticosa)苗木根系形态及养分承载对氮沉降的应激响应,以1年生盆栽灌木铁线莲为对象,分别采用单接种和混合接种,即：单接种根内根孢囊霉(Rhizophagus intraradices,以下简称+R),单接种摩西斗管囊霉(Funneliformis mosseae,以下简称+F);混合接菌(上述2菌种菌剂按体积1∶1混合,以下简称+RF)的菌根苗。以非菌根苗(未接菌,以下简称-M)为对照。氮沉降处理设置4个梯度(不施氮(0N,0 g·m^-2·a^-1)、低氮(LN,3 g·m^-2·a^-1)、中氮(MN,6 g·m^-2·a^-1)、高氮(HN,9 g·m^-2·a^-1)),1年后测定各处理细根形态(直径≤0.5 mm的总根长、总表面积、总体积、根尖数量)、菌根侵染率、土壤孢子密度及根、茎、叶各器官的养分(碳、氮、磷)含量等指标。(1)在+R和+RF处理下,LN处理的苗...     

Interaction between root growth allocation and mycorrhizal fungi in soil with patchy P distribution

Aims and Background

Many plants preferentially grow roots into P-enriched soil patches, but little is known about how the presence of arbuscular mycorrhizal fungi (AMF) affects this response.

Methods

Lotus japonicus (L.) was grown in a low-P soil with (a) no additional P, (b) homogeneous P (28 mg pot^?1), (c) low heterogeneous P (9.3 mg pot^?1), and (d) high heterogeneous P (28 mg pot^?1). Each P treatment was combined with one of three mycorrhiza treatments: no mycorrhizae, Glomus intraradices, indigenous AMF. Real-time PCR was used to assess the abundance of G. intraradices and the indigeneous AMF G. mosseae and G. claroideum.

Results

Mycorrhization and P fertilization strongly increased plant growth. Homogeneous P supply enhanced growth in both mycorrhizal treatments, while heterogeneous P fertilization increased biomass production only in treatments with indigenous AMF inoculation. Preferential root allocation into P-enriched soil was significant only in absence of AMF. The abundance of AMF species was similar in P-enriched and unfertilized soil patches.

Conclusion

Mycorrhization may completely override preferential root growth responses of plants to P- patchiness in soil. The advantage of this effect for the plants is to give roots more freedom to forage for other resources in demand for growth and to adapt to variable soil conditions.     

Soil respiration response to three years of elevated CO2 and N fertilization in ponderosa pine (Pinus ponderosa Doug. ex Laws.)

We measured growing season soil CO₂ evolution under elevated atmospheric [CO₂] and soil nitrogen (N) additions. Our objectives were to determine treatment effects, quantify seasonal variation, and compare two measurement techniques. Elevated [CO₂] treatments were applied in open-top chambers containing ponderosa pine (Pinus ponderosa L.) seedlings. N applications were made annually in early spring. The experimental design was a replicated factorial combination of CO₂ (ambient, + 175, and +350 L L^–1 CO₂) and N (0, 10, and 20 g m^–2 N as ammonium sulphate). Soils were irrigated to maintain soil moisture at > 25 percent. Soil CO₂ evolution was measured over diurnal periods (20–22 hours) in October 1992, and April, June, and October 1993 and 1994 using a flow-through, infrared gas analyzer measurement system and corresponding pCO₂ measurements were made with gas wells. Significantly higher soil CO₂ evolution was observed in the elevated CO₂ treatments; N effects were not significant. Averaged across all measurement periods, fluxes, were 4.8, 8.0, and 6.5 for ambient + 175 CO₂, and +350 CO₂ respectively).Treatment variation was linearly related to fungal occurrence as observed in minirhizotron tubes. Seasonal variation in soil CO₂ evolution was non-linearly related to soil temperature; i.e., fluxes increased up to approximately soil temperature (10cm soil depth) and decreased dramatically at temperatures > 18°C. These patterns indicate exceeding optimal temperatures for biological activity. The dynamic, flow-through measurement system was weakly correlated (r = 0.57; p < 0.0001; n = 56) with the pCO₂ measurement method.