Population responses of oribatids and enchytraeids to ectomycorrhizal and saprotrophic fungi in plant-soil microcosms

The oribatid mites Oppiella nova, Tectocepheus velatus and Nothrus silvestris and the enchytraeid worm Cognettia sphagnetorum are four common animal species in boreal forest soils. According to the literature, they respond differently to clear-cutting of forest stands. O. nova responds with population decreases, T. velatus and N. silvestris with small changes and C. sphagnetorum with population increases. We hypothesised that the presence/absence of ectomycorrhizal (EM) fungi is a major factor in explaining these reactions. The population responses of these soil animals to inoculation of five species of EM fungi growing in symbiosis with their host tree (Pinus sylvestris L.) and one saprotrophic fungus, Hypholoma capnoides, growing on wood were tested in pot microcosms with artificial soil (peat and vermiculite) for 70-84 days. Additionally, plants without inoculation of EM fungi, plants growing in forest soil (FS) and plant-free peat and vermiculite (PV) were included. O. nova increased significantly in abundance in the treatments with the EM fungi Suillus variegatus and Paxillus involutus, but not in the other treatments. T. velatus increased significantly in abundance in FS, but declined in most of the other treatments, and N. silvestris showed a similar, albeit not significant, response. C. sphagnetorum did not increase in abundance in any of the treatments with EM fungi but increased its abundance four times in both FS and PV. The results show that the fungivore O. nova preferentially feeds on certain EM fungi, especially S. variegatus, whereas the EM fungus Piloderma fallax and the saprotrophic H. capnoides are not preferred. In contrast, C. sphagnetorum is restricted in its population growth by the EM fungi studied, and this dominant microbi-detritivore is clearly favoured by the absence of EM fungi.     

Microbial biomass and microbial C:N ratio in bulk soil and buried bags for evaluating in situ net N mineralization in agricultural soils

The in situ net nitrogen mineralization (N_net) was estimated in five agricultural soils under different durations of organic farming by incubating soil samples in buried bags. Simultaneously, soil microbial C and N was determined in buried bags and in bulk soil under winter wheat and after harvest. The aim was to check for variations in soil microbial biomass contents and microbial C:N ratios during the incubation period, and their importance for N_net rates. Microbial C and N contents were highest in soils that had been organically farmed for 41 years, whereas N_net rates were highest in a short‐term organically managed soil that had been under grassland use until 36 years ago. The mean coefficient of variation in the bulk soil for microbial C estimates ranged from 5 to 12 %. Microbial N contents were similar inside buried bags and in the bulk soil at the end of the incubation periods. Under winter wheat during the incubation period until harvest, microbial C contents and microbial C:N ratios (in 10—27 cm depth only) decreased more strongly inside buried bags than in the bulk soil. Following harvest of winter wheat and ploughing, microbial biomass increased while in situ N_net decreased, presumably due to N immobilization. The N_net rates were not correlated with microbial N contents or changes in microbial N contents inside buried bags. At the end of the vegetation period of winter wheat, N_net rates were negatively correlated with microbial C:N ratios. Because these ratios concurrently decreased more inside buried bags than in the bulk soil, the N_net estimates of the buried bag method may differ from the N_net rates in the bulk soil at that time.     

Effects of compost addition on extra-radical growth of arbuscular mycorrhizal fungi in Acacia tortilis ssp. raddiana savanna in a pre-Saharan area

We studied the influence of added compost, consisting of Acacia cyanophylla leaves, on the production of extra-radical mycelia of arbuscular mycorrhizal (AM) fungi in natural stands of Acacia tortilis, which forms a desert savanna. Four different plots with different soil characteristics in terms of nutrient level and water-holding capacity were included in the study. The production of AM fungi was measured as the increase in the amount of the phospholipid fatty acid (PLFA) 16:1ω5 and the neutral lipid fatty acid (NLFA) 16:1ω5 in mesh bags placed in the root zone of A. tortilis trees. The production of AM mycelia was much higher at the site with the highest nutrient level and highest water holding capacity. Principal component analysis revealed that mesh bags from this plot had proportionally more PLFA 16:1ω5 than the other plots, indicating that this plot contained proportionally more AM fungi in the microbial community. Compost addition enhanced the production of AM mycelia in all plots although the response was greatest in the plot with the highest proportion of AM fungi. The beneficial effect of compost addition on growth of the AM fungal biomass found in this study could be one way to improve survival of planted seedlings in arid regions. We suggest that indigenous AM fungi, which are adapted to the limiting conditions in the plots, are the preferable source of inoculum for improving the growth of A. tortilis in plantations in pre-Saharan ecosystems.     

Uptake of Sr from microcline and biotite by ectomycorrhizal fungi in a Norway spruce forest

Ectomycorrhizal (EM) fungi growing in symbiosis with tree seedlings have been found in laboratory experiments to stimulate weathering and the uptake of nutrients from silicate minerals. In the present study, we used the natural abundance of strontium isotopes to confirm that these fungi obtain strontium from biotite and microcline under field conditions. Minerals enriched in radiogenic ⁸⁷Sr were introduced into fungal in-growth mesh bags and placed under a 5-10 cm thick humus layer developed on boulders in a Norway spruce (Picea abies (L.) Karst) forest in south-western Sweden. EM root tips were sampled above the mesh bags and the ⁸⁷Sr/⁸⁶Sr ratio was used to calculate the fraction of Sr in the root tips that originated from the minerals. In the EM root tips sampled above the mesh bags containing the different minerals, 1.5% of the Sr originated from biotite and 6.4% from microcline. The amount of Sr was more than 300 times higher in the mesh bags containing microcline than in those containing biotite, indicating that proportionally more Sr was released from the biotite. This study demonstrates that EM fungi have the potential to take up measurable amounts of nutrients, such as Ca and K, from microcline and biotite in the field.     

Microbial residues as indicators of soil restoration in South African secondary pastures

Prolonged intensive arable cropping of semiarid grassland soils in the South African Highveld resulted in a significant loss of C, N and associated living and dead microbial biomass. To regenerate their soils, farmers converted degraded arable sites back into secondary pastures. The objective of this study was to clarify the contribution of microorganisms to the sequestration of C and N in soil during this regeneration phase. Composite samples were taken from the topsoils of former arable land, namely Plinthustalfs, which had been converted to pastures 1-31 years ago. Amino sugars were determined as markers for microbial residues in the bulk soil and in selected particle-size fractions. The results showed that when C and N contents increased during the secondary pasture usage, the amino sugar concentration in the bulk soil (0-5 cm) recovered at similar magnitude and reached a new steady-state level after approximately 90 years, which corresponded only to 90% of the amino sugar level in the primary grassland. The amino sugar concentration in the clay-sized fraction recovered to a higher end level than in the bulk soil, and also at a faster annual rate. This confirms that especially the finer particles contained a high amount of amino sugars and were responsible, thus, for the restoration of microbially derived C and N. The incomplete recovery of amino sugars in bulk soil can only in parts be attributed to a slightly coarser texture of secondary grassland that had lost silt through wind erosion. The soils particularly had also lost the ability to restore microbial residues below 5 cm soil depth. Overall, the ratios of glucosamine to muramic acid also increased with increasing duration of pasture usage, suggesting that fungi dominated the microbial sequestration of C and N whereas the re-accumulation of bacterial cell wall residues was less pronounced. However, the glucosamine-to-muramic acid ratios finally even exceeded those of the primary grassland, indicating that there remained some irreversible changes of the soil microbial community by former intensive crop management.     

Soil particulate organic matter effects on nitrogen availability after afforestation with Eucalyptus globulus

We examined the hypothesis that changes in the quality and/or quantity of soil particulate organic matter (POM) after afforestation of pasture land with Eucalyptus globulus Labill. plantations caused increased nitrogen (N) immobilization and a decline in N availability. The quantity of POM was measured on soils from 10 paired pasture/plantation sites in south-western Australia. Net mineralization of C and N were measured over a 14-day incubation of POM, whole soil, and a mix of POM (33%) and whole soil (67%) at 25 °C and optimal moisture content (matric potential of 25 kPa). There was no significant difference in total organic C between pasture and plantation. However, the POM fraction C was higher in plantation soils (75%) than under pasture (62%), reflecting the coarser nature of organic inputs under plantation. Total soil N concentration was 20% lower under plantation compared to pasture, and the proportion in the POM was higher (74% compared to 57% for pasture soil). The C:N ratios in POM under both pasture and plantation, and in the whole soil under plantation were around 19, but C:N ratios of whole soil under pasture was 17. Average C mineralization was 13% lower in plantation relative to that in pasture soil. The isolated POM fraction had 18% higher C mineralization rate than that in whole soil. The change in net N mineralization with afforestation was marked, with 50% lower net N mineralization in plantation than pasture whole soils. Net N mineralization in the isolated POM fraction was also about 50% of that in the whole soil for both pasture and plantation soils. Although, the pasture and plantation POM had similar C:N ratios, the net N mineralization was 2-fold greater in pasture POM than in plantation POM, suggesting that biochemical characteristics other than the C:N ratio had the main influence on net N mineralization rates. The POM fraction did not significantly immobilize N from the whole soil when placed in a mixture of POM and whole soil, suggesting that N immobilization was not the main mechanism for POM to influence N availability in these soils.     

Microbial characteristics of ectomycorrhizal mat communities in Oregon and California

Summary Specialized ectomycorrhizal fungi form dense mats in forest soils that have different enzyme levels, higher respiration rates, more biomass, different soil fauna, and different soil chemistry compared with adjacent soils not obviously colonized by these mats. In this study, mats formed by two genera of fungi collected in three locations were compared with a wide range of measurements. Per cent moisture, pH, chloroform fumigation-flush C, anaerobic N mineralization, exchangeable ammonium, and respiration, N₂ fixation, and denitrification rates were compared between soils or litter colonized by ectomycorrhizal mat-forming fungi and adjacent non-mat material. Significant differences were observed between the two genera of mat-forming fungi and also between mats formed primarily in mineral soil and those formed in litter. These differences suggest that different mat-forming fungi perform different functions in forest soils and that these fungi function differently in mineral soil compared with litter.Published as Technical Paper 9496, Oregon Agricultural Experiment Station     

Fraxinus-Glomus-Pisolithus symbiosis: Plant growth and soil aggregation effects

It has been established that arbuscular mycorrhizal (AM) fungi are involved in the conservation of soil structure. However, the effect of ectomycorrhizal (EM) fungi alone or in interaction with AM fungi in soil structure has been much less studied. This experiment evaluated EM and AM fungi effects on soil aggregation and plant growth. Ash plants (Fraxinus uhdei) were grown in pots, and were inoculated with Glomus intraradices and Pisolithus tinctorius separately but also in combination. Our results showed that F. uhdei established a symbiotic association with EM and AM fungi, and that these organisms, when interacting, showed synergistic and additive effects on plant growth compared to singly inoculated treatments. EM and AM fungi prompted changes in root morphology and increased water-stable aggregates. AM fungi affect mainly small-sized macroaggregates, while EM and EM-AM fungi interaction mainly affected aggregates bigger than 0.5 mm diameter. These results suggest that ectomyccorrhizal as well as arbuscular mycorrhizal fungi should be considered in restoration programs with Fraxinus plants.     

Seasonal changes in microbial biomass in soils cropped with palmarosa (Cymbopogon martinii L.) and Japanese mint (Mentha arvensis L.) in subtropical India

Changes in microbial C, N, and P were investigated for 1 year in two soils with similar physicochemical properties but supporting different crops under subtropical conditions. One was cropped with palmarosa (Cymbopogon martinii L.) and the other with Japanese mint (Mentha arvensis L.). Both the season and the type of cropping had a significant influence on changes in the soil microbial biomass. In general, soil microbial biomass C, N, and P were highest in summer months and lowest in midwinter. Soil microbial biomass levels and microbial C:N and C:P ratios were higher and N:P ratios lower under palmarosa soil than under mint.     

Plant regulation of microbial enzyme production in situ

Soil extracellular enzymes regulate the rate at which complex organic forms of nitrogen (N) become bio-available. Much research has focused on the limitations to heterotrophic enzyme production via lab incubations, but little has been done to understand the limitations to enzyme production in situ. We created root and symbiotic mycelia exclusion treatments using mesh in-growth bags in the field to isolate the effect of roots and other portions of the microbial community on enzyme production. When fertilized with complex protein N we found increases in N-degrading enzyme concentrations only when root in-growth was allowed. No response was observed when complex N was added to root-free treatments. Expanding on economic rules of microbial element limitation theory developed from lab incubation data, we suggest this is due to active transport of labile carbon (C) from roots to associated microbial communities in root bags. Roots alleviate C limitation of microbial enzyme synthesis, representing a tradeoff between plants and microbes–plant C for microbially-derived N.     

Long‐term effects of leguminous cover crops on microbial indices and their relationships in soils of a coconut plantation of a humid tropical region

In this study, leguminous crops like Atylosia scarabaeoides, Centrosema pubescens, Calopogonium mucunoides, and Pueraria phaseoloides. grown as soil cover individually in the interspaces of a 19‐yr‐old coconut plantation in S. Andaman (India) were assessed for their influence on various microbial indices (microbial biomass C, biomass N, basal respiration, ergosterol, levels of ATP, AMP, ADP) in soils (0–50 cm) collected from these plots after 10 years. The effects of these cover crops on . CO₂ (metabolic quotient), adenylate energy charge (AEC), and the ratios of various soil microbial properties viz., biomass C : soil organic C, biomass C : N, biomass N : total N, ergosterol : biomass C, and ATP : biomass C were also examined. Cover cropping markedly enhanced the levels of organic matter and microbial activity in soils after the 10‐yr‐period. Microbial biomass C and N, basal respiration, . CO₂, ergosterol and levels of ATP, AMP, ADP in the cover‐cropped plots significantly exceeded the corresponding values in the control plot. While the biomass C : N ratio tended to decrease, the ratios of biomass N : total N, ergosterol : biomass C, and ATP : biomass C increased significantly due to cover cropping. Greater ergosterol : biomass C ratio in the cover‐cropped plots indicated a decomposition pathway dominated by fungi, and high . CO₂ levels in these plots indicated a decrease in substrate use efficiency probably due to the dominance of fungi. The AEC levels ranged from 0.80 to 0.83 in the cover‐cropped plots, thereby reflecting greater microbial proliferation and activity. The ratios of various microbial and chemical properties could be assigned to three different factors by principal components analysis. The first factor (PC1) with strong loadings of ATP : biomass C ratio, AEC, and . CO₂ reflected the specific metabolic activity of soil microbes. The ratios of ergosterol : biomass C, soil organic C : total N, and biomass N : total N formed the second factor (PC2) indicating a decomposition pathway dominated by fungi. The biomass C : N and biomass C : soil organic C ratios formed the third principal component (PC3), reflecting soil organic matter availability in relation to nutrient availability. Overall, the study suggested that Pueraria phaseoloides. or Atylosia scarabaeoides were better suited as cover crops for the humid tropics due to their positive contribution to soil organic C, N, and microbial activity.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Net nitrification rate and presence of Nitrosospira cluster 2 in acid coniferous forest soils appear to be tree species specific

The impact of four coniferous tree species and their corresponding soil factors on N transformation rates and presence of ammonia-oxidising bacteria (AOB) was studied in an acid pine forest soil (Appelscha, The Netherlands). Pine soil had a relatively low net nitrification rate, while spruce, fir and larch soils showed high net nitrification rates. 16S rRNA and amoA sequences were only found in soils with high nitrification rates and belonged solely to Nitrosospira cluster 2. We conclude that tree species, possibly through their effects on soil C/N ratios, determines the presence of Nitrosospira cluster 2. Whenever AOB are present, however, the AOB community composition appears to be similar.     

Microbial populations involved in the suppression of Rhizoctonia solani AG1-1B by lignin incorporation in soil

Rhizoctonia solani causes worldwide losses in numerous crops. Sclerotia of R. solani remain viable for several years in soil and are an important source of primary infection. In this study the effect of soil incorporation of Kraft pine lignin, a side product of the paper industry, on viability of R. solani AG1-1B sclerotia was investigated. The efficacy of lignin was assessed in a sandy loam (Oppuurs) and a silt loam soil (Leest) collected from commercial fields in Belgium. Evaluating sclerotial viability after 4 weeks incubation in the two soils amended with 1% (w/w) Kraft pine lignin demonstrated a soil-dependent effect. In Leest soil the addition of lignin resulted in a significantly reduced sclerotial viability, together with an increased mycoparasitism by Trichoderma spp.; in Oppuurs soil, on the other hand, only a slight and insignificant reduction in sclerotial viability was observed. Based on phospholipid fatty acid analysis, different changes in microbial community structure upon lignin amendment were detected in the two soils. Both amended soils showed a significant increase in Gram negative bacteria. In Leest soil this increase was accompanied with a significantly higher increase in fungi and actinomycetes compared with Oppuurs soil. In addition, Kraft pine lignin resulted in both soils in a small but significant increase in manganese peroxidase activity and this increase tended to be higher in Leest soil. Manganese peroxidase produced by lignin-degrading basidiomycetes has previously been shown to degrade melanin, which protects the sclerotia against biotic and abiotic stress. We hypothesize that lignin-degrading fungi increased the susceptibility of the sclerotia to sclerotial antagonists such as Trichoderma, Gram negative bacteria and actinomycetes. Clearly, the effect observed here did not rely on the stimulation of one microbial group, but is the result of an interaction of different groups.     

Soil respiration characteristics of tropical soils from agricultural and forestry land-uses at Wondo Genet (Ethiopia) in response to C, N and P amendments

In most parts of tropical Africa, conversion of forests into agricultural lands is often accompanied by drastic changes in soil properties. However, little study has been done to examine changes in biological properties of soils from different land-uses in response to addition of C and nutrients. We conducted this study with the aim of investigating nutrient limitations for microbial activity in soils from agricultural (farm) and forest land-uses at Wondo Genet (Ethiopia) after amendment with C and limiting nutrients. We measured CO₂ respiration rates from the soils incubated in the laboratory before and after addition of glucose-C together with N and/or P in excess and/or limiting amounts. Based on the respiration kinetics, we determined the basal respiration (BR), substrate-induced respiration (SIR), specific-microbial growth rate (μ), respiration maxima (R_max), % of glucose-C respired, and microbially available N and P in the soils. We found that N was more limiting than P for the micro-biota in the soils considered, suggesting the presence of ample amounts of indigenous P that could be extracted by the micro-biota, if provided with C. Addition of P resulted in a respiration pattern with two peaks, presumably reflecting different N pools being available over time. The SIR, Respiration maxima, μ and microbially available P were higher in soils from the farm, while %C respired was higher in the forest, suggesting increased C costs for micro-biota to be able to utilize nutrients that are strongly bound to organic-matter or clay minerals. Depending on land-use, about 49-69% of added glucose-C was respired during two and a half weeks time, but differences between N or P additions were not significant. The correlation between soil physical and chemical properties and respiration parameters, however, depended on whether N or P was limiting. We concluded that examining the soil respiration kinetics could provide vital information on nutritional status of micro-organisms under different land-uses and on potential availability of nutrients to plants.     

Soil microbial and nutrient dynamics in a wet Arctic sedge meadow in late winter and early spring

Microbial activity is known to continue during the winter months in cold alpine and Arctic soils often resulting in high microbial biomass. Complex soil nutrient dynamics characterize the transition when soil temperatures approach and exceed 0 °C in spring. At the time of this transition in alphine soils microbial biomass declines dramatically together with soil pools of available nutrients. This pattern of change characterizes alpine soils at the winter-spring transition but whether a similar pattern occurs in Arctic soils, which are colder, is unclear. In this study amounts of microbial biomass and the availability of carbon (C), nitrogen (N) and phosphorus (P) for microbial and plant growth in wet peaty soils of an Arctic sedge meadow have been determined across the winter-spring boundary. The objective was to determine the likely causes of the decline in microbial biomass in relation to temperature change and nutrient availability. The pattern of soil temperature at depths of 5-15 cm can be divided into three phases: below −10 °C in late winter, from −7 to 0 °C for 7 weeks during a period of freeze-thaw cycles and above 0 °C in early spring. Peak microbial biomass and nutrient availability occurred early in the freeze-thaw phase. Subsequently, a steady decrease in inorganic N occurred, so that when soil temperatures rose above 0 °C, pools of inorganic nutrients in soils were very low. In contrast, amounts of microbial C and soluble organic C and N remained high until the end of the period of freeze-thaw cycles, when a sudden collapse occurred in soluble organic C and N and in phosphatase activity, followed by a crash in microbial biomass just prior to soil temperatures rising consistently above 0 °C. Following this, there was no large pulse of available nutrients, implying that competition for nutrients from roots results in the collapse of the microbial pool.     

外生菌根真菌对土壤无机磷迁移的影响

Ectomycorrhizal(EM) fungi could form symbiosis with plant roots and participate in nutrient absorption; however, many EM species commonly found in forest soils, where phosphorus(P) concentration and availability are usually very low, particularly in tropical and subtropical areas, have not yet been investigated for their efficiencies to mobilize soil P. In this study, fungal growth, P absorption,efflux of protons and organic acids, and soil P depletion by four isolates of EM fungi isolated either from acidic or calcareous soils were compared in pure liquid culture using soil as a sole P source. Boletus sp. 7(Bo 7), Lactarius deliciosus 3(Ld 3), and Pisolithus tinctorius 715(Pt 715) from acidic and P-deficient soils of southwestern China showed higher biomass and P concentration and accumulation than Cenococcum geophilum 4(Cg 4) from a calcareous soil of Inner Mongolia, northern China, after 4 weeks of liquid culture. Oxalate, malate, succinate, acetate, and citrate concentrations in the culture solutions varied significantly with fungal species,and oxalate accounted for 51.5%–91.4% of the total organic acids. Organic acids, particularly oxalate, in the culture solutions may lead to the solubilization of iron-bound P(Fe-P), aluminum-bound P(Al-P), and occluded P(O-P) from soil phosphates. Fungal species also varied greatly in proton efflux, which decreased the culture solution pH and may dissolve calcium-bound P(Ca-P) in soil.This could be the reason for the increment of both inorganic P in the culture solutions and Olsen P in the soil when EM fungi were present. Total inorganic P, the sum of Al-P, Fe-P, O-P, and Ca-P, in the culture solutions was positively correlated with the total concentration of organic acids in the culture solutions(r = 0.918*, n = 5), but negatively with both the total inorganic P in soil(r =-0.970**, n = 5) and the culture solution pH(r =-0.830*, n = 5). These suggested variable efficiencies of EM fungal species to mobilize inorganic P fractions from soil, which could make EM trees to utilize inorganic P in the same way like EM fungi and adapt to the soils with various P concentrations and availabilities.     

Variations in nitrogen,zinc, and sugar concentrations in Chinese fir seedlings grown on shrubland and plowed soils in response to arbuscular mycorrhizae-mediated process

One-year-old seedlings of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) are not colonized with ectomycorrhizal (EM) fungi, but often colonized with arbuscular mycorrhizal (AM) fungi. The AM fungi could be important for nutrient acquisition in Chinese fir seedlings. Previous studies show that feedbacks between aboveground and belowground plant tissues play a fundamental role in controlling the interaction between plants and AM fungi. Our results indicate significant feedback in seedlings grown on shrubland soils, but not on plowed soils. The amounts of sugar in fir leaves in the shrubland soils were significantly lower than those in plowed soils. Leaf zinc (Zn) and nitrogen (N) concentrations were significantly higher in seedlings in shrubland soils than in plowed soils. In mycorrhizal seedlings growing in shrubland soils, leaf N:P ratios were significantly higher than those in plowed soils, likely because of enhanced N absorption through AM-mediated process. Leaf N:P ratios in seedlings grown on plowed soils were below the threshold levels, because of low metabolic activity of feedback in AM-mediated process. The results suggested that the presence of feedback between Chinese fir seedlings and AM fungi should be benefit in transplanting Chinese fir seedlings.     

Grazing and plant-canopy effects on semiarid soil microbial biomass and respiration

The major objectives of this study were to determine the influence of grazing on the soil microbial biomass and activity in semiarid grassland and shrubland areas and to quantify the canopy effect (the differences in soil microbial biomass and activities between soils under plant canopies and soils in the open between plants). We also quantified changes in microbial biomass and activity during seasonal transition from dry to moist conditions. Chronosequences of sites withdrawn from grazing for 0, 11, and 16 years were sampled in a grassland (Bouteloua spp.) area and a shrubland (Atriplex canescens) area on and near the Sevilleta National Wildlife Reguge in central New Mexico, USA. Samples were obtained from beneath the canopies of plants (Yucca glauca in the grassland and A. canescens in the shrubland) and from open soils; they were collected three times during the spring and summer of a single growing season. Organic C, soil microbial biomass C, and basal respiration rates (collectively called the soil C triangle) were measured. We also calculated the microbial: organic C ratio and the metabolic quotient (ratio of respiration to microbial C) as measures of soil organic C stability and turnover. Although we had hypothesized that individual values of the soil C triangle would increase and that the ratios would decrease with time since grazing, differences in microbial parameters between sites located along the chronosequences were generally not significant. Grazing did not have a consistion effect on organic C, microbial C, and basal respiration in our chronosequences. The microbial: organic C ratio and the metabolic quotient generally increased with time since grazing on the shrubland chronosequence. The microbial: organic C ratio decreased with time since grazing and the metabolic quotient increased with time since grazing on the grassland chronosequence. The canopy effect was observed at all sites in nearly all parameters including organic C, microbial C, basal respiration, the microbial: organic C ratio, and the metabolic quotient which were predominantly higher in soils under the canopies of plants than in the open at all sites. Microbial biomass and activity did not increase during the experiment, even though the availability of moisture increased dramatically. The canopy effects were approximately equal on the shrubland and grassland sites. The microbial: organic C ratios and the metabolic quotients were generally higher in the shrubland soils than in the grassland soils.     

A possible role for saprotrophic microfungi in the N nutrition of ectomycorrhizal Pinus resinosa

We determined whether Pinus resinosa, selected ectomycorrhizal and saprotrophic microfungi have access to various organic nitrogen sources commonly found in the forest. Vector analysis demonstrated nitrogen limitation of the P. resinosa in the plantation from which most of the fungi were isolated, establishing this study's relevance. Nonmycorrhizal P. resinosa seedlings did not absorb significant N from amino acids. The ectomycorrhizal fungi, including Pisolithus tinctorius, Suillus intermedius and Tylopilus felleus, obtained substantial N from amino acids, a limited amount of N from glucosamine, and essentially no N from protein-tannin complex. In contrast, Penicillium and Trichoderma readily acquired N from protein-tannin and glucosamine. Thus, there was an increasing ability to obtain N from complex organic N sources from plant to ectomycorrhizal fungi to saprotrophic fungi. Furthermore, N mineralization from an organic N source by Penicillium depended on the C:N ratio. We conclude that acquisition of relatively simple organic N sources by P. resinosa is likely to be largely indirect via ectomycorrhizal fungi, and that more complex organic N sources may become accessible to ectomycorrhizal fungi (and thus possibly their host plants) following mineralization by saprotrophic fungi such as Penicillium or Trichoderma when C:N ratios are sufficiently low.