Effects of nitrogen and phosphorus fertilization on mycorrhizal formation of two poplar clones (Populus trichocarpa and P. tremula x tremuloides)

Mineral fertilization is a common management practice in short rotation forestry. The mycorrhizal formation of trees can be affected by fertilizer applications, however, very little is known on such effects in arable soils. The effects of a nitrogen (N) and phosphorus (P) fertilization on mycorrhizal formation of two poplar clones (Populus trichocarpa and P. tremula x tremuloides) were investigated at the plantation Abbachhof (South Germany). We determined the ectomycorrhizal colonization and the abundance of VAM spores in the soil during three years, and the species richness of sporocarps during one growing season. Approximately 26 to 73% of the fine roots of P. trichocarpa and 41 to 82% of the fine roots of P. tremula x tremuloides were colonized with ectomycorrhizal fungi. The percentage of ectomycorrizal colonization on P. tremula x tremuloides was significantly reduced after both fertilization treatments. On P. trichocarpa only the P‐fertilization reduced the ectomycorrhizal colonization. The composition of ectomycorrhizal morphotypes was significantly affected by the N and P fertilization on P. tremula x tremuloides, but not on P. trichocarpa. Sporocarps of 12 ectomycorrhizal fungi species were found at the plantation. Cortinarius uliginosus, Lactarius controversus and Krombholziella aurantiaca produced sporocarps only on control plots, whereas Cortinarius croceocaeruleus, Inocybe umbrina, Laccaria tortilis, Paxillus involutus and Rhizopogon roseolus produced sporocarps only on fertilized plots. Inocybe geophylla, I. glabripes, Laccaria laccata and Tuber borchii produced sporocarps on both control and fertilized plots. The density of VAM spores was lower in the rooting zone of Populus trichocarpa than under P. tremula x tremuloides. In an efficient management of these short rotation plantations mineral fertilizer applications must be low enough to avoid undesired suppressions of mycorrhizal formation.     

Rapid incorporation of carbon from ectomycorrhizal mycelial necromass into soil fungal communities

Ectomycorrhizal mycelial necromass is an important source of carbon for free-living microorganisms in forest soils, yet we know little either of its fate when it enters soil or of the identity of microbes that are able to utilise mycelium as their energy source. Here we used ¹³C-labelled mycelium of the ectomycorrhizal fungus Pisolithus microcarpus in laboratory incubations in combination with DNA-stable isotope probing (SIP) to determine the identity of functionally active soil fungi that can utilise dead mycelium. We also used solid-state nuclear magnetic resonance (NMR) spectroscopy to detect parallel changes in the abundance of key biochemical constituents of soil. A decrease in bulk soil ¹³C concentration together with rapid loss of glycogen and chitin-glucan during the 4 week incubations suggested that dead mycelium was rapidly turned over. Further, ¹³C was incorporated into fungal DNA within 7 days of addition to soil. DNA-SIP also revealed a dynamic community of functionally active soil fungi. By applying DNA-SIP and NMR in parallel, our data show that carbon from decaying ectomycorrhizal mycelium is rapidly transformed and incorporated into free-living soil fungi. This finding emphasises that dead extra-matrical mycelium is an important source of labile carbon for soil microorganisms.     

Allelopathic effects of artemisinin on ectomycorrhizal fungal isolates in vitro

The anti-malarial drug artemisinin is extracted from the leaves of Artemisia annua L. The release of artemisinin into forest soils could produce a potential risk for forest ecosystems, including effects on ectomycorrhizal fungal nutrient uptake, in areas where commercial and continual cultivation of the medicinal plant A. annua L. is practiced. Therefore, growth, proton and oxalate efflux, and nutrient uptake (nitrogen, phosphorus and potassium) of three isolates of Suillus luteus (S. luteus 1, S. luteus 13, and S. luteus 11) and of one isolate of Suillus subluteus (S. subluteus 12) were compared in culture solutions with different nominal artemisinin concentrations. The results showed that artemisinin inhibited significantly the growth of all studied fungi. With 25 mg artemisinin L⁻¹ added, fungal biomass was decreased by 78.6% (S. luteus 1), 96.7% (S. luteus 13), 77.8% (S. luteus 11) and 86.8% (S. subluteus 12) compared with the control (without artemisinin). This could explain, at least in part, why ectomycorrhizal fungal sporocarps in forests are consistently not found near cultivated A. annua L. fields. The amount of proton efflux by the fungal isolates also decreased as nominal artemisinin concentrations increased, indicating the limited ability of ectomycorrhizal fungi to mobilize nutrients from soil minerals. However, nominal artemisinin significantly increased the rate of fungal oxalate efflux, suggesting membrane damage and the abnormal opening of anion channels on hyphae cell membranes. Nominal artemisinin also decreased the uptake of nitrogen, phosphorus and potassium by the fungal isolates, which may not benefit from the nutrient uptake by ectomycorrhizae. Therefore, artemisinin released from large A. annua L. plantations may inhibit ectomycorrhizal fungal growth, nutrition and functions in forest ecosystems in Southwest China.     

Fatty acid composition of various ectomycorrhizal fungi and ectomycorrhizas of Norway spruce

Whole cell fatty acid (WCFA) compositions of three different structures of ectomycorrhizal (ECM) fungi: sporocarps, pure culture mycelia and ectomycorrhizas were analysed to evaluate the potential use of fatty acid profiles as biomarkers for ECM fungi and ectomycorrhiza-associated bacteria. Sporocarps of Amanita muscaria, Amanita rubescens, Lactarius rufus, Lactarius thejogalus, Leccinum scabrum, Paxillus involutus, Russula foetens, Russula rosea, Russula vesca, Suillus grevillei, Tylopilus felleus, Xerocomus badius, Xerocomus subtomentosus, pure cultures of A. muscaria, P. involutus, X. badius, X. subtomentosus, Suillus bovinus Suillus luteus and seven ectomycorrhizal morphotypes of Norway spruce were examined. Our results revealed species-specific composition of fatty acids of fungal sporocarps and pure culture mycelia. Ectomycorrhizal morphotypes distinguished and identified by morphological and molecular methods (PCR-RLFP and sequencing) created specific fatty acid profiles. The dominating fatty acids in pure cultures and sporocarps were 18:2ω6,9, 18:1ω9 and 16:0, whereas ectomycorrhizas also contained plant and bacterial specific fatty acids. Especially, fatty acids specific to Gram-positive bacteria 15:0 anteiso and 17:0 anteiso were present in relatively high amounts and suggested that these bacteria are dominating in the examined Norway spruce mycorrhizosphere. In conclusion, our results show that fatty acid based methods can be useful in studies of ectomycorrhizal fungi, both as a quick method for differentiation of fungal species and also in studies of mycorrhiza-associated microorganisms in the field.     

Can ectomycorrhizal fungi circumvent the nitrogen mineralization for plant nutrition in temperate forest ecosystems?

Nitrogen (N) limits plant growth in many forest ecosystems. The largest N pool in the plant-soil system is typically organic, contained primarily within the living plants and in the humus and litter layers of the soil. Understanding the pathways by which plants obtain N is a priority for clarifying N cycling processes in forest ecosystems. In this review, the interactions between saprotrophic microorganisms and ectomycorrhizal fungi in N nutrition with a focus on the ability of ectomycorrhizal fungi to circumvent N mineralization for the nutrition of plants in forest ecosystems will be discussed. Traditionally, it is believed that in order for plants to fulfill their N requirements, they primarily utilize ammonium (NH₄⁺) and nitrate (NO₃⁻). In temperate forest ecosystems, many woody plants form ectomycorrhizas which significantly improves phosphorus (P) and N acquisition by plants. Under laboratory conditions, ectomycorrhizal fungi have also been proven to be able to obtain N from organic sources such as protein. It was thus proposed that ectomycorrhizal fungi potentially circumvent the standard N cycle involving N mineralization by saprotrophic microorganisms. However, in many forest ecosystems the majority of the proteins in the forest floor form complexes with polyphenols. Direct access of N by ectomycorrhizal fungi from a polyphenol-protein complex may be limited. Ectomycorrhizal fungi may depend on saprotrophic microorganisms to liberate organic N sources from polyphenol complexes. Thus, interactions between saprotrophic microorganisms and ectomycorrhizal fungi are likely to be essential in the cycling of N within temperate forest ecosystems.     

An ecosystem-scale radiocarbon tracer to test use of litter carbon by ectomycorrhizal fungi

The degree to which ectomycorrhizal fungi rely on decomposing litter as a carbon source in natural ecosystems is unknown. We used a radiocarbon (¹⁴C) tracer to test for uptake of litter carbon by ectomycorrhizal fungi as part of the Enriched Background Isotope Study (EBIS) in Oak Ridge Reservation, Tennessee. In EBIS, leaf litter from a highly ¹⁴C-labeled Quercus alba (white oak) forest was reciprocally transplanted with litter from a nearby low-labeled forest that had not been as strongly exposed to ¹⁴C. These litter transplants were conducted yearly. We measured Δ¹⁴C signatures of ectomycorrhizal fungi collected from each forest four months and 2.25 years after the first litter transplant. The ectomycorrhizas were associated with white oak trees. We found no significant differences in ¹⁴C signatures of ectomycorrhizal fungi exposed to low-labeled versus high-labeled litter, indicating that less than 2% of the carbon in ectomycorrhizal biomass originated from transplanted litter. In contrast, ectomycorrhizal Δ¹⁴C signatures from the high-labeled forest were 117-140‰ higher than those from the low-labeled forest. This pattern suggests that ectomycorrhizal fungi acquired most (or all) of their carbon from their host plants, probably via direct transfer of photosynthate through the roots.     

Carbon partitioning in ectomycorrhizal Scots pine seedlings

The complete carbon budget and the turnover rate of assimilated carbon of ectomycorrhizal Scots pine seedlings growing on natural humus were determined in microcosm conditions. The main aim was to improve understanding of the partitioning of the assimilated carbohydrates within seedlings associated with multiple ectomycorrhizal fungi, and to discover carbon dynamics of the mycorrhizosphere.Plant photosynthesis and below-ground respiration were measured in order to obtain the actual carbon assimilation and respiration rates at the time of measurements. Soon after the photosynthesis and respiration rate measurements the seedlings were pulse-labeled with ¹⁴CO₂ to follow carbon allocation to different plant, fungal and soil compartments and rhizosphere respiration. Long-term carbon allocation during the entire life span of the seedlings was estimated by measuring plant and mycorrhizal root-tip biomass. The ectomycorrhizal community was analyzed using morphotyping and ITS-sequencing.The ¹⁴C label was detected in rhizosphere respiration after 12 h and it peaked between 36 and 60 h after labeling. More than half of the assimilated carbon was allocated below-ground as biomass or respiration and higher mycorrhizal biomass increased the below-ground carbon turnover. The presence of Suillus variegatus affected the plant carbon balance in several ways. When S. variegatus was present, the below-ground respiration increased and this carbon loss was compensated by higher photosynthetic activity. Other fungal species did not differ between each other in their effects on carbon balance. Our findings indicate that some root-associated mycorrhizal fungal symbionts can significantly alter plant CO₂ exchange, biomass distribution, and the allocation of recently photosynthesized plant-derived carbon.     

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     

Reforestation of burned stands: The effect of ectomycorrhizal fungi on Pinus pinaster establishment

The area occupied by Pinus pinaster in Portugal is rapidly diminishing because of forest fires. Ectomycorrhizal fungi form obligate, mutually beneficial associations with P. pinaster which improve plant growth and resistance to adverse conditions. The aim of this work was to assess whether native ectomycorrhizal fungi could be a useful tool in the reforestation of burned areas. The work was conducted in a forest nursery greenhouse, where P. pinaster seedlings were inoculated with compatible ectomycorrhizal fungal isolates: Suillus bovinus, Pisolithus tinctorius, Rhizopogon roseolus, and a mixture of the three fungi, using burned and unburned forest soil as substrate. Inoculation significantly enhanced the growth of P. pinaster, with R. roseolus proving to be the most effective in burned soil, with an 8-fold increase in plant fresh weight. Overall, inoculation stimulated growth most in burned than in unburned soil.This study suggests that inoculation with selected ectomycorrhizal fungi in containerised nurseries can be an advantageous approach for the successful establishment of P. pinaster in burned soil. The obtained results point out to the interest of extending these studies into fire-impacted areas, using ectomycorrhizal fungi as a biological tool.     

Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

Effects of inoculation with ectomycorrhizal fungi on microbial biomass and bacterial functional diversity in the rhizosphere of Pinus tabulaeformis seedlings

This study investigated the effects of inoculation with three individual ectomycorrhizal (ECM) fungal species on soil microbial biomass carbon and indigenous bacterial community functional diversity in the rhizosphere of Chinese pine (Pinus tabulaeformis Carr.) seedlings under field experimental conditions. The results showed that ECM fungal inoculation significantly increased the ectomycorrhizal colonization compared with non-inoculated seedlings. ECM fungal inoculations have higher soil microbial biomass carbon than that of control, ranging from 49.6 μg C g^?1 dry soil in control to 134.02 μg C g^?1 dry soil in treatment inoculated with Boletus luridus Schaeff ex Fr. Multivariate analyses (PCA) of BIOLOG data revealed that the application of ECM fungi significantly influenced bacterial functional diversity in the rhizosphere of P. tabulaeformis seedlings. The highest average well-color development (AWCD) and functional diversity indices were also observed in treatment inoculated with B. luridus. A wider range of sole carbon sources were utilized by the bacterial community in the rhizosphere of inoculated seedlings. The data gathered from this study provides important information for utilization of ECM fungi in forest restoration project in the Northwestern China. The present study will also significantly broaden our understanding of practical importance in the application of ECM fungal inoculum to promote soil microbial community diversity of soil.     

Alleviation of P limitation makes tree roots competitive for N against microbes in a N-saturated conifer forest: A test through P fertilization and 15N labelling

Chronic N deposition to forests may induce N saturation and stand decline, leading to reduced ecosystem N retention capacity, triggered by a shift from N limitation of trees to limitation by another nutrient. We conducted a ¹⁵N soil labelling experiment in non-fertilized and P-fertilized plots at two elevations in an N-saturated Mediterranean-fir (Abies pinsapo) forest in southern Spain which shows P limitation symptoms. Root-exclusion was applied to identify the relative contributions of roots (plus mycorrhizal fungi) uptake, and heterotrophic immobilization by free-living microbes, to N retention. Overall ¹⁵N recovery from the litter, 0–15-cm soil and root-uptake components was c.a. 35% higher in P-fertilized than in non-fertilized plots at both elevations. In non-fertilized plots, soil was the biggest sink for added ¹⁵N. Phosphorus fertilization increased the competitive ability of tree roots for soil N resulting in equal importance of the autotrophic (roots plus associated mycorhizal fungi) and heterotrophic (free-living microbes) components with respect to total ¹⁵N recovery in P-fertilized plots. Phosphorus addition increased litter and soil N immobilization only if roots had been excluded. By combining in situ fertilization, root-exclusion and isotope labelling we have demonstrated that reduced N retention capacity and dominance of soil microbial over plant immobilization in a N-saturated forest results from a shift from N to P limitation of trees, while alleviation of P limitation makes tree roots and associated mycorrhizal fungi competitive for N against free soil microorganisms.     

Bacterial and fungal contributions to soil nitrogen cycling under Douglas fir and red alder at two sites in Oregon

A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. ¹⁵N isotope dilution experiments, with ammonium (NH₄⁺) and nitrate (NO₃^?), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH₄⁺ consumption and nitrification rates decreased in all treatments except for NH₄⁺ consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH₄⁺ in forest soils with higher N availability. Cycloheximide consistently increased NH₄⁺ consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH₄⁺ combined with the inhibition of NH₄⁺ assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils.     

Effects of NH4 + on the growth and K+ (86Rb) uptake of various ectomycorrhizal fungi in pure culture

As a result of considerable deposition of ammonium sulphate on vegetation and soil the NH₄/K ratios in the soil water extracts of pine forests in the Netherlands are increasing. Increasing the NH₄/K ratio from 1 to 40 in the solid nutrient medium of isolated ectomycorrhizal fungi will in general result in a 60 to 80% inhibition of the lateral growth. On increasing the NH4/K ratio the biomass of most of the fungi examined shows a maximum at ratios of approximately 10 to 20, although the colony diameter at those ratios remains rather small. NH₄ ⁺ also exerts a pronounced effect on the uptake rate of Rb⁺ inRhizopogon luteolus.At low Rb concentrations the inhibitory effect of NH₄ ⁺ is maximal, whereas on increasing the Rb+ concentration the inhibitory effect of NH₄ ⁺ is abolished. This means that NH₄ ⁺ competitively inhibits the Rb⁺ uptake inR. Iuteolus.     

Ectomycorrhizal fungi introduced with exotic pine plantations induce soil carbon depletion

Exotic pine plantations are promoted for their presumed capacity to provide a net sink of atmospheric C. Millions of hectares worldwide will be subjected to conversion into plantations during the next decades. However, pine introductions are known to result in a marked depletion of soil C, a phenomenon which has remained unexplained. We studied plantations in paramo grasslands of Ecuador, where the effect of the exotic introduction of radiata pines (Pinus radiata) and their accompanying ectomycorrhizal fungi can be studied in isolation from other ecosystem disturbances. We suggest that ectomycorrhizal fungi can extract C previously accumulated by paramo grasslands based on (a) a drastic simplification of the ectomycorrhizal community shown by direct DNA identification, (b) a loss of up to 30% soil C within <20 years of plantation, (c) stable C isotope values in fungal fruitbodies which are closer to grassland than pine values, and (d) radiocarbon dating of fruitbodies indicating relatively old C sources for fruitbody formation. Species number in the ectomycorrhizal guild drops to only three fungal species per plantation compared to approximately 100 in comparable native pine stands. Our results provide evidence for a dynamic role of ectomycorrhizal fungi in soil C processing, and question the strategy of introducing pine plantations as a general solution to reduce mounting atmospheric CO₂ levels.     

Interactions between bacteria and ectomycorrhizal fungi

Interactions between eight ectomycorrhizal fungi and eight bacteria were tested on five laboratory media and in the rhizoplane of Pinus radiata. Depression of growth of the fungi by the bacteria in laboratory media was dependent on the medium and bore little relation to effects in the rhizoplane. In the rhizoplane, different bacteria could depress, have no effect or even stimulate growth of mycorrhizal fungi. Competition and antagonism are suggested as mechanisms for depression of the fungi. Some bacteria gave protection against the depressive effects of other bacteria. Considerable differences occurred between ectomycorrhizal fungi in their colonization of the rhizoplane in the absence of bacteria and also in their presence. The common mycorrhizal fungi Rhizopogon luteolus and Thelephora terrestris generally colonized roots well but the strain of Pisolithus tinctorius studied colonized poorly. Direct microscopy showed the percentage cover of the root by microorganisms was usually only 10–20%.It is proposed that interactions of ectomycorrhizal fungi with soil organisms are important in determining the successful introduction and persistence of inoculated ectomycorrhizal fungi. Fungi should be selected for compatibility with a wide range of soil microflora as well as efficiency in plant stimulation.     

Tracking Collembola feeding strategies by the natural C signal of fatty acids in an arable soil with different fertilizer regimes

Compound specific stable isotope analysis (¹³C/¹²C ratio of fatty acids) was used to assess the allocation of plant carbon in soil microbiota, and to identify the trophic links to microbial grazers in an arable field with long-term mineral and organic fertilizer amendments. The feeding strategy of two dominant Collembola species, epedaphic Isotoma viridis and euedaphic Willemia anophthalma was determined. The investigation was conducted following a shift to amaranth, a C₄ plant, after 27 years of continuous C₃ crop rotation. The influence of new C₄ plant carbon was observed in microbial phospholipids (PLFAs) with higher δ¹³C recorded in C₄ amaranth than in C₃ clover soils. The strongest enrichment occurred in the fungal PLFA 18:2ω6,9c and bacterial PLFA 18:1ω9t with 11.2‰ and 6.6‰, respectively. However, other bacterial PLFAs showed no isotopic change, suggesting that the microbial community simultaneously utilized “new” and “old” plant carbon. The δ¹³C of Collembola fatty acids displayed species specific lipid pattern, which was affected by crop type, but not fertilizer amendments. Isotopic separation of Collembola lipids from amaranth and clover plots was more distinct in I. viridis than W. anophthalma. With up to 18‰, the enrichment in Collembola lipids was stronger than in microbial PLFAs, pointing to a distinct incorporation of carbon resources originating from the actual plant residues. The δ¹³C pattern in I. viridis indicated trophic links with bacteria, saprotrophic fungi and plant tissues, while saprotrophic fungi and plant tissues were accountable for the patterns observed in W. anophthalma.     

Carbon dynamics determined by natural C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-plant) since 1961 with and without NPK fertilization from the long-term experiment ‘Ewiger Roggen’ in Halle, Germany, were incubated for 230 days at 8 °C and irrigated with 2 mm 10⁻² M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance. The percentage of maize-derived carbon was highest in CO₂ (42-79%), followed by C_mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO₂-C and C_mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO₂-C and 1.9-2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO₂-C and 0.3-0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r≥0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r≤0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil.     

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.     

Variables controlling denitrification from earthworm casts and soil in permanent pastures

Summary Denitrification (using the acetylene block method) was determined in earthworm casts and soils from permanent, drained or undrained pasture plots fertilized with 0 or 200 kg N ha^-1 year^-1 as ammonium nitrate. Rates of N₂O production from soil cores were about three times higher from the fertilized than from the unfertilized plots while drainage had a relatively small effect. Denitrification rates from casts were 3–5 times higher than those from soil irrespective of the drainage treatment. Casts generally had higher NO _inf3 ^sup- , NH _inf4 ^sup+ , and moisture contents, and higher microbial respiration rates than soil. Rates of N₂O production were determined primarily by NO _inf3 ^sup- supply, secondarily by moisture; available C did not appear to limit denitrification in these pastures. Estimates of the potential contribution of casts to denitrification ranges from 10.1% of 29.3 kg ha^-1 year^-1 from the unfertilized, drained plot to 22% of 82.5 kg ha^-1 year^-1 from the fertilized undrained plot.