Carbon and nitrogen allocation in ectomycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings

We studied carbon and nitrogen allocation in mycorrhizal and non-mycorrhizal Scots pine (Pinus sylvestris L.) seedlings grown in a semi-hydroponic system with nitrogen as the growth limiting factor. Three ectomycorrhizal fungi were compared: one pioneer species (Thelephora terrestris Ehrh.: Fr.) and two late-stage fungi (Suillus bovinus (L.: Fr.) O. Kuntze, and Scleroderma citrinum Pers.). By giving all plants in each treatment the same amount of readily available nitrogen, we ensured that the external mycelium could not increase the total nitrogen content of the plants, thereby guaranteeing that any change in carbon or nitrogen partitioning was a direct effect of the mycorrhizal infection itself. Carbon and nitrogen partitioning were measured at an early and a late stage of mycorrhizal development, and at a low and a high N addition rate. Although mycorrhizal seedlings had a higher net assimilation rate and a higher shoot/root ratio than non-mycorrhizal seedlings, they had a lower rate of shoot growth. The high carbon demand of the mycobionts was consistent with the large biomass of external mycelia and the increased belowground respiration of the mycorrhizal plants. The carbon cost to the host was similar for pioneer and late-stage fungi. Above- and belowground partitioning of nitrogen was also affected by mycorrhizal infection. The external mycelia of Scleroderma citrinum retained 32% of the nitrogen supplied to the plants, thus significantly reducing nitrogen assimilation by the host plants and consequently reducing their growth rate. By contrast, the external mycelia of T. terrestris and Suillus bovinus retained less nitrogen than the mycelia of Scleroderma citrinum, hence we attributed the decreased growth rates of their host plants to a carbon drain rather than a nitrogen deficiency.     

Synergistic effects between Suilllus luteus and Trichoderma virens on growth of Korean spruce seedlings and drought resistance of Scotch pine seedlings

Korean spruce(Picea koraiensis Sieb. El Zucc.)is one of the main afforestation species in northern China.Seedling quality is a critical factor at planting time. To test whether the synergistic growth enhancement of Scotch pine(P. sylvestris var. mongolica) seedlings brought by the plant beneficial fungus Trichoderma virens(J.H. Mill., Giddens and A.A. Foster) Arx and ectomycorrhizal fungus(Suillus luteus(L.) Roussel.) can also benefit Korean spruce seedlings, we examined the effects of S. luteus and T. virens on the growth of P. koraiensis seedlings and drought resistance of P. sylvestris var. mongolica in peat soils. The two fungi were added to sterilized peat soil in pots, and the plants were grown for 4 months. Seedling growth and physiological variables, including mycorrhizal colonization rate of roots,biomass, and chlorophyll content, were examined. The colonization rate of the mycorrhizal fungus on P. koraiensis exceeded 65 %, and the synergism between S. luteus and T.virens enhanced most of the variables for P. koraiensis seedlings after inoculation with S. luteus then 30 days later with T. virens as in our published results for seedlings of P.sylvestris var. mongolica. When seedlings of P. sylvestris var. mongolica were inoculated with this sequence, they became more drought tolerant. T. virens also induced S.luteus to produce-1,3-glucanase and chitinase. This inoculation sequence at planting can thus improve the quality of P. sylvestris var. mongolica and P. koraiensis seedlings and substantiates our previous results.     

Growth and nutrient uptake of ectomycorrhizal Pinus sylvestris seedlings in a natural substrate treated with elevated Al concentrations

Models of the effects of elevated concentrations of aluminum (Al) on growth and nutrient uptake of forest trees frequently ignore the effects of mycorrhizal fungi. In this study, we present novel data indicating that ectomycorrhizal mycelia may prevent leaching of base cations and Al. Mycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings were grown in sand obtained from the B-horizon of a local forest. In Experiment 1, non-mycorrhizal seedlings and seedlings inoculated with Hebeloma cf. longicaudum (Pers.: Fr.) Kumm. ss. Lange or Laccaria bicolor (Maire) Orton were provided with nutrient solution containing 2.5 mM Al. Aluminum did not affect growth of non-mycorrhizal seedlings or seedlings inoculated with L. bicolor. Seedlings colonized by H. cf. longicaudum had the highest biomass production of all seedlings grown without added Al, but the fungus did not tolerate Al. Shoots of seedlings colonized by L. bicolor had the lowest nitrogen (N) concentrations but the highest phosphorus (P) concentrations of all seedlings. The treatments had small but significant effects on shoot and root Al concentrations. In Experiment 2, inoculation with L. bicolor was factorially combined with the addition of a complete nutrient solution, or a solution lacking the base cations K, Ca and Mg, and solutions containing 0 or 0.74 mM Al. Seedling growth decreased in response to 0.74 mM Al, but the effect was significant only for non-mycorrhizal seedlings. Mycorrhizal seedlings generally had higher P concentrations than non-mycorrhizal seedlings. Aluminum reduced P uptake in non-mycorrhizal plants but had no effect on P uptake in mycorrhizal plants. Mycorrhizal colonization increased the pH of the soil solution by about 0.5 units and addition of Al decreased the pH by the same amount. We conclude that the presence of ectomycorrhizal mycelia decreased leaching of base cations and Al from the soil.     

Temporal allocation of (14)C to extramatrical hyphae of ectomycorrhizal ponderosa pine seedlings

Ponderosa pine seedlings were inoculated with Hebeloma crustuliniforme either in growth pouches before they were transplanted to root-mycocosms (P seedlings), or at the time of transfer to root-mycocosms (V seedlings). Uninoculated seedlings served as controls (U seedlings). The use of root-mycocosms allowed examination of portions of hyphae separate from roots and rooting substrate but still in symbiosis with the host. The results thus provided a quantitative basis for estimating hyphal mass and carbon allocation to extramatrical hyphae. The amount of (14)CO(2) fixed after a 2-h exposure was greatest for P seedlings and least for uninoculated seedlings. Four and nine days after exposure, (14)C content was greatest in uninoculated seedlings and least in inoculated seedlings. In isotope distribution and dry mass accumulation, V seedlings were more similar to U than to P seedlings. Calculated on a dry weight basis, the allocation of isotope to mycelium suggested that extramatrical hyphae of P seedlings were a stronger sink for carbon than extramatrical hyphae of V seedlings. Differences in inoculation methods resulted in differences in carbon allocation and physiology of extramatrical hyphae that could affect seedling establishment and survival. Seedlings inoculated by one method cannot serve as surrogates for mycorrhizal seedlings produced by other inoculation techniques.     

Mycorrhizal status of an ozone-sensitive poplar clone treated with the antiozonant ethylene diurea

The antiozonant ethylene diurea is proven to prevent growth reductions in forest trees induced by ozone. The community of mycorrhizal fungi could be useful indicator of environmental stress. In this study, response of mycorrhizal fungi and fine roots to a 4-year exposure to ambient ozone and treatment with antiozonant was investigated in ozone-sensitive poplar clone under field conditions. The community of ectomycorrhizal fungi and root length colonization with ectomycorrhizal, arbuscular mycorrhizal fungi, and root endophytic fungi was analyzed in antiozonant-treated poplar plants and in poplar plants irrigated with water. In general, plants protected by antiozonant showed higher total number of fine roots, number of ectomycorrhizal types, Shannon–Weaver diversity index, and Species richness index compared to the plants treated with water. The ectomycorrhizal community shifted from contact exploration type in the trees irrigated with water to short-distance exploration type in ethylene diurea-treated trees. Ozone protectant may beneficially affect the belowground community of mycorrhizal fungi colonizing roots of ozone-sensitive poplar clone.     

Effect of different nitrogen and carbon sources and concentrations on the mycelial growth of ectomycorrhizal fungi under in-vitro conditions

Effect of different nitrogen and carbon sources and their concentrations in liquid media on the mycelial growth of six different ectomycorrhizal (ECM) fungal species was studied. Differences were found in the utilization of the different nitrogen and carbon sources between the fungal species. All the species showed better mycelial growth in the medium containing ammonium as nitrogen source. Growth was low in all species in medium in which nitrogen was supplied in nitrate form. All the ECM isolates investigated showed reduced growth in the medium containing maleic acid as the carbon source. The effect of glucose and di-ammonium hydrogen orthophosphate concentrations on mycelia growth of all the six fungal species was studied with ranges for glucose of 2–40 g/l, and for di-ammonium hydrogen orthophosphate of 2–20 g/l. Cortinarius fulvoconicus and Cortinarius flexipes showed maximal mycelial growth at a glucose concentration greater than 20 g/l. Suillus luteus, Scleroderma citrinum, Laccaria laccata, and Tricholoma aurantium showed maximal growth at a glucose concentration of 20 g/l. All six species showed maximal mycelial growth at 5–10 g/l of di-ammonium hydrogen orthophosphate concentration and with increased concentration mycelial growth in all species decreased.     

Effects of lime and ash treatments on ectomycorrhizal infection of Pinus sylvestris L. seedlings planted in a pine forest

A pine forest in the south of Sweden was treated with lime and wood ash. In early June, 12 months after the ash treatment and 18 months after the lime treatment, one year old Pinus syhestris L. seedlings were planted. Four months later six différent ectomycorrhizal types had infected the seedlings in all the treatments. A mycorrhizal type designated “pink”; was more than twice as common in the lime treatments as in the control and ash treatments. Piloderma croceum Erikss. & Hjorts. was significantly more abundant in limed soil than in ash treated soil. The results were compared to those from a bioassy performed in the laboratory, where P. syhestris seedlings had been grown in soil from the same forest. Similar soil pH values in the two studies resulted in different relative infection rates of the mycorrhizal types found. One additional mycorrhizal type, designated “white”; was found in the field experiment. This suggests that mycelial connections to the mature host plants may significantly alter the ability of different fungi to colonize host plant roots in competition with each other compared to when the fungi infect from propagules in the soil.     

Ectomycorrhizal colonisation of roots and ash granules in a spruce forest treated with granulated wood ash

Granulated wood ash has been proposed as a slow release fertiliser suitable for forest soils. In this study ectomycorrhizal colonisation of roots and ash granules was studied in a 40-year-old spruce forest treated with 0, 3 or 6 t ha⁻¹ granulated wood ash. We used PCR-RFLP methods for ITS-typing and identification of ectomycorrhizal fungi. In total 20 different ITS-types were recognised on roots in the organic soil horizon. Five of these were identified to species and two to genus. Six species, Tylospora fibrillosa Donk, Cortinarius sp. 3, Piloderma sp. 1, and three unidentified ITS-types (Ve-95-1, Tö-95-3 and Ve-95-9) each occurred on over 5% of the total root tips analysed. Together these comprised 55% of the ectomycorrhizal community on the screened roots.

Ash granules collected from the fertilised plots were normally colonised by fungal mycelia. PCR-RFLP analysis of these mycelia revealed the presence of four ITS-types. Three of these (Piloderma sp. 1, Ha-96-3 and Tor-97-1) were also present on the mycorrhizal roots. Piloderma sp. 1 was the most abundant species colonising roots. A possible role of ectomycorrhizal mycelia in the direct mobilisation of nutrients from ash granules is discussed.     

辽河三角洲盐碱地树种优化配置模式研究

在辽河三角洲盐碱地台田上,布置小胡杨纯林、白蜡纯林、小胡杨×刺槐混交林、白蜡×刺槐混交林4种树种配置模式,对不同配置模式下的树木生长和土壤水、盐、温度进行了研究。结果表明,小胡杨、白蜡、刺槐在辽河三角洲盐碱地上造林均能成活,成活率达92%～98%。混交配置模式对树木生长具有显著的促进作用,优于纯林,其中小胡杨×刺槐混交模式好于白蜡×刺槐模式。与CK相比,4种模式均能改善土壤水分状况,降低土壤盐分,调控土壤温度,防止盐分表聚,4种模式间比较顺序是小胡杨×刺槐混交林>白蜡×刺槐混交林>白蜡纯林>小胡杨纯林。可见,小胡杨×刺槐混交模式可作为该地区防护林营建的首选优化配置模式。     

外生菌根真菌彩色豆马勃优良菌株营养源的研究*

通过Ｐ．ｔ．优良菌株９１０９在不同培养基上生长速度、培养特性及其生长规律的研究，筛选出生长最佳培养基作为基础培养基，进行Ｃ、Ｎ营养研究，试验结果表明：Ｐ．ｔ．９１０９不仅能利用单糖、低聚糖和无机Ｎ等速效Ｃ、Ｎ源，而且能利用多糖、多元醇和有机Ｎ等长效Ｃ、Ｎ源。其菌丝生长最适Ｃ源是玉米粉和甘露糖；最适Ｎ源是蛋白胨和氯化铵。在无机Ｎ利用中，对硝态Ｎ和铵态Ｎ的利用无显著差异。应用Ｌ１６（４５）正交设计确定了Ｐ．ｔ．９１０９菌丝生长的最佳营养配方，菌丝增殖高达８６０倍，不但提高菌丝生物产量，而且节省了原料，降低了成本，为改进Ｐ．ｔ．菌剂生产配方和研制新型菌剂提供了最佳营养条件。     

The effects of soil and air temperature on CO2 exchange and net biomass accumulation in Norway spruce, Scots pine and silver birch seedlings

Soil temperature is proposed to affect the photosynthetic rate and carbon allocation in boreal trees through sink limitation. The aim of this study was to investigate the effect of temperature on CO(2) exchange, biomass partitioning and ectomycorrhizal (ECM) fungi of boreal tree species. We measured carbon allocation, above- and below-ground CO(2) exchange and the species composition of associated ECM fungi in the rhizosphere of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies K.) and silver birch (Betula pendula Roth) seedlings grown in soil maintained at 7-12, 12-15 and 16-22 °C. We found increased root biomass and photosynthetic rate at higher soil temperatures, but simultaneously with photosynthesis rate, higher temperature generally increased soil respiration as well as shoot, and root and rhizosphere respiration. The net CO(2) exchange and seedling biomass did not increase significantly with increasing temperature due to a concomitant increase in carbon assimilation and respiration rates. The 2-month-long growth period in different soil temperatures did not alter the ECM fungi species composition and the below-ground carbon sink strength did not seem to be directly related to ECM biomass and species composition in any of the tree species. Ectomycorrhizal species composition and number of mycorrhiza did not explain the CO(2) exchange results at different temperatures.     

13C and 15N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation

¹⁵N and ¹³C natural abundances of foliage, branches, trunks, litter, soil, fungal sporophores, mycorrhizas and mycelium were determined in two forest stands, a natural forest and a Norway spruce plantation, to obtain some insights into the role of the functional diversity of saprotrophic and ectomycorrhizal fungi in carbon and nitrogen cycles. Almost all saprotrophic fungi sporophores were enriched in ¹³C relative to their substrate. In contrast, they exhibited no or very little shift of δ¹⁵N. Judging from the amount of C discrimination, ectomycorrhizal fungi seem to acquire carbon from their host or from dead organic matter. Some ectomycorrhizal species seem able to acquire nitrogen from dead organic matter and could be able to transfer it to their host without nitrogen fractionation, while others supply their host with ¹⁵N-depleted nitrogen. Moreover ectomycorrhizal species displayed a significant N fractionation during sporophore differentiation, while saprotrophic fungi did not.     

Binucleate Rhizoctonia (Ceratorhiza spp.) induce adventitious root formation in hypocotyl cuttings of Pinus sylvestris L.

Adventitious rooting of Scots pine (Pinus sylvestris L.) hypocotyl cuttings was promoted by binucleate Rhizoctonia (BnR) and to a lesser extent by ectomycorrhizal fungi. Four BnR isolates (251, 266, 268 and 269) differentiated root meristems and significantly induced adventitious rooting in young derooted seedlings. Rooting rates were significantly higher in BnR treatments than in either indol-3-butyric acid (IBA) pretreatment (200 µm) or co-cultivation with the ectomycorrhizal fungi Suillus bovinus or Laccaria bicolor. In pre-IBA-treated cuttings, adventitious root numbers were higher in the treatments with isolates 251 and particularly 268, while more similar in other BnR treatments. In the 251, roots emerged from distal positions along the hypocotyl and not from the cut base as in the other three BnR isolate treatments. Loss of turgor and hypocotyl wilt was commonly detected up to the point of root initiation. Lateral roots and dichotomous short roots subsequently appeared at high frequency from primary adventitious roots. Mechanisms involved in root meristem differentiation, e.g. auxin production, wound response and oligosaccharide signals, are discussed with respect to host-fungal signalling mechanisms. These beneficial BnR and ectomycorrhizal fungi could provide promising new tools in the development of efficient clonal propagation methodology for this highly recalcitrant Pinus species.     

不同碳氮源对紫丁香蘑菌丝生长的对比研究

分别采用不同碳源和不同氮源对紫丁香菌丝体进行培养,对比研究其对紫丁香蘑菌丝体生长的影响,分别观察紫丁香蘑菌丝体在培养基中菌落长势、大小、菌丝生长指数及速率。本研究为今后研究紫丁香蘑菌种扩繁、大规模发酵及椴木接种提供了实验依据。     

Effects of slash retention and wood ash addition on fine root biomass and production and fungal mycelium in a Norway spruce stand in SW Sweden

In the study reported here we examined the short-term effects (1–3 years) of slash retention (SR) and the long-term effects (13–15 years) of wood-ash application (A) on fine roots and mycorrhizae in a 40-year-old Norway spruce forest in southwest Sweden. Soil cores were used to obtain estimates of the biomass (g m⁻²) of roots in three diameter classes (<0.5, 0.5–1 and 1–2 mm), root length density (RLD), specific root length (SRL) and mycorrhizal root tip density (RTD). Fine root (<1 mm) length production and mortality, and mycelium production, were estimated using minirhizotron and mesh bag techniques, respectively. Compared with the control plots (C), the biomass of fine roots in diameter classes <0.5 mm and 0.5–1 mm was significantly higher in A plots, but lower in SR plots. In addition, RLD was significantly lower in the humus layer of SR plots than in the humus layers of C and A plots, but not in the other layers. None of the treatments affected the SRL. In all soil layers, the SR treatment resulted in significant reductions in the number of ectomycorrhizal root tips, and the mycelia production of fungi in mesh bags, relative to the C treatment, but the C and A treatments induced no significant changes in these variables. Fine root length production in the C, A and SR plots amounted to 94, 87 and 70 mm tube⁻¹ during the 2003 growing season, respectively. Fine root mortality in treated plots did not change over the course of the study. We suggest that leaving logging residues on fertile sites may result in nitrogen mineralisation, which may in turn induce reductions in root biomass, and both root and mycelium production, and consequently affect nutrient uptake and the accumulation of organic carbon in soil derived from roots and mycorrhizae.     

The importance of wild plant species as potential inoculum reservoirs of white root rot disease

Rosellinia necatrix is a soilborne plant pathogenic fungus belonging to the phylum Ascomycota. The fungus is distributed widely in temperate zones and often causes huge economic losses on numerous crop plants. To determine the potential for wild plants to serve as inoculum reservoirs of white root rot disease, eight woody and two herbaceous plant species belonging to nine families were inoculated with two R. necatrix isolates. The species used were Actinidia polygama, Broussonetia kazinoki, Camellia sinensis, Castanea crenata, Hydrangea serrata, Magnolia obovata, Pteridium aquilinum, Petasites japonicus, Quercus serrata and Rosa multiflora, all common flora in the cool temperate forests of Japan. The mortality of plants varied between species, independent of the effect of fungal isolates. Although some of the plants survived fungal infection, all plant species tested were more or less susceptible, having rotting lesions on the inner bark and cortex. For the woody plants, most of the mycelial strands in the diseased tissues became plumose mycelia, growing compressed between the outer and inner barks. However, mycelia found in diseased tissues of the two herbaceous plants were not in the form of plumose mycelia. All plant species harboured mycelial strands on the surface of their root systems and underground main stem, and the amount of the mycelial strands did not correlate with the mortality of the plants. Even if plants exhibited no external symptoms, some of them harboured abundant mycelial strands. In conclusion, wild plants can serve as alternative host plants for R. necatrix and function as persistent inoculum reservoirs, irrespective of their apparent health status. Thus, assessments of such latent host plants must be considered in the course of managing white root rot.     

Influence of elevated CO2 and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua

An understanding of root system capacity to acquire nitrogen (N) is critical in assessing the long-term growth impact of rising atmospheric CO2 concentration ([CO2]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO2] on root ammonium (NH4+) and nitrate (NO3-) uptake capacity in sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.). Mycorrhizal treatments included inoculation of seedlings with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith in sweetgum and the ectomycorrhizal (EM) fungus Laccaria bicolor (Maire) Orton in loblolly pine. These plants were then equally divided between ambient and elevated [CO2] treatments. After 6 months of treatment, root systems of both species exhibited a greater uptake capacity for NH4+ than for NO3-. In both species, mycorrhizal inoculation significantly increased uptake capacity for NO3-, but not for NH4+. In sweetgum, the mycorrhizal effect on NO3- and NH4+ uptake capacity depended on growth [C02]. Similarly, in loblolly pine, the mycorrhizal effect on NO3- uptake capacity depended on growth [CO2], but the effect on NH4+ uptake capacity did not. Mycorrhizal inoculation significantly enhanced root nitrate reductase activity (NRA) in both species, but elevated [CO2] increased root NRA only in sweetgum. Leaf NRA in sweetgum did not change significantly with mycorrhizal inoculation, but increased in response to [CO2]. Leaf NRA in loblolly pine was unaffected by either treatment. The results indicate that the mycorrhizal effect on specific root N uptake in these species depends on both the form of inorganic N and the mycorrhizal type. However, our data show that in addressing N status of plants under high [CO2], reliable prediction is possible only when information about other root system adjustments (e.g., biomass allocation to fine roots) is simultaneously considered.     

毛白杨苗期VA菌根菌与外生菌根菌接种的研究

1998～1999年在田间条件下布置了毛白杨VA菌根(包括根内球囊霉菌和单孢球囊霉菌)与外生菌根硬皮马勃混合接种试验.结果表明接种菌根菌对毛白杨苗高生长量有促进作用,且以根内球囊霉菌作用最明显,单孢球囊霉菌处理次之,硬皮马勃处理最差.水涝大大降低人工接种菌根菌的效果,严重影响VA菌根和外生菌根菌单独接种及混合接种的效应,根内球囊霉菌甚至会抑制毛白杨苗木的高生长,VA菌根和外生菌根间的负交互作用也不对苗高生长产生影响.菌根真菌的感染率随时间变化出现较大的波动和变化.     

Role of metal proteinases in the fruit-body formation ofHypsizygus marmoreus

We investigated the possible role of metal proteinase on the fruit-body formation ofHypsizygus marmoreus. The addition of a specific metal proteinase inhibitor, phosphoramidon, to the culture medium (10g/ml) completely inhibited fruit-body formation. Metal proteinase activity in both the medium and the mycelia of this fungus increased markedly during vegetative mycelial growth, and activity was maximal 25 days after inoculation. When phosphoramidon was added to the culture medium during vegetative mycelial growth, the metal proteinase activity in the mycelium decreased to 56% of the control (without inhibitor) level. Isoelectric focusing analysis showed that two kinds of metal proteinases with a pl of 7.7 and 8.4, respectively, were obtained from 29-day-old mycelia. Uptake of phosphoramidon into the mycelia was confirmed as the result of inhibition of thermolysin activity by the mycelial extracts. The degree of inhibitor uptake into mycelia was about 2.0% and was independent of the initial concentration of the inhibitor administered. The addition of peptone and amino acids to medium treated with phosphoramidon resulted in fruit-body dry weight yields that were about 50% that of the control.     

Soil DIC uptake and fixation in Pinus taeda seedlings and its C contribution to plant tissues and ectomycorrhizal fungi

Plants can acquire carbon from sources other than atmospheric carbon dioxide (CO(2)), including soil-dissolved inorganic carbon (DIC). Although the net flux of CO(2) is out of the root, soil DIC can be taken up by the root, transported within the plant, and fixed either photosynthetically or anaplerotically by plant tissues. We tested the ability of Pinus taeda L. seedlings exposed to (13)C-labeled soil DIC and two NH(4)(+) availability regimes to take up and fix soil DIC. We also measured the concentration and distribution of the fixed soil DIC within the plant and mycorrhizal tissues, and quantified the contribution of soil DIC to whole-plant carbon (C) gain. Seedlings exposed to labeled DIC were significantly enriched in (13)C compared with seedlings exposed to unlabeled DIC (6.7 versus -31.7 per thousand). Fixed soil DIC was almost evenly distributed between above- and belowground biomass (55 and 45%, respectively), but was unevenly distributed among tissues. Aboveground, stem tissue contained 65% of the fixed soil DIC but represented only 27% of the aboveground biomass, suggesting either corticular photosynthesis or preferential stem allocation. Belowground, soil DIC had the greatest effect (measured as (13)C enrichment) on the C pool of rapidly growing nonmycorrhizal roots. Soil DIC contributed approximately 0.8% to whole-plant C gain, and approximately 1.6% to belowground C gain. We observed a slight but nonsignificant increase in both relative C gain and the contribution of soil DIC to C gain in NH(4)(+)-fertilized seedlings. Increased NH(4)(+) availability significantly altered the distribution of fixed soil DIC among tissue types and increased the amount of fixed soil DIC in ectomycorrhizal roots by 130% compared with unfertilized seedlings. Increased NH(4)(+) availability did not increase fixation of soil DIC in nonmycorrhizal roots, suggesting that NH(4)(+) assimilation may be concentrated in ectomycorrhizal fungal tissues, reflecting greater anaplerotic demands. Soil DIC is likely to contribute only a small amount of C to forest trees, but it may be important in C fixation processes of specific tissues, such as newly formed stems and fine roots, and ectomycorrhizal roots assimilating NH(4)(+).