Lead mineral transformation by fungi.

Pyromorphite (Pb5(PO4)3Cl), the most stable lead mineral under a wide range of geochemical conditions [1], can form in urban and industrially contaminated soils [2] [3] [4] [5]. It has been suggested that the low solubility of this mineral could reduce the bioavailability of lead, and several studies have advocated pyromorphite formation as a remediation technique for lead-contaminated land [3] [5] [6], if necessary using addition of phosphate [6]. Many microorganisms can, however, make insoluble soil phosphate bioavailable [7] [8] [9] [10], and the solubilisation of insoluble metal phosphates by free-living and symbiotic fungi has been reported [11] [12] [13] [14] [15]. If pyromorphite can be solubilised by microbial phosphate-solubilising mechanisms, the question arises of what would happen to the released lead. We have now clearly demonstrated that pyromorphite can be solubilised by organic-acid-producing fungi, for example Aspergillus niger, and that plants grown with pyromorphite as sole phosphorus source take up both phosphorus and lead. We have also discovered the production of lead oxalate dihydrate by A. niger during pyromorphite transformation, which is the first recorded biogenic formation of this mineral. These mechanisms of lead solubilisation, or its immobilisation as a novel lead oxalate, have significant implications for metal mobility and transfer to other environmental compartments and organisms. The importance of considering microbial processes when developing remediation techniques for toxic metals in soils is therefore emphasised.     

Zinc Phosphate and Pyromorphite Solubilization by Soil Plant-Symbiotic Fungi

Acidolysis, complexolysis and metal accumulation were involved in solubilization of zinc phosphate and pyromorphite by a selection of soil fungi representing ericoid and ectomycorrhizal plant symbionts and an endophytic/entomopathogenic fungus, Beauveria caledonica. Zinc phosphate was much more readily solubilized than pyromorphite. According to the relationship between metal mobilization and pH, acidolysis (protonation) was found to be the major mechanism of both zinc phosphate and pyromorphite dissolution for most of the fungi examined. In general, the more metal tolerant fungal strains yielded more biomass, acidified the medium more and dissolved more of the metal mineral than less tolerant strains. However, B. caledonica 4 excreted a substantial amount of oxalic acid (～ 0.8 mM) in the presence of pyromorphite that coincided with a dramatic increase in lead mobilization providing a clear example of complexolysis. Organic acid excretion by fungi was inter- and intraspecific and was strongly influenced by the presence of the toxic metal minerals. When grown on zinc phosphate or pyromorphite, Hymenoscyphus ericae DGC3(UZ) accumulated the lowest metal concentration, but Thelephora terrestris accumulated the highest metal concentration in the biomass. The ability to accumulate water-soluble lead species, representing mainly cytosolic and vacuolar pools, seemed to be connected with pyromorphite-solubilizing ability. B. caledonica 4, which demonstrated the highest ability to dissolve pyromorphite, accumulated the highest water-soluble fraction and total lead concentration in the mycelium. Generally, isolates with a higher zinc-tolerance accumulated significantly less total zinc from zinc phosphate (including the sum of water-soluble and NaCl-extractable zinc) than non-tolerant strains.     

Transformation of galena to pyromorphite produces bioavailable sulfur for neutrophilic chemoautotrophy

The aqueous concentration of lead [Pb(II)] in geochemical environments is controlled by the solubility of Pb‐bearing minerals and their weathering products. In contaminated soils, a common method for in situ stabilization of Pb(II) is the addition of phosphate to convert more redox sensitive sulfide minerals into sparingly soluble pyromorphite [Pb₅(PO₄)₃X]. In this study, we conducted experimental studies to investigate the fate of reduced sulfur during the conversion of galena [PbS] to chloropyromorphite [Pb₅(PO₄)₃Cl]. Powder X‐ray diffraction analysis indicated that the reaction of phosphate with galena under oxic conditions resulted in the oxidation of sulfide and formation of elemental sulfur [S₈]. Under oxic abiotic conditions, the S₈ was retained in the solid phase, and negligible concentrations of sulfur as sulfide and thiosulfate were detected in the aqueous phase and only a small amount of sulfate. When PbS reacted in the presence of the chemoautotrophic organism Bosea sp. WAO, the S₈ in the secondary mineral was oxidized to sulfate. Strain WAO produced significantly more sulfate from the secondary S₈ than from the primary galena. Microscopic analysis of mineral–microbe aggregates on mineral‐embedded slide cultures showed that the organism was colocalized and increased in biomass over time on the secondary mineral surface supporting a microbial role. The results of this study indicate that stimulation of sulfur‐oxidizing activity may be a direct consequence of phosphate amendments to Pb(II)‐contaminated soils.     

Fungal biogeochemistry: a central role in the environmental fate of lead

Fungi play major roles in biogeochemistry and are responsible for many metal transformations during mineral weathering. A recent finding that fungi transform lead to chloropyromorphite highlights the importance of fungi in biogeochemical processes.     

Fungal Bioweathering of Mimetite and a General Geomycological Model for Lead Apatite Mineral Biotransformations

Fungi play important roles in biogeochemical processes such as organic matter decomposition, bioweathering of minerals and rocks, and metal transformations and therefore influence elemental cycles for essential and potentially toxic elements, e.g., P, S, Pb, and As. Arsenic is a potentially toxic metalloid for most organisms and naturally occurs in trace quantities in soil, rocks, water, air, and living organisms. Among more than 300 arsenic minerals occurring in nature, mimetite [Pb₅(AsO₄)₃Cl] is the most stable lead arsenate and holds considerable promise in metal stabilization for in situ and ex situ sequestration and remediation through precipitation, as do other insoluble lead apatites, such as pyromorphite [Pb₅(PO₄)₃Cl] and vanadinite [Pb₅(VO₄)₃Cl]. Despite the insolubility of mimetite, the organic acid-producing soil fungus Aspergillus niger was able to solubilize mimetite with simultaneous precipitation of lead oxalate as a new mycogenic biomineral. Since fungal biotransformation of both pyromorphite and vanadinite has been previously documented, a new biogeochemical model for the biogenic transformation of lead apatites (mimetite, pyromorphite, and vanadinite) by fungi is hypothesized in this study by application of geochemical modeling together with experimental data. The models closely agreed with experimental data and provided accurate simulation of As and Pb complexation and biomineral formation dependent on, e.g., pH, cation-anion composition, and concentration. A general pattern for fungal biotransformation of lead apatite minerals is proposed, proving new understanding of ecological implications of the biogeochemical cycling of component elements as well as industrial applications in metal stabilization, bioremediation, and biorecovery.     

Transformation of vanadinite [Pb5(VO4)3Cl] by fungi

Saprotrophic fungi were investigated for their bioweathering effects on the vanadium‐ and lead‐containing insoluble apatite group mineral, vanadinite [Pb₅(VO₄)₃Cl]. Despite the insolubility of vanadinite, fungi exerted both biochemical and biophysical effects on the mineral including etching, penetration and formation of new biominerals. Lead oxalate was precipitated by Aspergillus niger during bioleaching of natural and synthetic vanadinite. Some calcium oxalate monohydrate (whewellite) was formed with natural vanadinite because of the presence of associated ankerite [Ca(Fe²⁺,Mg)(CO₃)₂]. Aspergillus niger also precipitated lead oxalate during growth in the presence of lead carbonate, vanadium(V) oxide and ammonium metavanadate, while abiotic tests confirmed the efficacy of oxalic acid in solubilizing vanadinite and precipitating lead as oxalate. Geochemical modelling confirmed the complexity of vanadium speciation, and the significant effect of oxalate. Oxalate–vanadium complexes markedly reduced the vanadinite stability field, with cationic lead(II) and lead oxalate also occurring. In all treatments and geochemical simulations, no other lead vanadate, or vanadium minerals were detected. This research highlights the importance of oxalate in vanadinite bioweathering and suggests a general fungal transformation of lead‐containing apatite group minerals (e.g. vanadinite, pyromorphite, mimetite) by this mechanism. The findings are also relevant to remedial treatments for lead/vanadium contamination, and novel approaches for vanadium recovery.     

高山森林土壤微生物群落结构和功能对模拟增温的响应

将高山森林土壤装入PVC管中(土壤有机层在上、矿质土壤层在下)培养10周,以高山森林土壤年均温为对照,采用室内人工气候箱分别模拟增温2和4 ℃,研究土壤微生物群落和土壤酶活性对温度升高的响应.结果表明: 温度升高显著降低了土壤有机层中细菌、矿质土壤层中革兰氏阴性菌(G^-)PLFAs含量,但对土壤真菌无显著影响.温度升高引起革兰氏阳性菌和阴性菌比值(G⁺/G^-)升高,改变了微生物群落结构.增温对漆酶、β-葡萄糖酶、酸性磷酸酶和N-乙酰葡糖胺糖苷酶活性没有显著影响.土壤微生物群落之间呈现出协同增长的趋势,真菌、细菌、G⁺、G^-等微生物群落之间均呈显著正相关.土壤有机层中β-葡萄糖苷酶与土壤微生物群落对碳源利用的竞争,导致β-葡萄糖苷酶活性与土壤有机层细菌、真菌、G⁺呈显著负相关.高山森林不同土壤微生物类群对增温的响应不同,细菌比真菌对温度的响应更敏感,真菌对增温有一定的耐受能力.     

Fungal biotransformation of zinc silicate and sulfide mineral ores

In this work, several fungi with geoactive properties, including Aspergillus niger, Beauveria caledonica and Serpula himantioides, were used to investigate their potential bioweathering effects on zinc silicate and zinc sulfide ores used in zinc extraction and smelting, to gain understanding of the roles that fungi may play in transformations of such minerals in the soil, and effects on metal mobility. Despite the recalcitrance of these minerals, new biominerals resulted from fungal interactions with both the silicate and the sulfide, largely resulting from organic acid excretion. Zinc oxalate dihydrate was formed through oxalate excretion by the test fungi and the mineral surfaces showed varying patterns of bioweathering and biomineral formation. In addition, calcium oxalate was formed from the calcium present in the mineral ore fractions, as well as calcite. Such metal immobilization may indicate that the significance of fungi in effecting metal mobilization from mineral ores such as zinc silicate and zinc sulfide is rather limited, especially if compared with bacterial sulfide leaching. Nevertheless, important bioweathering activities of fungi are confirmed which could be of local significance in soils polluted by such materials, as well as in the mycorrhizosphere.     

Suitable Chemical Properties of Animal Manure Compost to Facilitate Pb Immobilization in Soil

Chemical immobilization using animal manure compost is one of the most useful for low-cost, in-situ soil remediation techniques. The present study aimed to determine suitable chemical properties of animal manure compost to facilitate lead (Pb) immobilization in soil. The level of mobile Pb in soil amended with swine compost was higher than that amended with cattle compost during the early stage of incubation. However, the level of mobile Pb was almost the same in soil amended with both types of compost on day 184 of incubation. The ratio of the residual fraction after sequential extraction was enhanced in soil amended with both types of compost, particularly swine compost. X-ray diffractometer (XRD) results demonstrated the precipitation of Pb phosphate minerals, such as pyromorphite, in Pb-sorbed composts, particularly swine compost. Amendment using swine compost could reduce Pb solubility even when it had a high content of water-soluble organic matter because it significantly lowered Pb phase solubility. The amendment with swine compost improved plant growth and microbial activity. This study suggests that composts with high phosphorus (P) content are suitable for Pb immobilization amendment even if they have a high water-soluble organic matter content.     

高山林线土壤微生物群落结构对模拟增温的响应

研究土壤微生物群落结构对模拟增温的响应,对预测全球气候变化背景下土壤碳氮磷循环具有重要意义.采用开顶式生长室(OTC)模拟增温,研究了土壤有机质层和矿质土壤层真菌(F)、细菌(B)、革兰氏阳性菌(G⁺)和革兰氏阴性菌(G^-)PLFAs微生物量,以及真菌/细菌(F/B)和革兰氏阴性菌/革兰氏阳性菌(G^-/G⁺)比值对模拟增温的响应.结果表明: OTC模拟增温使空气温度增加0.87 ℃,土壤有机质层温度增加0.5 ℃,矿质土壤层温度增加0.23 ℃.土壤有机质层微生物群落组成比矿质土壤层对模拟增温的响应更敏感.细菌比真菌对模拟增温的响应更加敏感,模拟增温显著影响了土壤有机质层的F/B和G^-/G⁺比值,对矿质土壤层的所有PLFAs含量或比值均没有显著影响.微生物的PLFAs含量及真菌/细菌和G^-/G⁺比值总体呈现非生长季低于生长季前期和生长季后期.冗余分析表明,土壤中的碳含量(可溶性有机碳DOC 12.1%、凋落物可溶性碳DC 9.5%和全碳TC 3%)是微生物群落结构的决定性因素,可溶性组分(DOC和DC)对微生物群落结构的影响大于全量养分(全碳和总氮).     

杉木叶片内生真菌定殖对凋落物分解及其微生物活性的影响

【背景】植物内生真菌在植物组织由衰亡转入腐生过程中发挥重要的作用,但这种作用可能因植物及其内生真菌种类不同而具有差异。【目的】分析不同优势度种类内生真菌定殖对于凋落物分解及其相应微生物活性的影响。【方法】采用凋落物分解袋法,选取优势树种杉木凋落叶作为分解底物。【结果】不同优势度类型的内生真菌单菌或组合定殖在分解过程前期几乎均显著加速了凋落物分解,而在分解后期,除了Irpex lacteus和Colletotrichum sp.,这种加速效应几乎均在减弱,甚至抑制了分解过程。微生物活性各变量对内生真菌定殖处理的响应与失重率并不完全一致,这依赖于分解时期。CO2释放速率在前期与失重率相关性不强,而后期则密切相关;分解前期羧甲基纤维素酶(carboxymethylcellulase,Cx酶)对失重率贡献较大,然而后期漆酶与过氧化物酶对失重率的贡献升高。总之,内生真菌定殖对凋落物分解及相应的微生物活性均产生了较大影响。【结论】对内生真菌定殖效应的研究有助于人们对森林生态系统土壤碳库平衡和养分循环维持机制的理解,同时对于贫瘠人工林土壤肥力的恢复研究也具有重要的意义。     

Antioxidant enzymes activities and bilirubin level in adult rat treated with lead

Lead toxicity is closely related to its accumulation in several tissues and its interference with bioelements, whose role is critical for several biological processes. Recently, oxidative stress has been proposed as a possible mechanism involved in lead toxicity. This study was carried out to investigate the effect of dose-dependent lead exposure on haematological and oxidative stress parameters. Adult male 'Wistar' rats (150-200 g) were divided into three groups: group [Pb 5] and group [Pb 15] received respectively 5 mg Pb(2+) (n=16) and 15 mg Pb(2+)/kg b.w. (n=16) as lead acetate solution i.p. for a period of seven days. Group [T] (n= 16) served as control and received 15 mg Na(+)/kg b.w. as sodium acetate solution i.p. for the same period. All animals were sacrificed 24 h after the last injection. Blood superoxide dismutase (SOD) and blood glutathione peroxidase (GPx) activities and plasma bilirubin level were measured. Liver was quickly excised for the estimation of alteration in lipid peroxidation indices (MDA). Lead exposure induces, in both treated groups, a marked decline in haematocrit and haemoglobin levels (p<0.01) when compared to control. The results show also a significant decrease (p<0.01) in SOD activity, but only in group [Pb 15]. SOD activity did not decrease in group [Pb 5] in comparison with control (p>0.05). However, lead exposure caused a light increase in GPx activity in group [Pb 15], which remains non-significant (p>0.05) compared to control. Group [Pb 5] did not record significant changes in the activity of GPx. Lead exposure for a period of seven days resulted in a significant (p<0.05) increase in bilirubin level in group [Pb 15] compared to control. The bilirubin level from rats of group [Pb 5] did not reach a statistical significance. Changes in liver MDA content in lead-exposed rats from [Pb 5] and [Pb 15] groups did not reach a statistical (p<0.05) significance. It is concluded that lead induces oxidative stress in a dose-dependent manner. No dose-dependent response was observed in blood GPx activity and liver MDA content. These results could be due to the short duration of the treatment.     

Seasonality of VAM infection in three populations of Agrostis capillaris (Gramineae) on soil with or without heavy metal enrichment

Three populations of the perennial grass Agrostis capillaris, growing on limestone derived clay with and without natural enrichment of the heavy metals cadmium, lead, and zinc, and on a sandy soil polluted by a metal smelter have been investigated with regard to the percentage and seasonality of infection with vesicular-arbuscular mycorrhizal (VAM) fungi and its impact on mineral nutrition.In all populations VAM infection was lowest during winter, and highest during late summer and autumn. The population at the metal smelter site was less infected by VAM fungi than both other populations. The concentration of mineral nutrients for the three populations was clearly related to the soil concentration, but hardly modified by the degree of VAM infection.     

Effect of lead stress on mineral content and growth of wheat (Triticum aestivum) and spinach (Spinacia oleracea) seedlings

Lead (Pb) is the most common heavy metal contaminant in the environment. Pb is not an essential element for plants, but they absorb it when it is present in their environment, especially in rural areas when the soil is polluted by automotive exhaust and in fields contaminated with fertilizers containing heavy metal impurities. To investigate lead effects on nutrient uptake and metabolism, two plant species, spinach (Spinacia oleracea) and wheat (Triticum aestivum), were grown under hydroponic conditions and stressed with lead nitrate, Pb(NO₃)₂, at three concentrations (1.5, 3, and 15 mM).Lead is accumulated in a dose-dependent manner in both plant species, which results in reduced growth and lower uptake of all mineral ions tested. Total amounts and concentrations of most mineral ions (Na, K, Ca, P, Mg, Fe, Cu and Zn) are reduced, although Mn concentrations are increased, as its uptake is reduced less relative to the whole plant’s growth. The deficiency of mineral nutrients correlates in a strong decrease in the contents of chlorophylls a and b and proline in both species, but these effects are less pronounced in spinach than in wheat. By contrast, the effects of lead on soluble proteins differ between species; they are reduced in wheat at all lead concentrations, whereas they are increased in spinach, where their value peaks at 3 mM Pb.The relative lead uptake by spinach and wheat, and the different susceptibility of these two species to lead treatment are discussed.     

Association of microbial community composition and activity with lead,chromium, and hydrocarbon contamination

Microbial community composition and activity were characterized in soil contaminated with lead (Pb), chromium (Cr), and hydrocarbons. Contaminant levels were very heterogeneous and ranged from 50 to 16,700 mg of total petroleum hydrocarbons (TPH) kg of soil(-1), 3 to 3,300 mg of total Cr kg of soil(-1), and 1 to 17,100 mg of Pb kg of soil(-1). Microbial community compositions were estimated from the patterns of phospholipid fatty acids (PLFA); these were considerably different among the 14 soil samples. Statistical analyses suggested that the variation in PLFA was more correlated with soil hydrocarbons than with the levels of Cr and Pb. The metal sensitivity of the microbial community was determined by extracting bacteria from soil and measuring [(3)H]leucine incorporation as a function of metal concentration. Six soil samples collected in the spring of 1999 had IC(50) values (the heavy metal concentrations giving 50% reduction of microbial activity) of approximately 2.5 mM for CrO(4)2- and 0.01 mM for Pb2+. Much higher levels of Pb were required to inhibit [14C]glucose mineralization directly in soils. In microcosm experiments with these samples, microbial biomass and the ratio of microbial biomass to soil organic C were not correlated with the concentrations of hydrocarbons and heavy metals. However, microbial C respiration in samples with a higher level of hydrocarbons differed from the other soils no matter whether complex organic C (alfalfa) was added or not. The ratios of microbial C respiration to microbial biomass differed significantly among the soil samples (P < 0.05) and were relatively high in soils contaminated with hydrocarbons or heavy metals. Our results suggest that the soil microbial community was predominantly affected by hydrocarbons.     

Earthworm invasions of ecosystems devoid of earthworms: effects on soil microbes

Recent studies document North American earthworm invasions and their profound effects on the structure of the soil profile, which is the habitat for soil microorganisms (mainly fungi and bacteria). Dramatic alterations made to these layers during earthworm invasion significantly change microbial community structure and therefore microbial activities such as C transformations. Understanding the impacts of earthworm invasion on the microbes themselves will give insight into earthworm effects on microbial activities. Bacterial and actinomycete communities in earthworm guts and casts have not been studied in environments recently invaded by earthworms. Earthworm invasion tended to decrease fungal species density and fungal species diversity and richness. The presence of earthworms decreased zygomycete species abundance probably due to disruption of fungal hyphae. Physical disruption of hyphae may also explain decreased mycorrhizal colonization rates, decreased mycorrhizal abundance and altered mycorrhizal morphology in the presence of earthworms. Mixing of organic layers into mineral soil during earthworm invasion tended to decrease microbial biomass in forest floor materials while increasing it in mineral soil. In newly invaded forest soils, microbial respiration and the metabolic quotient tended to decline. In forests where either the microbial community has had time to adapt to earthworm activities, or where the destruction of the forest floor is complete, as in invasions by the Asian Amynthas hawayanus, the presence of earthworms tends to increase the metabolic quotient indicating a shift to a smaller, more active microbial community.     

Effect of heavy metal pollution on mycorrhizal colonization and function: physiological, ecological and applied aspects

 High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed. Accepted: 23 June 1997     

两种代表性蚯蚓对设施菜地土壤微生物群落结构及理化性质的影响

不同生态型蚯蚓的取食偏好和生境有所差异,因此蚯蚓的生态型差异可能关乎其对土壤性质的不同影响;有关不同生态型蚯蚓对土壤性质尤其是微生物学性质影响的研究有助于了解蚯蚓生态功能的作用机制。在野外调控试验的第4年采集土壤,研究了牛粪混施和表施处理下内层种威廉腔环蚓(Metaphire guillelmi)和表层种赤子爱胜蚓(Eisenia foetida)对设施菜地土壤微生物群落结构和主要理化性质的影响。结果表明,土壤微生物群落结构同时受到蚯蚓种类和牛粪施用方式的影响。牛粪表施时,两种蚯蚓均显著降低了菌根真菌、真菌生物量和原生动物生物量(P0.05);牛粪混施时,不同蚯蚓的影响有所差异,威廉腔环蚓明显增加了菌根真菌、真菌生物量和放线菌生物量,而赤子爱胜蚓的作用不明显。此外,两种蚯蚓均提高了土壤孔隙度、团聚体稳定性和土壤p H、矿质氮以及微生物生物量碳氮水平,但提高幅度取决于蚯蚓种类和牛粪施用方式。冗余分析表明蚯蚓影响下土壤微生物群落结构的变化与团聚体稳定性、p H、速效磷、矿质氮呈正相关,而与土壤容重呈负相关。     

植物菌根共生磷酸盐转运蛋白

大多数植物能和丛枝菌根（arbuscular mycorrhiza, AM）真菌形成菌根共生体。AM能够促进植物对土壤中矿质营养的吸收,尤其是磷的吸收。磷的吸收和转运由磷酸盐转运蛋白介导。总结了植物AM磷酸盐转运蛋白及其结构特征,分析其分类及系统进化,并综述了AM磷酸盐转运蛋白介导的磷的吸收和转运过程及其基因的表达调控。植物AM磷酸盐转运蛋白属于Pht1家族成员,它不仅对磷的吸收和转运是必需的,而且对AM共生也至关重要,为进一步了解菌根形成的分子机理及信号转导途径提供了理论基础。     

The role of fungi in biogenic weathering in boreal forest soils

In this article we discuss the possible significance of biological processes, and of fungi in particular, in weathering of minerals. We consider biological activity to be a significant driver of mineral weathering in forest ecosystems. In these environments fungi play key roles in organic matter decomposition, uptake, transfer and cycling of organic and inorganic nutrients, biogenic mineral formation, as well as transformation and accumulation of metals. The ability of lichens, mutualistic symbioses between fungi and photobionts such as algae or cyanobacteria, to weather minerals is well documented. The role of mycorrhizal fungi forming symbioses with forest trees is less well understood, but the mineral horizons of boreal forests are intensively colonised by mycorrhizal mycelia which transfer protons and organic metabolites derived from plant photosynthates to mineral surfaces, resulting in mineral dissolution and mobilisation and redistribution of anionic nutrients and metal cations. The mycorrhizal mycelia, in turn provide efficient systems for the uptake and direct transport of mobilised essential nutrients to their host plants which are large sinks. Since almost all (99.99 %) non-suberised lateral plant roots involved in nutrient uptake are covered by ectomycorrhizal fungi, most of this exchange of metabolites must take place through the plant–fungus interface. This idea is still consistent with a linear relationship between soil mineral surface area and weathering rate since the mycelia that emanate from the tree roots will have a larger area of contact with minerals if the mineral surface area is higher. Although empirical models based on bulk soil solution chemistry may fit field data, we argue that biological processes make an important contribution to mineral weathering and that a more detailed mechanistic understanding of these must be developed in order to predict responses to environmental changes and anthropogenic impact.