Assuming that tunneling can be taken as an indicator for overall weathering activity, it remains unclear whether the greater weathering activity in the lower fertility sites is due to lower pH, greater nutrient demand on the part of the host, or greater ectomycorrhizal colonization. Hoffland et al. assessed tunneling activity across a northern Sweden podzol sequence and found that the occurrence of tunnels in feldspar grains coincided with the disapearence of easily weatherable cation sources such as biotite. Taken together, these tunnel studies imply a correlative, but not a causative, link between weathering activity by ectomycorrhizal fungi and host nutrient demand. Wilson at al. used magnetic separation to segregate readily weatherable cation sources such as biotite and orthopyroxene from more cation poor K feldpars. They then used a variety of molecular and microscopic methods to asses the density of microbial colonization and weathering state of these minerals. They found significantly more mycelial colonization of readily weatherable cation sources such as biotite and orthopyroxene than on more cation poor K feldpars, but noticed only slightly increased weathering of the biotite compared to the feldspar minerals. In the aforementioned field studies, there is evidence that ectomycorrhizal fungi may increase mineral foraging and colonization in response to increased demand for phosphorus. There is also evidence that weathered tunnels coincide with increased demand for mineral elements other than phosphorus and that ectomycorrhizal hyphae can preferentially colonize mineral fragments which are good sources of mineral nutrients other than phosphorus. However,frambueso en maceta there is no direct evidence in field studies that foraging for and weathering of K, Mg, or Ca sources by ECM can respond to demand for these nutrients. There are many reports in the literature of forest ecosystems dominated by ectomycorrhizal hosts which, possibly due to anthropogenic acid deposition, are now limited by base cation availability and not nitrogen or phosphorus. The mesh bag approach employed by Wallander and others in Swedish forests may be a good method for examining how the mycorrhizal role in nutrient acquisition has changed with the changing nutrient status of these forests. Especially good sites to use this approach would be the sharp N depositional gradients near industrial or agricultural sites.Microcosm studies allow weathering to be quantified and can focus on the weathering of a single mineral or any desired mineral mix. Microcosms can be used to examine the weathering potential of individual ectomycorrhizal species and can be employed to isolate the weathering activity of the ectomycorrhizal fungus from that of the plant root. Microcosm studies also allow the researcher to isolate the effects of the availability of just one nutrient on weathering activity. In relatively sterile microcosm experiment it is also much easier to assay for readily decomposable weathering agents, particularly low molecular weight organic acids , and examine how LMWOA production affects weathering rates. In soils, measured bulk solution LMWOA concentrations are generally too low to significantly enhance mineral weathering due to their rapid degradation by soil microbiota. However in semi sterile microcosms and at the fungus mineral interface in natural soils, LMWOA concentrations may be high enough to greatly enhance weathering rates via proton promoted and ligand promoted dissolution. Van Scholl et al. looked at how organic acid production was influenced by nutrient deficiency of Mg, N, P, and K. Decreasing P or N increased organic acid production, while reducing Mg or K either had no effect or slightly decreased overall LMWOA, although reducing Mg did increase oxalate production in some treatments. There were also significant differences between individual fungal species organic acid exudation profiles and how they reacted to different nutrient deficiencies. Paris et al. conducted a series of studies examining how weathering activity of ectomycorrhizal fungi in azenic culture is affected by nutrient availability. They found that Ca , K, and Mg had no effect of weathering activity when one element was deficient, however when Mg and K were simultaneously deficient both phlogopite weathering and oxalic acid production increased. In order to test whether weathering activity can respond to nutrient demand there must be a nutrient sufficient treatment and a nutrient deficient treatment, both with added minerals. The great majority of microcosm studies investigating ectomycorrhizal weathering fail to have both a nutrient deficient and a nutrient sufficient treatment. Only the work by van Scholl et al. and Paris et al. , explicitly tested whether weathering activity can respond to nutrient demand. From these studies it does appear that there is potential for the ectomycorrhizal fungus alone, or the ectomycorrhizal seedling to respond to deficiencies in P, Mg, or K by enhancing weathering activity, however the study by van scholl et al. had no added minerals and thus doesn’t actually measure weathering, and the studies by Paris et al. are azenic pure culture studies. More studies are clearly needed to address this specific question. If the ectomycorrhizal fungus is also below its optimal level for a particular nutrient, then increases in weathering or nutrient uptake observed by ectomycorrhizae in a –nutrient treatment are not necessarily a reaction to host plant nutrient demand. Increased weathering may be a reaction to ectomycorrhizal nutrient demand only. Having separate mycorrhizal and rooting compartments would help to resolve this question as would an additional treatment in which the growth medium is kept very nutrient poor but the plant is foliarly fertilized. The two compartment system used in Jentschke et al. would be a very effective way to segregate the ecomycorrhizal nutrient demand from plant nutrient supply. While they do not explicitly test whether weathering activity can respond to changing nutrient status, a number of other studies can offer insight into the study of ectomycorrhizal weathering and some discussion of them is warranted in this review. Ectomycorhizae have been found to increase weathering in a number of microcosm studies ,planta de arandanos en maceta while others have not found any increase in weathering with ectomycorrhizal colonization . Many of these studies find increased weathering with one ectomycorrhizal species but not another or with one nutrient treatment or mineral type but not another. Generally, studies that deny P or K and add a weatherable P or K source such as apatite or biotite do find increased weathering with ectomycorrhizal colonization. The same cannot be said for Mg; no study has yet looked at how weathering by ectomycorrhizal plants is affected by Ca status.Wallander found that all 3 ECM strains tested increased weathering rates above the non mycorrhizal control, but the mechanism of increased weathering was different for each strain: decreasing solution pH , oxalic acid prodution and greater P uptake . The most commonly proposed mechanism for ectomycorrhizal enhancement of mineral weathering is greater nutrient uptake and transport away from the mineral surface . Organic acid production by seedlings is generally found to be altered, though not necessarily increased, by ectomycorrhizal colonization. Organic acid exudation does not respond in a consistent way to nutrient demand or to the presence of certain minerals, nor is it generalizable across different ectomycorrhizal species. When one LMWOA is linked to increased weathering rates it is most commonly oxalic acid. Oxalic acid is produced in particularly large quantities by P. involutus, which also happens to be the most commonly used ectomycorrhizal species in weathering experiments. Ochs et al. found that there were strong weathering agents in the root exudates of H.& crustiliniforme, present in very low concentrations which were likely not LMWOA’s. The work of Calvaruso et al. and Uroz et al. give convincing evidence for a key role that bacteria may play in ectomycorrhizal weathering. They found that bacteria isolated from the symbiotic mantle of ectomycorrhizosphere of oak mycorrhizas have significantly higher weathering capacity than phylogenetically closely related bacteria isolated from the adjacent bulk soil . Calvaruso et al. demonstrated that one of these bacteria has the potential to enhance non mycorrhizal seedling growth by alleviating Mg and K limitation by stimulating biotite weathering. These results strongly suggest that further research into the field of mycorrhizal helper bacteria and ectomycorrhizal weathering is warranted. It also suggests that some of the highly reductionist experiments with either no bacteria or a much simplified bacterial community may fail to account for a key mechanism of ectomycorrhizal weathering. Often the rooting area in pot or microcosm studies is quite small such that the roots are far more densely packed than they would be in a natural setting. As a result, the ectomycorrhizosphere is no larger than the rhizosphere in nonmycorrhizal treatments. This eliminates one of the major proposed advantages of mycorrhizal colonization, and possibly a key mechanism by which ectomycorrhizae may confer a greater weathering ability on root systems: greater mineral surface contact and uptake of weathering products directly from mineral surfaces. The majority of microcosm experiments employ an artificial rooting medium and/or an inorganic nutrient solution, both of which may be a poor recreation of the nutrient environment of field settings. Nutrient starvation may be achieved when minor nutrient limitation, more representative of field conditions, is desired. Most microcosm studies also have either no or a highly simplified bacterial community, which may significantly alter weathering dynamics from natural settings. Another key drawback of microcosm studies is that the carbon and nutrient exchange dynamics of isolated seedlings in a laboratory may bear little resemblance to that of seedlings or mature trees in the field. In field settings hyphal networks may allow seedlings to avoid some of the initial carbon investment involved in establishing mycorrhizal colonization. Mature ectomycorrhizal trees are generally considered to be dependent on ectomycorrhizal communities for survival, while seedlings in the lab often experience growth reductions in response to mycorrhizal colonization and uncolonized seedlings can be far larger and more vigorous. Calculating the weathering rates in forest soils is critically important to forest managers, air quality policy, and models of forest productivity. Any removal of timber from a forest represents a removal of mineral nutrients; understanding how quickly those nutrients are replenished by atmospheric deposition or mineral weathering is a key component of a sustainable harvesting cycle . Mineral weathering rates determine a soil’s buffering capacity and are the single most important properties determining an ecosystem’s ability to buffer the effects of acidifying pollutants . Mineral weathering rates in soils are also the single most poorly constrained component of models designed to calculate acceptable airborne pollutant loads of nitrogen and sulfur deposition from power generation, transport, and agriculture . Accurate estimates of net primary productivity of forests over the course of the next century are critically important to global carbon models. Forest productivity is predicted to increase due to elevated CO2 . The extent of this negative feedback to elevated CO2 levels is largely dependant on forest trees’ ability to meet their increased carbon availability and water use efficiency with increased nutrient uptake . As the effects of anthropogenic nitrogen deposition continue to accumulate, large areas of forest are limited by base cation availability , which is a function of mineral dissolution. In coniferous trees, elevated CO2 has been shown to increase the ratio of root to shoot biomass and allocation to mycorrhizal symbionts . To understand how forest productivity and forest carbon stocks will be affected by global change we must first understand whether increased carbon allocation to nutrient uptake organs actually results in increased nutrient uptake and whether this increased carbon allocation is a result of increased nutrient demand. Most forest trees of the temperate and boreal biomes are dependant on ectomycorrhizae for their survival . Ectomycorrhizal fungi are symbionts that form an intimate association with the fine roots of trees and some woody shrubs. Increased nutrient uptake is generally considered to be the most beneficial effect of EMF on forest trees , though EMF have also been shown to increase water uptake , provide resistance to aluminum and other toxic metals , and increase pathogen resistance . EMF take up nitrogen from the soil and provide their host plant significant amounts of it; up to 80 % of total plant N uptake is from EMF .
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