When grazing of AM fungal hyphae does occur, animals often only clip the hyphae, severing connections to the root but not ingesting mycelial mass . Thus changes in micro-arthropod numbers might not have a major impact on C flux from AM hyphae to soil organic matter. Instead, death rates of live hyphae determine the flux of C from the live to the dead hyphal pool. At this point, dead tissue is distributed between active and slow soil organic matter pools as a function of tissue quality . Active soil organic matter includes sugars and other metabolites that are processed relatively quickly by decomposers; slow soil organic matter consists of recalcitrant components such as chitin and glomalin, and might last from years to decades in the soil. In plant tissues, higher N content generally speeds decomposition rates . Finally, as soil organic matter decomposes, a portion of C remains in decomposer tissues, and the rest returns to the atmosphere. Each of these fluxes and pools might be affected directly by elevated CO# and N deposition, or indirectly through changes in the composition of the mycorrhizal community.Groups of mycorrhizal fungi differ in several factors, including growth rate, that could influence C cycling. For example, isolates of ECM fungi vary markedlyin productivity, both within species and among species . In AM fungi, Sanders et al. observed significant differences in hyphal biomass among three Glomus species. In addition, after 16 wk growth, total hyphal lengths of Acaulospora denticulata and Scutellospora calospora were significantly greater than those of two Glomus species in a glasshouse experiment with Artemisia tridentata . If mycorrhizal communities are altered by climate change,blueberry in container then variation in growth and biomass among groups could affect the amount of atmospheric C that is initially drawn into the pool of live hyphae.
Mycorrhizal groups also differ in tissue qualities that might affect the rate at which this mycorrhizal C is returned to the atmosphere. Wallander et al. found that five morphotypes of ECM fungi on field-grown Pinus sylvestris varied more than twofold in chitin concentration. Likewise, glomalin content in AM hyphae differed between Gigaspora and Glomus , and significantly between Glomus caledonium and Glomus intraradices . In addition, mean N concentrations in the hyphae of four isolates of Paxillus involutusranged from 5 to 9% when grown in culture .These results suggest that the identity, as well as the amount, of mycorrhizal fungi might be important in soil C dynamics.As most mycorrhizal structures are relatively delicate and often below ground, measurements of mycorrhizal biomass, growth rate or turnover present some challenges. Most mycorrhizal studies under elevated CO# or N deposition have quantified changes in mycorrhizal colonization . This measure might be an appropriate index for nutrient transfer to the host plant . However, because extraradical hyphae account for a large portion of fungal biomass , direct measures of hyphal length are a valuable indicator of the mycorrhizal C pool . Furthermore, root colonization does not necessarily increase linearly with hyphal biomass, and environmental changes might alter relationships between the two variables. For instance, the ratio of AM hyphal length : total root length colonized by AM varied nearly twofold among CO# and N treatments in Gutierrezia sarothrae , and was nearly three times greater under ambient versus elevated CO# in a serpentine grassland . In an additional study, Staddon et al. noted a decrease in this ratio with elevated CO# in Plantago lanceolata and Trifolium repens. For this reason we focus primarily on hyphal length or biomass in this review.
We emphasize that hyphal length per unit soil area is a particularly meaningful variable in field studies, and might be used eventually to scale biomass to the ecosystem or regional level. The life stage of hyphae is also an important consideration when measuring fungal productivity. Few CO# or N studies have made the distinction between live and dead hyphae when determining hyphal length. The size of this combined pool is a function of productivity, survival rate and initial decomposition rate, and therefore changes in any combination of these factors might influence the results. Although the magnitude of these two pools provides useful information regarding immobilization of C, the influence of underlying mechanisms must be interpreted with caution. To draw conclusions about productivity, the growth rate of live hyphae must be directly followed. Several techniques enable us to focus on this live pool. For example, ergosterol concentrations are thought to indicate relative amounts of living fungal cytoplasm , and are an appropriate measure when no nonmycorrhizal fungi are present. In addition, stains such as fluorescein diacetate, differential fluorescent stain and immunofluorescent antibodies can distinguish between live and dead hyphae. Immunofluorescent antibodies are also specific to different genera and can be used to characterize the community composition of hyphae . This information, together with live and dead hyphal biomass, can be critical in analyses of mycorrhizal C dynamics.Mycorrhizal fungi might provide a negative feedback on anthropogenic CO# emissions by responding to rising concentrations of this trace gas. Overall, elevated CO# tends to increase or produce no change in hyphal biomass or growth on both ECM and AM fungi . These variables are determined independently of plant growth or biomass, and are therefore not an indication of relative allocation between plant and fungal tissue.
Studies have reported variation among mycorrhizal species, plant communities, N treatments or harvest dates; however none has detected a significant decrease in hyphal length or growth. This trend suggests a possible increase in global mycorrhizal biomass as atmospheric CO# levels rise, although the magnitude of this response might vary regionally and among species. Long-term field-based studies of mycorrhizal biomass under elevated CO# are rare but critical in predicting responses of natural systems. Rillig et al. found increases in AM hyphal biomass in a serpentine grassland after ” 5 yr CO# treatment. This change could be due to indirect effects of shifts in plant or fungal communities, or direct effects on plant C status. A similar increase occurred in AM fungi associated with trembling aspen after 14 months . Likewise, Tingey et al. noted a rise in the presence of ECM root tips and visible hyphae after 2.5 yr enrichment. With the exception of the sandstone grassland, these results are not consistent with the hypothesis that hyphal lengths in soils are already at a maximum under ambient CO# and will not increase as CO# levels rise . Each of these studies focused on changes in the incidence of live and dead hyphae combined. Additional field studies have indicated that the quantity of soil organic matter derived from mycorrhizal tissue might rise under CO# enrichment. Rillig et al. reported an increase in glomalin concentrations in soil from a chaparral system exposed to elevated CO# for 3 yr. In large macro-aggregates from the same ecosystem, the length of live AM hyphae increased 10-fold as CO# treatments varied from 250 to 650 ppm CO# . This increase in mycorrhizal biomass was accompanied by a 30- fold rise in C allocation to these macro-aggregates. These field-based studies suggest that the combined influences of elevated CO# on mycorrhizal C dynamics could ultimately produce an increase in the amount of C sequestered in intact hyphae and their residual components. However, many more studies of this nature are required before we can make general statements with any certainty. In addition, it is not clear whether these increases in hyphal biomass or residues will be maintained at equilibrium levels after the system has adjusted to the sudden rise in CO# that occurred at the onset of the experiment. Controlled,collection pot smaller-scale experiments provide insights into the mechanisms underlying the increase in hyphal biomass in field systems. For example, Rouhier & Read directly followed hyphal growth of two ECM fungi and noted a positive response in both under CO# enrichment. These measurements of actual growth rates are rare. Most investigations have been conducted in growth chambers or glasshouses for periods from several weeks to months . Many of these have reported augmentation of hyphal length under CO# enrichment. However, the duration and scale of these experiments impose some limitations in scaling up to ecosystem-level dynamics or in assessing underlying mechanisms .
As the plants and fungi were not grown in an intact community, they were not necessarily subject to competition or interactions from higher trophic levels. In addition, the bacterial community might not have been representative of those found in natural systems. Decomposition might also be a factor in experiments lasting more than a few weeks, and could introduce some unknown degree of error into interpretation of growth rates. Klironomos et al. used differential fluorescent staining to restrict biomass measurements to live hyphae, and found a more than twofold increase with elevated CO# . This response might also have been affected somewhat by changes in lifespan of the fungi. Nevertheless, the general trend toward increases in hyphal biomass under elevated CO# in pot experiments indicates that the abundance of mycorrhizal hyphae could rise in a number of AM and ECM fungal species and, potentially, ecosystems. Notably, the hyphal lengths of ECM fungi do not appear to demonstrate a greater frequency or magnitude of response to CO# than do hyphae of AM fungi, as suggested by O’Neill .Mycorrhizal groups vary in the magnitude of their responses to elevated CO# , resulting in shifts in the mycorrhizal community structure . In AM fungi, hyphal lengths of Acaulospora denticulata and Scutellospora calospora increased in response to CO# enrichment, while those of two Glomus species did not . These genera were each grown separately in pots and did not compete for resources. In a complementary chaparral-based field experiment, the abundance of hyphae from four AM genera was evenly distributed in chambers exposed to CO# concentrations of 250–350 ppm, but Acaulospora and Scutellospora dominated at 450–650 ppm . The results of these two studies indicate that Acaulospora and Scutellospora might become more prevalent as CO# levels rise. Inter specific variation and shifts in community composition have also been documented in ECM fungi. For example, Rouhier & Read reported that the biomass of P. involutus responded more strongly to a doubling of CO# concentrations than did that of Suillus bovinus. Likewise, inmycorrhizal root tips of Betula pendula seedlings Leccinum dominated at elevated CO# , but at ambient CO# species were more evenly distributed . In addition, in Betula papyrifera the relative abundance of ECM morphotypes changed significantly under elevated CO# , with a shift toward morphotypes with higher numbers of associated hyphae and rhizomorphs . Finally, in young Pinus sylvestris trees, CO# enrichment reduced by half the presence of the dominant morphotype , although this response could have been strictly morphological. As mycorrhizal groups vary in tissue quality and growth rate, these shifts in the mycorrhizal community might feed back to affect several processes involved in the cycling of mycorrhizal C.The effects of elevated CO# on the life span or turnover of individual mycorrhizal hyphae are not known. Rygiewicz et al. used minirhizotrons to track the length of time between formation and disappearance of mycorrhizal root tips on seedlings of Pinus ponderosa and found that elevated CO# had no significant effect. Moreover, as high CO# increased production of new root tips, a greater C flux through mycorrhizas was implied. Single hyphae might respond in a similar manner, and studies of their dynamics are required. Additionally, nutrient concentrations might be related to decomposition rates in mycorrhizal tissue , and P concentrations in ECM tips of P. involutus and S. bovinus declined significantly with CO# enrichment . Nitrogen content also decreased in P. involutus. Finally, we note that elevated CO# can increase fine-root mortality , which might be followed by a decrease in the life span of relatively long-lived ECM root tips and rhizomorphs.Although approaches and scopes have varied widely among studies of elevated CO# on mycorrhizal dynamics, several lines of evidence indicate that pools of mycorrhizal C might increase as CO# levels rise. With exceptions, growth rate and biomass of total live and dead hyphae tend to increase for both AM and ECM fungi both at plant and ecosystem level. This response is probably related to concurrent increases in plant biomass or to changes in plant or fungal community composition.