Seasonality of arbuscular mycorrhizal hyphae and glomalin in a western Montana grassland

In order to more fully understand the basic biology of arbuscular mycorrhizal fungi (AMF), and their role in natural ecosystems, it is necessary to document seasonal changes of various aspects of the life history of these fungi. Due to their unique position at the root-soil interface, AMF have been described as `keystone mutualists' in ecosystems. Despite the importance of AMF in ecosystems, few studies exist that examine the seasonality of external hyphae and their exuded products (e.g. glomalin), the AMF variables directly related to ecosystem function through their contributions to soil aggregation. This study examined seasonal dynamics of several soil variables, with a specific interest in the seasonality of external hyphae and glomalin, a glycoprotein produced by AMF, which is correlated with soil aggregate stability. Here we measured glomalin concentrations and external AMF and non-AMF hyphal length, as well as soil moisture, percent fungal root colonization (AMF and non-AMF), and root length in soil in an intermountain grassland in western Montana over one growing season (13 time points). Of the glomalin pools and hyphal lengths measured, significant seasonal changes occurred for total glomalin (TG; 24.5% change), immunoreactive easily extractable glomalin (IREEG; 53.8% change), and AM hyphal length (107% change). Prior studies on glomalin in natural systems have not considered seasonal effects. The small seasonal change in glomalin pools lends further support to the hypothesis that glomalin is relatively stable in soils, and suggests that one-time sampling may be sufficient to satisfactorily capture this response variable. However, the generality of this observation has yet to be tested in a wider range of ecosystems.     

Artificial climate warming positively affects arbuscular mycorrhizae but decreases soil aggregate water stability in an annual grassland

Despite the importance of arbuscular mycorrhizae to the functioning of terrestrial ecosystems (e.g. nutrient uptake, soil aggregation), and the increasing evidence of global warming, responses of arbuscular mycorrhizal fungi (AMF) to climate warming are poorly understood. In a field experiment using infrared heaters, we found effects of warming on AMF after one growing season in an annual grassland, in the absence of any effects on measured root parameters (weight, length, average diameter). AMF soil hyphal length was increased by over 40% in the warmed plots, accompanied by a strong trend for AMF root colonization increase. In the following year, root weight was again not significantly changed, and AMF root colonization increased significantly in the warmed plots. Concentration of the soil protein glomalin, a glycoprotein produced by AMF hyphae with importance in soil aggregation, was decreased in the warmed plots. Soil aggregate water stability, measured for five diameter size classes, was also decreased significantly. In the following year, soil aggregate weight in two size classes was decreased significantly, but the effect size was very small. These results indicate that ecosystem warming may have stimulated carbon allocation to AMF. Other factors either influenced glomalin decomposition or production, hence influencing the role of these symbionts in soil aggregation. The observed small changes in soil aggregation, if widespread among terrestrial ecosystems, could have important consequences for soil carbon storage and erosion in a warmed climate, especially if there are cumulative effects of warming.     

Does percent root length colonization and soil hyphal length reflect the extent of colonization for all AMF?

Percent root length colonization may not be an appropriate measure of root colonization by arbuscular mycorrhizal fungi (AMF) in all cases. We suggest that AMF will differ in how well percent root length colonization measures the amount of AMF colonization in the root due to differences among AMF in hyphal structure and hyphal aggregation. Although soil hyphal length accounts for hyphal density, we suggest that it does not consider differences in hyphal structure in measurements of external colonization and thus might also misrepresent the true amount of AMF in the soil. To test these suggestions, we measured and compared percent root length colonization and soil hyphal length with root ergosterol and soil ergosterol, respectively, for 21 different species of AMF from three families in a greenhouse experiment. Percent root length colonization predicted intra-radical colonization best for Glomaceae and Acaulosporaceae isolates, while soil hyphal length best represented soil ergosterol for Gigasporaceae isolates. The results show that conventional methods for estimating AMF colonization are not universal for all AMF. Caution is advised when drawing inferences for different groups of AMF.     

Elevated carbon dioxide and irrigation effects on water stable aggregates in a Sorghum field: a possible role for arbuscular mycorrhizal fungi

While soil biota and processes are becoming increasingly appreciated as important parameters for consideration in global change studies, the fundamental characteristic of soil structure is a neglected area of research. In a sorghum [Sorghum bicolor (L.) Moench] field experiment in which CO₂[supplied using free‐air CO₂ enrichment (FACE) technology] was crossed factorially with an irrigation treatment, soil aggregate (1–2 mm) water stability increased in response to elevated CO₂. Aggregate water stability was increased by 40% and 20% in response to CO₂, at ample and limited water supply treatments, respectively. Soil hyphal lengths of arbuscular mycorrhizal fungi (AMF) increased strongly (with a threefold increase in the dry treatment) in response to CO₂, and the concentrations of one fraction (easily extractable glomalin, EEG) of the AMF‐produced protein glomalin were also increased. Two fractions of glomalin, and AMF hyphal lengths were all positively correlated with soil aggregate water stability. The present results further support the hypothesis that AMF can become important in global change scenarios. Although in this field study a causal relationship between hyphal length, glomalin and aggregate stability cannot be demonstrated, the present data do suggest that AMF could mediate changes in soil structure under elevated CO₂. This could be of great importance in agricultural systems threatened by erosional soil loss.     

Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils

The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect contributors through their influence on soil aggregation, plant physiology, and plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered glycoprotein produced by AMF hyphae, was detected in tropical soils in concentrations of over 60 mg cm^–3. Along a chronosequence of soils spanning ages from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consistent with the hypothesis that this protein accumulates in soil. Carbon dating of glomalin indicated turnover at time scales of several years to decades, much longer than the turnover of AMF hyphae (which is assumed to be on the order of days to weeks). This suggests that contributions of mycorrhizae to soil carbon storage based on hyphal biomass in soil and roots may be an underestimate. The amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils. Our results thus indicate that microbial (fungal) carbon that is not derived from above- or below-ground litter can make a significant contribution to soil carbon and nitrogen pools and can far exceed the contributions of soil microbial biomass (ranging from 0.08 to 0.2% of total C for the oldest soils).     

The influence of plant species,fertilization and elevated CO2 on soil aggregate stability

We tested the effects of plant species, fertilization and elevated CO₂ on water-stable soil aggregation. Five annual grassland species and a plant community were grown in outdoor mesocosms for 4 years, with and without NPK fertilization, at ambient or elevated atmospheric CO₂ concentrations. Aggregate stability (resistance of aggregates to slaking) in the top 0.15 m of soil differed among plant species. However, the more diverse plant community did not enhance aggregate stability relative to most monocultures. Species differences in aggregate stability were positively correlated with soil active bacterial biomass, but did not correlate with root biomass or fungal length. Plant species did not affect aggregate stability lower in the soil profile (0.15–0.45 m), where soil biological activity is generally decreased. Elevated CO₂ and NPK fertilization altered many of the factors known to influence aggregation, but did not affect water-stable aggregation at either depth, in any of the plant treatments. These results suggest that global changes will alter soil structure primarily due to shifts in vegetation composition.     

球囊霉素相关土壤蛋白的分布及环境功能研究进展

球囊霉素相关土壤蛋白(GRSP)是由丛枝菌根真菌(AMF)在土壤中产生的一种糖蛋白,其在土壤中大量存在,可分为总球囊霉素、易提取球囊霉素、免疫反应性总球囊霉素、免疫反应性易提取球囊霉素.土地利用方式、施肥条件、AMF及宿主类型、外界环境条件等均会影响土壤中GRSP的含量及分布.GRSP能改善土壤团聚体的水稳定性、降低陆地生态系统土壤中CO₂排放、促进土壤中碳贮存、降低土壤中重金属的有效性、减弱重金属的植物毒害.GRSP的提取及定量表征技术仍是限制人们深入了解其在土壤中分布及环境功能的瓶颈.今后有关GRSP的研究应重视以蛋白及其编码该蛋白的基因为基础,阐释GRSP在土壤生态系统中的分子生物学作用及机制,以及GRSP对土壤中有机污染物环境行为的影响.     

Soil mycorrhizal and nematode diversity vary in response to bioenergy crop identity and fertilization

The mandate by the Energy Independence and Security Act of 2007 to increase renewable fuel production in the USA has resulted in extensive research into the sustainability of perennial bioenergy crops such as switchgrass (Panicum virgatum) and miscanthus (Miscanthus× giganteus). Perennial grassland crops have been shown to support greater aboveground biodiversity and ecosystem function than annual crops. However, management considerations, such as what crop to plant or whether to use fertilizer, may alter belowground diversity and ecosystem functioning associated with these grasslands as well. In this study, we compared crop type (switchgrass or miscanthus) and nitrogen fertilization effects on arbuscular mycorrhizal fungal (AMF) and soil nematode abundance, activity, and diversity in a long‐term experiment. We quantified AMF root colonization, AMF extra‐radical hyphal length, soil glomalin concentrations, AMF richness and diversity, plant‐parasitic nematode abundance, and nematode family richness and diversity in each treatment. Mycorrhizal activity and diversity were higher with switchgrass than with miscanthus, leading to higher potential soil carbon contributions via increased hyphal growth and glomalin production. Plant‐parasitic nematode (PPN) abundance was 2.3 ×  higher in miscanthus plots compared to switchgrass, mostly due to increases in dagger nematodes (Xiphinema). The higher PPN abundance in miscanthus may be a consequence of lower AMF in this species, as AMF can provide protection against PPN through a variety of mechanisms. Nitrogen fertilization had minor negative effects on AMF and nematode diversity associated with these crops. Overall, we found that crop type and fertilizer application associated with perennial bioenergy cropping systems can have detectable effects on the diversity and composition of soil communities, which may have important consequences for the ecosystem services provided by these systems.     

不同丛枝菌根真菌侵染对土壤结构的影响

为了定量化比较研究接种丛枝菌根真菌后,根际、菌根际和菌丝际土壤结构的变化,采用四室分根装置,比较中性紫色土接种不同AM真菌后,菌根际、根际、菌丝际和非根际土壤平均重量直径(MWD)、几何平均直径(GMD)和大于0.25mm团聚体总量(R_0.25)的变化。结果表明:接种3个菌种后菌丝际EEG和有机质含量均呈高于菌根际的趋势。菌丝密度和易提取球囊霉素相关蛋白(EEG)与MWD、GMD和R_0.25呈显著正相关,菌根际和菌丝际土壤水稳性R_0.25与菌丝密度显著正相关,相关系数分别为0.777和0.671。接种G. mosseae的菌根际土壤R_0.25值显著高于其它分室土壤,而接种G.etunicatum的菌丝际土壤R_0.25值显著高于其它分室土壤。试验结果在一定程度上说明不同菌种对土壤结构均有不同程度的影响,反映了丛枝菌根真菌生态功能的多样性。     

Relating foliar dehydration tolerance of mycorrhizal Phaseolus vulgaris to soil and root colonization by hyphae

Mycorrhizal symbiosis can modify plant response to drying soil, but little is known about the relative contribution of soil vs. root hyphal colonization to drought resistance of mycorrhizal plants. Foliar dehydration tolerance, characterized as leaf and soil water potential at the end of a lethal drying episode, was measured in bean plants (Phaseolus vulgaris) colonized by Glomus intraradices or by a mix of arbuscular mycorrhizal fungi collected from a semi-arid grassland. Path analysis modeling was used to evaluate how colonization rates and other variables affected these lethal values. Of several plant and soil characteristics tested, variation in dehydration tolerance was best explained by soil hyphal density. Soil hyphal colonization had larger direct and total effects on both lethal leaf water potential and soil water potential than did root hyphal colonization, root density, soil aggregation, soil glomalin concentration, leaf phosphorus concentration or leaf osmotic potential. Plants colonized by the semi-arid mix of mycorrhizal fungi had lower lethal leaf water potential and soil water potential than plants colonized by G. intraradices. Our findings support the assertion that external, soil hyphae may play an important role in mycorrhizal influence on the water relations of host plants.