Josef Kohler

Arbuscular mycorrhizal fungi – short‐term liability but long‐term benefits for soil carbon storage?

The interaction between plants and mycorrhizal fungi represents a major link between atmospheric and soil-contained carbon (C). In order to estimate the fate of atmospheric CO2 under the projected increases in the upcoming century, ranging from an increase of 20% to > 200% compared with current concentrations (Pachauri & Reisinger, 2007), it is crucial to understand how plants and mycorrhizal fungi either buffer or exacerbate atmospheric CO2 rises through their effects on soil C sequestration. Indirect evidence suggests that arbuscular mycorrhizal fungi (AMF) generally stimulate soil carbon pools (Wilson et al., 2009), and experience enhanced growth under elevated CO2 (eCO2) (Antoninka et al., 2011), leading to the assumption that they will buffer atmospheric CO2 increases. However, long-term experiments under eCO2 show both increased carbon storage (Iversen et al., 2012) and accelerated decomposition (negating the effect of the increase of soil carbon inputs; Phillips et al., 2012), leaving the question as to whether soils will buffer against CO2 increases wide open.While there is a dearth of direct empirical evidence regarding the involvement of AMF in soil C storage processes under conditions of global change, there is uncertainty about how component processes leading to soil C storage will be affected. Recently, Cheng et al. (2012) presented a compelling body of evidence to suggest that AMF may diminish rather than enhance soil C pools in the topsoil. Their findings are based on the observation that, in the presence of AMF, fresh above-ground plant litter decomposes faster, in particular at eCO2 and increased nitrogen (N) concentrations (Cheng et al., 2012). This observation suggests that AMF can accelerate decomposition and can even lead to a loss of soil C pools, at least in the short term. However, we feel that other parts of the soil C equation will need to be resolved in order to fully understand how AMF affect long-term soil C-sequestration potential. This is because short-term experiments do not account for potential increases in organic matter (OM) of plant or microbial origin triggered by increased decomposition; long-term (decadal scale) effects of soil biota such as AMF can be qualitatively different from short-term effects; and pulse increases of CO2 and N affect soils in a way that may not represent a system where CO2 and N are at consistently higher concentrations. Soil C sequestration is the net build-up of C in the entire soil profile through accumulation ofOM from plant, fungal (and other microbial) and animal origins. Decomposition of OM is an ongoing process, and snapshot rate assessments must therefore be interpreted with caution. If a particular nutrient (e.g. C or N) is elevated, this may lead to accelerated decomposition, but conclusions about soil C gain or loss can only be drawn if the effect of biomass increases of all biota is also incorporated into the equation (Fig. 1a). This becomes apparent in a simplemodel where AMF are allowed to produce recalcitrant compounds (such as various polysaccharides (K€ogel-Knabner, 2002) and glomalin, in line with experimental observations; Rillig, 2004) that contribute to the future OM fraction (see Fig. 1b). In the short term, an AMFmediated increase in decomposition of labile plant littermay lead to a reduction of soil C. However, the C balance is offset by a longterm gain in recalcitrant compounds (Fig. 1a). Contributions of AMF are likely to be further amplified through physically protecting OM from decomposition by means of soil aggregation (Rillig, 2004) and via a general increase in plant productivity and hence significantly higher litter input (Hoeksema et al., 2010). The principal mechanism by which AMF are proposed to stimulate soil C efflux is through priming of decomposers, which is a commonly observed soil-biotic response to increased (labile) OM deposition (de Graaff et al., 2010). However, whether this stimulation of soil saprobes is a permanent effect will require further study: C pulses and the resulting soil fungal community responses are a well-appreciated side-effect of sudden-onset CO2 exposure designs, which disappearwhenCO2 is gradually increased (Klironomos et al., 2005). Such sudden increases in atmospheric CO2 concentration are unlikely to happen in the near future. By contrast, other parameters will likely change under permanently altered amounts of resources, for instance litter quality. Decomposability of plant litter is known to decrease following plant exposure to eCO2 (Norby et al., 2001), and has the potential to buffer soil C concentrations against effects predicted from shortterm experiments. Thus the magnitude of priming effects through AMF under permanent eCO2 (as opposed to pulse elevation) must be scaled against indirect effects on litter quality to fully appreciate the contribution of AMF to plant-derived soil C concentrations. A way in which short-term experimental studies could control for some of these effects is to include additional treatments where soil and OM (thus controlling for factors such as soil aggregation and quantity and quality of litter) have been preconditioned, to the extent feasible, according to experimental treatments of interest (e.g. ambient vs eCO2; low vs high N; + vs AMF) in a factorial manner. Another highly useful addition might be a treatment where plant roots but not AMF can access plant litter generated under ambient vs eCO2 concentrations (a true nonAMF treatment). Even though these approaches do not resolve all fundamental issues arising from predicting long-term processes with short-term experiments, decomposition in the eCO2 and AMF treatments can be compared between ‘uniform’ and ‘preconditioned’ (according to treatment) plant and soil material. This way

Interactive effects of root endophytes and arbuscular mycorrhizal fungi on an experimental plant community.

Plant-soil microbial interactions have moved into focus as an important mechanism for understanding plant coexistence and composition of communities. Both arbuscular mycorrhizal (AM) as well as other root endophytic fungi co-occur in plant roots, and therefore have the potential to influence relative abundances of plant species in local assemblages. However, no study has experimentally examined how these key root endosymbiont groups might interact and affect plant community composition. Here, using an assemblage of five plant species in mesocosms in a fully factorial experiment, we added an assemblage of AM fungi and/or a mixture of root endophytic fungal isolates, all obtained from the same grassland field site. The results demonstrate that the AM fungi and root endophytes interact to affect plant community composition by changing relative species abundance, and consequently aboveground productivity. Our study highlights the need to explicitly consider interactions of root-inhabiting fungal groups in studies of plant assemblages.

Unraveling the role of hyphal networks from arbuscular mycorrhizal fungi in aggregate stabilization of semiarid soils with different textures and carbonate contents

AimsOur study was intended to elucidate the involvement of three species of arbuscular mycorrhizal fungi (AMF) in the formation and stabilization of aggregates in semiarid soils with different textures and calcium carbonate contents.MethodsWe used a root-hyphae compartment approach to compare the effect of three AMF (Rhizophagus irregularis, Septoglomus deserticola, and Gigaspora gigantea) on the structural stability of the hyphosphere (root-free hyphae) and mycorrhizosphere (hyphaeu2009+u2009root) soil of Olea europaea plants grown in two soils differing in their texture (sandy loam and silty loam) and calcium carbonate content.ResultsOnly the R. irregularis strain significantly increased the percentage of stable aggregates in both types of soil, being the increases higher in the hyphosphere compartment (on average, about 30xa0% compared to non-inoculated soil). In the hyphosphere compartment of both soils, the hyphal length developed by plants inoculated with R. irregularis was 81xa0% greater than that of non-inoculated plants. The effect of the AMF on soil aggregation was mediated by mechanical entanglement of mycorrhizal fungal hyphae but without a contribution of labile carbohydrates.ConclusionThe ability of extraradical hyphae to improve soil structure was independent of the soil texture and content of carbonates.

Arbuscular mycorrhizal fungi negatively affect soil seed bank viability

Abstract Seed banks represent a reservoir of propagules important for understanding plant population dynamics. Seed viability in soil depends on soil abiotic conditions, seed species, and soil biota. Compared to the vast amount of data on plant growth effects, next to nothing is known about how arbuscular mycorrhizal fungi (AMF) could influence viability of seeds in the soil seed bank. To test whether AMF could influence seed bank viability, we conducted three two‐factorial experiments using seeds of three herbaceous plant species (Taraxacum officinale, Dactylis glomerata, and Centaurea nigra) under mesocosm (experiments 1 and 2) and field conditions (experiment 3) and modifying the factor AMF presence (yes and no). To allow only hyphae to grow in and to prevent root penetration, paired root exclusion compartments (RECs) were used in experiments 2 and 3, which were either rotated (interrupted mycelium connection) or kept static (allows mycorrhizal connection). After harvesting, seed viability, soil water content, soil phosphorus availability, soil pH, and hyphal length in RECs were measured. In experiment 1, we used inoculation or not with the AMF Rhizophagus irregularis to establish the mycorrhizal treatment levels. A significant negative effect of mycorrhizal hyphae on viability of seeds was observed in experiments 1 and 3, and a similar trend in experiment 2. All three experiments showed that water content, soil pH, and AMF extraradical hyphal lengths were increased in the presence of AMF, but available P was decreased significantly. Viability of seeds in the soil seed bank correlated negatively with water content, soil pH, and AMF extraradical hyphal lengths and positively with soil P availability. Our results suggest that AMF can have a negative impact on soil seed viability, which is in contrast to the often‐documented positive effects on plant growth. Such effects must now be included in our conceptual models of the AM symbiosis.

Assessing soil ecosystem processes – biodiversity relationships in a nature reserve in Central Europe

Background and aimsPlant diversity – ecosystem processes relationships are essential to our understanding of ecosystem functioning. We aimed at disentangling the nature of such relationships in a mesotrophic grassland that was highly heterogeneous with regards to nutrient availability.MethodsRather than targeting primary productivity, like most existing reports do, we focused our study on belowground ecosystem processes. We tested three, largely mutually exclusive, hypotheses of ecosystem processes relationships: the redundancy hypothesis, the insurance hypothesis and the centrifugal model hypothesis. We sampled the grassland twice within a single plant growing season in a spatially explicit way and assayed the soil for nitrification, urease activity, relative bacterial activity and a microbial community profile based on respiration while we simultaneously assessed plant diversity.ResultsResults supported the centrifugal model. We justify the lack of support for the other two hypotheses on the basis of having conducted an observational study in an environmentally heterogeneous site.ConclusionsThe centrifugal model hypothesis appears to be a very good predictive model for explaining diversity in observational, heterogeneous studies. The specific study represents one of the few observational studies that consider measures of ecosystem functioning other than primary productivity.