Robert L. Bradley

A trait-based framework to understand life history of mycorrhizal fungi

Despite the growing appreciation for the functional diversity of arbuscular mycorrhizal (AM) fungi, our understanding of the causes and consequences of this diversity is still poor. In this opinion article, we review published data on AM fungal functional traits and attempt to identify major axes of life history variation. We propose that a life history classification system based on the grouping of functional traits, such as Grimes C-S-R (competitor, stress tolerator, ruderal) framework, can help to explain life history diversification in AM fungi, successional dynamics, and the spatial structure of AM fungal assemblages. Using a common life history classification framework for both plants and AM fungi could also help in predicting probable species associations in natural communities and increase our fundamental understanding of the interaction between land plants and AM fungi.

Changes to mineral N cycling and microbial communities in black spruce humus after additions of (NH4)2SO4 and condensed tannins extracted from Kalmia angustifolia and balsam fir

Abstract Mechanisms responsible for conifer growth “check” on cutovers invaded by Kalmia angustifolia L. in central Newfoundland were studied by examining effects of added Kalmia and balsam fir ( Abies balsamea (L.) Mill) condensed tannins on black spruce humus N dynamics and microbial community development over 10 weeks using microcosms. Because of the silvicultural implications, interactions of tannins with fertiliser N, applied as (NH 4 ) 2 SO 4 , were also studied. Both tannin types significantly reduced NH + 4 –N leaching, whereas only Kalmia tannins reduced NO − 3 –N leaching, and then only from non-fertilised humus. Tannins did not significantly affect mineral N leaching from fertilised humus. Fertiliser N increased gross N mineralisation rates such that the increase in actively cycling N was many times greater than the increase in N leaching due to fertiliser N addition. Gross N mineralisation rates were higher in fertilised humus amended with tannins, suggesting possible toxicity of tannins on microbes at high N concentrations. Recovery of added tannins in leachate and in post-treatment humus samples was low. Net anaerobic N mineralisation decreased with tannin additions but increased with fertiliser N additions. There were few significant treatment effects on microbial properties derived from humus respirometry. Microbial biomass and basal respiration rates of all treatments declined by 30 and 37% respectively, indicating a general loss of available C during the experiment. The ratio of C mic -to-N mineralised as well as the nutrient deficiency index was lowest in humus amended with Kalmia tannins, suggesting higher microbial N deficiency in this treatment. Utilisation rates of various C sources by microbial communities showed distinctive patterns between pre-treatment and post-treatment humus samples, but did not reveal distinctive patterns among different treatments. Overall, results suggested that (1) condensed tannins decreased mineral N cycling abiotically by binding to and sequestering organic N sources, (2) fertiliser N counteracted negative effects of condensed tannins on humus N cycling, (3) microbial communities were N limited, which prevented abundant leaching of fertiliser N while maintaining fertiliser N in an active pool, and (4) the physiology and functional diversity of soil heterotrophic communities were controlled by C availability but were unaffected by tannin or fertiliser N additions. Further work is needed to determine the ecological importance of Kalmia tannins, relative to tannins produced by other plants, in reducing humus N availability on spruce cutovers.

Growth of paper birch (Betula papyrifera) seedlings increases soil available C and microbial acquisition of soil-nutrients

Abstract We compared the flux of energy and nutrients in a mineral forest soil in which paper birch ( Betula papyrifera ) seedlings had grown with soils that had been exposed to one of five other tree species over a 22 week growing period. Soil basal respiration rate, metabolic quotient, soil available C ( AC ), and the affinity of soil microoganisms for substrate-C left in the soil after harvest all increased significantly, in soils treated with birch root systems. Concentrations of AC in birch-treated soils related to the energy-only limited microbial biomass (MB E ), but not the nutritionally limited microbial biomass (MB N ). Amounts of rhizosphere activity, described as root-supported MB E per unit root and per unit fine root, were one order of magnitude higher in the birch rhizosphere. Plant uptake of soil-N during the growing period was high while the soil mineral-N pool was low in birch experimental units relative to those of other species, suggesting that birch competed well against soil microorganisms for available mineral-N. Anaerobic N mineralization rates were significantly higher while the MB N -to-MB E ratio, which describes the degree of nutritional limitation of the microbial biomass, was significantly lower in birch-treated soils. Significant negative correlations were found between the MB N -to-MB E ratio and both AC and MB E . These results suggest that high amounts of root labile C compounds in conjunction with rapid mineral-N uptake by birch roots can stimulate microbial communities to acquire nutrients from the native soil.

Using ecological network theory to evaluate the causes and consequences of arbuscular mycorrhizal community structure

Arbuscular mycorrhizal fungi (AMF) are widespread and their symbiotic interactions involve the majority of terrestrial plant species (Wang & Qiu, 2006). These obligate biotrophs generally improve the nutrition and vigor of the host, thereby affecting individual plant traits (van der Heijden et al., 1998) as well as the composition and functioning of entire plant communities (Moora & Zobel, 1996; Hartnett & Wilson, 1999; Bever, 2002). Studies on individual plant traits are useful in determining fitness benefits to the plant (e.g. increased growth, resistance to pathogens, etc.), whereas studies on community-level interactions can potentially explain constraints on host–symbiont web architecture (e.g. Bluthgen et al., 2007). Community-level studies have been limited, however, to small subsets of natural plant communities, because processing and identifying AMF species associated with numerous plant root systems have proven costly and painstaking. Recent advances in next-generation sequencing technologies (Margulies et al., 2005) have removed this hurdle and improved the detection of rare AMF species (Opik et al., 2009). This increased capacity in describing whole plant–AMF networks provides an opportunity to identify the causes, and assess the functional consequences, of symbiotic network architectures (i.e. topology). Network theory, originally developed to describe the flow of information within computational and social networks (Emerson, 1972), has more recently been applied to ecological studies of various mutualistic systems (Jordano et al., 2003; Olesen et al., 2007; Joppa et al., 2010). The major advantage of an ecological network approach is that topological metrics can be quantified for any given network involving two or more groups of interacting organisms (e.g. plants and pollinators, food webs, etc.). For example, ecological networks may be described in terms of their ‘nestedness’. High nestedness occurs when specialist species interact with a subset of partners with which generalist species also interact. For example, a specialist pollinator would tend to specialize on a generalist plant, and vice versa (Fig. 1a). This absence of reciprocal specialization was shown to be a pervasive feature of pollination networks (Bascompte et al., 2003; Joppa et al., 2009, 2010) that potentially favors diversity and stability of ecological communities (Memmott et al., 2004; Burgos et al., 2007; Bastolla et al., 2009; Thébault & Fontaine, 2010). Ecological networks can also be described according to their ‘modularity’, that is, the tendency of species to be grouped into modules in which interactions are more frequent than with the rest of the community (Fig. 1b). Thompson (2005) suggested that communities may assemble into distinct modules based on the functional complementarity of their traits, and this may offer some insight into coevolutionary dynamics between symbiotic species (Guimarães et al., 2007). In this Letter, we argue that an ecological network approach could provide a framework by which to characterize and compare plant–AMF communities from different environments or at different successional stages. This, in turn, could improve our understanding of mechanisms structuring mycorrhizal communities and bring mycorrhizal science to a more predictive level (Johnson et al., 2006). In a recent study, Opik et al. (2009) used pyrosequencing to describe AMF communities associated with 10 plant species in a forest understory community. Here, we have used their published data set to demonstrate the applicability of ecological network theory to characterize plant–AMF communities. Our exercise revealed that this particular plant–AMF network was both highly nested and modular. We discuss possible reasons and implications for such topological features, Forum

A kinetic parameter describing soil available carbon and its relationship to rate increase in C mineralization

Abstract No standard rapid method reliably assesses the potential energy supply to soil microorganisms. We found that the metabolic quotient q CO 2 failed to adequately reflect repeated weekly glucose-C additions corresponding to 25 and 50% of soil respiratory CO 2 C loss. We addressed the concept of an available C pool in soil using a kinetic parameter ( AC ) derived from a modified Michaelis-Menten equation. The new parameter AC correlates significantly and positively to soil microbial biomass, to the Michaelis constant and negatively to the initial potential rate of increase in C mineralization ( R 0 ). A negative correlation between the AC -to-biomass ratio, which we have called the specific C reserve (SCR), and R 0 was found to be highly significant ( P q CO 2 of soil microorganisms did not correlate significantly ( P > 0.05) with any of the indices discussed. Based on its theoretical soundness, its correlative value and its applicability to realistic soil-plant systems, we propose that AC becomes a standard measure of the available C pool in soil.

Relationships between Stand Composition and Ectomycorrhizal Community Structure in Boreal Mixed-Wood Forests

We investigated the community structure of ectomycorrhizal fungi under varying overstory tree compositions in the southern mixed-wood boreal forest of Quebec. Sampling took place at two locations of differing postfire ages and nine 100-m2 plots were sampled per location. The dominant overstory tree species in the plots were trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.) or white spruce [Picea glauca (Moench) Voss], and balsam fir [Abies balsamea (L.) Mill.]. Mycorrhizae were analyzed using morphological as well as molecular methods, employing fungal-specific primers to amplify ribosomal DNA for subsequent cloning and sequencing. A total of 1800 mycorrhizal root tips collected from the 18 plots were morphologically classified into 26 morphotypes, with Cenococcum geophilum dominating (36% of root tips). A second set of root tips, selected from the same 18 samples on which the morphological analysis was based, were analyzed using molecular methods. From this analysis, 576 cloned polymerase chain reaction products were screened by restriction fragment length polymorphism analysis and a total of 207 unique types were found. No one type dominated the system and 159 occurred only once. Sequence analysis of the types that occurred more than once revealed that Piloderma sp., Russula sp., Cortinarius sp., and Lactarius sp. were the most common mycorrhizae. The ectomycorrhizal fungal community structure revealed by the rDNA analysis differed from that observed using morphological methods. Canonical correspondence analyses of the sequenced restriction types and % overstory composition indicate that the distributions of ectomycorrhizal fungi are influenced by the relative proportions of host tree species. The distinct fungal assemblages found in the different plots supported by the different combinations of host tree species provides further support for the need to conserve stand diversity in the southern boreal forest.

The phenology of fine root growth in a maple-dominated ecosystem: relationships with some soil properties

A two-year study was undertaken in a maple-dominated watershed of southern Québec, Canada, to examine relationships between trends in fine root growth, stem diameter growth, soil moisture, soil temperature, mineralized-N and extractable-P. Until September, soil temperature was consistently higher in 1995 than in 1994. Apart from the first sampling in mid-May, soil moisture was higher in 1994 than in 1995. In 1994, most fine roots were produced before leaf expansion, whereas in 1995, fine root production peaked in July. Annual fine root production was estimated to be 2.7 times higher in 1994 than in 1995. Stem growth was strongly associated with the seasonal and annual variation in soil temperature. Root and diameter growth were asynchronous in 1994 but not in 1995. Fine root production was associated with two groups of variables: a soil fertility (mineralized-N and extractable-P) group and a physical soil environment (moisture and temperature) group. Our results are consistent with the negative effect of high soil-N fertility on fine root production but are inconclusive as to the positive effect of high soil-P fertility. Soil conditions that are detrimental to root growth such as high N availability and anaerobiosis could modify the normal dynamics of fine root growth.

How does a tree species influence litter decomposition? Separating the relative contribution of litter quality, litter mixing, and forest floor conditions

Litter quality is often considered the main driver of decomposition rate. The objective of this study was to investigate the relative contribution of two other tree-driven mechanisms, litter mixing...

Sequestration of soil nitrogen as tannin-protein complexes may improve the competitive ability of sheep laurel (Kalmia angustifolia) relative to black spruce (Picea mariana).

The role of litter tannins in controlling soil nitrogen (N) cycling may explain the competitive ability of Kalmia relative to black spruce (Picea mariana), although this has not been demonstrated experimentally. Here, the protein-precipitation capacities of purified tannins and leaf extracts from Kalmia and black spruce were compared. The resistance to degradation of tannin-protein precipitates from both species were compared by monitoring carbon (C) and N dynamics in humus amended with protein, purified tannins or protein-tannin precipitates. The purity of the precipitates was verified using solid-state (13)C nuclear magnetic resonance (NMR) spectra. The ability of mycorrhizal fungi associated with both species to grow on media amended with tannin-protein complexes as the principal N source was also compared. The protein precipitation capacity of Kalmia tannins was superior to those of black spruce. Humus amended with protein increased both mineral and microbial N, whereas humus amended with tannin-protein precipitates increased dissolved organic N. Mycorrhizal fungi associated with Kalmia showed better growth than those associated with black spruce when N was provided as tannin-protein precipitates. These data suggest that Kalmia litter increases the amount of soil N sequestered as tannin-protein complexes, which may improve the competitive ability of Kalmia relative to black spruce by favouring N uptake by mycorrhizas associated with the former.

Prediction of in situ root decomposition rates in an interspecific context from chemical and morphological traits.

BACKGROUND AND AIMS We quantitatively relate in situ root decomposition rates of a wide range of trees and herbs used in agroforestry to root chemical and morphological traits in order to better describe carbon fluxes from roots to the soil carbon pool across a diverse group of plant species. METHODS In situ root decomposition rates were measured over an entire year by an intact core method on ten tree and seven herb species typical of agroforestry systems and were quantified using decay constants (k values) from Olsons single exponential model. Decay constants were related to root chemical (total carbon, nitrogen, soluble carbon, cellulose, hemicellulose, lignin) and morphological (specific root length, specific root length) traits. Traits were measured for both absorbing and non-absorbing roots. KEY RESULTS From 61 to 77 % of the variation in the different root traits and 63 % of that in root decomposition rates was interspecific. N was positively correlated, but total carbon and lignin were negatively correlated with k values. Initial root traits accounted for 75 % of the variation in interspecific decomposition rates using partial least squares regressions; partial slopes attributed to each trait were consistent with functional ecology expectations. CONCLUSIONS Easily measured initial root traits can be used to predict rates of root decomposition in soils in an interspecific context.