Richard Jeannotte

Nutrient Input and Carbon and Microbial Dynamics in an Ombrotrophic Bog

Slow rates of plant production and decomposition in ombrotrophic bogs are believed to be partially the result of low nutrient availability. To test the effect of nutrient availability on decomposition, carbon dioxide (CO2) flux dynamics, microbial biomass, and nutrients, we added nitrogen (N) with phosphorus (P) and potassium (K), to prevent limitation of the latter 2 nutrients, over 2 growing seasons to plots at Mer Bleue peatland, Ontario, Canada. After the first growing season, increasing N fertilization (with constant P and K) decreased in vitro CO2 production potential and increased microbial biomass measured with a chloroform fumigation-extraction technique in the upper peat profile, while by the end of the second season, CO2 production potential was increased in response to N plus PK treatment, presumably due to more easily decomposable newly formed plant material. In situ CO2 fluxes measured using chamber-techniques over the second year corroborated this presumption, with greater photosynthetic CO2 uptake and ecosystem respiration (ER) during high N plus PK treatments. The more efficient microbial community, with slower CO2 production potential and larger biomass, after the first year was characterized by larger fungal biomass measured with signature phospholipid fatty acids. The majority of N was likely quickly sequestered by the vegetation and transferred to dissolved organic forms and microbial biomass in the upper parts of the peat profile, while additional P relative to controls was distributed throughout the profile, implying that the vegetation at the site was N limited. However, in situ CO2 flux data suggested the possibility of P or NPK limitation. We hypothesize that nutrient deposition may lead to enhanced C uptake by altering the microbial community and decomposition, however this pattern disappears through subsequent changes in the vegetation and production of more readily decomposable plant tissues.

Negative feedback on a perennial crop: Fusarium crown and root rot of asparagus is related to changes in soil microbial community structure

The dynamic equilibrium of an ecosystem is driven by mutual feedback interactions between plants and soil microorganisms. Asparagus exerts a particularly strong influence on its soil environment through abundant production of persistent phenolic acids, which impact selectively soil microorganisms and may be involved in Fusarium crown and root rot (FCRR) of asparagus. In a survey of 50 asparagus plantations of the province of Québec, we found that FCRR was associated with a profound cultivar-specific, reorganization of the soil microbial community, as revealed by phospholipid fatty acid (PLFA) profiling. According to PLFA indicators, microbial biodiversity as well as bacterial and fungal abundance dropped sharply with the onset of FCRR in fields planted with the cultivar Guelph Millenium. This drop was followed by a similar drop in the arbuscular mycorrhizal population. Biodiversity and microbial population size then increased to finally reach a new equilibrium. Discriminant analysis of PLFA profiles obtained from soil samples also indicated a shift in soil microbial community structure associated with FCRR development in fields planted with the cultivar Jersey Giant. Different soil biological conditions, as indicated by microbial biomass C and N and soil enzyme activities, were associated with different cultivars. Preceding crop, manure application, geographical location and tillage depth also influenced the structure of soil microbial communities in asparagus plantations, as determined by PLFA profiling. If higher FCRR incidence is a consequence of the soil microbial community reorganization, means to reduce FCRR incidence in asparagus plantations may be found among practices such as soil organic fertilization, soil tillage and intercropping strategies that would dilute the negative influence of asparagus on the soil microbial community. Finally, FCRR outbreaks were generally promoted by a previous crop of maize. It seems that maize and asparagus host a F. proliferatum teleomorph (Gibberella fujikoroi) of the same mating type.

Factors Associated with Fusarium Crown and Root Rot of Asparagus Outbreaks in Quebec

ABSTRACT The Fusarium spp. causing Fusarium crown and root rot (FCRR) are ubiquitous and abundant in soils, but in contrast, disease expression is localized and sporadic. Previous studies have related FCRR infection to phenolic acids released by asparagus, to the repression of Mn-reducers in soil, and to various soil physicochemical conditions. Fifty commercial asparagus plantations were surveyed using an exploratory approach in order to pinpoint the ecological conditions associated with FCRR development. Twenty-eight variables were used to describe the soil environments of the asparagus crops as well as the influence of crop management practices used locally. The data set was analyzed both as a whole and parsed by main cultivars (Jersey Giant and Guelph Millenium). Both field conditions and percentage of field area affected by FCRR varied widely between asparagus plantations. Planting depth was positively correlated with percentage of field area affected by FCRR and, hence, deep planting may favor FCRR infection. Plantation age was positively correlated with percentage of field area affected by FCRR, while soil available Mn was inversely correlated. Most importantly, soil Mn availability decreased with increasing plantation age, supporting the hypothesis of an asparagusmediated negative impact on Mn-reducing bacteria and of the involvement of reduced Mn availability in FCRR development. Improving the availability of Mn could provide a solution to the problem of FCRR in asparagus plantations.

Patterns of Fusarium community structure and abundance in relation to spatial, abiotic and biotic factors in soil

Members of the Fusarium genus are important components of many plant-soil systems worldwide and are responsible for many crop diseases. Knowledge of the relative influence of biotic and abiotic factors on this genus is therefore of broad economic and ecological importance. In order to address this issue, we examined Fusarium communities in soils nearby apparently healthy and symptomatic asparagus plants in 50 fields scattered in four agricultural regions of Québec, Canada. Fusarium community structure and abundance were assessed using genus-specific PCR-denaturing gradient gel electrophoresis and CFU counts, respectively. Multivariate statistical analyses were used to detect community patterns related to spatial, abiotic and biotic factors. Results suggested that Fusarium community structure (i.e. the presence and absence of the different Fusarium sequence variants in the samples) in soil is mainly related to biotic factors (arbuscular mycorrhizal fungi and bacterial community structure), whereas Fusarium abundance is more closely related to abiotic factors (mainly clay, organic matter, NH(4), Na and Cu). Some degree of influence of spatial patterns was also observed on both Fusarium community structure and abundance with, for instance, a large regional variation in Fusarium community structure. However, Fusarium community structure was not directly related to the disease status of nearby asparagus plants.

Comparison of solvent mixtures for pressurized solvent extraction of soil fatty acid biomarkers.

The extraction and transesterification of soil lipids into fatty acid methyl esters (FAMEs) is a useful technique for studying soil microbial communities. The objective of this study was to find the best solvent mixture to extract soil lipids with a pressurized solvent extractor system. Four solvent mixtures were selected for testing: chloroform:methanol:phosphate buffer (1:2:0.8, v/v/v), chloroform:methanol (1:2, v/v), hexane:2-propanol (3:2, v/v) and acetone. Soils were from agricultural fields and had a wide range of clay, organic matter and microbial biomass contents. Total lipid fatty acid methyl esters (TL-FAMEs) were the extractable soil lipids identified and quantified with gas chromatography and flame ionization detection. Concentrations of TL-FAMEs ranged from 57.3 to 542.2 nmole g(-1) soil (dry weight basis). The highest concentrations of TL-FAMEs were extracted with chloroform:methanol:buffer or chloroform:methanol mixtures than with the hexane:2-propanol or acetone solvents. The concentrations of TL-FAMEs in chemical groups, including saturated, branched, mono- and poly-unsaturated and hydroxy fatty acids were assessed, and biological groups (soil bacteria, mycorrhizal fungi, saprophytic fungi and higher plants) was distinguished. The extraction efficiency for the chemical and biological groups followed the general trend of: chloroform:methanol:buffer> or =chloroform:methanol>hexane:2-propanol=acetone. Discriminant analysis revealed differences in TL-FAME profiles based on the solvent mixture and the soil type. Although solvent mixtures containing chloroform and methanol were the most efficient for extracting lipids from the agricultural soils in this study, soil properties and the lipid groups to be studied should be considered when selecting a solvent mixture. According to our knowledge, this is the first report of soil lipid extraction with hexane:2-propanol or acetone in a pressurized solvent extraction system.

Lipid profiles in wheat cultivars resistant and susceptible to tan spot and the effect of disease on the profiles.

Lipid profiles in wheat leaves and the effects of tan spot on the profiles were quantified by mass spectrometry. Inoculation with Pyrenophora tritici-repentis significantly reduced the amount of leaf lipids, including the major plastidic lipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), which together accounted for 89% of the mass spectral signal of detected lipids in wheat leaves. Levels of these lipids in susceptible cultivars dropped much more quickly during infection than those in resistant cultivars. Furthermore, cultivars resistant or susceptible to tan spot displayed different lipid profiles; leaves of resistant cultivars had more MGDG and DGDG than susceptible ones, even in noninoculated plants. Lipid compositional data from leaves of 20 noninoculated winter wheat cultivars were regressed against an index of disease susceptibility and fitted with a linear model. This analysis demonstrated a significant relationship between resistance and levels of plastidic galactolipids and indicated that cultivars with high resistance to tan spot uniformly had more MGDG and DGDG than cultivars with high susceptibility. These findings suggest that lipid composition of wheat leaves may be a determining factor in the resistance response of cultivars to tan spot.