Do growth yield efficiencies differ between soil microbial communities differing in fungal:bacterial ratios? Reality check and methodological issues

Soil communities dominated by fungi such as those of no-tillage (NT) agroecosystems are often associated with greater soil organic matter (SOM) storage. This has been attributed in part to fungi having a higher growth yield efficiency (GYE) compared to bacteria. That is, for each unit of substrate C utilized, fungi invest a greater proportion into biomass and metabolite production than do bacteria. The assumption of higher fungal efficiency may be unfounded because results from studies in which fungal and bacterial efficiencies have been characterized are equivocal and because few studies have measured microbial GYE directly. In this study, we measured microbial GYE in agricultural soils by following ¹³C-labeled glucose loss, total CO₂-C, and ¹³CO₂-C evolution at 2 h intervals for 20 h in two experiments (differing in N amendment levels) in which the fungal:bacterial biomass ratios (F:B) were manipulated. No differences in efficiency were observed for communities with high versus low F:B in soils with or without added inorganic N. When calculated using ¹³CO₂-C (in contrast to total CO₂-C) evolution, growth yield efficiencies of soils having high and low F:B were 0.69±0.01 and 0.70±0.01, respectively. When soils were amended with N, soils with high and low F:B had growth yield efficiencies of 0.78±0.01 and 0.76±0.01, respectively. Our experiments do not support the widely held assumption that soil fungi have greater growth efficiency than soil bacteria. Thus, claims of greater fungal efficiency may be unsubstantiated and should be evoked cautiously when explaining the mechanisms underlying greater C storage and slower C turnover in fungal-dominated soils.     

Root exudation,organic acids,and element distribution in roots of Norway spruce seedlings treated with aluminum in hydroponics

Seedlings of Norway spruce (Picea abies [L.] Karst.), which had been grown under sterile conditions for three months, were treated for one week in a hydroculture system with either 500 μM AlCl₃ or 750 μM CaCl₂ solutions at pH 4. Organic acids were determined in hot‐water extracts of ground root tissue. Oxalate (3.3—6.6 μmol (g root dry weight)^—1) was most abundant. Malate, citrate, formate, acetate, and lactate concentrations ranged between 1—2 μmol (g root dry weight)^—1. Organic substances and phosphate found in the treatment solutions at the end of the experimental period were considered to be root exudates. Total root exudation within a 2‐day period ranged from 20—40 μmol C (g root weight)^—1. In root exudates, organic acids, and total carbohydrates, total amino acids, and total phenolic substances were quantified. Citrate and malate, although present in hot‐water extracts of root tissue, were not detected in root exudates. Phosphate was released from Ca‐treated plants. In Al treatments, there was indication of Al phosphate precipitation at the root surface. Oxalate and phenolics present in the exudates of Norway spruce seedlings are ligands that can form stable complexes with Al. However, concentrations of these substances in the treatment solutions were at micromolar levels. Their importance for the protection of the sensitive root apex under natural conditions is discussed.     

SCAR–based PCR primers to detect the hybrid pathogen <Emphasis Type="Italic">Phytophthora alni</Emphasis> and its subspecies causing alder disease in Europe

Since the 1990s, a new Phytophthora species hybrid has been jeopardizing the natural population of alders throughout Europe. This new Phytophthora, P. alni, has been suggested as a natural hybrid between two closely related species of Phytophthora. Little is known about the epidemiology of this pathogen, because its direct isolation is not always satisfactory. In this study we developed three pairs of Polymerase Chain Reaction (PCR) primers derived from Sequence Characterized Amplified Regions (SCAR) that allow discrimination among the three subspecies of P. alni: P. alni subsp. alni, P. alni subsp. uniformis and P. alni subsp. multiformis. These molecular tools were successfully used to detect P. alni directly in different substrates such as infested river water and soil, and necrotic alder bark, without the need for any prior baiting or isolation stages. An Internal Amplification Control (IAC) was included to help discriminate against false negative samples due to the potential presence of inhibitory compounds in DNA extracts. These molecular tools should be useful for epidemiological studies on this emerging disease.