Frances L. Walley

Landscape-scale variability of N mineralization in forest soils

Abstract Our understanding of the controls on N-cycling and availability in forest soils following disturbance is limited. A comparative study was conducted to examine the spatial distribution of soil N in forest soils and assess the effects of site disturbance. Sampling grids were established within a 120 × 120 m representative area at a native site, and at recently (i.e. 4-year-old) burned and clear-cut sites. A three-dimensional classification of landscape form was used to stratify each landscape into distinct landform elements. The spatial distribution of inorganic-N was not related to landform element, irrespective of site disturbance, indicating an absence of topographic control at the scale studied. However, a narrowing of the NH4+-to-NO3− ratio at the clear-cut site compared to the native site suggests that N-cycling was influenced by site disturbance. Similarly, an increase in the size of the microbial biomass at the clear-cut site, coupled with a widening of the microbial biomass C-to-N ratio, suggest that disturbance altered both the size and composition of the microbial biomass. Potential N and C mineralization, and net nitrification in the forest floor and surface mineral horizons representing two distinct landform complexes were studied in a controlled aerobic 8-wk incubation experiment. Accumulation of NH4+ and NO3− differed markedly between sites although the effects of topographic position were generally non-significant. Inorganic-N accumulated principally as NH4+ in soils from the native site due to an extended lag in nitrification. In contrast, NH4+ accumulation in soils from the recently disturbed sites remained limited, whereas NO3− accumulation predominated. Thus, although topography did not markedly influence N distribution at the scale studied, site disturbance had a direct effect on N-cycling processes in these forest soils.

Failure to decontaminate Glomus clarum NT4 spores is due to spore wall-associated bacteria

Abstract Exposure of spores of Glomus clarum NT4 to solutions of chloramine-T (2.5–10% w/v) for 10–120 min failed to fully decontaminate all spores. Scanning electron microscopy did not show the presence of contaminants on treated spores, but transmission electron microscopy revealed bacterial cells embedded within the outer spore wall layer. Bacteria that remained protected within the spore walls were detected only when the spores were placed on appropriate media. Nutrient agar and tryptic soy agar supported relatively high levels of contaminant growth and were regarded as good media for assessing contamination, whereas the detection of contaminant growth on water agar required prolonged incubation. Contamination and germination of G. clarum NT4 spores following decontamination treatments were dependent on spore age. Generally, lower concentrations of chloramine-T and shorter incubation periods were required to reduce contamination of freshly harvested spores than of mature spores. Exposure to 10% chloramine-T for 120 min was required to reduce the levels of contamination of mature spores to ≤10%. Unfortunately, spore germination was compromised by rigorous decontamination treatments, thus the success of any decontamination procedure should be evaluated prior to its routine use. Moreover, if the interpretation of experimental results rests on the assumption of true surface sterility of VAMF spores, we suggest that the axenic condition of spores be confirmed prior to experimentation on a medium that encourages contaminant growth.

The influence of tillage and crop rotation on nitrogen fixation in lentil and pea

Direct seeding into standing stubble and crop diversification are two practices that are becoming widely adopted in western Canada. This study was conducted to determine: i) the influence of zero and conventional tillage on N-fixation in lentil (Lens culinaris Medikus) and pea (Pisum sativum L.), and ii) the effect of cropping history on N-fixation in lentil. Data were obtained from a crop rotation experiment being conducted on a silty clay soil in east-central Saskatchewan, which included six cereal-oilseed-cereal-pulse rotations, each managed using zero and conventional tillage practices. The finding showed that N-fixation was 10% higher by lentil and 31% higher by pea when grown using zero tillage as compared to conventional tillage practices. On average, lentil grown in highly diversified crop rotations fixed 12% more nitrogen than when grown in less diversified crop rotations. Key words: Nitrogen fixation, lentil (Lens culinaris Medikus), pea (Pisum sativum L.), zero tillage, conventional tillage

Estimating the viability of vesicular-arbuscular mycorrhizae fungal spores using tetrazolium salts as vital stains

The tetrazolium salts 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide and 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride were evaluated as vital stains for est...

Isotopic fractionation during N2 fixation by chickpea

Abstract Desi and Kabuli chickpea (Cicer arietinum L) inoculated with Mesorhizobium ciceri strain CP39 or a mixture of strains 27A2, 27A7 and 27A9 were grown in N-free nutrient solution in a growth chamber. At the flowering stage shoot N was significantly higher for the Kabuli plants inoculated with strain CP39 than those inoculated with the multi-strain but did not differ between the inoculant strains in the Desi chickpea. For both chickpea types, seed N concentrations were not different between the inoculant strains. The total N of the shoots at flowering and seed harvested at physiological maturity for each host-Mesorhizobium combination was depleted in 15N relative to the atmospheric N2 (0‰). The 15N natural abundance (δ15N) in the shoots at flowering, ranged from −1.3067 to −2.8225‰, and those for the seed ranged from −0.5475 to −0.9062‰. Consequently, measures of isotopic fractionation (β) during N2 fixation ranged from 1.0013 to 1.0028 for the shoots at flowering and 1.0005–1.0009 for the seed harvested at physiological maturity. The isotopic effect was not influenced by the infecting rhizobial strain at the flowering stage in the Desi chickpea, but the effect was dependent on the rhizobial strain in the Kabuli chickpea. In contrast, whereas the β values for the harvested Desi seed differed between strain CP39 and the mixed strains (27A2, 27A7 and 27A9), those for the harvested Kabuli seed were not different.