O'Halloran, I. P., von Bertoldi, A. P. and Peterson, S. 2004. Spatial variability of barley (Hordeum vulgare) and corn (Zea mays L.) yields, yield response to fertilizer N and soil N test levels. Can. J. Plant Sci. 84: 307-316. Identification of management units for the variable application of fertilizer N is a critical component for the implementation of a site-specific N management program. Field studies were conducted to examine the spatial variability of soil nitrate levels, spring barley (Hordeum vulgare) and corn (Zea mays L.) yields and yield responses to fertilizer N applications on two sites in southwestern Ontario, Canada. Soil sampling on a 3 × 10 m grid indicated that soil NO 3 -N test values had a log-normal distribution and varied considerably at both sites with CVs exceeding 57% on the untransformed data. Ranges of spatial correlation varied from 20 to 95 m with 30 to 80% of the total variance of the ln-transformed data existing as either random or unsampled variance, and these parameters were not temporally stable. Although NO 3 -N tended to increase at lower slope positions in two of the 3 site-years, considerable within-slope variability of soil NO 3 -N levels was also observed. Spatial variations in soil N test levels, crop yields and crop yield responses to applied fertilizer N were not strongly related to one another indicating that it would be unlikely that either soil N test level or yield would adequately delineate management zones for the variable application of N fertilizer at these sites. 307-316. La mise en oeuvre d'un programme de gestion du N adapté au site exige l'identification des unités dont on se servira pour modifier la quantité d'engrais azoté appliquée. Les auteurs ont effectué des essais sur le terrain pour vérifier la variabilité spatiale de la concentration de nitrate dans le sol, le rendement de l'orge (Hordeum vulgare L.) et du maïs (Zea mays L.) ainsi que de la réaction du rendement aux applications d'engrais N à deux endroits, dans le sud-ouest de l'Ontario (Canada). Après prélève-ment d'échantillons de sol dans un quadrillage de 3 × 10 m, on constate que la concentration de N-NO 3 relevée lors des essais suit une distribution logarithmique normale et varie considérablement aux deux endroits, le coefficient de variabilité dépassant 57 % pour les données non transformées. La corrélation spatiale varie de 20 à 95 m et 30 à 80 % de la variance globale des données après transformation logarithmique se retrouve sous forme de variance aléatoire ou non échantillonnée. En outre, les paramètres ne sont pas stables dans le temps. Bien que la concentration de N-NO 3 ait tendance à augmenter aux emplacements plus bas dans la pente deux années-site sur trois, on remarque aussi une variabilité considérable de ce facteur dans la pente même. La variation spatiale de la concentration de N dans le sol, le rendement des cultures et la réaction du rendement aux applications d'engrais azoté ne sont pas très corrélés, signe que la concentration de N dans le sol ou le rendement s'avéreraien...

Examining factors that influence seedling establishment is essential for predicting the impacts of climate change on tree species' distributions. Seedlings originating from contrasting climates differentially express functional traits related to water and nutrient uptake and drought resistance that reflect their climate of origin and influence their responses to drought. Soil microbes may improve seedling establishment because they can enhance water and nutrient uptake and drought resistance. However, the relative influence of soil microbes on the expression of these functional traits between seedling families or populations from contrasting climates is unknown. To determine if soil microbes may differentially alter functional traits to enhance water and nutrient uptake and drought resistance between dry and wet families, seeds of loblolly pine families from the driest and wettest ends of its geographic range (dry, wet) were planted in sterilized sand (controls) or in sterilized sand inoculated with a soil microbial community (inoculated). Functional traits related to seedling establishment (germination), water and nutrient uptake and C allocation (root:shoot biomass ratio, root exudate concentration, leaf C:N, leaf N isotope composition (d 15 N)), and drought resistance (turgor loss point, leaf carbon isotope composition (d 13 C)) were measured. Then, plants were exposed to a drought treatment and possible shifts in photosynthetic performance were monitored using chlorophyll fluorescence. Inoculated plants exhibited significantly greater germination than controls regardless of family. The inoculation treatment significantly increased root: shoot biomass ratio in the wet family but not in the dry family, suggesting soil microbes alter functional traits that improve water and nutrient uptake more so in a family originating from a wetter climate than in a family originating from a drier climate. Microbial effects on photosynthetic performance during drought also differed between families, as photosynthetic performance of the dry inoculated group declined fastest. Regardless of treatment, the dry family exhibited a greater root:shoot biomass ratio, root exudate concentration, and leaf d 15 N than the wet family. This indicates that the dry family allocated more resources belowground than the wet and the two family may have used different

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