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Onions are highly responsive to endomycorrhizae and improved plant growth is among the benefits associated with this symbiosis, particularly in low phosphorus soils. Although this crop is grown extensively in organic soils, few studies have been conducted on these. Onion plants (Allium cepa L. cv. Norstar) were inoculated with Glomus aggregatum Schenck & Smith, Glomus clarum Nicolson & Schenck, Glomus fasciculatum Gerdemann & Trappe, Glomus intraradix Schenck & Smith and Glomus versiforme (Karst.) Berch and grown in the field and under controlled conditions to evaluate their efficiency in newly reclaimed organic soil of low P fertility. Three species were selected to evaluate the effects of introduced endomycorrhizal fungi effects and P fertilization (equivalent to 0, 24 and 48 kg P ha$ sp{-1}$) on the growth and mineral nutrition of onion plants. / Introducing endomycorrhizal fungi in non-sterile soil did not affect the growth and mineral nutrition of onion. Crop maturity was advanced when plants were inoculated, however. Increasing levels of P fertilization did not depress root colonization and onion growth was increased significantly at the highest rate only. These results suggest that higher levels than recommended could be used in this soil. Inoculation in $ gamma$-irradiated (10 kGy, $ sp{60}$Co) soil alleviated excessive Mn absorption by onion plants. Adding P fertilizer depressed growth and root colonization when plants were inoculated with G. clarum and G. intraradix and was related to the low irradiance levels used in this study. G. versiforme appeared to be the most efficient of the introduced species.

Under field conditions, vesicles were the most frequently observed mycorrhizal structures in sugar maple, while greenhouse grown seedlings formed more arbuscules. Seasonal fluctuations of vesicular-arbuscular mycorrhizae existed. Mycorrhizal associations formed within 30 days in the greenhouse. Arbuscules were usually formed from hyphal coils and occasionally from linear hyphae spreading from cell to cell. Degenerating arbuscules were not observed. The addition of basic cations increased the number of vesicles formed and decreased the overall infection rates and seedling growth. The uptake of calcium, magnesium, and nitrogen decreased, and potassium uptake increased as fertilization rates increased. Positive correlations existed between the incidence of arbuscules and plant growth and health and between the incidence of arbuscules and the uptake of calcium, magnesium, nitrogen and phosphorus, and with the uptake ratios and these elements with potassium. This suggests that vesicular-arbuscular mycorrhizae may in some way be regulating ionic balance in these seedlings.

Sugar maple trees were fertilized in June 1988 on an individual tree basis in an attempt to improve tree vigour. Fertilizers included K$ sb2$SO$ sb4$, a phosphate and base cations mix, and a commercial organic mix (Maplegro). No effect of fertilization was found on foliar nutrient concentrations and water status of trees during the 1990 growing season. This is possibly the result of dilution in tree biomass, the addition of non limiting nutrients or the experimental design. A positive effect of fertilization on growth was found for one site and, at another, fertilization was found to impair growth. Thus fertilization requirements of declining sugar maple are site specific. Soil Mg was higher around trees fertilized with Maplegro, and trees fertilized with K$ sb2$SO$ sb4$ had low leaf buffering capacities.

Soybean growth with respect to N fertilizer rates, plant population densities and two cultivars was investigated on three Quebec soils at four sites. Soybean nodulation, growth, grain yields and nutrient uptake at three developmental stages were investigated. Soil nitrate levels after harvest were also studied. / N fertilizer application depressed soybean nodulation consistently, but improved soybean growth where initial soil nitrate levels were low. Grain yield was increased at one site with added N, where soybean growth was stressed by low initial soil nitrate levels (below 17 kg N/ha) and severe summer drought. Soybean N and K uptake were increased with increased N fertilizer but P uptake was not affected. Residual soil nitrate content in the 0-50 cm depth in the fall of the crop year increased linearly and this effect carried over to the following spring. / Plant population had little effect on individual plant nodulation but increased fresh nodule mass per unit area. Plant biomass, grain yield and nutrient uptake were increased with increased population densities. / The cultivar Apache had better nodulation potential and grain yield potential and was better adapted to intensive management practices with high plant populations than the cultivar Maple-Arrow.

Studies were undertaken to determine the effects of fertilizer and liming materials on soil and fine root chemistry and fine root biomass, and on the starch content of larger diameter sugar maple (Acer saccharum Marsh.) roots. Responses in fine root biomass, fine root chemistry, and soil chemistry were observed using root-free ingrowth cores containing an organic based fertilizer (3-4-8 plus dolomitic limestone at a rate of 800 kg/ha), an inorganic fertilizer (0-3-29 plus calcite and dolomite at a rate of 1370 kg/ha), or no fertilizer. The cores were installed in three mature sugar maple stands for one year. Significant differences (p $ leq$ 0.05) in many root and soil chemical properties existed among sites. Greatest treatment response in soil and fine root chemistry occurred at the site with the poorest nutrient status. Fine root biomass decreased due to treatment at one site. No other response in fine root biomass was observed. / Larger diameter roots were sampled at two sites from sugarbush fertilization plots treated with base rich fertilizers. At one site, an acidifying treatment was also applied. Significant differences (p $ leq$ 0.05) in root starch content were observed between the acidifying treatment and the most base enriched treatments several years following fertilization.

Sugar maple foliage litters from fertilized and unfertilized plots on a base-poor site and from a naturally base-rich site were incubated in litterbags of 1 and 3 mm mesh on fertilized and unfertilized plots at the base-poor site. Mass loss of unfertilized litter was slower in fertilized than unfertilized plots, suggesting a negative effect of fertilization on decomposers. Mass loss of fertilized litter was faster than unfertilized litter in the same plots, indicating that changes in litter quality due to fertilization enhanced microbial decomposition. Mass loss was higher in large mesh than in small mesh size bags, suggesting that larger soil fauna played a significant role in litter decomposition. / Potassium appeared to be rapidly leached, whereas Ca and Mg were released at rates more closely related to litter mass loss. Nitrogen was mineralized from N-rich Arboretum litter only; all other litters immobilized N. Release of Ca and Mg was reduced significantly on fertilized plots. Large soil fauna enhanced Ca release, while they delayed N-mineralization in Arboretum litter.

Nitrous oxide (N2O) produced from agricultural activities must be determined if management procedures to reduce emissions are to be established. From 1994 to 1996, N20 emissions were determined under continuous corn and corn-legume rotations in different soils of Quebec. Continuous corn was studied on four sites, two from a long-term experiment, a Ste. Rosalie heavy clay (Humic Gleysol) and a Chicot sandy loam (Gray-Brown Luvisol), at 0, 170,285 and 400 kg N ha-1, and two from a corn rotation study, a Ste. Rosalie clay (Humic Gleysol) and an Ormstown silty clay loam (Humic Gleysol). Treatments in the rotation study included no-till (NT) and conventional tillage (CT), monocropped corn, monocropped soybean-, corn-soybean; and soybean-corn-alfalfa phased rotations. Nitrogen rates of 0, 90, and 180 kg N ha-1 for corn and 0, 20, and 40 kg N ha-1 for monocropped soybean were used, and soybean/alfalfa with no fertilizer N following corn. Rates of N2O emissions were significantly affected by N fertilization, tillage and soil moisture content. Generally, N2O emissions were higher in the NT systems, with corn, and increased with increasing N rates. Increased nitrogen applications led to linear increases in N2O emission over the three years for both Ste. Rosalie (2) and Ormstown soils at a rate of 1.0 to 1.6 percent of added N. The N 2O emission rates were significantly related to soil denitrification rates, water filled pore space, and soil NH4+ and NO3-- concentrations in all three years. A corn system using conventional tillage, legumes in rotation and reduced N fertilizer would decrease N2O emission from agricultural fields.

Nitrogen fertilizer use suffers from low efficiencies and increased residual NO$ sb3$ in the soil. The purpose of this study was to compare N fertilizer efficiency by a comparison of three N sources, three rates of N fertilization, and three application times of N fertilizer. Grain corn yields increased with increases in N rates, and split applications of N were superior to preplant incorporated (PPI) fertilizer. Grain N uptake was associated to grain yield and N uptake. Stover yields increased with increased N and varied with application times of N, but the effects were not as pronounced as with grain yields. Soil NO$ sb3$ generally increased with increases of added N but not with split application of N compared to PPI N. Soil NO$ sb3$ in the spring in control plots was related to grain yield response to added N but not to stover yield response.

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