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Grain corn (Zea mays L.) production occupies more farmland than any other annual crop in Quebec and is expanding demand from the livestock sector and the emerging bioethanol industry. Corn production requires high nutrient inputs and intensive tillage (IT). Many producers have thus switched to conservation tillage systems like ridge-tillage (RT) to overcome soil compaction and erosion problems that are common in IT systems. However, fertilizer guidelines developed for IT soils are used in RT although RT adoption greatly modifies phosphorus (P) dynamics. Lower fertilizer P requirements are expected because arbuscular mycorrhizal (AM) symbiosis known to enhance plant P nutrition are minimally disrupted under RT, compared to IT systems. A two-year study was conducted on a commercial farm in the Monteregie region (Quebec, Canada) to investigate the effects of P fertilizer rates and soil P-saturation status on corn early growth, nutrition and yield. Surface soil plant-available P was monitored in situ with anionic exchange membranes (P AEM) from seeding to the end of July. The effects of indigenous AM fungi on corn parameters and rhizosphere soil P pools were also investigated in untreated (AMNI) or AM-inhibited fungicide-treated (AMI) soils. Quebec's P fertilizer guidelines underestimated the soil P fertility in studied soils. Adding inorganic P (Pi) did not improve the early corn P nutrition, growth or yield, and had little impact soil PAEM. Variations in PAEM were better explained by climatic variables. AMNI corn had similar early development and high yield, regardless of whether P fertilizer was applied, whereas AMI plants needed P fertilizer to produce optimal yield. AMNI corn had reduced dependence on Pi inputs due to more efficient uptake of soil solution P i in surface and rhizosphere soils during the first 22 days after seeding (DAS), In the rhizosphere, available-Pi pool appeared to be used first to replenish the resin-P pool, but over the longer term, AM symbiosis also enhanced NaHCO3-Po mobilization through a mechanism that remains unclear. Deeper understanding of the mechanisms by which AM fungi alter soil P dynamics will contribute to the development of more sustainable P fertilizer programs for RT systems.

The use of crop growth simulation models such asthose incorporated into Decision Support Systems for Agrotechnology Transfer (DSSAT) are useful tools for assessing the impacts of various management practices on crop productivity. The maize growth model of DSSAT is CERES-maize. To evaluate its predictive capability under western Quebec conditions, data on corn (Zea mays L.) yield and above-ground biomass were collected from a field-scale water table management experiment conducted during the 1996 and 1997 growing seasons at St. Emmanuel, Soulanges County, Quebec. The model was calibrated using various crop growth and development data observed during the 1996 growing season. Cultivar-specific coefficients were determined during the calibration. All measured data on phenology, grain yield and biomass collected in 1997 were used for model validation. / Validation results showed good agreement between predicted and measured yields. In 1996, the Root Mean Square Error (RMSE) was 0.64 and 0.66 Mg/ha for the free drainage plots with 200 kg/ha N (FD200) and 120 kg/ha N (FD120), respectively. In 1997, the RMSE for FD200 and FD120 were 1.07 and 1.23 Mg/ha respectively. However, the model overpredicted the biomass for 1997, and grain number for both FD200 and FD120 treatments in both years. The mean difference (MD) between simulated and observed biomass at maturity was statistically significant at (P < 0.01) for both treatments. Sensitivity analyses showed that the CERES-Maize model was most sensitive to changes in air temperature. The model was also sensitive to soil water and crop genetic parameters.

Earthworms have an important role in the decomposition of organic matter, mineralization of nutrients and physical mixing of soils. Despite a large number of laboratory and greenhouse-level studies investigating how earthworms modify soil properties and promote soil fertility, we lack reliable methods to scale-up and quantify earthworm contributions to nutrient cycling at the agroecosystem level. The objective of this thesis is to determine the influence of earthworm communities on nitrogen (N) transformations in soils and to quantify their contribution to nitrogen flux through soils for soybean and maize cropping systems of Quebec. Laboratory growth rates were used to predict how earthworm growth responded to seasonal fluctuations in soil temperature and moisture. The relationships between earthworm populations, soil-N pools and annual crop production were evaluated in a field experiment. When favourable conditions occurred in 2004 (temperatures <20°C, and rainfall at least once a week), a positive relationship was found between earthworm numbers and the plant available-N, including soil mineral-N, microbial biomass-N and total-N removed in soybean grain. In 2005, soil conditions were unfavourable (temperatures > 20°C and little or no rainfall) to earthworm survival and growth, and no relationship was found between earthworm populations, soil N pools and corn production. These data permitted me to make assumptions about earthworm activity and life histories under field conditions, which were used to estimate N flux through earthworm communities with two models. The models were tested for their sensitivity to varying parameter values within the range reported in the scientific literature. During a crop growing period with favourable climate conditions, a large earthworm population (100 g fresh weight biomass m-2 or greater) is predicted to cycle as much as 120 kg N ha-1. Model predictions were very sensitive to input parameters and did not correspond to the partial N budget calculated at the site. Accurate predictions of N mineralization by earthworms require more species- and site-specific parameter values. Further investigation using stable 15N isotopes as tracers would help us to follow the N transformations and evaluate the N flux mediated by earthworms at the field scale.

Anthropogenic greenhouse gas emissions are the main cause of increasing global temperatures. Climate change will affect crop production in Canada and, in turn agriculture has the potential to mitigate climate change. Analysis of historical climate and corn yield data, and field and greenhouse experiments were carried out in order to study the effect of climate variability and changes on corn yield, the adaptability of cropping systems to climate change conditions, the effect of tillage on soil greenhouse gas emissions (CO2 and N2O) associated with corn and soybean production, and the potential of Nod factors to increase biomass as way to mitigate CO2 emissions. In southwestern Quebec, corn yield variability has been strongly associated to July temperature and May precipitation during the last three decades. Field studies showed that switchgrass and sorghum-sudangrass were best adapted to dry and warm climate events. No-till improved corn yield when spring temperatures were warmer than normal. Soil CO2 fluxes were associated with temperature, while soil N2O fluxes were associated with precipitation. Conventional tillage (CT) had greater CO2 emissions than no-till (NT), particularly after disking in the spring. Both tillage systems had large N2O emission peaks during the wettest part of the season. In corn, peaks of N 2O occurred after nitrogen fertilizer application. NT had greater N 2O emissions than CT in the corn production systems; however, CT had greater N2O fluxes than NT in the soybean production system. Nod factors sprayed on soybean increased photosynthesis and biomass under controlled conditions. In the field, yield was increased by Nod factors under CT, but not under NT, and drought reduced the soybean response to Nod factors. / Les émissions anthropogènes de gaz à effet de serre sont la cause principale de l'augmentation globale des températures. Les changements climatiques vont affecter la production agricole au Canada, et en retour, l'agriculture pourrait limiter les changements climatiques. L'analyse de données historiques du climat et des rendements de maïs, combinés avec des expériences de serre et en champ ont été entreprises pour étudier l'effet de la variabilité et des changements de climat sur le rendement de maïs, l'adaptabilité des systèmes agricoles aux changements climatiques, l'effet du travail du sol sur les émissions de gaz à effet de serre (C02 et N20) associées avec la production de maïs et de soya, et le potentiel des facteurs Nod d'augmenter la biomasse pour limiter les émissions de CO2. L'analyse des données historiques ont démontré qu'au sud-ouest du Québec, la variabilité des rendements de maïs est fortement associée avec les températures de juillet et les précipitations de mai pendant les dernières trois décennies. Les expériences au champ ont démontré que le panic raide, et le sorghum-sudangrass sont les mieux adaptés aux conditions chaudes et sèches. Le semis direct a augmenté les rendements de maïs lorsque les températures printanières étaient plus chaudes que la normale. Les flux de C02 étaient associés avec la température, mais les flux de N20 étaient associés avec les précipitations. Le travail du sol conventionnel (CT) a produit plus d'émissions de CO2 que le semis direct (NT), particulièrement après le disquage au printemps. Les deuxsystèmes ont montré un large pic d'émission de N20 pendant les périodes les pluspluvieuses. Dans le maïs, les pics de N20 ont été détectés après la fertilisation enazote. NT a montré des émissions de N20 plus importantes que CT en productionde maïs, mais CT a montré des flux de N20 plus important que NT en productionde soya. Les facteurs Nod vaporisés sur le soya ont augmenté la photosynthèse etla biomasse sous conditions controllées. Au champ, le rendement a été augmentépar les facteurs Nod sous CT, mais pas sous NT, et la sécheresse a réduit laréponse du soya aux facteurs Nod.

The purpose of this study was to investigate the effect of maize (Zea mays L.) hybrids, plant population, rates of fertilization and irrigation in 1984 and 1985 on soil test levels, soil pH, yield and yield components as well as nutrient uptake. / High rates of fertilization increased soil test P and K but decreased soil test Ca and Mg and soil pH. Irrigation decreased soil test K, Ca and Mg. In both years maximum grain yields (at 15.5% moisture) were obtained when all factors were at high levels. Hybrid Pioneer 3925 and and hybrid CO-OP 2645 produced a maximum yield of 11.9 and 15.2 Mg ha$ sp{-1}$ in 1984 and 1985, respectively. Increasing plant population alone increased grain yield from 9.6 to 10.4 Mg ha$ sp{-1}$ in 1984, bu increases in grain yield with high plant population in 1985 could only be obtained when high rate of nutrients and supplemental irrigation were applied. / Nutrient uptake was usually greater for hybrid 2645 than hybrid 3925 in 1985, and greater for hybrid 3925 than for hybrid 3949 in 1984. Increasing plant population, rate of fertilization as well as available soil moisture generally brought about greater nutrient uptake. Ear leaf nutrient concentration at silking time was influenced by these factors but the differences obtained were not agronomically significant.

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