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Earthworms are well known to increase decomposition of organic matter and release of plant available nutrients. They can also increase CO 2 and N2O fluxes from the soil by stimulating respiration, denitrification, and nitrification caused by soil microorganisms. The objective of this thesis was to examine the influence of different earthworm species and population numbers on CO2 and N2O fluxes from a corn agroecosystem. In the field study, earthworm treatments had a significant effect on CO2 fluxes, but there was no difference between CO 2 fluxes from the two species (Lumbricus terrestris L., Aporrectodea caliginosa Savigny) or from the two population levels (1x and 2x the naturally-occuring population). Also, the earthworm treatments had no significant effect on N2O fluxes. Since all treatments contained mixed species and similar population levels at the end of the study, it is likely that CO2 and N2O fluxes in the field were affected more by soil temperature and moisture fluctuations than by the earthworm treatments. The study was repeated in laboratory microcosms under environmental control. Again, earthworm treatments had a significant effect on CO2 fluxes, but not on N2O fluxes. Interestingly, the N 2O fluxes from microcosms containing L. terrestris came solely from denitrification, while the N2O fluxes from A. caliginosa microcosms were produced mostly by nitrification. It is not known why these species stimulate different groups of microorganisms that can produce N2O, and this remains to be investigated.

Concerns for environmental quality stimulate the development of various management strategies that mitigate nutrient losses to the environment. / Field experiments were conducted at St. Emmanuel, Quebec, from 1998 to 2000 to investigate the combined effects of water table management and N fertilizer application rates on corn yield, concentrations of NO3- -N in the soil profile and tile subsurface drainage water, denitrification and N2O production rates, and N2O:N2O+N 2 production ratios in the soil profile. There were two water table treatments: free drainage (FD) with open drains at a 1.0 m depth from the soil surface and subirrigation (SI) with a water table depth of 0.6 m below the soil surface, and two N fertilization rates: 120 kg N ha-1 (N120) and 200 kg N ha-1 (N 200) arranged in a split-plot design. Compared to FD, subirrigation reduced NO3--N concentration in the soil by up to 50% and in drainage water by 55 to 73%. Water table had little effect on corn yield during the study period. Greater denitrification rates under SI were not accompanied with greater N2O emissions as ratios of N2O:N2O+N2 were lower under SI than in FD plots. Denitrification rate, N2O emissions, and their ratios were unaffected by N rate. / A second field experiment was initiated from 1999 to 2000 to assess impacts of tillage systems on NO3--N, denitrification, N2O, and ratios of denitrification end-products (N2O:N 2O+N2). The experiment was conducted on long-term momocropped corn experimental plots under conventional tillage (CT), reduced tillage (RT), and no-till (NT), located at the Macdonald Research Farm, McGill University. Soil NO3--N concentrations tended to be lower under RT than under NT or CT. Denitrification and N2O were similar among tillage systems. / Approximately 50% of soil denitrification activity was measured within the 0.15--0.45 m soil layer. Consequently, we propose that sampling the 0--0.15 m soil layer alone, as is usually done, may not give an accurate picture of soil denitrification activity. Dissolved organic carbon concentrations remained high in all soil depths sampled, but was not affected by water table, N rate or tillage system.

The impact of tillage and residue management on soil microorganisms was studied over the maize (Zea mays L.) growing season in southwestern Quebec. Tillage and residue treatments were imposed on a sandy loam to loamy sand soil in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (-R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial nitrogen (SMB-N) and phospholipid fatty acid (PLFA) concentrations were measured four times, at two depths (0--10 cm, 10--20 cm), over the 2001 growing season. Sample periods were: May 7th (pre planting), June 25 th, July 16th, and September 29th (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed no seasonal change (160 mug C g-1 soil); SMB-N was responsive to mineral nitrogen fertilizer; and PLFA data showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling period, and depth, than treatments. Of the two treatments, the effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (6.1% and 96%) in +R plots compared to -R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical, chemical conditions, and nutrient supply.

Due to recent research suggesting that water table management (WTM) can significantly reduce nitrate (NO3") loads in agricultural drainage, a study was carried out in 2001 and 2002 in Coteau-du-Lac, 60 km west of Montreal, to investigate the effect of water table management on the migration of phosphorus (P) via tile drainage and surface runoff. The second main objective was to study the influence of WTM on grain corn yields. Two drainage treatments were compared: conventional free drainage and WTM (combined controlled drainage and subirrigation) with a design water table depth at 0.6 m below the ground surface. Tile drainage and surface runoff were monitored and sampled automatically. Increased outflow volumes and concentrations - and therefore increased P loads - were measured in drainage water from plots under WTM. Plots under WTM also generally exhibited higher P loads in surface runoff. Higher P concentrations in surface runoff from plots under WTM were observed in surface runoff, especially during winter. Phosphorus loads from combined tile drainage and surface runoff were low compared with literature data (<0.4 kg/ha/year). However, the mean P concentrations in tile drainage were above Quebec's surface water quality standard of 0.03 mg total P/L during both growing seasons in plots under WTM, but not in plots with conventional free drainage. Mean P concentrations in surface runoff water routinely exceeded the criteria, except in plots with conventional free drainage in winter 2002. Therefore, P from tile drainage and surface runoff could contribute to the eutrophication of surface water. Based on these results, WTM increases P loads from the field, both in tile drainage and surface runoff. However, the well water used for subirrigation was found to contain P concentrations above Quebec's surface water quality standard, and this could partly explain the higher P concentrations found in water from plots under WTM. Water table management increased grain corn yields by 35% in both years. The growing seasons of 2001 and 2002 were among the driest ever recorded in Canada.

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A study was initiated in 1989 to examine the effects of applying excess liquid hog manure with mineral fertilizers to corn and forage crops. Manure was applied yearly at twice the recommended level either in the spring, fall or a combination of both spring and fall applications. Mineral fertilizers were applied at recommended levels to plots receiving only mineral fertilizers and to manure treatment plots. Two control plots received no fertilizers. / During the summer of 1999, soil samples were taken at a depth of 0--2 cm and analysed using different phosphorus extractants. Six runoff events were sampled and analysed for different phosphorus fractions. / Strong correlations were found in corn plots between average dissolved reactive phosphorus concentrations in runoff and soil test phosphorus concentrations. Timing of manure application had a significant impact on both soil and runoff phosphorus concentrations. Runoff from forage plots had significantly higher concentrations of dissolved phosphorus, but phosphorus loads were greater from corn plots due to higher runoff volumes. Corn plots released significantly higher particulate phosphorus concentrations than forage plots because of higher sediment loads in runoff from corn plots.