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.

A two year study was initiated in 2001 on a 2.4 ha site of mostly St. Amable loamy sand and shallow loamy sand at the Macdonald Campus Research Farm. This study sought to assess the effects of long term tillage and residue practices on soil physical properties and to relate these properties to their influence on total carbon, total nitrogen and soil microbial biomass-carbon. The site was set up as a factorial experiment with three tillage practices (no till---NT; reduced till---RT; and conventional till---CT) and two residue practices (with residue (grain corn), +R; without residue (silage corn), -R). Soil physical properties measured were bulk density, macroporosity at -6 kPa, saturated hydraulic conductivity (Ksat ), dry aggregate distribution, soil moisture and soil temperature. Wheel versus non-wheel track bulk density was also determined in the NT plots. Total porosity, soil water characteristic curves and pore volume distribution were derived from field samples. Crop yield was also assessed. Tillage alone had an effect on total porosity, bulk density, pore diameter and macroporosity. No individual physical property was significantly affected by the residue treatments; however, the tillage and residue interaction was significant for Ksat, soil moisture and soil temperature. Dry aggregate distribution was not affected by either tillage, residue or the combination of the two. Tillage affected total carbon and total nitrogen in the 0--0.10 m depth. Soil microbial biomass-carbon was affected by residue inputs. Total carbon and nitrogen were influenced by the bulk density, but none of the other soil physical properties. Soil microbial biomass-carbon was not influenced by any of the physical properties. Tillage and residue practices had no significant effect on crop yield for either year.

A comparison of the soil fertility and relationships between soil fertility and early regenerating vegetation were examined following clearcut harvesting and wildfire in the black spruce-feathermoss zone of west-central Quebec. During the summer of 1997, sampling was conducted in wildfires burnt 2, 14, and 21 years ago, stands clearcut within +/-3 years of each fire, and undisturbed control stands. At each site an estimation of vegetative cover of each species present and a volumetric sample of the forest floor (FH), and mineral soil (0--10 cm) were collected from at least 8 fire and cut sites and at least 4 control sites in each study area. The comparison between the soil fertility of stands clearcut and burnt suggested that important differences exist following these two disturbance types in the boreal forest. The forest floor of clearcut sites had greater dry mass, mass of total nutrients and mineralized N than fire or control sites, whereas fire sites generally had higher pH and concentrations of total nutrients than clearcut or control sites. Partitioning of the variance of the vegetation data between soil fertility and general site characteristic variables was carried out to evaluate the direct influence of post-disturbance soil fertility on the composition of regenerating vegetation. All explanatory variables accounted for 53.6% of the variance, of which 23.2% was exclusively attributed to soil fertility variables. Disturbance severity is suggested as an important factor, directly affecting the initial composition of the regenerating vegetation, and indirectly affecting soil fertility and stand productivity in the later stages of regeneration.

Hybrid poplars are fast-growing trees, well-suited for the production of bioenergy and wood products. They are often planted on marginal agricultural lands where soil fertility is low. The application of nitrogen (N) fertilizers is expected to replenish soil N reserves and support hybrid poplar productivity throughout the short-rotation growth period. The objective of this thesis was to assess the effect of two organic N fertilizers, papermill biosolids and pig slurry, on tree growth, nutrient uptake and soil fertility in a Populus trichocarpa x Populus deltoides hybrid poplar plantation. I also evaluated how these organic amendments influenced N transformation rates and denitrification losses. Hybrid poplar growth was greater when trees were fertilized with mixed biosolids: pig slurry fertilizer treatments compared to biosolids or pig slurry alone. Assessment of foliar nutrition with the critical value approach (CVA) and compositional nutrient diagnosis (CND) methods revealed that foliar N and P concentrations were limiting to tree growth. Vector analysis (VA) reflected growth responses in fertilized trees, relative to an unfertilized control, but may not be useful as a diagnostic tool due to the indeterminate growth habit of hybrid poplars. Nevertheless, surface application of papermill biosolids and pig slurry had the potential to improve soil fertility by increasing soil pH and extractable nutrients in the hybrid poplar plantations. Nutrient use efficiency (NUE) was greater in plots receiving the mixed biosolids:pig slurry treatment than biosolids or pig slurry alone. Laboratory incubations and 15N isotope pool technique revealed that pig slurry was a source of readily-mineralizable N one month after fertilizer application, and stimulated the gross nitrification rate and immobilization, while papermill biosolids contributed to a larger mineral N pools at the end of the season. Our results suggest that denitrification will be greater in biosolids-treated soils, which have a higher extractable Ca concentration and soil pH, as well as more NO3-N and available C for denitrifiers. Further investigation under field conditions would help us to evaluate the N transformations and losses mediated by microbial activities following organic fertilizer application in a hybrid poplar plantation.

In 1991, a project was initiated at Quinn and Ormstown, two sites with soils representative of Quebec's present-day agriculture. Three field management techniques were studied related to their possible impact on the soils: tillage, rotation, and N fertilization. The three treatments were randomly assigned in two blocks of a split-split-plot design. The main treatment, tillage, had two levels, conventional tillage (CT) and no-till (NT). The sub-treatment was represented by five 4-year rotations, starting from 1991 as follows: CCCC (continuous corn), CSAC (corn-soybean-alfalfa-corn), CSCS (corn-soybean-corn-soybean), SCSC (soybean-corn-soybean-corn) and SSSS (soybean monocrop). The third factor was N fertilization, at three different rates: 0, 90, and 180 kg/ha, excepting the continuous soybeans which received lower rates of, respectively, 0, 20 and 40 kg N/ha. Soil sampling was performed in 1994, during the last year of the 4-year rotations, at two different moments over the growing season, June and August, for all analysed soil properties, except aggregate stability and organic matter for which samples were taken only in September. (Abstract shortened by UMI.)

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