Increasing levels of nitrate (NO3-) in groundwater have become a major environmental and health concern. In situations where NO3-concentrations in the soil-water system pose an environmental hazard, water table management may be a desirable practice to reduce such pollution. Careful management of N applications is also believed to reduce NO3- levels. / A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Quebec, about 30 km South-West of Macdonald Campus of McGill University, to investigate the effect of water table management (WTM) and N fertilizer combinations on potentially leachable NO3- and denitrification rates in the top soil layer (0--0.15 m). The field was planted with monocrop corn (Zea mays. L) in both years. Treatments consisted of a factorial combination of two water table managements, free drainage (FD) and subirrigation (SI) (about 1.0 m and 0.6 m, respectively, below the soil surface) and two N fertilizer rates, 200 kg ha-1 (N200) and 120 kg ha-1 (N120). / Water table management had a significant effect on reducing NO3 - concentrations in the soil profile. Subirrigation treatment reduced NO3- in the top soil layer by 41% and 15% in 1996 and 1997, respectively. Similarly, NO3 - levels were 50% and 20% lower in N120 compared to N200 treatment. / Climatic conditions (rainfall and temperature) played a large role in regulating denitrification rates. Due to drier and cooler conditions in 1997, denitrification rates were lower compared to 1996, leaving more NO3 - in the soil profile. Following harvest, this high NO 3- concentration may be subject to leaching.

The overall goal of this study was to investigate the possibility of reducing NO3-- concentration in the lower soil horizon by promoting denitrification. The study looked at an inexpensive remediation practice for subsurface-drained fields in order to degrade N0j' and consequently diminish NO3-- pollution. The experiments were conducted on a corn field at the Macdonald Campus Farm. The field was composed of sandy loam soil underlined by a clay layer. / In this study, sucrose was injected at a concentration of 20 mg L --1 within the subirrigation water. Furthermore, the water table was maintained at approximately 70 cm from the soil surface in order to create an anaerobic environment that allows for denitrification. The purpose was to furnish dissolved organic carbon (DOC) to the resident microorganisms in order to carryout a higher amount of denitrification. / The NO3-- levels in ground water were monitored weekly. Gas samples were taken from the field to observe whether N2O emissions increased as a result of the treatment This was considered to be important since N2O is a greenhouse gas. / It was found that the addition of sucrose significantly resulted in a faster rate of denitrification. The higher biological activity severely damped the NO3-- peaks in the subsoil, which arose following major rainfall events. / The addition of sucrose at 20 mg L--1 was found to be significant in lowering the amounts of released N2O. Therefore, adding DOC in the subirrigation water not only helped remediate water contamination but also served as a remedy to atmospheric pollution. / In addition, the hydraulic conductivity of the soil was monitored to ensure that no bioclogging arose from an anticipated rise in the bacterial population due to the addition of sucrose. It was found that the addition of sucrose at 20 mg L--1 did not contribute in anyway to reduce the soil's hydraulic conductivity.

The fate of fertilizer N is of primary concern for both agricultural productivity and environmental quality. Concerns include denitrification, leaching losses, mineralization of organic N as plant available N. Denitrification is an important source of N$ sb2$O, a greenhouse gas but field measurements are difficult. Two methods of measuring denitrification are soil core (SC) incubation and closed chamber (CC) methods. These methods were assessed on soil under monoculture corn, monoculture soybean, and alfalfa in a corn soybean alfalfa rotation. Greater concentrations were found in the CC method than the SC method. Denitrification rates ranged from less than 15 g N ha$ sp{-1}$h$ sp{-1}$ to nearly 2000 g N ha$ sp{-1}$h$ sp{-1}$. The CC method was more sensitive to treatment effects. The denitrification rates were dependent on the soil type, being higher on soils with high clay content. The variables that had the highest degree of relationship with denitrification were water filled pore space, soil NH$ sb4$-N and NO$ sb3 $,-N concentrations. Higher rates of N increased denitrification. As to assessment of available soil N, this was accomplished N and C mineralization measurements. Potentially mineralizable N(N$ sb0$) ranged from 144 mg N kg$ sp{-1}$ to 30.3 mg N kg$ sp{-1}$. Higher rates of organic amendment resulted in higher measured values on Brandon soil while higher rates of inorganic N on Chicot and Ste. Rosalie soils caused no change in mineralizable N or respired C. Total N, organic C, water soluble organic C (WSOC) and microbial biomass C (MBC) increased with increasing amounts of organic or inorganic N amendment on Brandon soil. Higher rates of inorganic N resulted in lower WSOC and MBC on Chicot and Ste. Rosalie soils. Nitrogen mineralized, C respired, total N, organic C, WSOC and MBC were all related to soil texture. MBC and WSOC were found to have a strong positive relationship with potentially mineralizable N.

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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 field study was carried out in western Quebec to investigate methods of predicting and reducing NO3--N leaching. Corn (Zea mays L.) was planted on the expel site and them were four treatments: water table controlled at 600 mm and 120 kg/ha of N fertilizer (WT120), water table controlled at 600 mm and 200 kg/ha of N fertilizer (WT200), free drainage and 120 kg/ha of N fertilizer (FD120), and free drainage and 200 kg/ha of N fertilizer (FD200). Drain flow was monitored and water samples were taken and analyzed for NO3- -N. Soil NO3- levels were measured along with led chlorophyll and denitrification throughout the two growing seasons. / Drain flow was dependent on both rainfall and the soil moisture content. In 1996, water table control decreased drain flow. However, in 1997 (a drier year), the drain flows for all treatments were similar. NO3 --N was reduced significantly in the controlled water table plots. In 1996, there was 59.2% less NO3--N in the controlled water table plots than in the free drainage plots and in 1997 this increased to 75.9% less NO3--N in the controlled water table compared to the fire drainage plots. In 1996, denitrification was enhanced by the controlled water table plots, with 72.2% more denitrification occurring in the controlled water table plots than in the free drainage plots. In 1997, there was a 93.2% increase in denitrification occurring in the water table plots than in the free drainage plots. / The controlled water table plots had no effect on plant chlorophyll levels. Chlorophyll contents of the corn plants were higher where fertilizer was applied at 200 kg/ha. Overall, it was shown that water table management can significantly decrease NO3--N pollution in drainage water. (Abstract shortened by UMI.)

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