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.

The effect of management practices used by smallholder farmers to improve soil quality and increase maize yield was examined in an 80 ha. micro-watershed of central Malawi. Because of the complexity inherent in smallholder farming systems, this research proposed the combination of participatory methods with analytical techniques developed in field ecology, such as multivariate and spatial analysis. During a Participatory Rural Appraisal (PRA), farmers identified factors potentially influencing soil quality and maize yield. One hundred and seventy-six (176) plots were located in twenty-nine (29) fields and characterized for management practices and biophysical characteristics. Soil samples were collected at each plot and analysed for a suite of properties. The maize yield was measured for both 1996--97 and 1997--98 seasons. A formal survey was used to gather information on household characteristics. Results showed that management practices that were promoted by a previous extension project, such as alley cropping and the planting of grass on contour ridges, were strongly correlated and found mainly in fields located closer to house compounds. Farmers with a higher proportion of their land under wetland gardens tended to use less agroforestry. Food security was associated with households that were able to purchase inorganic fertilizers, had larger landholding size, and owned livestock and woodlots. The effect of management practices on maize yield and soil quality was partially confounded with characteristics of the plot, such as slope, degradation level, number of years under cultivation or pest damage. Higher maize yield was observed in plots that were better managed, as expressed by the combination of different management practices, lower pest incidence, fewer erosion signs and higher soil fertility. Some positive effects of alley cropping on soil quality were observed in plots that were cultivated for a longer period and located on flatter land. This study demo

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Studies were conducted in 1990 and 1991 to investigate two kinds of soil insects that attack field corn in Virginia. Corn rootworms, the most important of which is the western corn rootworm (WCR), feed on corn roots and have been sampled effectively using yellow sticky traps. The spatial distribution of weekly counts of WCR on yellow sticky traps was investigated over three consecutive weeks in July of each year using Pherocon A.M. traps. The traps were cut in half, placed in a seven by seven grid, and separated by an average distance of 18.3 m. Data were collected in a total of 24 corn fields. Dispersion indices showed that the trap counts of WCR were aggregated. Further analysis using geostatistics revealed that the trap counts were nonrandomly distributed at least half the time at all population densities; however, the presence and type of spatial pattern was density-independent. Pests of germinating corn seeds comprise a guild of insects including seedcorn maggots, wireworms, and white grubs. No one method has yet been developed to sample all of the insects in the guild. A new method, the baited wire trap (BWT), was evaluated in 47 fields in 1990 and 1991. No significant differences were found between the proportion of seeds damaged by pests of germinating seeds in the BWT and seeds planted by hand at conventional plant spacings. The proportion of damaged seeds in the BWT was significantly less than that in the commercially-planted fields; however, the proportion of damaged seeds in the BWT was significantly greater than the stand loss in these same fields. Linear regression revealed a significant relationship between the proportion of damaged seeds in the BWT and simulated plantings, and the conventionally planted fields. Because of low coefficients of determination (R²), the BWT could not be used to accurately predict damage by the germinating seed pest guild. However, the BWT detected damage by all three members of the pest guild and was found to be significantly faster to use than the bait station method. / Master of Science

Field experiments were conducted during the 1985 growing season on six soils in the Atlantic Coastal Plain region and one from the Piedmont region to determine the response of irrigated corn (<i>Zea mays</i> L.) to boron application. Two treatments applied to each of the soils consisted of 1) a control and 2) both band and foliar boron applied at rates of 2.0 and 0.5 kg ha⁻¹, respectively, as Solubor when corn plants were in the V5 to V6 growth stage. Corn grain yields averaged 13,485 kg ha⁻¹ across treatments on the seven soils. Based on published calibration data for the hot water soluble boron procedure, a corn grain yield response to boron application was not expected on any of the seven soils. Nevertheless, boron application increased corn grain yield from 13,485 kg ha⁻¹ on the control treatment to 14,300 kg ha⁻¹ on the plus boron treatment for one of the seven soils, an Altivista loamy sand. This soil had a hot water soluble boron concentration of 0.56 mg kg⁻¹ and a Mehlich 3 extractable boron concentration of 0.78 mg kg⁻¹. The six soils for which boron application did not increase yields had hot water soluble boron concentrations from 0.55 to 0.92 mg kg⁻¹ and Mehlich 3 extractable boron concentrations from 1.02 to 1.33 mg kg⁻¹. The boron concentration in whole corn plants sampled at the V5 to V6 growth stage was 6.7 mg kg⁻¹ on the Altivista soil where boron application increased corn grain yields and ranged from 9.6 to 103.1 mg kg⁻¹ on the six soils where boron application did not increase yields. The boron concentrations in ear leaves sampled at the early silk growth stage ranged from 8.5 to 18.6 mg kg⁻¹ and in corn grain from 0.66 to 3.31 mg kg⁻¹ on the control treatments of the seven soils. Ear leaf and grain boron concentrations were 10.7 and 0.74 mg kg⁻¹ for plants grown on the control treatment on the Altivista loamy sand, respectively. It can be concluded that, of the plant tissue and soil test procedures evaluated, boron concentration in whole young plants and Mehlich 3 extractable B were the most promising for diagnosis of boron deficiency in corn plants. Nevertheless, research is required on more soils before boron concentrations in young plants and Mehlich 3 extractable B can be used as a routine procedure for detection of boron deficiency in corn plants. / M.S.

Mechanisms of root growth under variable field conditions were investigated by observing corn ( Zea mays L.) root growth and distribution in the field and by observing the influence of soil physical stresses on corn seedling root growth in controlled environments. The field soil was Groseclose silt loam (clayey, mixed, mesic Typic Hapludult). Groseclose A horizon material was used for the growth chamber experiments where corn was grown in a range of aggregate sizes, bulk densities, low and high soil moisture levels, and temperatures. Rooting patterns in the field were altered by drought. Root length density decreased in the dry surface soil and proliferated in the moist subsurface soil. Distribution of roots length densities was skewed. A few samples contained many roots and many samples contained few roots because roots were restricted to interpedal voids. In the growth chamber experiments, roots were not able to penetrate large aggregates and were restricted to interaggregate zones. This tortuous path of root growth led to transitory impedances as roots were deflected around aggregates. Corn roots were able to push small aggregates out of their path. An equation was developed to describe this impedance as a function of aggregate size, root diameter, and deflection angle. Mechanical impedance, oxygen stress, lower temperatures, and moisture stress reduced seedling root elongation to some extent, but the influence of reduced temperature was the most dramatic. At 6 days corn root length at 21°C was 20% of that at 25°C while root length at 17°C was only 5% of that at 25°C. Mechanical impedance and reduced temperatures also increased root diameter. In wet soil, oxygen stress was the most immediate factor affecting root growth, but after 4 days root elongation was stimulated suggesting other unknown factors. Two semi-empirical models were developed. One was based on the exponential growth rate of the root system and the other based on the linear growth rate of each root member. These models accounted for the reduction in root growth rate due to the soil physical stresses. / Ph. D.

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.