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No description available.

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.

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.