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.

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