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The top-down effects of consumers and bottom-up effects of resource availability are important in determining community structure and ecological processes. I experimentally examined the roles of consumers — both detritivores and predators — and habitat context in affecting nutrient cycling using the detritus-based insect community in bromeliad leaf wells. I also investigated the role of multiple resources in limiting plant productivity using meta analyses. The insect community in bromeliads only increased nitrogen release from leaf detritus in the presence of a predator trophic level. When only detritivores were present, the flow of stable isotope-labeled nitrogen from detritus to bromeliads was statistically indistinguishable from that in bromeliads lacking insects. I suggest that emergence of adult detritivores constitutes a loss of nitrogen from bromeliad ecosystems, and that predation reduces the rate of this nutrient loss. Hence, insects facilitate nutrient uptake by the plant, but only if both predators and detritivores are present. Moreover, predators can affect nutrient cycling by influencing the spatial scale of prey turnover. This mechanism results in a pattern opposite to that predicted by classic trophic cascade theory. Increasing habitat complexity can have implications for nutrient cycling by decreasing the foraging efficiency of both predators and their prey, and by affecting the vulnerability of predators to intraguild predation. Along a natural gradient in bromeliad size, I found that, depending on the relationship between community composition and habitat size, habitat complexity interacts with the changing biotic community to either complement or counteract the impact of predators on nutrient uptake by bromeliads. In contrast to the existing emphasis on single-resource limitation of primary productivity, meta-analyses of a database of 653 studies revealed widespread limitation by multiple resources, and frequent interaction between these resources in restricting plant growth. A framework for analyzing fertilization studies is outlined, with explicit consideration of the possible role of multiple resources. I also review a range of mechanisms responsible for the various forms of resource limitation that are observed in fertilization experiments. These studies emphasize that a wider range of predator and nutrient impacts should be considered, beyond the paradigm of single resource limitation or classic trophic cascades.

A principal concern in crop agriculture is yield, and a key factor for crop growth is the availability of nitrogen. The large amount of nitrogen fertilizer required by plants is a major cost to farmers. Moreover, environmental issues such as groundwater pollution arise from the utilization of nitrogen fertilizers. Therefore, improvement in the nitrogen use efficiency (NUE) of plants is of urgent importance for sustainable and efficient agriculture. Although hybrid varieties have increased crop yields in low N conditions, the molecular mechanism of plant adaptation to N stress is not completely understood. Herein, the study of responses to N limitations in the natural signalling pathways of model plants facilitates the understanding of complex responses in plants to N stress, and this information can be used to further improve NUE. In this research, the transcriptomes of three model plants Arabidopsis, maize, and rice were compared under diverse N growth conditions. An evaluation of the response of the three plants to varying N levels was also conducted. From a statistical point of view, three distinct methods of detecting differential expression were utilized to reduce the likelihood of false positives due to the tens of thousands of genes simultaneously studied. Furthermore, the performance of three statistical approaches was compared during detection of the N-responsive genes. Finally, a clustering analysis (agglomerative hierarchical clustering) was performed on the genes that significantly responded to N levels as identified by a more biologically intuitive method called Rank Products (RP).

Immunization against gonadotropin-releasing hormone (GnRH) reduces boar taint occurrence in male pigs and likely alters the rate and composition of body weight gain. Serial slaughter and nitrogen-balance studies were conducted to evaluate dynamics and hormonal control of body protein deposition (PD) and body lipid deposition (LD) in entire male pigs (EM), entire male pigs immunized against GnRH with IMPROVEST (IM), conventional, early castrates (EC), and male pigs surgically castrated between 25 and 40 kg body weight (late castrates; LC). Growth performance and physiology were similar in EC and LC. The PD was highest and LD was lowest in EM, while LD was highest in IM. Following immunization, PD in IM gradually changed from that in EM to that in EC and LC. Changes in PD and LD were consistent with changes in the animal’s physiology. Dynamics of PD and LD should be considered when optimizing feeding programs for IM. / Pfizer, Swine Innovation Porc, Ontario Pork, OMAFRA

An improved understanding of nitrogen metabolism in the gastrointestinal tract of the pig is required in order to provide accurate estimates of nitrogen and amino acid (AA) bioavailability in feed ingredients and for adequate diet formulation. Research objectives were to estimate the extent of fermentative AA catabolism (FAAC) in the upper gut of pigs. Further objectives were to determine the impact of lower gut nitrogen absorption on measures of apparent ileal digestibility of AA and nitrogen, whole-body nitrogen balance, and urea kinetics in pigs fed a valine-limiting diet. It was determined that simple isotope dilution calculations are inappropriate for determining ileal ammonia flux and FAAC from a continuous infusion of labelled ammonia and urea. A static model with two-pools (blood urea and digesta ammonia) was then developed to determine possible value ranges for FAAC in the upper gut of pigs. Maximum estimated FAAC based on this model was lower when dietary protein content was decreased (P < 0.001). The model presented is limited to minimum and maximum estimates of FAAC due to the sampling and isotope infusion protocol used. Refinements to the model and experimental protocol could allow for more accurate estimates of FAAC. Infusion of casein or urea into the lower gut of pigs did not affect measures of apparent ileal digestibility of AA or nitrogen. These results further validate the methodology available for determination of ileal digestibility and the use of ileal digestibility to estimate bioavailability of AA and nitrogen. Infusion of casein or urea into the lower gut resulted in an increase in nitrogen balance and urea flux in growing pigs fed a valine-limiting diet. Nitrogen absorbed from the lower gut of pigs is likely in the form of ammonia which is converted to urea. Lower gut nitrogen can contribute to whole-body protein deposition via urea recycling and microbial AA production in the upper gut. Lower gut nitrogen absorption should be accounted for when estimating bioavailability of nitrogen in feeds and foods. / Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA); Ontario Pork; Canadian Swine Research and Development Cluster; Natural Sciences and Engineering Research Council of Canada (NSERC); Evonik Industries AG

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Several nitrogen fixers were isolated from the roots and rhizosphere of Quebec crops. Many of these nitrogen fixers were discarded when the production of N(,2)O in the presence of C(,2)H(,2), as a test for denitrifying ability, was included as a selected characteristic. Further characterization of the Nir('+) strains showed that they were Azospirillum lipoferum. / The cytochrome composition of Azospirillum brasilense (a denitrifier) grown under various conditions in a defined medium was investigated. Optical absorbance difference spectra of the particulate fraction of cells grown under aerated conditions indicated the presence of cytochromes of type b, c and a+a(,3). Under low aeration there was a quantitative increase in cytochromes b and c with a concomitant decrease in the a+a(,3)-type cytochrome. At high aeration, a CO spectrum indicated the possible participation of an o-type cytochrome. / At both high and low oxygen concentrations, the supernatant fraction revealed only one c-type cytochrome. Its abundance was increased at low oxygen concentrations. / Cytochrome spectra of anaerobically grown cells using different nitrogen oxides (NO(,3)('-), NO(,2)('-) and N(,2)O) as final electron acceptors revealed the presence of the different cytochromes involved in anaerobic respiration. The reduction of NO(,2)('-) was associated with the cytochrome cd (peak at 620 nm) found only in the supernatant fraction of NO(,2)('-)-grown cells. / Growth on NO(,3)('-) was characterized by a diauxic type of curve in which the first logarithmic phase corresponded to the reduction of NO(,3)('-). The second logarithmic phase corresponded to the reduction of NO(,2)('-). / Growth of Azospirillum brasilense with NO(,2)('-) and N(,2)O as final electron acceptor was possible only when a small amount of NO(,3)('-) was present initially. In contrast with other bacteria, growth of Azospirillum brasilense with tungstate instead of molybdate did not result in NO(,3)('-) reductase-deficient cells. / The NO(,2)('-) accumulation observed with NO(,3)('-)-grown cells possibly resulted from the different NO(,3)('-)- and N(,2)('-)-reductase specific activities. However, the longer lag in the NO(,2)('-) reduction when higher concentrations of NO(,3)('-) were used may be due to a direct effect of NO(,3)('-) on the synthesis or activity of the NO(,2)('-) reductase.

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