Graduation date: 1993

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.

Abstract This research investigated the spatial and temporal distributions of Trichodesmium spp. and Richelia intracellularis in the South China Sea. The surveys covered the period from July 2000 to July 2002. A total of eight cruises, including spring, summer and fall were conducted. The sampling stations located between 18~22o N and 115~122 o E , over the continental shelf, the slope, and the basin of the northern South China Sea. Trichodesmium biomass was higher in summer and fall than spring. There was no significant difference in biomass among shelf , slope and basin. The averaged biomass was 69

Continuous multi-phase feeding of broiler chickens was evaluated to optimize broiler nutrition and minimize environmental impact related to excess nitrogen in poultry manure. Four experiments were conducted. Experiments 1 and 2 studied effects of continuous multi-phase feeding during a 3-week starting period using battery brooders while experiments 3 and 4 evaluated multi-phase feeding during a traditional 7-week growing period using both battery brooders and floor pens. In the first and second experiments, the nutrient content of the multi-phase diets was changed every 24 hours in comparison to single-phase feeding. Results indicated that during the starter period, continuous multi-phase feeding had no significant influence on feed consumption, daily gain, feed to gain ratio or fecal nitrogen. In the third and fourth experiments, a four phase industry type broiler feeding program was compared to intensive multi-phase feeding programs created by linearly blending three different diets based on typical industry nutrient values and a commercial nutrient modeling computer program (EFG Natal®). In both intensive multi-phase feeding programs, the diets were changed every three days over a 7-week growth period. Broilers in experiment 3 were raised in Petersime battery brooders to primarily access nitrogen balance while birds in experiment 4 were raised in a floor pen on pine shaving litter to resemble commercial broiler production. The results indicated that intensive multi-phase feeding improved body weight gain and feed to gain ratio only in weeks 5 and 6 but not during the overall 7-week period. Nitrogen excretion and nitrogen retention were unaffected by the intensive multi-phase feeding systems. Economic analysis indicated that intensive multi-phase feeding programs could potentially lower feed costs per kilogram of gain. However, the high cost of implementing a continuous multi-phase feeding system may not justify the relatively small gain in lower feed cost per kilogram of gain. In conclusion, continuous multi-phase feeding of broiler chickens using corn-soy diets does not appear to be justified by either increased performance or reduced nitrogen excretion.

Thesis (M.S.)--Oregon State University, 2003. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.S.)--Pennsylvania State University, 2009. / Mode of access: World Wide Web. Thesis advisor: Erica A.H. Smithwick.

Thesis (M.S.)--Marshall University, 2003. / Title from document title page. Document formatted into pages; contains ix, 140 p. including illustrations. Includes vita. Includes bibliographical references (p. 104-108).

Thesis (Ph. D.)--University of Queensland, 2003. / Includes bibliographical references.

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, and the Woods Hole Oceanographic Institution), 2003. / Includes bibliographical references.

Rising water costs and concern for groundwater contamination are forcing melon growers to improve irrigation and nitrogen fertilization efficiency. The research objectives were: 1) to determine quantities of nitrogen and water applied through a subsurface drip irrigation system to cantaloupe and watermelon which would optimize fruit yield while minimizing losses of nitrogen and; 2) to develop specific monitoring techniques for assessing the water and nitrogen status of melon crops throughout the growing season. Four years of field research were conducted from 1988 to 1991 at the University of Arizona Maricopa Agricultural Center, Maricopa, AZ. The first two years of research determined which varieties of cantaloupe and watermelon would grow best under drip irrigation and the feasibility of using plant tissue tests to aid in N fertilizer scheduling. The last two years of research used a factorial design with levels of N and target soil water tension to determine response surfaces for fruit yield and net return. Information from tensiometers was used to schedule irrigations. Watermelon petiole nitrate levels at critical growth stages were used to recommend application rates of nitrogen fertilizer. In 1991, an N difference method was used to estimate N which was unaccounted for in this watermelon cropping system. Petiole nitrate levels were highly responsive to N fertilizer treatments and accurately quantified visual observations of crop N status. Petiole nitrate results also indicated that preliminary nitrogen fertilizer management guidelines using a tissue nitrate test was reasonably accurate in predicting optimum nitrogen 'management. Market able yield showed a soil water tension by N interaction. Maximum marketable yield was estimated to be 101 Mg ha⁻¹ at 7.2 kPa tension and 336 kg of applied N ha⁻¹. Maximum net return was estimated to be $8 250 ha⁻¹ when average soil water tension was 10.6 kPa and applied N was 243 kg N ha⁻¹. Unaccountable N was estimated to be 300 kg N ha⁻¹ when N rates were 500 kg ha⁻¹ and average soil water tension was 4 kPa. A response surface was estimated through the transformation and summation of yield, net return and unaccounted for N response variables. The optimum average soil water tension and rate of applied N were 12.6 kPa and 181 kg N ha⁻¹, respectively.