A Novel Technique to Label Cover Crop Biomass Using Stable Isotopes

Stable isotopes can be used as tracers for carbon and nitrogen pathways being a great tool to track nutrients in integrated systems. The objective of this experiment was to understand the partitioning of 15N and 13C within cover crop plants when they were labeled with stable isotopes, using chambers under field conditions. Cover crops were planted at the University of Florida, North Florida Research and Education Center-Marianna, located in Marianna, FL. Treatments were four cover crops, in which one was considered a typical cover crop system and the other three consisted of an integrated crop-livestock system with or without the inclusion of legume or different nitrogen fertilizer rates grazed every two weeks. All treatments were replicated three times in a randomized complete block design. Two chambers were built and placed in each plot to label the cover crop plants. For the 15N labeling, 15N2-labeled urea (98 atom% 15N) was applied at a rate of 0.5 kg N ha-1 only once. The target amount of 13CO2 (99 atom% 13C) was determined considering a 20% enrichment of the CO2 concentration present inside the chamber’s volume. The 13CO2 labeling was performed for 28 consecutive days. The labeling technique using chambers and stable isotopes to enrich cover crop species worked under field conditions for both, grass and legume species. Moving forward, this labeling technique can be a useful tool to track nutrient pathways, especially litter decomposition in diversified integrated crop and livestock systems under different management practices

Masonry dams : analysis of the historical profiles of Sazilly, Delocre and Rankine

The significant advances in masonry dam design that took place in the second half of the 19th century are analyzed and discussed within the context of the historical development of dam construction. Particular reference is made to the gravity dam profiles proposed by Sazilly, Delocre and Rankine, who pioneered the application of engineering concepts to dam design, basing the dam profile on the allowable stresses for the conditions of empty and full reservoir. These historical profiles are analyzed taking into consideration the present safety assessment procedures, by means of a numerical application developed for this purpose, based on limit analysis equilibrium methods, which considers the sliding failure mechanisms, the most critical for these structures. The study underlines the key role of uplift pressures, which was only addressed by Lévy after the accident of Bouzey dam, and provides a critical understanding of the original design concepts, which is essential for the rehabilitation of these historical structures.This work has been funded by FCT (Portuguese Foundation for Science and Technology) through the PhD grant SFRH/BD/43585/2008, for which the first author is grateful