1,085 research outputs found

    The viability of cattle ranching intensification in Brazil as a strategy to spare land and mitigate greenhouse gas emissions

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    Alternating current driven instability in magnetic junctions

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    An effect is considered of alternating (high-frequency) current on the spin-valve type magnetic junction configuration. The stability with respect to small fluctuations is investigated in the macrospin approximation. When the current frequency is close to the eigenfrequency (precession frequency) of the free layer, parametric resonance occurs. Both collinear configurations, antiparallel and parallel ones, can become unstable under resonance conditions. The antiparallel configuration can become unstable under non-resonant conditions, also. The threshold current density amplitude is of the order of the dc current density switching the magnetic junction.Comment: 8 page

    Effects of the Structure of Water Rights on Agricultural Production During Drought: A Spatiotemporal Analysis of California\u27s Central Valley

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    California’s Central Valley region has been called the “bread-basket” of the United States. The region is home to one of the most productive agricultural systems on the planet. Such high levels of agricultural productivity require large amounts of fresh water for irrigation. However, the long-term availability of water required to sustain high levels of agricultural production is being called into question following the latest drought in California. In this paper, we use Bayesian multilevel spatiotemporal modeling techniques to examine the influence of the structure of surface water rights in the Central Valley on agricultural production during the recent drought. California is an important place to study these dynamics as it is the only state to recognize the two dominant approaches to surface water management in the United States: riparian and appropriative rights. In this study, Bayesian spatiotemporal modeling is employed to account for spatial processes that have the potential to influence the effects of water right structures on agricultural production. Results suggest that, after accounting for spatiotemporal dependencies in the data, seniority in surface water access significantly improves crop health and productivity on cultivated lands, but does not independently affect the ability to maintain cultivated extent. In addition, agricultural productivity in watersheds with more junior surface water rights show less sensitivity to cumulative drought exposure than other watersheds, however the extent of cultivation in these same watersheds is relatively more sensitive to cumulative drought exposure

    Genetic and Functional Diversification of Small RNA Pathways in Plants

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    Multicellular eukaryotes produce small RNA molecules (approximately 21–24 nucleotides) of two general types, microRNA (miRNA) and short interfering RNA (siRNA). They collectively function as sequence-specific guides to silence or regulate genes, transposons, and viruses and to modify chromatin and genome structure. Formation or activity of small RNAs requires factors belonging to gene families that encode DICER (or DICER-LIKE [DCL]) and ARGONAUTE proteins and, in the case of some siRNAs, RNA-dependent RNA polymerase (RDR) proteins. Unlike many animals, plants encode multiple DCL and RDR proteins. Using a series of insertion mutants of Arabidopsis thaliana, unique functions for three DCL proteins in miRNA (DCL1), endogenous siRNA (DCL3), and viral siRNA (DCL2) biogenesis were identified. One RDR protein (RDR2) was required for all endogenous siRNAs analyzed. The loss of endogenous siRNA in dcl3 and rdr2 mutants was associated with loss of heterochromatic marks and increased transcript accumulation at some loci. Defects in siRNA-generation activity in response to turnip crinkle virus in dcl2 mutant plants correlated with increased virus susceptibility. We conclude that proliferation and diversification of DCL and RDR genes during evolution of plants contributed to specialization of small RNA-directed pathways for development, chromatin structure, and defense

    Sediment source and mixing and the cycle of sediment transport: an example from NE Negev Desert, Israel.

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    Alluvial terraces represent the end product of sedimentary cycles; each includes sediment generation, transport, accumulation, and the ultimate incision that forms abandoned alluvial surfaces. We examine the middle Pleistocene to recent drainage system evolution in Makhtesh Hazera, Negev Desert, southern Israel and compare the characteristics of erosion and sediment transport in the present system with those expressed by the alluvial terraces. The Hazera drainage basin lies at the margins of the arid to hyper-arid Dead Sea rift (DSR). Makhtesh (crater) Hazera is a deeply incised erosional structure (5X7 km) that has been excavated since the early Pliocene into the crest of the Hazera asymmetric anticline. The Makhtesh floor is surrounded by cliffs rising more than 400 meters high. The cliffs are built of Upper Cretaceous hard carbonates caprocks overlying Lower Cretaceous friable quartz sandstones. Bedrock knickpoints isolate the drainage basin in the Makhtesh and above it from a direct influence of the terminal base level of the DSR. Thus, the accumulation of sediment and abandonment of terraces are controlled by climate and bedrock barriers located at the Makhtesh outlet. We use cosmogenic isotope concentrations to determine bedrock denudation rates, ages of alluvial terraces, and basin wide erosion rates in different channels throughout the basin. The use of cosmogenic isotopes enables us to determine sediment sources and reconstruct sedimentary cycles. OSL dating was used to determine the accumulation ages of alluvial sediment in alluvial terraces. These two methods enable quantitative evaluation of fluvial processes. Bedrock erosion rates suggest a strong dependence of erosion on lithology. While the Lower Cretaceous sandstone erodes at >100 mm ky-1, the overlying hard carbonate caprock yielded cosmogenic isotope concentrations that correspond to erosion rates of 1-3 mm ky-1. This significant difference in erosion rates maintains the dramatic relief of the Hazera drainage basin. We find that the quartz sediment in the present fluvial system of Makhtesh Hazera originates from two predominant sources. One is the Lower Cretaceous sandstone that crops out along the base of the Makhtesh cliffs. The second source are un consolidated Miocene sands that fill the syncline which is located north west of the Makhtesh and is drained into it. 10Be concentrations in successive samples indicate that the Miocene sand is gradually diluted by Lower Cretaceous sand as it flows down stream and the mixing of sediment from both sources is good. Alluvial terraces and bedrock units exposed inside the Makhtesh do not contribute a significant amount of sediment to the present drainage system. Three major alluvial terrace levels were identified. The highest terrace level (MKT0) was abandoned at 279±19 ky. This level probably covered most of the Makhtesh surface. The deposition of the two lower levels, MKT1 and MKT2 (which were abandoned at 160±6 and 47±9 ky, respectively), was confined to the present drainage system. Analysis of cosmogenic depth profiles from the terraces suggests significant recycling of sediment within the Mekhtesh. This is in contrast to the present system that lacks recycled sediment. We explain this difference by the fact that the terraces are the final product of a sedimentary cycle while the present drainage system presents a “snapshot” in time which does not represent the entire cycle only the present state of the system which is expressed by rapid incision and very little lateral migration.European Geosciences Unio