Elastica-based strain energy functions for soft biological tissue

Continuum strain energy functions are developed for soft biological tissues that possess long fibrillar components. The treatment is based on the model of an elastica, which is our fine scale model, and is homogenized in a simple fashion to obtain a continuum strain energy function. Notably, we avoid solving the full fourth-order, nonlinear, partial differential equation for the elastica by resorting to other assumptions, kinematic and energetic, on the response of the individual, elastica-like fibrils.Comment: To appear in J. Mech. Phys. Solid

Quasi-rigidity: some uniqueness issues

Quasi-rigidity means that one builds a theory for assemblies of grains under a slowly changing external load by using the deformation of those grains as a small parameter. Is quasi-rigidity a complete theory for these granular assemblies? Does it provide unique predictions of the assembly's behavior, or must some other process be invoked to decide between several possibilities? We provide evidence that quasi-rigidity is a complete theory by showing that two possible sources of indeterminacy do not exist for the case of disk shaped grains. One possible source of indeterminacy arises from zero-frequency modes present in the packing. This problem can be solved by considering the conditions required to obtain force equilibrium. A second possible source of indeterminacy is the necessity to choose the status (sliding or non-sliding) at each contact. We show that only one choice is permitted, if contacts slide only when required by Coulomb friction.Comment: 14 pages, 3 figures, submitted to Phys Rev E (introduction and conclusion revised

Degradation mechanisms and consequences for SOC stocks for the world's largest alpine pastoral ecosystem on the Tibetan Plateau

Approximately 1.5 million km² of the Tibetan Plateau are covered with grasslands. Thereof one third is occupied by the world’s largest pastoral alpine ecosystem (Kobresia pastures). Paleo-records indicate the grazing-induced origin of this ecosystem since more than 8000 years or at least since yak domestication since 4000 years. Long-term moderate grazing by yak and sheep increased belowground C allocation of Kobresia pygmaea, caused the development of dense root-mats and finally lead to an accumulation of soil organic carbon (SOC) and nutrients such as nitrogen (N) and phosphorus (P) in the topsoil. These pastures, however, are increasingly affected by large-scale degradation caused by overgrazing of these highly sensitive ecosystems. Loss of the topsoil threatens several ecosystem functions: i.e. SOC and nutrient storage, biodiversity, provision of grazing-ground and supply of clean water for large parts of SE-Asia. Here, we present a conceptual model and results of degradation processes combining anthropogenic and natural amplifications. To evaluate losses of SOC and nutrients we synthesize field observations and surveys in the highlands and validates this with own analyses in the Kobresia core area. We show that drought- and frost-induced polygonal cracking opens the root-mats, already weakened by overgrazing. This initiates a dying of the Kobresia turf, extends the surface cracks, triggers soil erosion and promotes SOC mineralization and leaching losses. Soil erosion caused further high losses of SOC and nutrients from the topsoil (i.e. 0-10 cm: ~5.1 kg C m-2), whereas SOC loss beneath the surface cracks was primary caused by both, decreasing C-input and SOC mineralization (mineralization-derived SOC loss: ~2.5 kg C m-2). The root biomass decreased with intensity of pasture degradation and lower C input constrains the ecosystem recovery. A negative δ13C shift of SOC reflected intensive decomposition and corresponded to a relative enrichment of 13C depleted lignin components. In sum, degradation triggered high SOC loss (up to 70% of intact soil in 0-30 cm: ~7.6 kg C m-2) from this ecosystem with profound consequences for carbon sequestration, atmospheric CO2, water quality and ecosystem stability

Does EO NDVI seasonal metrics capture variations in species composition and biomass due to grazing in semi-arid grassland savannas?

Most regional scale studies of vegetation in the Sahel have been based on Earth observation (EO) imagery due to the limited number of sites providing continuous and long term in situ meteorological and vegetation measurements. From a long time series of coarse resolution normalized difference vegetation index (NDVI) data a greening of the Sahel since the 1980s has been identified. However, it is poorly understood how commonly applied remote sensing techniques reflect the influence of extensive grazing (and changes in grazing pressure) on natural rangeland vegetation. This paper analyses the time series of Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI metrics by comparing it with data from the Widou Thiengoly test site in northern Senegal. Field data include grazing intensity, end of season standing biomass (ESSB) and species composition from sizeable areas suitable for comparison with moderate – coarse resolution satellite imagery. It is shown that sampling plots excluded from grazing have a different species composition characterized by a longer growth cycle as compared to plots under controlled grazing or communal grazing. Also substantially higher ESSB is observed for grazing exclosures as compared to grazed areas, substantially exceeding the amount of biomass expected to be ingested by livestock for this area. The seasonal integrated NDVI (NDVI small integral; capturing only the signal inherent to the growing season recurrent vegetation), derived using absolute thresholds to estimate start and end of growing seasons, is identified as the metric most strongly related to ESSB for all grazing regimes. However plot-pixel comparisons demonstrate how the NDVI/ESSB relationship changes due to grazing-induced variation in annual plant species composition and the NDVI values for grazed plots are only slightly lower than the values observed for the ungrazed plots. Hence, average ESSB in ungrazed plots since 2000 was 0.93 t ha−1, compared to 0.51 t ha−1 for plots subjected to controlled grazing and 0.49 t ha−1 for communally grazed plots, but the average integrated NDVI values for the same period were 1.56, 1.49, and 1.45 for ungrazed, controlled and communal, respectively, i.e. a much smaller difference. This indicates that a grazing-induced development towards less ESSB and shorter-cycled annual plants with reduced ability to turn additional water in wet years into biomass is not adequately captured by seasonal NDVI metrics

МНОГОФАЗНО-ОДНОФАЗНыЕ РЕВЕРСИВНыЕ ЭЛЕКТРОМАШИННО-ВЕНТИЛЬНыЕ ПРЕОБРАЗОВАТЕЛИ БЕСКОНТАКТНыХ МАШИН ДВОЙНОГО ПИТАНИЯ

Розглянуто процеси в багатофазно-однофазних реверсивних електромашинно-вентильних перетворю- вачах безконтактних машин подвійного живлення. Рассмотрены процессы в многофазно-однофазных реверсивных электромашинно-вентильных преобра- зователях бесконтактных машин двойного питания

Fate of Organic and Inorganic Nitrogen in Crusted and Non-Crusted Kobresia Grasslands

Copyright © 2016 John Wiley & Sons, Ltd.A widespread pattern of the Tibetan plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non-crusted and crusted patches in Kobresia grasslands remains unknown. We reported on a field 15N-labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29 days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto soil surface. Crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil than plants in both patches 29 days after the labeling. In non-crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly because of higher root biomass. Microorganisms in non-crusted patches were N-limited because of more living roots and competed strongly for N with roots. Inorganic N input to non-crusted patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant + soil) for inorganic N forms. Compared to non-crusted patches, microorganisms in crusted patches were more C-limited because of depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N-glycine than inorganic N. We conclude that overgrazing-induced crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N. Copyright © 2016 John Wiley & Sons, Ltd

Fate of Organic and Inorganic Nitrogen in Crusted and Non-Crusted Kobresia Grasslands

© 2016 John Wiley & Sons, Ltd.A widespread pattern of the Tibetan plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non-crusted and crusted patches in Kobresia grasslands remains unknown. We reported on a field 15N-labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto soil surface. Crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil than plants in both patches 29days after the labeling. In non-crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly because of higher root biomass. Microorganisms in non-crusted patches were N-limited because of more living roots and competed strongly for N with roots. Inorganic N input to non-crusted patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant+soil) for inorganic N forms. Compared to non-crusted patches, microorganisms in crusted patches were more C-limited because of depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N-glycine than inorganic N. We conclude that overgrazing-induced crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N

Revisiting afro-alpine Lake Garba Guracha in the Bale Mountains of Ethiopia:Rationale, chronology, geochemistry, and paleoenvironmental implications

Abstract: Previous paleolimnological studies demonstrated that the sediments of Garba Guracha, situated at 3950 m asl in the afro-alpine zone of the Bale Mountains of Ethiopia, provide a complete Late Glacial and Holocene paleoclimate and environmental archive. We revisited Garba Guracha in order to retrieve new sediment cores and to apply new environmental proxies, e.g. charcoal, diatoms, biomarkers, and stable isotopes. Our chronology is established using 210Pb dating and radiocarbon dating of bulk sedimentary organic matter, bulk n-alkanes, and charcoal. Although bedrock was not reached during coring, basal ages confirm that sedimentation started at the earliest ~ 16 cal kyr BP. The absence of a systematic age offset for the n-alkanes suggests that “pre-aging” is not a prominent issue in this lake, which is characterised by a very small afro-alpine catchment. X-ray fluorescence scans and total organic carbon contents show a prominent transition from minerogenic to organic-rich sediments around 11 cal kyr BP coinciding with the Holocene onset. While an unambiguous terrestrial versus aquatic source identification seems challenging, the n-alkane-based Paq proxy, TOC/N ratios, δ13C values, and the sugar biomarker patterns suggest a predominantly autochthonous organic matter source. Supraregional climate events, such as the African Humid Period, the Younger Dryas (YD), a 6.5 cal kyr BP short drying event, and the 4.2 cal kyr BP transition to overall drier climate are recorded in our archive. The Garba Guracha record suggests that northern hemisphere forcings played a role in the Eastern African highland paleoclimate