170,717 research outputs found
On Veldkamp Lines
No full textOne says that Veldkamp lines exist for a point-line geometry Γ if, for any three distinct (geometric) hyperplanes A, B and C (i) A is not properly contained in B and (ii) A ∩ B ⊆ C implies A ⊂ C or A ∩ B = A ∩ C. Under this condition, the set V of all hyperplanes of Γ acquires the structure of a linear space – the Veldkamp space – with intersections of distinct hyperplanes playing the role of lines. It is shown here that an interesting class of strong parapolar spaces (which includes both the half-spin geometries and the Grassmannians) possess Veldkamp lines. Combined with other results on hyperplanes and embeddings, this implies that for most of these parapolar spaces, the corresponding Veldkamp spaces are projective spaces. The arguments incorporate a model of partial matroids based on intersections of sets
Soil organic carbon dynamics in pastures established after deforestation in the humid tropics of Costa Rica
No full textCurrently, rates of deforestation in the tropics are probably higher than ever before in the past. As a consequence, changes in the earth's physical and chemical environments are proceeding at unprecedented rates. Increasing atmospheric concentrations of CO 2 , N 2 O and other trace gases, caused by enhanced emissions from soils after forest clearing, show that deforestation in tropical areas is of global importance. Recent estimates suggest a net release of carbon from the world's tropics, due to deforestation, of between 0.42 and 1.60 Pg C yr -1(1 Pg = 10 15g) of which 0.1 to 0.3 Pg C yr -1are attributed to decreases in soil organic matter content. This carbon release from tropical areas is second only to the global release from the burning of fossil fuels (which is about 5.3 Pg C yr -1).The main objective of this thesis was to quantify the changes in soil organic carbon storage and the resulting release of CO 2 after the conversion of tropical rain forest to pasture on two contrasting soil types in the humid tropics of Costa Rica. To study changes in soil organic carbon storage, sites of an Andisol and an Inceptisol, cleared at different times in the past (deforestation sequences) were compared. A deforestation map, based on aerial photographs from the period 1952 - 1984, was made for a part of the Atlantic Zone of Costa Rica, providing a well documented history of forest clearing. Using GIS techniques, this deforestation map was combined with an available soil map to select the study sites. Analysis of deforestation patterns on the map demonstrated a close relation of deforestation rate with accessibility and soil quality.Soil organic matter levels are the result of complex production and decomposition processes. The input of carbon from grass plant roots into the soil was quantified, using pulse labelling with 14C. The pulse labelling experiment revealed that root dry matter production of an improved pasture like Brachiaria (12 Mg ha -1yr -1) was about twice the root production of a low-productive species like Axonopus (6 Mg ha-1 yr-1). Root biomass of Brachiaria was about three times the root biomass of Axonopus due to higher residence time of carbon in the root biomass of Brachiaria as compared to Axonopus . Root exudates of grass plants were found to have a minor direct contribution to the longer term carbon dynamics, either because exudation rate was small or because decomposition was fast and complete.Decomposition of soil organic matter was measured using the δ 13C method, which uses differences in natural 13C isotope levels in vegetation (C3 and C4 vegetation) and soil organic matter to calculate changes in soil organic carbon. The method is applicable in soil organic matter studies where a change from C3 to C4 vegetation has occurred (or vice versa). It was demonstrated that for a correct application of the method, detailed information of changes in bulk densities accompanying changes in land use was vital. An uncertainty analysis of the δ 13C method demonstrated that the output of the δ 13C method in soil organic matter studies was highly variable due to variations in the input data. Spatial variability was the main source of the uncertainty in input data. However, variations due to sampling error and short scale variability were considerable and should not be ignored.Information on carbon input and decomposition was integrated, using a simple structured soil organic carbon (SOC) model which included carbon isotope fractionation during decomposition and depth dependent decomposition and humification rates. With this model, the observed changes in soil organic carbon and corresponding δ 13C levels during the conversion from a humid tropical forest to a cattle pasture were simulated successfully for the two soil types. With the calibrated model the cumulative net C02 release was calculated. The cumulative net release of CO 2 for pastures with low productive grass species (Axonopus compressus), varied from 31.5 (Humitropept) to 60.5 Mg C ha -1(Hapludand) in the first 20 years after forest clearing. These cumulative emissions could be reduced to 12.0 and 24.7 Mg C ha -1respectively, if higher productive grass species (e.g. Brachiaria dictyoneura ) would be introduced into the area.Decomposition rates were strongly influenced by depth. Inclusion of deeper layers in soil organic carbon simulation studies and considering carbon isotopes will probably improve the performance of SOC models in long-term studies
Soil organic carbon dynamics in pastures established after deforestation in the humid tropics of Costa Rica
No full textCurrently, rates of deforestation in the tropics are probably higher than ever before in the past. As a consequence, changes in the earth's physical and chemical environments are proceeding at unprecedented rates. Increasing atmospheric concentrations of CO 2 , N 2 O and other trace gases, caused by enhanced emissions from soils after forest clearing, show that deforestation in tropical areas is of global importance. Recent estimates suggest a net release of carbon from the world's tropics, due to deforestation, of between 0.42 and 1.60 Pg C yr -1(1 Pg = 10 15g) of which 0.1 to 0.3 Pg C yr -1are attributed to decreases in soil organic matter content. This carbon release from tropical areas is second only to the global release from the burning of fossil fuels (which is about 5.3 Pg C yr -1).The main objective of this thesis was to quantify the changes in soil organic carbon storage and the resulting release of CO 2 after the conversion of tropical rain forest to pasture on two contrasting soil types in the humid tropics of Costa Rica. To study changes in soil organic carbon storage, sites of an Andisol and an Inceptisol, cleared at different times in the past (deforestation sequences) were compared. A deforestation map, based on aerial photographs from the period 1952 - 1984, was made for a part of the Atlantic Zone of Costa Rica, providing a well documented history of forest clearing. Using GIS techniques, this deforestation map was combined with an available soil map to select the study sites. Analysis of deforestation patterns on the map demonstrated a close relation of deforestation rate with accessibility and soil quality.Soil organic matter levels are the result of complex production and decomposition processes. The input of carbon from grass plant roots into the soil was quantified, using pulse labelling with 14C. The pulse labelling experiment revealed that root dry matter production of an improved pasture like Brachiaria (12 Mg ha -1yr -1) was about twice the root production of a low-productive species like Axonopus (6 Mg ha-1 yr-1). Root biomass of Brachiaria was about three times the root biomass of Axonopus due to higher residence time of carbon in the root biomass of Brachiaria as compared to Axonopus . Root exudates of grass plants were found to have a minor direct contribution to the longer term carbon dynamics, either because exudation rate was small or because decomposition was fast and complete.Decomposition of soil organic matter was measured using the δ 13C method, which uses differences in natural 13C isotope levels in vegetation (C3 and C4 vegetation) and soil organic matter to calculate changes in soil organic carbon. The method is applicable in soil organic matter studies where a change from C3 to C4 vegetation has occurred (or vice versa). It was demonstrated that for a correct application of the method, detailed information of changes in bulk densities accompanying changes in land use was vital. An uncertainty analysis of the δ 13C method demonstrated that the output of the δ 13C method in soil organic matter studies was highly variable due to variations in the input data. Spatial variability was the main source of the uncertainty in input data. However, variations due to sampling error and short scale variability were considerable and should not be ignored.Information on carbon input and decomposition was integrated, using a simple structured soil organic carbon (SOC) model which included carbon isotope fractionation during decomposition and depth dependent decomposition and humification rates. With this model, the observed changes in soil organic carbon and corresponding δ 13C levels during the conversion from a humid tropical forest to a cattle pasture were simulated successfully for the two soil types. With the calibrated model the cumulative net C02 release was calculated. The cumulative net release of CO 2 for pastures with low productive grass species (Axonopus compressus), varied from 31.5 (Humitropept) to 60.5 Mg C ha -1(Hapludand) in the first 20 years after forest clearing. These cumulative emissions could be reduced to 12.0 and 24.7 Mg C ha -1respectively, if higher productive grass species (e.g. Brachiaria dictyoneura ) would be introduced into the area.Decomposition rates were strongly influenced by depth. Inclusion of deeper layers in soil organic carbon simulation studies and considering carbon isotopes will probably improve the performance of SOC models in long-term studies
Regional variation in soil carbon and δ13 C in forests and pastures of northeastern Costa Rica
No full textRecent studies suggest that the direction and magnitude of changes in soil organic carbon (soil C) pools following forest-to-pasture conversion in the tropics are dependent upon initial soil conditions and local factors (e.g. pre-conversion soil C content, soil texture, vegetation productivity, and management practices). The goal of this study was to understand how landscape-scale variation in soil-forming factors influenced the response of soil C pools to forest clearing and pasture establishment in northeastern Costa Rica. We measured soil C and its stable isotopic composition in 24 paired pasture and reference forest sites distributed over large gradients of edaphic characteristics and slope throughout a 1400 km2 region. We used the large difference in stable C isotopic signatures of C3 vegetation (rain forest) versus C4 vegetation (pasture grasses) as a tracer of soil C dynamics. Soil C pools to 30 cm depth ranged from 26% lower to 23% higher in pastures compared to paired forests. The presence of non-crystalline clays and percent slope explained between 27 and 37% of the variation in the direction and magnitude of the changes in soil C storage following pasture establishment. Stable carbon isotopes (δ13C) in the top soil (0–10 cm) showed a rapid incorporation of pasture-derived C following pasture establishment, but the vegetation in these pastures never became pure C4 communities. The amount of forest-derived soil C in pasture topsoils (0–10 cm) was negatively correlated to both pasture age and the concentrations of non-crystalline iron oxides. Together these results imply that site factors such as soil mineralogy are an important control over soil C storage and turnover in this region
The role of dissolved organic carbon, dissolved organic nitrogen, and dissolved inorganic nitrogen in a tropical wet forest ecosystem
No full textAlthough tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha–1 y–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha–1 y–1 below the litter layer to around 50 kg C kg C ha–1 y–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO3−). The dominance of NO3– relative to the total amount nitrate of N leaching from the soil shows that NO3– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000230804500001&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=6e41486220adb198d0efde5a3b153e7dVoR - Version of Recor
Magic Three-Qubit Veldkamp Line and Veldkamp Space of the Doily
No full textA magic three-qubit Veldkamp line of , i.\,e. the line comprising a hyperbolic quadric , an elliptic quadric and a quadratic cone that share a parabolic quadric , the doily, is shown to provide an interesting model for the Veldkamp space of the latter. The model is based on the facts that: a) the 20 off-doily points of form ten complementary pairs, each corresponding to a unique grid of the doily; b) the 12 off-doily points of form six complementary pairs, each corresponding to a unique ovoid of the doily; and c) the 15 off-doily points of -- disregarding the nucleus of -- are in bijection with the 15 perp-sets of the doily. These findings lead to a conjecture that also parapolar spaces can be relevant for quantum information
Objective image quality assessment in X-ray breast imaging
No full textContains fulltext :
212645.pdf (Publisher’s version ) (Open Access)Radboud University, 12 december 2019Promotor : Karssemeijer, N. Co-promotores : Broeders, M.J.M., Veldkamp, W.J.H., Sechopoulos, I
Veldkamp en het onderzoek op het terrein van de sociale zekerheid
No full textINLEIDING. Op verzoek van de redactie zullen wij in dit artikel aandacht schenken aan de bijdragen van prof.dr G.M.J. Veldkamp op het terrein van de sociale zekerheid. Denkend over een nadere afbakening, leek het ons goed in het bijzonder aandacht aan de relatief beperkte omvang van het onderzoek op het terrein van de sociale zekerheid te besteden. Ook Veldkamp baarde dit verschijnsel zorgen. Hij sprak in dit verband van een stiefmoederlijke bedeling van het sociale zekerheidsonderzoek (Veldkamp, 1984, p. 74-75). In eerdere publikaties hebben wij hiervoor overigens ook al aandacht gevraagd (zie bijv.: Van den Bosch, Petersen, 1983). In paragraaf 2 wordt in dit verband kort de bijdrage van Veldkamp zelf geschetst. Daarna volgt een beschouwing over de wisselwerking tussen onderzoek en beleid in de sociale zekerheid en de betekenis van de onderlinge afhankelijkheidsrelaties daarbij in paragraaf 3. In het verlengde hiervan wordt in paragraaf 4 aan de hand van een case studie betreffende evaluatieonderzoek sociale zekerheid, ingegaan op de relatie besluitvorming - onderzoek sociale zekerheid. De samenvatting en conclusies volgen in paragraaf 5
Long-term CO2 production from deeply weathered soils of a tropical rain forest: evidence for a potential positive feedback to climate warming
No full textCurrently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO2) diffusing out of soils is an important flux in the global carbon. Although soil CO2 efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO2 and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5-year study in which we measured soil CO2 production on two deeply weathered soil types at different depths in an old-growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO2 efflux varied between 2.8–3.6 μmol CO2-C m−2 s−1 (old alluvium) and 3.4–3.9 μmol CO2-C m−2 s−1 (residual). More than 75% of the CO2 was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO2 production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO2 production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO2 production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming
FIG. 1 in Schmidiella Veldk., gen. nov., an enigmatic new genus of Gramineae from Laos
No full textFIG. 1. — Schmidiella maxwellii Veldk., gen. nov., sp. nov.: A, habit; B, sheath/ligule/blade;C, spikelet;D, Palea; E, anthers/ovary/style; F, caryopsis/style (L3963931). Scale bars: A, 1 cm; B-F, 1 mm.Published as part of Veldkamp, Jan-Frits, 2018, Schmidiella Veldk., gen. nov., an enigmatic new genus of Gramineae from Laos, pp. 55-58 in Adansonia (3) 40 (4) on page 57, DOI: 10.5252/adansonia2018v40a4, http://zenodo.org/record/460207
- …
