Sombric horizon: five decades without evolution

The sombric horizon is a diagnostic subsurface horizon defined in the soil classification system of the United States (Soil Taxonomy) and FAO (WRB), used to classify the soil at different categorical levels. The sombric horizon has a soil color darker than the overlying surface(s) horizon(s), and must show illuvial humus accumulation features, though they are not associated with aluminum (Al), as in the spodic horizon, nor associated with sodium (Na), as in the natric horizon. There are also criteria to distinguish it from buried A horizons. However, since the first references and proposed concept of the sombric horizon in African soils made by Sys and co-workers in the 1960s, and adopted by the Soil Taxonomy edition of 1975, few modifications have been made to its definition. Moreover, the pedogenic process involved in illuvial humus accumulation in these horizons remains inadequately clarified, making the distinction between the sombric and spodic or buried A horizon difficult and unclear. This review reports the historical evolution of the sombric horizon concept, its definition and inconsistencies under different soil classification systems, and the current hypothesis, together with its fragilities, proposed to explain the soil illuvial humus accumulation. Although it is recognized that further research is necessary, alternative criteria are proposed for the definition of the sombric horizon in the Brazilian System of Soil Classification

Učinak bakra na toksičnost i genotoksičnost kadmija u vodenoj leći (Lemna minor L.)

We investigated interactions between copper (in the concentrations of 2.5 μmol L-1 and 5 μmol L-1) and cadmium (5 μmol L-1) in common duckweed (Lemna minor L.) by exposing it to either metal or to their combinations for four or seven days. Their uptake increased with time, but it was lower in plants treated with combinations of metals than in plants treated with either metal given alone. In separate treatments, either metal increased malondialdehyde (MDA) level and catalase and peroxidase activity. Both induced DNA damage, but copper did it only after 7 days of treatment. On day 4, the combination of cadmium and 5 μmol L-1 copper additionally increased MDA as well as catalase and peroxidase activity. In contrast, on day 7, MDA dropped in plants treated with combinations of metals, and especially with 2.5 μmol L-1 copper plus cadmium. In these plants, catalase activity was higher than in copper treated plants. Peroxidase activity increased after treatment with cadmium and 2.5 μmol L-1 copper but decreased in plants treated with cadmium and 5 μmol L-1 copper. Compared to copper alone, combinations of metals enhanced DNA damage after 4 days of treatment but it dropped on day 7. In conclusion, either metal given alone was toxic/genotoxic and caused oxidative stress. On day 4 of combined treatment, the higher copper concentration was more toxic than either metal alone. In contrast, on day 7 of combined treatment, the lower copper concentration showed lower oxidative and DNA damage. These complex interactions can not be explained by simple antagonism and/or synergism. Further studies should go in that direction.U svrhu istraživanja interakcija između bakra kao esencijalnog elementa te kadmija kao neesencijalnog i toksičnog metala, vodenu leću Lemna minor L. uzgajali smo na podlogama s kadmijem (5 μmol L-1) odnosno s bakrom (2,5 μmol L-1 i 5 μmol L-1) te s njihovim kombinacijama. Unos metala u biljke povećavao se s trajanjem pokusa, a kod kombinacije metala u biljkama je izmjerena niža količina kadmija nego u onima uzgajanima samo na kadmiju. U biljkama tretiranim pojedinačnim metalom došlo je do povećanja sadržaja malondialdehida (MDA) te aktivnosti katalaze i peroksidaze u odnosu na kontrolne biljke. Također, primijećeno je oštećenje DNA iako kod bakra tek sedmog dana tretmana. Količina MDA i aktivnost obaju enzima dodatno se povećala na tretmanu kombinacijom kadmija i bakra (5 μmol L-1) nakon četvrtog dana pokusa, dok se količina MDA smanjila nakon sedmog dana kod kombinacije kadmija i 2,5 μmol L-1 bakra. U tim biljkama primijećena je i veća aktivnost katalaze, dok je aktivnost peroksidaze porasla na tretmanu kadmijem i 2,5 μmol L-1 bakrom, ali se smanjila na tretmanu kadmijem i 5 μmol L-1 bakrom. Oštećenje DNA koje je bilo veće kod kombinacije metala nakon četvrtog dana, osobito u usporedbi sa samim bakrom, smanjilo se nakon sedmog dana pokusa. Iz ovih rezultata može se zaključiti da su oba metala u istraživanim koncentracijama toksična i genotoksična za vodenu leću i da uzrokuju oksidacijski stres. Kadmij u kombinaciji s bakrom više koncentracije bio je toksičniji od pojedinačnih metala nakon četvrtog dana pokusa, dok su u biljaka tretiranih kombinacijom kadmija i bakra niže koncentracije toksični učinci bili manji. Budući da su primijećene interakcije vrlo kompleksne i ne uključuju samo antagonizam odnosno sinergizam potrebna su daljnja istraživanja

Impact of bleaching conditions on the components of non-photochemical quenching in the zooxanthellae of a coral

Mass coral bleaching events are a worldwide phenomenon, which generally occur during periods of elevated sea surface temperature and intense sunlight. These conditions result in a decline in photochemical efficiency of symbiotic microalgae (zooxanthellae) which ultimately leads to the expulsion of these symbionts. The physiological mechanism which triggers the release of the zooxanthellae has yet to be adequately determined. Under bleaching conditions, non-photochemical quenching (NPQ) is used to dissipate excess energy from photosystem II (PSII). NPQ was partitioned into three components, (energy dependent quenching [qE], state transition quenching [qT] and photoinhibitory quenching [qI]), based on relaxation kinetics upon addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and darkening. This investigation revealed that for corals not exposed to bleaching stress, qE was the principle means of energy dissipation (∼60% of the total NPQ). In corals exposed to either high-light (475 μmol photons m-2 s-1 and 25°C) or elevated temperature (225 μmol photons m-2 s -1 and 32°C) treatments, the dominant component of NPQ was qE and the relative proportions did not change during the exposure period (1-8 h). When exposed to bleaching conditions (475 μmol photons m-2 s -1 and 32°C) the contribution of the different components changed after 4 h and the total NPQ increased. At this time, the contribution of qT to the total NPQ significantly increased to equal that of qE (40%), suggesting state transitions become more important under such conditions. Throughout the exposure period in all treatments, no change in the proportion of qI was observed. © 2005 Elsevier B.V. All rights reserved

An Occurrence of Interstratified Kaolinite-smectite Minerals in a Red-black Soil Toposequence

ABSTRACT: Interstratified kaolinite-smectite minerals form the major part of a pedon transitional between 'red ' kaolinitic and black smectitic soils in a soil toposequence derived from basalt in Burundi, Central Africa. These interstratified kaolinite-smectites have many X-ray, morphological and chemical features incommon with 'tabular halloysite'. Modifications tothe 001 spacings of these minerals brought about by thermal nd chemical treatments, however, show that they are not halloysite but consist of a mixture of ~ 70~o kaolinite and 30~o smectite randomly interstratified. Field evidence indicates that hese clays can be considered asa transient stage in the overall reaction smectite--,kaolinite + ron oxides, which takes place in this landscape whenever the slope gradients give rise to better drainage conditions. When Wilson & Cradwick (1972) reported the first occurrence of interstratified kaolinite-smectite n soils, they suggested that, as X-ray criteria for identifying such clays had now been made available, further accounts of such occurrences could be expected. Although several subsequent papers described similar minerals occurring in sediment

Evolution de la stabilité structurale du sol sous l'influence des racines de trèfle (Trifolium pratense L.) et de ray-grass (Lolium multiflorum Lmk.). Observations pendant et après culture

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