Volcanic ash soils contain very large stocks of soil organic matter (SOM) per unit area. Consequently, they constitute potential sources or sinks for the greenhouse gas carbon dioxide. Whether soils become a net carbon source or sink with climate and/or land-use change depends on the stability of SOM against decomposition, which is influenced by stabilization mechanisms in the soil. To quantify organic carbon stocks and to clarify the importance of chemical and physical soil characteristics for carbon stabilization in volcanic ash soils, we applied selective extraction techniques, performed X-ray diffraction analysis of the clay fraction and estimated pore-size distribution of soils under natural upper montane forest and grassland (páramo) in the Ecuadorian Andes. Our results show that organic carbon stocks under both vegetation types are roughly twice as large as previously reported global averages for volcanic ash soils. SOM stabilization is suggested to be dominantly influenced by the following chemical and physical soil characteristics: (i) direct stabilization of SOM in organo-metallic (Al-humus) complexes, explaining at most 40% of carbon accumulation, (ii) indirect protection of SOM (notably aliphatic compounds) through low soil pH and toxic levels of Al, and probably also (iii) physical protection of SOM caused by a very large micro-porosity. Moreover, in the case of the forest soils, inherent recalcitrance of OM itself was responsible for substantial accumulation in ectorganic horizons. Both vegetation types contributed to soil acidification, thus increasing SOM accumulation.
Analysis of plant micro-and macrofossils from a sequence from Usselo (The Netherlands) allows a detailed reconstruction oflocal developments during a period of more than one and a half millennia, starting before the Belling period and lasting till the end of the Altered period. The sequence started in an oligotrophic shallow pool with a very low organic production in a barren sandy landscape. An early phase with G/oeotrichia type may be connected with the ability of N-fixation of these blue-green algae. This capacity may have initiated nutrient availability. Characeae played an important role on the initially sandy substrate. These pioneers opened up the site for other aquatics and helophytes, e.g. Potamogeton a/pinus, Carex rostrata, Phragmites australis, Equisetumfluviatile, followed by Menyanthes trifoliata, Myriophyllum spicatum and various Carex species. This sequence indicates a gradual eutrophication of the site, accompanied by an increase in production of organic matter (interrupted during the Older Dryas), as substantiated by the organic/inorganic ratio of the substrate. The tendency towards eutrophication ended when the vegetation at the mire surface lost its contact with the ground water. The sequence ended with a vegetation type dominated by Sphagnum and the occurrence of oligotraphentous Thecamoeba.These changes in local vegetation are explicable without resort to temperature fluctuations. The nutrient status of the habitat, and the water level(the latter influenced by sand and organic sediment deposition) were apparently the most important factors in the vegetation succession.
Fifteen populations of Azolla were sampled in the western and north-eastern parts of The Netherlands. After cultivation under identical conditions in a greenhouse they could be divided into two species which had previously been recorded from The Netherlands under the names of Azolla filiculoides Lam. and Azolla caroliniana Willd. Populations which appeared to be morphologically intermediate between these two species were eventually found to be true A.filiculoides. Although identification of certain forms in the field remains difficult, the two species can always be distinguished under a microscope by the number of cells forming the trichomes on the surface of the upper lobe of the leaflets, the septation of the glochidia and the surface structure of the megaspores. Other characteristics which have been reported in the literature are not always consistent. However, well-developed specimens of A.filiculoides can easily be recognised from their general appearance. In the autumn, when the plants become reddish as a result of anthocyanin production, A. filiculoides turns to a purer red than the plants of the other species which become more browny red. In both species the anthocyanins contain the relatively rare anthocyanidins luteolinidin and apigenidin. Anthocyanin production was found to be influenced by temperature and water composition but not by day-length. Well-developed forms of A. filiculoides generally occur in more eutrophic water than plants of the other species. According to the revised classification of the New World species of Azolla by SVENSON (1944), the Dutch specimens previously referred to as A. caroliniana do not belong to this species, but to either A. mexicana Presl or A. microphylla Kaulfuss.
Fifteen strains of subgenus Lemna, collected in ponds and ditches in the western part of The Netherlands. strain G3 (previously described as Lemna gibba) and strains 6573 and F (previously described as Lemna minor) were aseptically cultured on M-medium in the presence and absence of EDDHA. When cultivated on EDDHA medium the strains showed a marked variation in the degree of gibbosity. whereas in the absence of the chelate all strains were more or less flat. The strains could be divided into two groups as far as the degree of gibbosity in the presence of EDDHA was concerned. There were no consistent differences in morphology between the two groups if cultured on the nutrient medium devoid of EDDHA. In the light of the present investigation distinction between Lemna gibba and Lemna minor seems not always possible.
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