Browsing by Subject "Biogeochemistry"

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Distribution of Al, Fe, Si, and DOC between size fractions mobilised from topsoil horizons with progressing degree of podzolisation
(2022) Krettek, Agnes; Stein, Mathias; Rennert, Thilo
Aluminium, Fe, Si, and dissolved organic C (DOC) accumulate in the subsoil of Podzols after mobilisation in the topsoil. We conducted laboratory experiments with topsoil horizons with progressing degree of podzolisation by irrigation with artificial rainwater at varying intensity and permanence. We monitored the concentrations and distribution of mobilised Al, Fe, Si, and DOC between size fractions (< 1000 Dalton, 1 kDa– < 0.45 µm, and > 0.45 µm). Total eluate concentrations were increased at the onset of the experiments and after the first irrigation interruption, indicating non-equilibrium release. There was no statistical effect of the degree of podzolisation on element concentrations. Release of Al, Fe, and DOC was mostly dominant in the fraction 1 kDa– < 0.45 µm, indicating metals complexed by larger organic molecules and colloids. Silicon released was dominantly monomeric silicic acid < 1 kDa. Particularly with the least podzolised soils, Al and Si concentrations < 1 kDa might have been controlled by short-range ordered aluminosilicates, while their transport in colloidal form was unlikely. Our study pointed to both quantitative and qualitative seasonality of element release during podzolisation, to decoupling of Al and Si release regarding size, and to different minerals that control element release as a function of the degree of podzolisation.
Models for the representation of ecological systems? The validity of experimental model systems and of dynamical simulation models as to the interaction with ecological systems
(2001) Haag, Daniel; Kaupenjohann, Martin
Models guide the investigation of ecological phenomena and the managemant of man-environment interactions. Based on six papers, this thesis critically examines characteristic features, limitations and the scientific and societal role of experimental model systems (as well-tried instruments of knowledge production) and of dynamical simulation models (as representatives of relatively recent computer models). Experimental model systems are described as materially and conceptually closed systems with a limited number of parameters. They consist of a material component which is encoded into a formal (numerical) system through the measurement of defined parameters. The transfer of statements derived from model systems to natural systems is critically discussed. Dynamical systems - the paradigm for the representation of ecosystems - permit the simultaneous handling of a large number of parameters. Dynamical systems are conceptually closed systems and are based on the notion of an abstract state (focussing on 'being'). I contrast this view with an image of ecosystems as conceptually open systems ('becoming') which emphasizes the evolutionary openness of ecological systems, the internal production of novelty, and the emergence of system level properties. Taking the nitrogen cycle and its human alterations as an example, model concepts and limitations to the derivation of cause-effect-relationships in ecological systems are illustrated. Acknowledging the limited predictive capacity of simulation models and the intrinsic perspectivity of the identification of 'relevant' phenomena and parameters and drawing on new forms of knowledge production (as described by science studies), a modified role for model building and for simulation models - particularly with respect to science for policy - is sketched.