A new chemical mass balance technique has been developed for simultaneous mineralogical quantification and chemical characterization of soil clays. The procedure includes separation of the whole clay (<2 μm fraction) into six particle size fractions (<0.02, <0.06, <0.2, 0.02–0.06, 0.06–0.2, and 0.2–2 μm fractions), chemical analysis of the whole clay and each of the six fractions, and fitting of a nonlinear chemical mass balance model to the chemical analyses. As written, the chemical mass balance model is valid only for samples containing mixtures of quartz, kaolinite, illite, and mixed-layered smectiteillite. Samples containing carbonates and free iron compounds may be analyzed using the technique if these phases are chemically removed prior to particle size fractionation. Accuracy of the new technique was tested using synthetic data and found to depend on the quality of the input data; however, clay phase quantification within three percentage points of known values was readily achieved. Precision of the technique was evaluated by independently preparing and analyzing five samples of the same soil clay. Standard deviations for clay phase percentages (w:w) in the <2 μm fraction were all less than one percent. The new technique yields accurate determinations of chemistry for the smectitic and illitic phases in mixed-layered smectiteillite, and qualitative estimates for the chemistry of 10 Å-illite. The elemental compositions of quartz and kaolinite are assumed a priori and treated as constants within the non-linear chemical mass balance model.

Since Rosenzweig showed the destabilisation of exploited ecosystems, the so called Paradox of enrichment, severalmechanisms have been proposed to resolve this paradox. In this paper we will show that a feeding threshold in the functional response for predators feeding on a prey population stabilizes the system and that there exists a minimumthreshold value above which the predator-prey system is unconditionally stable with respect to enrichment. Two models areanalysed, the first being the classical Rosenzweig-MacArthur (RM) model with an adapted Holling type-II functional response to include a feeding threshold. This mathematical model can be studied using analytical tools, which gives insight into the mathematical properties of the two dimensional ode system and reveals underlying stabilisation mechanisms. The second model is a mass-balance (MB) model for a predator-prey-nutrient system with complete recycling ofthe nutrient in a closed environment. In this model a feeding threshold is also taken into account for the predator-prey trophic interaction. Numerical bifurcation analysis is performed on both models. Analysis results are compared between models and are discussed in relation to the analytical analysis of the classical RM model. Experimental data from the literature of a closed system with ciliates, algae and a limiting nutrient are used to estimate parameters for the MB model. This microbial system forms the bottom trophic levels of aquatic ecosystems and therefore a complete overview of its dynamics is essential for understanding aquatic ecosystem dynamics.