Limitation mineral nutrient

FIGURE 3 Expanding the model in Figure 1 by adding an organic form of the limiting mineral nutrient, assumed here to be amino acids only consumed by the heterotrophic bacteria. 

In the idealized framework outlined above, bacterial physiology is represented by two parameters only. The yield coefficient YBc gives the stoichiometric link between consumption of organic carbon and the limiting mineral nutrient, whereas the specific affinity constant aB defines the volume of water cleared for limiting mineral nutrient per unit bacterial biomass, per unit time. 

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

The formula for BCD in Eq. (2) was derived under the assumption that the bacterial growth rate was mineral nutrient limited. If the supply rate of labile organic carbon from allochthonous and autochthonous sources is insufficient to meet this demand, the pool of labile dissolved organic carbon (DOC) will eventually be depleted and the bacteria will become carbon... 

Values of q >1 thus correspond to steady states with carbon-limited bacterial growth rate, whereas values of q <1 correspond to mineral nutrient limitation. Equation (3) illustrates how everything depends on everything in a steady-state situation. The value of q is a function not only of the ratio between production rate of organic bacterial substrates and the product of loss rates of the bacterial predators and competitors to higher predators, but also of all the parameters representing physiological properties of bacteria, bacterial competitors, and bacterial predators. 

In the preceding discussions, we assumed labile DOC and DON to be produced at rates y/c and y/N, respectively, without discussing their sources and how the production rate and the composition of the produced material would be expected to vary with food web structure. The important differences among different models can be illustrated by some examples. One potential model is that DOC production is an overflow mechanism occurring in mineral-nutrient-limited phytoplankton not able to use the photo-synthetically produced organic carbon for biomass production due to lack... 

FIGURE 4 Integrated model for bacterial production illustrating qualitatively the suggested proportionality with the square of total plankton biomass for C-limited bacteria, and proportionality with the square of ciliate biomass for mineral-nutrient-limited bacteria. In this model, C-limited growth occurs for food web structures with a high ciliate total plankton biomass. 

Mineral nutrient limitation of microbial degradation has been put forward as an explanation for accumulation of carbon-rich DOM after a Phae-ocystis bloom (Thingstad and Billen 1994). The increase in carbohydrate/POC due to overflow metabolism will give rise to a substrate with a C/P and C/N ratio that is unfavorable to bacteria. Since the C/P ratio of bacteria may be considerably lower than that of phytoplankton (Vadstein et al. 1988), especially phosphate limitation may hamper microbial degradation (Thingstad et al. 

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