Long-term Nutrient Limitation

Ecosystems are open systems, with the potential for inputs and outputs of all biologically essential elements. In the long term, for any element to limit NPP or other ecosystem processes, one essential condition must be met  

The element must be lost from the system by some path-way(s) in addition to the loss of excess available nutrients (defined below) these additional losses must be large enough to balance element inputs at a point where the supply of that element (within the system) remains limiting to NPP. 

Some process(es) must constrain rates of biological Nj fixation to the extent that Nj fixers cannot respond to N deficiency sufficiently to eliminate it. 

While the first condition applies to all essential elements, the second is specific to N. Biological N, fixation is capable of adding tens to hundreds of kg ha year to ecosystems (Sprent and Sprent, 1990), more than enough to meet plant and microbial demand for N in a short time, and to overwhelm the capacity of 

With these conditions and c [uestions in mind, why is P more often limiting in lakes than on land, at least in the temperate zone P limits lake productivity because (a) unlike C and N, there are no mechanisms that can increase inputs of P when it is in short supply, as discussed above (b) P is relatively immobile within and through terrestrial ecosystems, so inputs of P to lakes are small and (c) lakes have an uncontrollable loss of P, in the sinking of particulate organic matter out of the euphotic zone. 

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Mg has no harmful effect on the environment and is an essential nutrient for animals and plants in small concentrations. UK occupational c5q)osure limits give a long-term exposure limit (8-hour TWA) of 4 mg/m for MgO dust and respirable fume (expressed as Mg). 

Both of them require at least limited model calibration. They do not stochastically estimate daily climate data for model evaluations or long-term changes in plant nutrient status and the resulting changes in plant growth and water balance. HYDRUS and UNSAT-H would be very useful and accurate if used in research however, they are difficult to use in engineering design of ET landfill covers and provide incomplete estimates of performance. 

Committees of experts organized by the Food and Nutrition Board of the National Academy of Sciences have compiled Dietary Reference Intakes (DRIs)—estimates of the amounts of nutrients required to prevent deficiencies and maintain optimal health. DRIs replace and expand on Recommended Dietary Allowances (RDAs), which have been published wih periodic revisions since 1941. Unlike the RDAs, the DRIs establish Lpper limits on the consumption of some nutrients, and incorporate the role of nutrients in lifelong health, going beyond deficiency diseases. Boh the DRIs and the RDAs refer to long-term average daily nutrient itakes, because it is not necessary to consume the full RDA every day. 

Long-term exposure of microbial populations to certain toxicants often is necessary for adaptation of enzymatic systems capable of degrading those toxicants. This was the case with the Exxon Valdez oil spill in Alaska in 1989. Natural microbial populations in Prince William Sound, Alaska, had developed enzyme systems that oxidize petroleum hydrocarbons because of long-term exposure to natural oil seeps and to hydrocarbons that leached from the pine forests in the area. Growth of these natural microbial populations was nutrient limited during the summer. Thus the application of nutrient formulations to the rocky beaches of Prince William Sound stimulated microbial growth and helped to degrade the spilled oil. 

Comins H. N. and McMurtrie R. E. (1993) Long-term response of nutrient-limited forests to CO2 enrichment—equilibrium behavior of plant-soil models. Ecol. Appl. 3(4), 666-681. 

In addition to limitation and acclimation in vegetation, soil processes play an important role in ecosystem responses to the increasing inpnt of both carbon dioxide and nitrogen. Here soil characteristics, responsible for the available nntrient pools, are of great importance. Even under increasing input of nitrogen with atmospheric deposition, nutrient concentrations can be depleted by increased uptake, especially in long term period. 

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