Study was made of the rates at which and the routes by which nitrogen once taken up by the plant moves into litter and soil compartments and from them to new plant growth. Field mini—plots were given an initial pulse treatment with 15N, following which destructive removals were made over the next 5 growing seasons. In the year oif application, 15N moved primarily to the green herbage; 0.58 of all 15N recovered in plant materials the 1st year was in such herbage. There was late season translocation of N from aboveground to belowground plant parts. The pulse N peaked in the aboveground dead compartment in the winter months following the first and preceding the second growth season. Transfer of 15N from the aboveground litter occurred principally during the second growing season. The 15N content of crowns and live roots did not change significantly during the 5 seasons; that of senescent and detrital roots increased significantly during the same interval. It was concluded that the annual N requirement in ungrazed blue grama is met by a combination of 4 nitrogen—supplying mechanisms. One of these is that of internal translocation whereby N of one season is stored over winter belowground and then moved to new growth in the next growing season. Another is that of mineralization of easily decomposable organic materials; among these are certain herbage components, roots exudates and exfoliates, and short—lived unsuberized roots. A third is mineralization of organic nitrogen synthesized by microorganisms that grow on energy—rich materials such as those named in the immediately preceding sentence. Finally, not all of the plant and microbially synthesized organic N undergoes quick release to the available N pool; a portion of it undergoes polymerization and becomes humic nitrogen, from which there is slow release to the available N pool. It is the first 3 mechanisms that promote quick recycling of N in the blue grama system. Once a given N atom makes its initial entry into the blue grama plant, there is greatly increased probability that the atom will again enter ner herbage growth in each of several following years. Nitrogen that enters the soil humus is very slowly recycled into new plant growth.
Summary1. The theta-logistic is a simple and flexible model for describing how the growth rate of a population slows as abundance increases. Starting at r m (taken as the maximum population growth rate), the growth response decreases in a convex or concave way (according to the shape parameter h) to zero when the population reaches carrying capacity. 2. We demonstrate that fitting this model to census data is not robust and explain why. The parameters h and r m are able to play-off against each other (providing a constant product), thus allowing both to adopt extreme and ecologically implausible values. 3. We use simulated data to examine: (i) a population fluctuating around a constant carrying capacity (K); (ii) recovery of a population from 10% of carrying capacity; and (iii) a population subject to variation in K. We show that estimates of extinction risk depending on this or similar models are therefore prone to imprecision. We refute the claim that concave growth responses are shown to dominate in nature. 4. As the model can also be sensitive to temporal variation in carrying capacity, we argue that the assumption of a constant carrying capacity is both problematic and presents a fruitful direction for the development of phenomenological density-feedback models.
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