Internal Cycling of Nitrogen in Shortgrass Prairie

Abstract

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 '5N, following which destructive removals were made over the next 5 growing seasons. In the year of 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 growing season. Transfer of '5N from the aboveground litter occurred principally during the second growing season. The '5N 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 micro- bially 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 new herbage growth in each of several following years. Nitrogen that enters the soil humus is very slowly recycled into new plant growth.