Standing State and Cycling of Nitrogen in Soilvegetation Components of Prairie Ecosystems

In the present paper, distribution of nitrogen in different plant compartments and in the top 20 cm soil from three grassland sites for a 3-year period at the time of peak standing crop has been studied. The sites represent short-grass, mixed-grass, and tall-grass prairies located, respectively, in Colorado, South Dakota, and Oklahoma. Partitioning, uptake, transfers, and release of nitrogen in the above-ground and below-ground compartments were evaluated with respect to the abiotic factors. A positive linear relationship was found with the total nitrogen content in shoots (g m⁻²) and the annual precipitation as well as with the annual actual evapotranspiration. On the other hand, a negative linear relationship was found with the nitrogen per gram dry weight in shoots, and the annual precipitation and evapotranspiration. Inter-seasonal and inter-site variations in the nitrogen content (g m⁻²) in different plant compartments indicated significant differences. The short-grass and mixed-grass prairies indicated greater nitrogen content in the above-ground and below-ground plant tissues as compared to the tall-grass prairie. However the total nitrogen content in shoots from the tall-grass prairie was greater than that of short-grass or mixed-grass prairies which was mainly due to greater shoot biomass in the tall-grass prairie. The major portion of nitrogen (over 90 per cent) in the system was retained in the soil while a fraction of nitrogen (4-8 per cent) resided in the biological material. The distribution of nitrogen in different plant compartments, the uptake, transfer, and release of nitrogen with respect to abiotic factors have been discussed in detail.