Nitrogen Budget of Agropyron dasystachyum in Canadian Mixed Prairie

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-Objectives of this research were to determine the extent of internal cycling of N in Agropyron dasystachyum (Hook.) Scribn. (northern wheatgrass) and to determine the impact of individual plant N balance on N cycling in a mixed grassland ecosystem dominated by this species. The soil-plant system contained 381 g N m-2, nearly all in the organic form. Plant-available N, about 1.4% of total N, was mainly in the form of NH4; NO3 was scarce, probably because of effective plant scavenging, and inhibition of nitrification by low soil moisture. Nitrogen retranslocation from senescing leaves to new leaves occurred over the entire growing season. The highest N concentration was observed in the newlyemerged leaves, regardless of the time of emergence. Nitrogen recycled within plants, calculated from seasonal changes of N and biomass in individual plant leaves was as much as 69% of the total N taken up by A. dasystachyum shoots. Nitrogen-use efficiency and N-conservation are well-developed in this ecosystem. INTRODUCTION Nitrogen supply has a strong impact on the productivity and functioning of natural grassland ecosystems (Coupland et al., 1974; Reuss and Innis, 1977; Lauenroth and Dodd, 1978; Risser and Parton, 1982). During the past two decades studies of N cycling in grassland ecosystems have examined inputs, outputs, flows through compartments and the effects of supplemental N on the structure and functioning of these ecosystems (Henzell and Ross, 1973; Coupland et al., 1974; Bokhari and Singh, 1975; Clark, 1977; Woodmansee, 1978; Risser and Parton, 1982). Information on the N balance of individual grass species, the efficiency of N use, and the effects of N form on plant growth in grassland ecosystems is more limited, particularly for the northern mixed grassland region of North America. Studies of seasonal retranslocation in shortgrass (Clark, 1977; Woodmansee et al., 1978) and tallgrass species (Adams, 1982; Hayes, 1985) indicate the importance of internal N recycling in perennial grasses. The objectives of the research were (1) to measure internal recycling of N in Agropyron dasystachyum (Hook.) Scribn. (northern wheatgrass) growing in the field and (2) to determine the impact of individual plant N balance on overall N cycling in an A. dasystachyumdominated ecosystem. In particular, N fluxes, including uptake, retranslocation and the relative contribution of different organs to overall N budget were examined. STUDY SITE AND METHODS This research was carried out during the 1985-1986 growing seasons at the Matador Field Station located about 170 km southwest of Saskatoon, Saskatchewan, Canada. Vegetation of the study site was classified by Coupland et al. (1974) as the Agropyron-Koeleria Faciation of the Mixed Prairie (Stipa-Bouteloua) Association. The dominant species are Agropyron dasystachyum, A. smithii, Koeleria cristata, Stipa viridula and Artemisia frigida, contributing 34, 7, 21, 14 and 2% of basal cover, respectively. Agropyron spp. accounted for 75% of the total biomass. Soils at the study site are classified as the Rego-Brown and Calcareous Brown Series in