The Effects of Nitrogen Fertilization on Water Use by Crested Wheatgrass

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The application of N fertilizer to crested wheatgrass on a dry rangeland site increased yields substantially. In the early part of the growing season when moisture was not limiting, soil moisture was withdrawn from the fertilized site at a higher rate than from the unfertilized plots. At later periods in the growing season the soil water potential curves paralleled each other with the fertilized crop growing under conditions of lower soil water potential. The decreased soil water potential was confirmed when the actual evapotranspiration, as measured by the energy balance method, was examined. The data indicate that for a period following rapid growth in the spring, the evapotranspiration of the fertilized block was less than that of the unfertilized. The soil water potential data indicate that seasonal evapotranspiration Was slightly higher on the fertilized plot than on the unfertilized. The water use efficiency, in terms of biomass produced per unit of water used, was much greater for fertilized crested wheatgrass and resulted in increased yields. The need for improved forage yields within the dry Interior grassland regions of British Columbia has been recognized for many years. In I974 an intensive project examining the effects of nitrogen (N) fertilization on rangeland seeded with crested wheatgrass (Agropyron clesertor-urn[Fisch. J Schutt.) was begun. As an integral part of this study, micrometeorological data on evaportranspiration were collected. The purpose of these data was to observe differences in how water was used by fertilized and unfertilized rangeland. The benefits of nitrogen fertilizer on seeded and natural rangeland have been discussed by numerous authors. Sneva et al. ( 1968) found higher yields, increased seed heads, and more rapid maturity on fertilized crested wheatgrass range. Studies on the effects of fertilizer on the water use characteristics of rangeland grasses are controversial because not all were made in the same way. Some measure the amount, others the rate of water use. Smika et al. (1965) found that seasonal water use by nonirrigated fertilized native range was not significantly different from that of unfertilized range. Similarly, Lauenroth and Sims (1976) noted that in Great Plains grasslands, N fertilization did not increase the amount of seasonal soil water used but it The \enior author is meteorologist, B.C. Ministry of the Environment, Resource