Evaluation of Water and Nitrogen Stress on Bromegrass Growth 1

Percent stress [reduction in growth rate of bromegrass (Bromus inermis L.) due to deficiency] was calculated by the method of Greenwood et al. to evaluate the effects of deficiencies of N, water, and N plus water combined. Dry matter yields of bromegrass receiving either (a) 270 kg/ha of N, (b) complete irrigation, (c) N plus irrigation, or (d) no treatment 3 weeks earlier were determined at periodic intervals in the field at low, medium, and relatively high levels of N availability. Percent stress due to N deficiency was greatest where N availability was lowest, generally increased as the grass approached maturity, but was insignificant as long as a relatively large pool of inorganic N remained in the soil. Percent stress due to water deficiency was affected by precipitation, and was generally greatest for plots low in available N, even though soil water content (to 120 cm depth) was greater in those plots. Generally, percent stress due to deficiencies of water and N combined was approximately equal to the sum of stresses for the two measured separately. The concept of percent stress appears to be useful for quantifying the effects of both N and water deficiencies, alone and in combination, on growth rates of grass. In connection with other measures of physiological condition or activity, this concept may have considerable utility in agronomic research. Additional index words: N x water interaction, Growth rates, Nitrogen deficiencies, Water deficiencies. L Contribution from the Northern Plains Branch, Soil and Water Conserv. Res. Div., ARS, USDA. Received Jan. 20, 1971. " Research Soil Scientist, USDA, Mandan, North Dakota 58554. D of both nitrogen and water limit growth of grasses in the Northern Great Plains (4, 5). Often, water is adequate early in the growing season, but diminishes later when evapotranspiration increases and precipitation decreases. Similarly, in unfertilized grasslands a deficiency of N usually exists at all times, but probably restricts growth most during the jointing to heading period when growth rates are highest. Consequently, deficiencies of both water and N may concurrently limit plant growth. However, we have little information to indicate the relative intensities of such effects, or how they interact. Recently, Greenwood et al. (1, 2) developed a method to quantify the effects of N deficiencies on the rate of plant growth. They defined the values so obtained as "nitrogen stress", and were able to calculate the percent nitrogen stress that occurred under different conditions. Basically, their technique involved growing grass on adjacent plots—one adequately N-fertilized and one not—for several weeks and then harvesting. This process was repeated on another set of plots each 2 weeks. From the data collected, N stress was calculated by: s = M ~ log x 100 [i] logM — log WJ A L J where S = % N stress, M = dry weight from unstressed subplots, and Wg and Wj = dry weights from stressed POWER: WATER AND NITROGEN STRESS ON BROMEGRASS 727 subplots at times t2 and tz, respectively (1). Consequently, percent N stress is defined as the percent reduction in relative growth rate resulting from a N deficiency. Greenwood et al. concluded that N stress could be as much as 40% before visual deficiency symptoms were apparent. Both this concept and technique were applied to the measurement of N stress only (1, 2). In the experiment reported here, the principle was applied to measure the effects simultaneous of both water and N deficiencies. Thus, quantitative data were obtained on the effects of N deficiencies, water deficiencies, and their interaction, on the growth rates of grass. Calculation of degree of stress from such data has not previously been reported in the literature.