Effects of Moisture Stress on Nitrogen Fertilizer Response in Dryland Wheat

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Abstract
Long‐term N fertilizer trials with dryland wheat (Triticum aestivum L.) are frequently confounded by large year‐to‐year variability in yields resulting from moisture stress fluctuations. To account for this variability, an N fertilizer study was conducted in which the magnitude of moisture stress was monitored. The experiment was carried out during 1983 and 1984 (389 and 136 mm of seasonal rainfall, respectively) on a Plinthustalf loamy fine sand soil, with wheat (cv. Betta), using a completely randomized block design with four replicates and five levels (N) of N fertilizer: 0, 30, 60, 90, and 120 kg N ha−1. The seasonal moisture stress index, S, was estimated from a weighted product of ET/ETp ratios (actual to potential evapotranspiration) obtained for the different phenological wheat growing periods. The ET was estimated from soil moisture (monitored by means of a neutron moisture meter), whereas ETp was calculated from meteorological measurements using Penman's equation. The response to N in both years was significant; the mean yields were 3.18 and 1.7 Mg ha−1, for the 1983 and 1984 seasons, respectively. The data were statistically analyzed by means of a postulated multiple regression model, with N and S levels as the explanatory variables. The model was validated using yield and meteorological data from several other 2‐yr N response studies. A good correlation (R2 = 0.63, standard error of the estimate = 0.27 Mg ha−1) was obtained between the predicted and the observed yields. The results of the study highlight the need for moisture stress measurements to be included in fertilizer trials conducted in areas of large seasonal rainfall variability.