The Effects of Prolonged Drought and Nitrogen Fertilizer on Root and Shoot Growth and Water Uptake by Winter Wheat

Abstract

AbstractWinter wheat growing on a silty clay loam soil was protected from rainfall by a mobile shelter for 100 days from tillering to maturity. During this time the crop was either irrigated according to demand or grew on stored soil water. The effects of this high and low water supply, in combination with a high and low N supply, on root and shoot growth and water uptake were studied.The crop given both N and water yielded 9.7 t/ha of grain (85 % DM), drought reduced this to 7.9 t/ha, low N to 4.3 t/ha and drought and low N to 3.8 t/ha. Yield reductions were mainly due to fewer grains being produced.Little root growth occurred in the topsoil during the drought but there was compensatory growth in the subsoil provided that N fertilizer was given. The droughted crops rooted to 160 cm, about 20 cm deeper than the irrigated crops, but the amount of root in the deep subsoil was very small, less than 0.1 cm/cm3 ai 140–160 cm, compared with 5–9 cm/cm3 in the topsoil.The crop demand for water at any given time was partitioned throughout the root system but atmospheric demand was only met whilst the topsoil was wet. The fertilized, droughted crop extracted all of the potentially available water to a depth of 80 cm and a mean rooting density of 1 cm/cm3 was necessary to achieve this. Uptake from below this depth was limited by root growth.The limiting value of the potential soil water deficit was 170 mm, and weather records showed that this would be exceeded one year in ten, on average. The likelihood of yield reduction due to drought could be reduced on this soil by improving root growth below 80 cm depth, although the chances of achieving this are low as root growth was probably limited by poor soil structure.