Impact of nitrogen and phosphorus fertilization on drought responses in Eucalyptus grandis seedlings

Water and nutrients are two of the most important factors controlling the growth of trees. Numerous studies show that fertilization is most effective when trees are not water-stressed and that irrigation is most effective when nutrients are not scarce. The goal of this work was to test the responses of Eucalyptus grandis seedlings to drought as affected by soil type and fertilization with nitrogen (N) or phosphorus (P). Seedlings of E. grandis were planted in pots with two different types of soil: a sandy soil and a clay (black) soil. Plants were fertilized with N or P, and half of them were water-stressed. Fertilization did not affect the growth reduction brought about by drought in the red sands; in the black soil, plants fertilized with P showed a moderate growth increase under drought, compared to non-fertilized plants. Dry mass allocation to roots was decreased by fertilization in water-stressed and well-watered plants in the sandy soil, and in well-watered plants of the black soil, but it increased in water-stressed plants fertilized with N in the black soil. Water use efficiency (WUE), i.e., the grams of water transpired per gram of dry mass accumulated, was not altered by fertilization or water stress in the black soil, although it increased with P fertilization in well-watered plants in the sandy soil. Osmotic adjustment was observed only in plants fertilized with P in the black soil, and in non-fertilized plants in the sandy soil. Osmotic adjustment in water-stressed plants was associated with an increased stomatal conductance. We conclude that water stress tolerance strategies are altered by fertilization depending on soil properties, and that fertilization with P is recommended in black soils even if a moderate drought is likely to occur, but on sandy soils fertilization is recommended only under good water supply. (c) 2005 Elsevier B.V. All rights reserved.