Biomass and N and P pools in above- and below-ground components of an irrigated and fertilisedEucalyptus grandisstand in South Africa

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Abstract
Eucalyptus grandis trees were planted in a highly weathered acid mineral soil in KwaZulu-Natal, South Africa, and subjected to four treatments: control (C), irrigation (I), fertilisation (F), and irrigation combined with fertilisation (IF). This study aimed to quantify mid-rotation values of nitrogen (N) and phosphorus (P) contained in the soil, forest floor and above- and below-ground biomass pools. At age 4 y, the growth of this E. grandis stand was constrained more by water availability than by nutrient supply. Irrigated (I and IF) treatments accumulated more foliage, total branch, bark, stem, coarse root and total above- and below-ground mass than non-irrigated (C and F) trees. The respective totals of the above- and below-ground biomass pools for the C, I, F and IF treatments were 73, 82, 78 and 85 t ha−1. Fertiliser application significantly increased foliage biomass, but the effect was not as great as that produced by irrigation. The lower root:shoot ratio in fertilised trees indicated that proportionally less mass was contained in their below-ground components than in those of non-fertilised trees. The irrigated trees allocated a significantly higher proportion (58%) of total above-ground biomass to stemwood relative to non-irrigated trees (56%). The faster-growing irrigated trees accumulated more N and P than trees that did not receive additional water. The effect of potential nutrient removal through a harvesting operation was examined in relation to estimates of readily plant-available soil nutrient pools. Harvesting of the stem (bark retained on site) from irrigated plots would potentially remove 22% of the available soil N pool, compared to 15% without irrigation. The corresponding values for the available soil P pool are 9% (with irrigation) and 8% (without irrigation). In the short term, the potential depletion of these elements from this site appears sustainable.