Nutritional sustainability of<i>Eucalyptus</i>plantations: A case study at Karkloof, South Africa

Synopsis The nutritional sustainability of a short-rotation Eucalyptus grandis plantation system was evaluated in a trial located at Karkloof, KwaZulu-Natal, South Mrica, by determining nutrient pools and fluxes. Nutrient pools in the forest floor and biomass (above- and below-ground) were assessed by destructive sampling. The size of nutrient pools in the soil that approximate to readily available and potentially available fractions was estimated from chemical extractions. An approximate nutrient input-output budget was constructed. The study has shown that large nutrient pools occur in the forest floor and below-ground biomass when compared to most short-rotation eucalypt cropping systems overseas. The readily available soil pools are moderately large (100 to 800 kg ha-1 ) when compared to similar systems in the tropics (Brazil & Congo). The potentially available nutrient pools are particularly rich in potassium (2.5) and magnesium (1.1 t ha1). This indicates that the system is well buffered against nutrient depletion in the short and long term. Estimates of nutrient fluxes revealed that fertilization and mineral weathering constitute small inputs, while atmospheric deposition makes a major contribution to the system (95% of total N inputs, 92% of total Ca and Mg and 82% of total K inputs). Sizeable nutrient losses were caused by slash burning (N), leaching (Ca & Mg), and both wood harvesting and firewood collection (N, K & Ca). The management regime and intensity of operations both have a pronounced effect on nutrient fluxes to and from the system, and hence, the net balance ofthe budget was calculated for different management regimes at representative intensities. Net fluxes were positive or near zero for most elements in the absence of firewood collection or slash burning and changed to losses of between ca. 5 to 10 kg ha-1 a-1 per individual nutrient when slash burning was incorporated in the regime. Despite the increases in nutrient loss with more intensive management, the indices of nutritional sustainability for all regimes tested still indicated a stable system with respect to nutrition. Nutritional sustainability has been gauged by other researchers using the ratio (nutrient export in harvesting)/(available nutrient pool size). An index of nutritional stability (piNS) is proposed, based on the negative logarithm of the ratio (net nutrient loss)/(nutrient pool) where the nutrient pool can be readily available soil pools or (long term) potentially available system pools. Although this index has intensive data requirements, it evaluates management intensity effects as well as the "buffer capacity" of the system more rigorously than previously proposed indices.

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