ESTIMATES OF SUB-NATIONAL NUTRIENT BALANCES AS SUSTAINABILITY INDICATORS FOR AGROECOSYSTEMS IN ECUADOR

Using a model, inputs and outputs of nitrogen, phosphorus and potassiu m were calculated for Ecuadorian agro-ecosystems, in order to assess s ustainability of different land use types in terms of a soil fertility balance. For 5 x 5 minute land cells of a homogeneous geographical gr id covering the Ecuadorian territory, the nutrient balance was calcula ted on the basis of the separate contributions of the inputs and outpu ts: Mineral fertilizer, organic fertilizer, atmospheric deposition, bi ological N-fixation, sedimentation, harvested product, removed crop re sidues, leaching, gaseous losses and erosion. The estimates were aggre gated to sub-national and national level. In general, the estimates sh ow a depletion of the soil nutrient stock in Ecuadorian agro-ecosystem s. At a national scale for the land use type temporary crops there is mainly a deficit of nitrogen (42 kg ha(-1) yr(-1)), while for permanen t crops both nitrogen and potassium balances are clearly negative (40 and 25 kg ha(-1) yr(-1), respectively). For grassland overall, losses are smaller. Erosion is a major cause of nitrogen loss, leaching and d enitrification also contribute significantly. In permanent crops relat ively large amounts of potassium leave the agro-ecosystem through harv ested products, due to high potassium concentrations in these products and high yields. At sub-national scale, nutrient depletion under curr ent land use is more severe in the Andean region than the coastal regi on, mainly as a result of higher erosion losses. In the Andean region, this situation is likely to worsen due to the exploitation of margina l lands under the high pressure on the land. The Amazon region is stil l largely unexploited but this study suggests that the current convers ion of forest to agricultural land, may cause serious nutrient balance problems at a local level. The presented approach allows the sub-nati onal assessment of soil nutrient balances as sustainability indicator. It appears a useful tool to indicate areas of interest for more detai led follow-up studies. Furthermore, it may assist in the exploration o f the effects of land use changes. (C) 1997 Elsevier Science B.V.