Using a model, inputs and outputs of nitrogen, phosphorus and potassium were calculated for Ecuadorian agro-ecosystems, in order to assess sustainability of different land use types in terms of a soil fertility balance. For 5 x 5 minute land cells of a homogeneous geographical grid covering the Ecuadorian territory, the nutrient balance was calculated on the basis of the separate contributions of the inputs and outputs: Mineral fertilizer, organic fertilizer, atmospheric deposition, biological N-fixation, sedimentation, harvested product, removed crop residues, leaching, gaseous losses and erosion. The estimates were aggregated to sub-national and national level. In general, the estimates show a depletion of the soil nutrient stock in Ecuadorian agro-ecosystems. 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 permanent 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 denitrification also contribute significantly. In permanent crops relatively large amounts of potassium leave the agro-ecosystem through harvested products, due to high potassium concentrations in these products and high yields. At sub-national scale, nutrient depletion under current land use is more severe in the Andean region than the coastal region, mainly as a result of higher erosion losses. In the Andean region, this situation is likely to worsen due to the exploitation of marginal lands under the high pressure on the land. The Amazon region is still largely unexploited but this study suggests that the current conversion of forest to agricultural land, may cause serious nutrient balance problems at a local level. The presented approach allows the sub-national assessment of soil nutrient balances as sustainability indicator. It appears a useful tool to indicate areas of interest for more detailed follow-up studies. Furthermore, it may assist in the exploration of the effects of land use changes. (C) 1997 Elsevier Science B.V.
