Agriculture expanded during the last 50 years from the Pampas to NW Argentina at the expense of natural forests and rangelands. In parallel, productivity was boosted through the increasing application of external inputs, modern technology and management practices. This study evaluated the impact of agricultural expansion between 1960 and 2005 by assessing the implications of land use, technology and management changes on (i) carbon (C), nitrogen (N) and phosphorous (P) stocks in soil and biomass, (ii) energy, C, N, P and water fluxes and (iii) water pollution, soil erosion, habitat intervention and greenhouse gas (GHG) emissions (impacts). Based on different data sources, these issues were assessed over $1.5 million km 2 (63% of Argentina), involving 399 political districts during three representative periods: 1956-1960, 1986-1990 and 2001-2005. The ecological and environmental performance of 1197 farming system types was evaluated through the AgroEcoIndex model, which quantified the stocks, fluxes and impacts mentioned above. Cultivation of natural ecosystems and farming intensification caused a noticeable increase of productivity, a strengthening of energy flux, an opening of matter cycles (C, N, P) and a negative impact on habitats and GHGs emission. However, due to the improved tillage practices and the application of less aggressive pesticides, erosion and pollution risk are today lower than those of the mid-20th century. The consistency of some assumptions and results were checked through uncertainty analysis. Comparing our results with international figures, some impacts (e.g. soil erosion, nutrient balance, energy use) were less significant than those recorded in intensive-farming countries like China, Japan, New Zealand, USA, or those of Western Europe, showing that farmers in Argentina developed the capacity to produce under relatively low-input/low-impact schemes during the last decades. [Correction added after online publication 4 October 2010: In the first sentence of the Abstract, NE was corrected to NW.]

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A vast body of literature demonstrated that anthropogenic disturbances such as overgrazing and fire are key drivers of abrupt transition between vegetation types in ecosystems. In this study, we propose that the hydrological context (described in terms of rainfall, evapotranspiration and water yield) is a first-order, primordial determinant of the propensity of ecosystems to undergo transition. This implies that the anthropogenic disturbance is a second-order determinant that is strongly conditioned by the first one. Through the meta-analysis of existing studies, a collection of 685 geo-referenced study cases was organized to study the hydrological characteristics of three climatic regions and three ecosystems that vary in their relation between woody and grassy plants. Thus, humid, sub-humid and dry climatic regions, respectively, receiving >1000, 500-1000 and <500 mm year À1 , were studied, and possible transition mechanisms among grasslands/savannas, shrublands and forests were analysed. The results showed that the ecohydrological context determines the probabilities of ecosystems transitions in different climatic regions and the prevalence of alternative transition mechanisms. We showed that transition of forests into other ecosystems is highly improbable in high-precipitation regions, more probable and likely subject to a bi-stable and reversible regime in sub-humid regions, and highly probable and irreversible in dry regions. Factors such as runoff, deep-water drainage, fire, flammable/ nonflammable biomass and overgrazing were considered as hypothetical transition mechanisms. As a novel finding, we demonstrate that ecohydrology, as a determinant of transition, is a factor that operates at a hierarchical level higher than that of the human-driven disturbance. A synthetic graphical model was proposed to characterize resilience (the capacity of ecosystems to withstand transition) in the three study climatic regions.