Soil acidity in the Argentine Pampas: Effects of land use and management

•Soil acidity increases to 0.75 m depth were detected in Pampean soils under forest.•Agriculture did not acidified soils at regional scale.•The area had a positive surface cation balance since 1870 to the present.•The proton balance was negative since 1870 to the present.•Modern agriculture practices may acidify some soils with low carbonate content. Soil acidification restricts crop yield in many regions. Our objective was to estimate what effect land use has had on soil acidity in the Pampean Region of Argentina. Sampling paired sites under contrasting land uses indicated that soils under trees had lower pH and much higher levels of exchangeable and total acidity than uncultivated controls under graminaceous vegetation up to 0.75 m depth. Cultivated soils did not present acidification as compared to controls. The meta-analysis of all available comparisons between agricultural soils and non-cultivated controls indicated that when agriculture advanced on soils that previously were under savanna-forest vegetation, pH increases of the topsoil occurred. Conversely, in soil previously under grassland, agriculture resulted in only minimum pH changes. The cation balance of cultivated soils was positive from 1870 to 2010. Only the potassium balance became negative since 1990 but its losses were compensated by the positive calcium, magnesium and sodium balances. The amount of protons produced by the agricultural use of soils increased exponentially from the 1970s due to the excess cation in an increasing crop production and a greater nitrogen fertilizer use. Conversely, the loss of organic matter from cultivated soils consumed protons, resulting in a negative proton balance in 140 years of cultivation, becoming positive only in recent years. In summary, trees produced a strong increase of soil acidity in Pampean soils, with a minimal effect of agriculture. Acidification under tree seems to be related to the alteration of the carbon cycle and is not permanent. The present proton budget would acidify cropped Pampean soils not buffered by carbonates.