Patterns and mechanisms of soil acidification in the conversion of grasslands to forests
Authors: JobbÃ¡gy, EG, RB Jackson
Forest-grassland conversions currently affect some of the worldâs most productive regions and have the potential to modify many soil properties. We used afforestation of native ecosystems in the Pampas, a temperate humid grassland in central Argentina, as an experimental system to 1) isolate forest and grassland imprints on soil acidity and base cation cycling and 2) evaluate the mechanisms explaining soil acidification. We characterized soil changes with afforestation using ten paired stands of native grassland and Eucalyptus plantations (10-100 years of age). Compared to grasslands, afforested stands had lower soil pH (4.6 vs. 5.6, p<0.0001) and ~40% lower exchangeable Ca (p<0.001). At three afforested stands, where we further characterized soil changes to one meter depth, soil became increasingly acidic from 5 to 35 cm depth but more alkaline below ~60 cm compared to adjacent grasslands. These changes corresponded with gains of exchangeable acidity and Na in intermediate and deeper soil layers. Ecosystem balances of cations (biomass + forest floor + first meter of mineral soil) revealed substantial vertical redistribution of Ca and Mn and a tripling of Na pools within the mineral soil after afforestation. Soil exchangeable acidity increased 0.5-1.2 kmolc.Ha-1.yr-1 across afforested stands, although no aboveground acidic inputs were detected in wet+dry deposition, throughfall and forest floor leacheate. Our results suggest that cation cycling and redistribution by trees, rather than cation leaching by organic acids and enhanced carbonic acid production in the soil, is the dominant mechanism of acidification in this system. The magnitude of soil alterations observed within 50 years of tree establishment emphasizes the influence of vegetation on soil formation and suggests potential feedbacks of vegetation change on biogeochemistry.