PARENT SESSION
Oral Session #76: Ecosystem Ecology: Larger scale processes, geomorphology, soils.
Presiding: W. Straw
Thursday, August 8. 9:00 AM to 11:30 AM. Gila Meeting Room, TCC.

Patterns and mechanisms of soil acidification in the conversion of grasslands to forests.

JOBBAGY, ESTEBAN^*,1,2, JACKSON, ROBERT^1,3, ¹ Department of Biology, Durham, NC² Facultad de Agronomia, Buenos Aires, Argentina³ Nicholas School of the Environment, Durham, NC

ABSTRACT- Forest-grassland shifts affect some of the most productive areas of the world still covered by native vegetation and have the potential to impulse further environmental changes through soil modifications. We used afforestation of native ecosystems in the Pampas of Argentina as an experimental system to 1) isolate the imprint of forest and grassland vegetation on soil acidity and base cation cycling and 2) evaluate the mechanisms that explain the acidification of grassland soils by trees. Soils in the temperate humid plains of the Pampas developed under grassland vegetation in the absence of any trees, and acid rain has been effectively absent in the region. We characterized soil changes with afforestation using ten paired stands of native grassland and eucalypt plantations (10 to 100 years of age) distributed throughout the Pampas. Compared to grasslands, afforested stands had lower soil pH (4.6 vs.5.6 on average, p<0.0001, n=5) and ~40% lower exchangeable Ca on average (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 in tree biomass, the forest floor, and the 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 at all sites. Soil exchangeable acidity increased at a rate of 0.5-1.2 kmol_c.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 soil acidification in this system. The magnitude of soil alterations that we observed within 50 years of tree establishment emphasizes the importance of vegetation type in soil formation and suggests dramatic potential feedbacks of vegetation change on soil fertility and biogeochemistry.

KEY WORDS: Eucalyptus, Afforestation, Calcium biogeochemistry, Soil acidity