The chemistry of two streams draining kaolinitic soils
Daniel
Markewitz, University of Georgia, dmarke@forestry.uga.edu
(Presenting)
Ricardo
de O
Figueiredo, Embrapa-Amazonia Oriental, ricardo@cpatu.embrapa.br
Eric
A
Davidson, The Woods Hole Research Center, edavidson@whrc.org
Mercedes
Maria Cunha
Bustamante, University of Brasilia, mercedes@unb.br
Lucilia
Parron, Embrapa -Cerrado, Parron@embrapa.br
Julio
Resende, University of Brasilia, resende@unb.br
The Amazon and Cerrado regions of Brazil share a predominance of highly weathered, clay-rich soils and an extended dry season. Geology in our two study areas within these regions differ, however, with the eastern Amazon location being tertiary sediments of the Amazon trough and the Cerrado location being Precambrian bedrock of the Brazilian shield. The Amazonian study site has been 80% deforested and converted to cattle pastures, whereas the Cerrado site is located in a preserve that maintains native vegetation. The Amazon watershed is ~13000 ha with baseflow discharge of 800 m3/sec, while that in the Cerrado is ~2000 ha with baseflow discharge of 100 m3//sec. Si and Al concentrations in the Amazon and Cerrado streams (Si: 135 and 87 uM; Al: 1.3 and 0.7 uM, respectively) indicate that kaolinite is most likely in an equilibrium mineral phase in both study areas. However, dissolved element concentrations in the Amazonian stream exceeds that in the Cerrado stream for all macro anions (HCO3: 56 vs 44; NO3: 7.9 vs 1.0; Cl: 202 vs 38; SO4: 1.2 vs 30 uM) and cations (NH4: 2.4 vs 0.8; Ca:78 vs 60; Mg: 84 vs. 19; K:22 vs 1.8; Na: 151 vs 9 uM). Despite concentration differences, HCO3 and Cl are the predominant charge balancing anions in both streams, and Ca is an important charge balancing cation. However, the contribution of Na to charge balance in the Amazonian stream (31%) exceeds that in the Cerrado stream (10%). The concentration discharge relationships also differ between the two streams: cation concentrations increase with increasing flow in the Amazonian stream and decrease with flow in the Cerrado stream. Mechanisms contributing cations to this Amazon stream during high flow may relate to greater land use conversion in its watershed compared to the Cerrado study site.