Controls of land-water nitrogen movement through small lowland Amazonian forest and pasture drainage basins in Rondônia
Christopher
Neill, Marine Biological Laboratory, cneill@mbl.edu
(Presenting)
Linda
A.
Deegan, Marine Biological Laboratory, ldeegan@mbl.edu
Alex
V.
Krusche, Centro de Energia Nuclear na Agricultura/USP, alex@cena.usp.br
Victoria
R.
Ballester, Energia Nuclear na Agricultura/USP, vicky@cena.usp.br
Helmut
Elsenbeer, University of Potsdam, helsenb@rz.uni-potsdam.de
Jorge
Marcos de
Moraes, Centro de Energia Nuclear na Agricultura/USP, jmmoraes@cena.usp.br
Reynaldo
L.
Victória, Centro de Energia Nuclear na Agricultura/USP, reyna@cena.usp.br
Suzanne
M.
Thomas, Marine Biological Laboratory, sthomas@mbl.edu
Christie
L.
Haupert, Marine Biological Laboratory, chaupert@mbl.edu
Marisa
Cássia
Piccolo, Centro de Energia Nuclear na Agricultura/USP, mpiccolo@cena.usp.br
We investigated nitrogen movement through small forest and pasture drainage basins in central Rondônia. The total dissolved nitrogen exported in streamwater from small forest and pasture basins was similar (1.6 to 2.3 kg N/ha/y). Similar N export occurred despite major differences in the way that water and different forms of N moved through soils, riparian zones and stream channels. In soils, modeled fluxes of dissolved N in soil solution to below 1 m depth were 15 kg N/ha/y in forest and 4 kg N/ha/y in pasture. Forest soil dissolved N fluxes were dominated by nitrate (10.5 kg N/ha/y) while pasture fluxes were dominated by ammonium (2.6 kg N/ha/y). Declines in nitrate concentrations from about 200 µmol/L to < 5 µmol/L across forest riparian zones suggested riparian nitrate removal. Comparable gradients in pasture riparian nitrate concentrations were not found because low amounts of nitrate were produced in pasture soils. Both pasture and forest riparian soils showed high rates of potential denitrification in laboratory incubations. Nitrate retention by riparian zones at the watershed depends on the extent to which the flowpaths of water arriving from the terra firme allow contact with riparian soils. Increased compaction caused by cattle decreases soil hydraulic conductivity and shifts the distribution of flowpaths in pastures to more surface and near-surface flows that bypass riparian contact. This higher proportion of near-surface runoff and potential bypassing of riparian zones in pastures has the potential to increase ammonium delivery to streams but has only small consequences for nitrate and total dissolved nitrogen delivery because of the absence of nitrate generation in pasture soils.
Submetido por Christopher Neill em 18-MAR-2004
Tema Científico do LBA: SH (Hidrologia e Química das Águas)