title: Solute and sediment export from Amazon forest and soybean headwater streams.
abstract: Intensive cropland agriculture commonly increases streamwater solute concentrations and export from small watersheds. In recent decades, the lowland tropics have become the world's largest and most important region of cropland expansion. Although the effects of intensive cropland agriculture on streamwater chemistry and watershed export have been widely studied in temperate regions, their effects in tropical regions are poorly understood. We sampled seven headwater streams draining watersheds in forest (n = 3) or soybeans (n = 4) to examine the effects of soybean cropping on stream solute concentrations and watershed export in a region of rapid soybean expansion in the Brazilian state of Mato Grosso. We measured stream flows and concentrations of NO3-, PO43-, SO42-, Cl-, NH4+, Ca2+, Mg2+, Na+, K+, Al3+, Fe3+, and dissolved organic carbon (DOC) biweekly to monthly to determine solute export. We also measured stormflows and stormflow solute concentrations in a subset of watersheds (two forest, two soybean) during two/three storms, and solutes and δ18O in groundwater, rainwater, and throughfall to characterize watershed flowpaths. Concentrations of all solutes except K+varied seasonally in streamwater, but only Fe3+concentrations differed between land uses. The highest streamwater and rainwater solute concentrations occurred during the peak season of wildfires in Mato Grosso, suggesting that regional changes in atmospheric composition and deposition influence seasonal stream solute concentrations. Despite no concentration differences between forest and soybean land uses, annual export of NH4+, PO43-, Ca2+, Fe3+, Na+, SO42-, DOC, and TSS were significantly higher from soybean than forest watersheds (5.6-fold mean increase). This increase largely reflected a 4.3-fold increase in water export from soybean watersheds. Despite this increase, total solute export per unit watershed area (i.e., yield) remained low for all watersheds (<1 kg NO3-N·ha-1·yr-1, <2.1 kg NH4+-N·ha-1·yr-1, <0.2 kg PO43--P·ha-1·yr-1, <1.5 kg Ca2+·ha-1·yr-1). Responses of both streamflows and solute concentrations to crop agriculture appear to be controlled by high soil hydraulic conductivity, groundwater-dominated hydrologic flowpaths on deep soils, and the absence of nitrogen fertilization. To date, these factors have buffered streams from the large increases in solute concentrations that often accompany intensive croplands in other locations.
authors.affiliation: Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts, 02543, USA.|Brown University, Institute at Brown for Environment and Society, Box 1951, Providence, Rhode Island, 02912, USA.
authors.forename: Shelby H
authors.initials: SH
authors.lastname: Riskin
authors.affiliation: Brown University, Institute at Brown for Environment and Society, Box 1951, Providence, Rhode Island, 02912, USA.|Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts, 02540, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Neill
authors.affiliation: Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts, 02543, USA.
authors.forename: KathiJo
authors.initials: K
authors.lastname: Jankowski
authors.affiliation: Universidade de São Paulo, Centro de Energia Nuclear na Agricultura, Piracicaba, SP, 13416 000, Brazil.
authors.forename: Alex V
authors.initials: AV
authors.lastname: Krusche
authors.affiliation: Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts, 02543, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: McHorney
authors.affiliation: University of Potsdam, Institute for Earth and Environmental Sciences, Potsdam, Germany.
authors.forename: Helmut
authors.initials: H
authors.lastname: Elsenbeer
authors.affiliation: Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts, 02540, USA.|Instituto de Pesquisa Ambiental da Amazônia (IPAM), Canarãna, MT, 78640 000, Brazil.
authors.forename: Marcia N
authors.initials: MN
authors.lastname: Macedo
authors.affiliation: Instituto de Pesquisa Ambiental da Amazônia (IPAM), Canarãna, MT, 78640 000, Brazil.
authors.forename: Darlisson
authors.initials: D
authors.lastname: Nunes
authors.affiliation: Brown University, Institute at Brown for Environment and Society, Box 1951, Providence, Rhode Island, 02912, USA.
authors.forename: Stephen
authors.initials: S
authors.lastname: Porder
mesh_terms: D000383:Agriculture; D001938:Brazil; D065928:Forests; D019015:Geologic Sediments / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D012621:Seasons; D013025:Glycine max / Q000254:growth & development
keywords: Brazil; agroecosystems; biogeochemistry; hydrology; land-use change; streams; watershed