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Effects of land use change on stream water chemistry in three meso-scale catchments in Eastern Amazonia

Ricardo de Oliveira Figueiredo, Embrapa Amaz˘nia Oriental, (Presenting)
Daniel Markewitz, University of Georgia,
Eric Atlas Davidson, The Woods Hole Research Center,
Marysol A.E. Schuler, Universidade Federal do Parß,
PatrÝcio de Sousa Silva, Universidade do Estado do Parß,
LÝvia Gabrig T. Rangel-Vasconcelos, Embrapa Amaz˘nia Oriental,
Renata Tuma Sabß Paes, Embrapa Amaz˘nia Oriental,
Lilianne Maia Lima, Embrapa Amaz˘nia Oriental,

We are measuring changes in water chemistry by upstream-downstream sampling along three streams from their headwaters in remnant mature forests, through pastures, secondary forests, and large fertilized fields of soy bean, rice and corn in the Paragominas region, in Eastern Amazonia. These three catchments have the following areas and percentages of forested cover: Cinquenta e quatro (Cq) watershed (130 km2) with 18% forest; Sete (St) watershed (150 km2) with 34% forest; and Paje˙ (Pj) watershed (45 km2) with 45% forest. Field campaigns of stream chemistry measurements were conducted monthly from April 2003 to October 2005, including discharge, pH, conductivity, turbidity, alkalinity, DO, DOC, DIC, Ca2+, Mg2+, K+, Na+, NH4+, NO3-, PO43-, SO42- and Cl-. Among the three streams, turbidity (Cq= 66.7; St=14.9; Pj=14.2 FTU) and pH (Cq=5.6; St=4.5; Pj=4.6) are highest in the most deforested watershed. Results from the watershed containing significant cropland at the lower reaches of the stream reveal higher nitrate concentrations near the cropland, which increase nitrate mean concentration in Cq (Cq=256; St=146; Pj= 84 Ág L-1). Because soils are relatively similar within the study area, we suspect land use conversion is an important factor affecting the observed trends in stream chemistry. In addition to the effects of land-uses on inputs of nutrients to the streams, in-stream processes are also important. In-stream dissolved oxygen concentration consistently decreased in on-farm retention ponds, and nitrate concentrations generally followed a similar pattern as DO concentrations. In Paje˙, for instance, average measured DO was 3.69 mg L-1, while in free flowing stream it was 5.04 mg L-1. Similarly, nitrate in Pj decreased from 95 Ág L-1 in free flowing areas to 67 Ág L-1 in retention ponds. In phase III of our project we are investigating the role of the riparian zones as a filter of nutrients and sediments arriving from upslope land covers of forest, pasture, or agricultural land and whether riparian zones and/or in-stream processes significantly modify or uncouple the upslope soil solution chemistry and the stream chemistry.

Science Theme:  ND (Nutrient Dynamics)

Session:  1B: Nutrient Cycling in Terrestrial and Aquatic Systems

Presentation Type:  Oral

Abstract ID: 85

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