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ND-03 Abstract

Linking Soil Biogeochemistry to Surface Water Chemistry in Small Drainage Basins of the Amazon

Linda Ann Deegan — Marine Biological Laboratory (US-PI)
Reynaldo Luiz Victoria — CENA - Centro de Energia Nuclear na Agricultura (USP) (SA-PI)

Objectives





We propose studies on the effects of conversion from rainforest to pasture on the

biogeochemistry of small streams in the Amazon basin. Questions about how organic matter

and nutrients move from uplands to streams, how land use alters those inputs and controls,

and how terrestrially-controlled inputs are reflected in successively larger streams, are

central to our understanding of the ecological functioning and material fluxes in drainage

basin networks Our research will focus on three questions about how the dominant land use

conversion now occurring in the Amazon Basin-the conversion of moist tropical forest to

cattle pasture-changes the biogeochemistry and ecological functioning of small streams in

the Amazon Basin:







  • Question 1: What is the role of uniform forest and pastureland use in the

    drainage basin in determining the biogeochemical composition of stream waters?


  • Question 2: What is the effect on stream organic carbon production of altered

    nutrient inputs from the drainage basin?


  • Question 3: Are patterns of nutrient concentrations, organic matter and control

    of organic matter production associated with forest or pasture land use identifiable and

    persistent in successively larger drainage basins?






We will measure stream chemistry in paired drainage basins with second order streams

and uniform land-use and in larger streams and rivers of mixed land-use. One drainage

basin in each pair will have land use exclusively of native forest, the other exclusively

of cattle pasture. Stream water measurements in paired basins will be made bi-weekly as a

minimum during two month-long field trips, one during the period of low flow and one

during the period of high flow. When possible, these periods will be supplemented with

collections during the wet-dry and dry-wet transitions. To characterize stream

biogeochemistry, we will measure pH, conductivity, NO3-, NH4+, PO43-

(as soluble reactive phosphate), dissolved organic N and P (DON, DOP), K+, Na+, Ca2+,

Mg2+, Al3+, Fe2+, Si, Sr, Cl- and SO42-,

total suspended solids, suspended chlorophyll a, and the C, N and P content of suspended

solids. We will add to these measurements the concentrations of total DOC and in different

size fractions determined by ultrafiltration. We will also measure the (13C

value of the POC and the DOC in the different fractions. To provide more detailed

information on stream discharges and element concentrations over a range of discharges, we

will install recording stage and temperature/conductivity sensors and autosamplers. To

test whether P limits algal growth in the forest streams if light limitation in removed,

we are deploying nutrient bioassays. We will measure the concentrations of inorganic

nutrients, other anions and cations, DOC and the (13C value of DOC in a series of slightly

larger streams.





Contribution to LBA-ECO





This research program will contribute LBA Ecology an understanding of the ways in which

changes in land affect stream biogeochemistry and influence downstream transport of

materials. The work in small basins will develop a detailed picture of the characteristic

concentrations of nutrients, other ions, DOC and (13C of DOC and POC derived from uniform

land-use forest and pasture watersheds. The work in larger streams, including streams

formed by the confluence of the small streams in our uniform land use basins, will allow

us to trace the persistence of patterns present in the uniform-use watersheds in

successively larger drainage. We will also conduct bioassay experiments of N and P

limitation in these larger streams draining mixed land use. These experiments will tell us

how changing nutrient concentrations and nutrient ratios influence stream organic matter

production and how much land use change in a mixed land-use basin must take place before

patterns of stream primary production are altered. This project will integrate an

understanding of upland- small stream linkages into a landscape and drainage-basin network

perspective that takes into account the extent of forest conversion in different-sized

basins and how the biogeochemical changes resulting from that forest conversion are

transmitted downstream to the larger rivers of the Basin.





Research Team Responsibilities







  • Linda Deegan: Algal bioassays & stream nutrient chemistry


  • Christopher Neill: Stream nutrient chemistry, soil chemistry and data-loggers for

    physical parameters.


  • Reynaldo Victoria: Concentration, size fractionation and d13C value of DOC.






Research Sites





We would like to continue our work at Fazenda Nova Vida and extend this work to the LBA

paired drainage basins. We have over 3 years of work on two paired drainage basins on

Fazenda Nova Vida, a 20,000-ha cattle ranch in central Rondônia. These basins contain

permanently flowing second-order streams and are matched closely in area and discharge.

The forested basins contain moist terra firme forest typical of Rondônia. They have soils

(Kandiudults and Paleudults) that are typical of large areas of the western Amazon Basin.

First and second order forest streams in the region are typically 1 to 4 m wide, with

clear water and sandy bottoms, and a pH of 5.5 to 6.8. Forest vegetation consists of open

perennially evergreen broadleaf trees with a high number of palms. Pastures have been

planted to grasses that are widespread in Amazonia, including colonião (Panicum maximum)

and several species of the genus Brachiaria. Soils are well-drained ultisols (red-yellow

podzolic latosols in the Brazilian classification). Clay content ranges from 10 to 40%.

Forest soil pH is ~5. We will also pursue the possibility of making many of the same kinds

of measurements in another pair of small basins of uniform land use that will be outfitted

for intensive hydrological studies (Martin Hodnett, Institute of Hydrology, Wallingford,

UK, personal communication).





Activities







  • We are assuming we can begin activities in mid-1998.


  • Stream chemistry and bioassays in paired basins - biweekly during one-month periods in

    the wet and dry seasons - June 1998 - Dec. 2000.


  • Stream bioassays for primary production nutrient limitation - June 1998 - Dec. 2000.


  • Stream chemistry in larger, mixed land-use streams - June 1998 - Dec. 2000.


  • DOC characterization in paired drainage basins and larger streams - June 1998 - Dec.

    2000.


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