A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Land use change alters the biogeochemistry and downstream movement of nitrogen insmall drainage basinsChristopher Neill 1 , Linda A. Deegan 1 , Alex V. Krusche 2 , Suzanne M. Thomas 1 , M. Victoria R.Ballester 2 , and Reynaldo L. Victoria 21. The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USAcneill@mbl.edu2. Centro de Energia Nuclear na Agricultura, Avenida Centenário, 303, Caixa Postal 96, CEP13416000, Piracicaba, SP, BrazilLand use change can influence nitrogen transformation and movement by alteringbiogeochemical transformations at key places in the landscape. Terra firme forests cycle highamounts of N and lose NO 3 - to soil solution. Forest clearing briefly increases NO 3 - losses butpasture establishment quickly reduces N cycling and NO 3 - movement to soil solution. In forests,stream riparian zones receive high amounts of NO 3 - from the uplands and appear to be veryefficient at removing NO 3 - by denitrification. In pastures, stream riparian zones also have highpotential for denitrification but appear to receive little NO 3 - from adjacent uplands. Soluteinjection experiments in forest streams show that spiraling distances for NH 4 + are approximately200 m, while spiraling distances for NO 3 - are very long (>10 km). This indicates that once NO 3-reaches or is produced by nitrification in small forest stream channels, it travels downstreamrelatively unprocessed. Low demand for inorganic N in forest streams was confirmed by lowrates of incorporation of added 15 NH 4 + in algal and bacterial surface films. Shorter spiralingdistances of NH 4 + and NO 3 - in pasture streams determined from solute injections indicate a lowertendency for direct downstream transport. Forest conversion to pasture appears to alter themovement of inorganic N downstream in watersheds by 1) decreasing NO 3 - moving to riparianzones and 2) increasing uptake demand for NH 4 + and NO 3 - and thus retention of N in smallstream channels. These changes have the potential to alter control of stream primary productionand decrease movement of inorganic N to larger rivers.