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Scaling changes in biogeochemistry of small streams to the landscape: Using 15N to Trace N Transformations and Transport

Linda A Deegan, The Ecosystems Center, Marine Biologial Laboratory, (Presenting)
Chris Neill, The Ecosystems Center, Marine Biologial Laboratory,
M. Victoria Ballester, CENA -Universidade de São Paulo,
Alex Krusche, CENA -Universidade de São Paulo,
Reynaldo Victoria, CENA -Universidade de São Paulo,

Our work suggests that deforestation for pasture around small streams has disproportionately altered N budgets in river networks. Small streams act as important sites in the landscape where nutrients arriving from adjacent uplands are retained, transformed, or released to larger rivers. Our work has shown that the uptake lengths for NO3- and PO43- were shorter in pasture streams compared with forest streams. Forest streams exported N, mostly in inorganic form (87%), while the same size pasture stream (2nd order) was highly retentive of N, storing almost 88%, primarily in the riparian grasses, and exporting less than 15% predominately as suspended particulate N. The overall effect of this change was to make small pasture watersheds more retentive of these nutrients than the original forested watersheds. In the Ji-Paraná watershed, current deforestation of less than 30% of basin area increased total basin N uptake in first- and second-order streams by 720% and total N retention by 1,630%. This occurred primarily because of 4x greater uptake rates of N in pasture streams compared with forest streams. More than 14,000 kg of N that would have been delivered to third-order streams are now retained in headwater pasture streams in the Ji-Paraná Basin. The ecological consequences of reduced delivery of inorganic N to larger rivers are largely unknown, but the changes caused by deforestation have potentially important consequences for stream ecological function.

Science Theme:  ND (Nutrient Dynamics)

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

Presentation Type:  Oral

Abstract ID: 92

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