Stream denitrification across biomes and its response to anthropogenic nitrate loading.
Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.
Mulholland
Patrick J
PJ
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. mulhollandpj@ornl.gov
Helton
Ashley M
AM
Poole
Geoffrey C
GC
Hall
Robert O
RO
Hamilton
Stephen K
SK
Peterson
Bruce J
BJ
Tank
Jennifer L
JL
Ashkenas
Linda R
LR
Cooper
Lee W
LW
Dahm
Clifford N
CN
Dodds
Walter K
WK
Findlay
Stuart E G
SE
Gregory
Stanley V
SV
Grimm
Nancy B
NB
Johnson
Sherri L
SL
McDowell
William H
WH
Meyer
Judy L
JL
Valett
H Maurice
HM
Webster
Jackson R
JR
Arango
Clay P
CP
Beaulieu
Jake J
JJ
Bernot
Melody J
MJ
Burgin
Amy J
AJ
Crenshaw
Chelsea L
CL
Johnson
Laura T
LT
Niederlehner
B R
BR
O'Brien
Jonathan M
JM
Potter
Jody D
JD
Sheibley
Richard W
RW
Sobota
Daniel J
DJ
Thomas
Suzanne M
SM
D000383:Agriculture; D001419:Bacteria / Q000378:metabolism; D003198:Computer Simulation; D017753:Ecosystem; D005843:Geography; D006802:Human Activities; D009566:Nitrates / Q000032:analysis* / Q000378:metabolism*; D009573:Nitrites / Q000032:analysis* / Q000378:metabolism*; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism; D009587:Nitrogen Isotopes; D010944:Plants / Q000378:metabolism; D045483:Rivers / Q000737:chemistry*; D014507:Urbanization
D009566:Nitrates; D009573:Nitrites; D009587:Nitrogen Isotopes; D009584:Nitrogen