Spatially robust estimates of biological nitrogen (N) fixation imply substantial human alteration of the tropical N cycle.
Biological nitrogen fixation (BNF) is the largest natural source of exogenous nitrogen (N) to unmanaged ecosystems and also the primary baseline against which anthropogenic changes to the N cycle are measured. Rates of BNF in tropical rainforest are thought to be among the highest on Earth, but they are notoriously difficult to quantify and are based on little empirical data. We adapted a sampling strategy from community ecology to generate spatial estimates of symbiotic and free-living BNF in secondary and primary forest sites that span a typical range of tropical forest legume abundance. Although total BNF was higher in secondary than primary forest, overall rates were roughly five times lower than previous estimates for the tropical forest biome. We found strong correlations between symbiotic BNF and legume abundance, but we also show that spatially free-living BNF often exceeds symbiotic inputs. Our results suggest that BNF in tropical forest has been overestimated, and our data are consistent with a recent top-down estimate of global BNF that implied but did not measure low tropical BNF rates. Finally, comparing tropical BNF within the historical area of tropical rainforest with current anthropogenic N inputs indicates that humans have already at least doubled reactive N inputs to the tropical forest biome, a far greater change than previously thought. Because N inputs are increasing faster in the tropics than anywhere on Earth, both the proportion and the effects of human N enrichment are likely to grow in the future.
Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT 59812; bsully613@gmail.com.
Benjamin W
BW
Sullivan
Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT 59812;
W Kolby
WK
Smith
Department of Ecology and Evolutionary Biology, Environmental Studies Program and Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309;
Alan R
AR
Townsend
Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT 59812;
Megan K
MK
Nasto
US Geological Survey, Southwest Biological Science Center, Moab, UT 84532; and.
Sasha C
SC
Reed
Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269-3043.
Robin L
RL
Chazdon
Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT 59812;
Cory C
CC
Cleveland
D000383:Agriculture; D018533:Biomass; D003364:Costa Rica; D017753:Ecosystem; D004777:Environment; D007887:Fabaceae / Q000378:metabolism*; D006802:Human Activities; D006801:Humans; D008962:Models, Theoretical; D058458:Nitrogen Cycle; D009586:Nitrogen Fixation; D013559:Symbiosis; D014197:Trees; D014329:Tropical Climate
adaptive cluster sampling; free-living nitrogen fixation; nitrogen deposition; symbiotic nitrogen fixation