COS 13-6 - Spatial heterogeneity can resolve the nitrogen paradox of tropical forests

Tuesday, August 9, 2016: 9:50 AM
304, Ft Lauderdale Convention Center
Duncan Menge, Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY and Simon A. Levin, Ecology and Evolutionary Biology, Princeton University, Princeton, NJ
Background/Question/Methods

Many tropical forests are characterized by large losses of bioavailable forms of nitrogen (N) such as nitrate.  Plants and microbes excel at soaking up bioavailable N when they are N limited, so large losses indicate N richness rather than N limitation.  Many tropical forests also have an abundance of plants capable of symbiotic biological N fixation (BNF).  These N-fixing plants have the capacity to drive N richness.  However, from the plants’ perspective, BNF is more expensive than using bioavailable N when there is plenty of bioavailable N in the soil.  Indeed, tropical trees seem to exhibit a facultative BNF strategy, whereby they down-regulate BNF when soil N supply is sufficient.  If facultative BNF is common, why might we see, simultaneously, high rates of BNF and large losses of bioavailable N (hereafter, we call this combination “N rich BNF”)?

Here, we use spatially explicit ecosystem models to analyze the conditions under which spatial heterogeneity can induce N rich BNF.  The models track N and phosphorus (P) in plants and soils, and assume that BNF is perfectly facultative, shutting off entirely when soil N supply meets plant N demand.  The main spatial component of the models is litter movement.

Results/Conclusions

Litter movement can maintain N rich BNF.  When N-fixers have higher litter N content than non-fixers, N-fixers export N rich litter, providing non-fixers with N to sustain large losses of bioavailable N, and import N poor litter, inducing the need for sustained BNF.  Rates of N rich BNF increase in proportion to the ratio of N-fixer litter N:P to non-fixer litter N:P, and in proportion to the fraction of litter transferred out of a tree’s rooting zone.  Stoichiometric variability also augments N rich BNF, whereas greater root overlap between neighbors and clumping of N-fixers diminish N rich BNF.

Finally, we examine how spatial litter transfer interacts with another mechanism that can sustain N rich BNF, incomplete down-regulation of BNF.  Incomplete down-regulators maintain low but positive rates of BNF even when they are N sufficient.  Spatial transfer and incomplete down-regulation can both sustain N rich BNF, and can be distinguished from each other: Incomplete down-regulation is characterized by greater N loss under N-fixing trees, whereas spatial litter transfer is characterized by greater N losses under non-fixing trees.  These results put forth testable hypotheses that could explain N rich BNF, and thus could help understand the N paradox of tropical forests.