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.