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Spatial heterogeneity can resolve the nitrogen paradox of tropical forests
Author(s) -
Menge Duncan N. L.,
Levin Simon A.
Publication year - 2017
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1002/ecy.1733
Subject(s) - litter , nitrogen fixation , ecosystem , plant litter , ecology , biology , spatial heterogeneity , species richness , abundance (ecology) , agronomy , bacteria , genetics
Many tropical forests are characterized by large losses of plant‐available forms of nitrogen (N), indicating that they are N rich, and by an abundance of plants capable of symbiotic N fixation. These N‐fixing plants can fix enough N to drive N richness. However, biological N fixation ( BNF ) is more expensive than using plant‐available N, so sustained BNF in N‐rich soils appears to be a paradox. Here, we use spatially explicit ecosystem models to analyze the conditions under which spatial heterogeneity can induce simultaneous BNF and loss of plant‐available N (hereafter, we call this combination “N‐rich BNF ”). Spatial movement of litter to neighboring plants’ rooting zones can maintain N‐rich BNF under a variety of conditions. For example, when N‐fixers have higher N demand than non‐fixers, N‐fixers export N‐rich litter to non‐fixers, inducing large losses of plant‐available N from the ecosystem, and receive N‐poor litter from non‐fixers, inducing BNF . BNF and N loss fluxes increase in proportion to the ratio of N‐fixer litter N:P to non‐fixer litter N:P, and also in proportion to the fraction of litter transferred out of a tree's rooting zone. Stoichiometric variability augments N‐rich BNF , as does increasing the fraction of the landscape occupied by N‐fixers, at least when they are rare. On the contrary, greater root overlap between neighbors and clumping of N‐fixers diminish N‐rich BNF . Finally, we examined how spatial litter transfer interacts with another mechanism that can sustain N‐rich BNF , incomplete down‐regulation of BNF . Spatial transfer and incomplete down‐regulation can both sustain N‐rich BNF , but they are compensatory rather than additive. These mechanisms can be distinguished by examining where N losses occur. Incomplete down‐regulation of BNF leads to greater N loss under N‐fixing trees, whereas spatial litter transfer leads to greater N loss under non‐fixing trees. Along with time lags in regulating BNF , these results comprise a series of hypotheses that could help understand the N paradox of tropical forests.