Phosphatase activity and nitrogen fixation reflect species differences, not nutrient trading or nutrient balance, across tropical rainforest trees

Abstract

A fundamental biogeochemical paradox is that nitrogen-rich tropical forests contain abundant nitrogen-fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen-fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen-for-phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen-for-phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen-fixing trees cannot provide a positive nitrogen-phosphorus-carbon feedback to alleviate nutrient limitation of the tropical carbon sink.

Drought Differentially Affects Growth, Transpiration, and Water Use Efficiency of Mixed and Monospecific Planted Forests

“…Even though T. amazonia in monocultures had significantly lower SA growth than T. amazonia in mixtures, they transpired the same amount as T. amazonia in mixtures, indicating that the mixed species interactions changed water use patterns for T. amazonia. Although not directly tested here, one possible explanation could be related to a facilitation effect from N-fixation by D. retusa, which has been shown to fix at a rate of 33.5 µmol N 2 m −2 ha −1 in our study area [34], and which could have potentially increased the ability of T. amazonia in mixtures to assimilate more CO 2 without losing as much water through its leaves. Specifically, increased foliar N and rubisco concentration, and hence, photosynthetic capacity could have allowed for greater efficiency of carbon and water transfer through its leaves.…”

“…In addition to having different rooting depths, D. retusa is dry season semi-deciduous which allows for greater water availability at the stand level during the driest part of the year. Further, D. retusa is a known nitrogen fixer that has been shown to be fixing in the study plantation [34], which may increase the availability of limiting nutrients to T. amazonia, thereby increasing WUE via increases of foliar N and, hence, photosynthetic capacity.…”

Soil nutrients and dispersal limitation shape compositional variation in secondary tropical forests across multiple scales

“…This is only the second study that quantitatively assessed how soil nutrients and successional age simultaneously affect the abundance of plant species across secondary forests in a tropical landscape (Werden et al, ). Our data can be used to select species with contrasting successional and soil resource associations for more detailed explorations of their nutrient strategies and associated traits and experimental studies on the mechanisms governing soil resource partitioning (Batterman et al, ; Palmiotto et al, ).…”

Nutrient acquisition strategies augment growth in tropical N2‐fixing trees in nutrient‐poor soil and under elevated CO2

“…To convert acetylene reduction rates into N 2 fixation rates, an ethylene : N conversion ratio of 2.8:1 was applied . Though this ratio was determined by sampling nodules across a presumably wide variety of N 2 -fixing species in a nearby rain forest, the ratio can vary by species (Batterman et al 2018). We, thus, urge caution when making inferences about our N 2 fixation rates.…”