Symbiotic nitrogen (N)-fixing trees are 10x more abundant equatorward of 35°N than at higher latitudes in the Americas. This seems counterintuitive given that many lower-latitude forests have high N availability, which should select against N-fixing trees. One explanation—the “differential regulation hypothesis”—suggests that a difference in the degree to which N fixers regulate symbiotic N fixation (SNF) can explain the abundance pattern of N-fixing trees. It states that N-fixing trees that rapidly regulate SNF to meet N demand (“facultative” fixers) should have abundance patterns similar to lower-latitude forests, whereas those that do not adjust SNF to meet demand (“obligate” fixers) should resemble higher-latitude forests. Are lower-latitude N-fixing trees facultative? Are higher-latitude N-fixing trees obligate? Some field observations suggest so, but experimental evidence is lacking. Here, we present results from identical field experiments in two lower-latitude (both in Hawaii) and two higher-latitude (Oregon and New York) sites that were designed to establish the degree of regulation of SNF. We planted six species of N-fixing trees along with paired non-fixing trees, fertilized them with isotopically-labeled N at three levels of N addition (0, 100, and 150 kg N ha-1 yr-1) for three years, and measured tree growth and N isotopic composition.
Results/Conclusions
For each species—Casuarina equisetifolia, Morella faya, Gliricidia sepium, Acacia koa, Alnus rubra, and Robinia pseudoacacia—we first established whether our experimental treatments were adequate. Three conditions had to be met: (1) We had to isotopically enrich the soil enough to distinguish fixed from soil-derived N. Our non-fixing plants were enriched at 100s-1000s‰, so this condition was met. (2) The N fixers in the low fertilization treatment had to be fixing N, to establish that symbiotic bacteria were available. This condition was also met: Most plants in the low N treatment acquired over 50% of their N from SNF. (3) Our N additions had to exceed the plants’ demand for N, which is necessary to distinguish between obligate vs. facultative vs. in-between SNF strategies. This condition was also met. Given that all three conditions were met, our experiment was able to detect the regulation of SNF. Across all species, regardless of latitude or taxonomy, the proportion of plants’ N from SNF declined slightly as fertilization levels increased, but remained well above 0 at the highest levels of fertilization. These results suggest that all of our species were obligate or intermediate N fixers, inconsistent with the differential regulation hypothesis.