Carbon accumulation in secondary tropical forests is substantial, and thought to be limited at least in part by nitrogen (N) availability. Slash and burn agriculture and grazing remove N from the system, however, the abundance of symbiotic N fixing (SNF) trees in young tropical forests suggests rapid N accumulation as forests regrow with subsequent down regulation of SNF in mature forests. This hypothesis, however, has rarely been tested. Furthermore, the diversity of legume species in tropical forests suggests there might be a phylogenetic influence on patterns of SNF. Our study site, the Mata Altântica, once stretched 1500 km along the east coast of Brazil, is currently 85% deforested, and is a target of national and international restoration efforts that rely heavily on the planting of legume species to facilitate forest regrowth. Determining the species responsible for the majority of fixation during secondary succession will help to improve species selection during active restoration efforts. Here we use stratified adaptive cluster sampling approach to quantify symbiotic and a-symbiotic N fixation across a chronosequence of re-growing tropical forests in the Mata Atlântica of Bahia, Brazil. We combine this approach with DNA barcoding of nodulated roots to determine species-specific rates of SNF.
Contrary to theory, we did not observe significantly higher SNF early in forest succession (F(3, 16)=0.51; p=0.68). However, rates of SNF were most variable early in forest succession, ranging from 1.06 to 22.75 kg N ha-1 yr-1 in 16-year-old forests (mean 10.29 +/- 4.20 kg N ha-1 yr-1) compared to 0.3 to 8.16 kg N ha-1 yr-1 in >100 years old forests (mean 4.61 +/- 1.65 kg N ha-1 yr-1). Across all sites, SNF were ~5 times lower than previous estimates of SNF inputs into mature tropical forests using different methodologies, but similar to measurements using the same sampling approach of intact forests in Costa Rica. In our study, SNF accounted for >99% of the total N inputs via biological N fixation. Several intriguing possibilities emerge from these data: 1) contrary to expectations, abundant legumes early in succession do not dramatically increase N inputs in these re-growing tropical forests, and 2) the hypothesis that N fixation is down regulated by facultative fixers once forests reach maturity is not consistent with our observations.