SYNTHESIS AND EMERGING IDEAS Using indirect methods to constrain symbiotic nitrogen fixation rates: a case study from an Amazonian rain forest

The Author(s) 2009. This article is published with open access at Springerlink.com Abstract Human activities have profoundly altered the global nitrogen (N) cycle. Increases in anthropo-genic N have had multiple effects on the atmosphere, on terrestrial, freshwater and marine ecosystems, and even on human health. Unfortunately, methodological limitations challenge our ability to directly measure natural N inputs via biological N fixation (BNF)—the largest natural source of new N to ecosystems. This confounds efforts to quantify the extent of anthropo-genic perturbation to the N cycle. To address this gap, we used a pair of indirect methods—analytical modeling and N balance—to generate independent estimates of BNF in a presumed hotspot of N fixation, a tropical rain forest site in central Rondônia in th