Nitrogen cycling in the tropical rain forest of French Guiana: comparison of two sites with contrasting soil types using δ15N

Abstract

The natural 15N abundance method for estimating symbiotic biological N2-fixation was tested on legume trees from two rain forests on contrasting soils (oxisols and spodosols) in French Guiana. When possible, the significance of N2-fixing species in the plant community was evaluated in terms of density, biomass and contribution of N2-fixation to the building up of the total nitrogen mass in the leaves. Of the two sites, the rain forest on spodosols was the less favourable for application of the δ15N method: the available soil nitrogen was isotopically similar to fixed-N2. Hence, the results showed that a reliable estimate of N2-fixation could not be obtained. A substantial contribution of fixed-N2 to the nitrogen nutrition of legumes was found on oxisols, with an average value of 54 % Ndfa (Nitrogen derived from the atmosphere). The contribution of the N2-fixing legumes to the biomass of the stand was estimated to be 2 t ha−1 for the leaf biomass and 136 t ha−1 for the total above-ground plant biomass. With 7.5 % of trees in the stand able to fix N2 (462 out of 6156), N2-fixation was estimated to be 7 kg ha−1 y−1. These results are the first use of the δ15N method to estimate nitrogen input by N2-fixing legumes to a natural rain forest. The inter-site variability observed in the δ15N of the non-fixing plants suggested different nitrogen-cycling processes in the two soils. The δ15N of the non-N2-fixing plants could be related to the soil nitrogen availability and be used as an indicator of efficient or non-efficient nitrogen-cycling rain forests. The spatial variability of the δ15N in the plant-available soil nitrogen pool and the nitrogen balance in tropical rain forests are discussed.

Estimation of biomass in a neotropical forest of French Guiana: spatial and temporal variability

“…The research station of Piste de Saint-Elie (5°30′N, 53°00′W) is located 16 km south of Sinnamary and has been much studied since 1976 (Lescure et al 1983, Lescure & Boulet 1985, Pélissier & Riéra 1993, Puig et al 1990, Roggy et al 1999. Annual rainfall is slightly above 3000 mm y −1 .…”

“…For comparison, we have used the allometric equations developed in Lescure et al (1983), which was used recently (Roggy et al 1999 Clark & Clark (2000).…”

Distribution and Diversity of Rhizobial Populations Associated with <i>Acacia senegal</i> (L.) Willd. Provenances in Senegalese Arid and Semiarid Regions

“…Many legume trees significantly contribute to the nitrogen balance of tropical wetlands and rainforests (Dommergues, 1995;Pons et al, 2007;Roggy et al, 1999), and thus play an important role in their function and ecology. The prevalence of legumes to inhabit different environmental conditions might be related to their ability to associate with rhizobia, which enable them to fix atmospheric nitrogen (Moreira et al, 1992).…”

Deforestation of watersheds of Panama: nutrient retention and export to streams

“…Where forest biomass is replaced with other land covers, such as pastures, lower biomass transpires less water, provides lower surface area for evaporative loss (Cleveland et al 1999;Zhang et al 2001;Nosetto et al 2005;Li et al 2007;Bahn et al 2010), and allows greater water flow-through (Lewis et al 1999;Lewis 2002). Loss of tropical forest cover lowers potential for microbial processes such as nitrogen fixation by diazotrophs on tree leaves, epiphytic orchids and bromeliads, litter, and soils (Downing et al 1999;Cleveland et al 1999;Roggy et al 1999;Borbor-Cordova et al 2006;Pons et al 2007;Cusack et al 2009), inputs that are difficult to measure (Cleveland et al 2010;Reed et al 2008). The litany of ecosystem-level effects can be expanded, but the foregoing suffices to suggest the fundamental impact of the pervasive tropical deforestation.…”