Background. Bamboos are typical plants in tropical forests acting as resource for different animals and influencing forest structure and dynamics, but our knowledge about their influence on ecosystem functioning remains in its infancy. Nitrogen (N) is an important macronutrient that exerts control over productivity of ecosystems. Biological Nitrogen Fixation (BNF) is one of the main N input to terrestrial systems, more specifically by free-living BNF in tropical forests. In these forests, the dominant presence of bamboo and the occurrence of free-living N-fixers in its leaf surfaces appear to play a relevant role in N cycling. Here, we explore the importance of a native bamboo to N cycle in a Neotropical forest to provide insights on bamboo’s role on ecosystem functioning.
Methods. The study was conducted in a pristine montane Atlantic Forest, Brazil. We select 100 sample units (100 m² each) and counted all bamboo clumps and live culms, estimated the leaf area, the litter production and its N content. We estimated the potential N input mediated by bamboo based on available data of free-living BNF rates for this species and then we contextualized it with information about N cycling components in the study area.
Results. We counted 4000 live culms ha-1 and estimated 4.3 x 104 m² ha-1 of bamboo leaf area for free-living N-fixers colonization. Considering the free-living BNF rates quantified for the same study area, the native bamboo may contribute up to 61.6 kg N ha-1 y-1 which represents a reduction in the N deficit of at least 25% in the Neotropical forest area we studied. Through its litter, the bamboo adds 9 kg N ha-1 y-1 to the system (15% of the total fixed on its leaf surface).
Discussion. The bamboo’s contribution for N input we found may be explained by free-living community composition on its leaf as previously indicated by other studies, as well as by the bamboo overabundance, and thus habitat availability for colonization. Such N input represents a reduction in N deficit in the study area where the rates of symbiotic BNF (0.2 kg N ha-1y-1) are lower than values in Amazon Forest (3 kg N ha-1y-1). This native bamboo may supply N to the system probably through the decomposition of its leaves which represents a return of 15% of the total N fixed. Although it is still necessary to evaluate the characteristics of bamboo that will further elucidate its role in the functioning of the forests, our findings suggest that it closely regulates N inputs and may better explain high diversity and carbon stocks of the area.