Deforestation and increasing drought intensity are increasing the susceptibility of tropical forests to fire over large areas of the Amazon. While phosphorus (P) is thought to ultimately limit net primary productivity in tropical forests, nitrogen (N) often limits secondary forest regrowth until forests mature. During burning, mineral and organic N are lost, but the N cycle gradually recovers after decades. In areas isolated from atmospheric N deposition, biological nitrogen fixation (BNF) is the primary mechanism to support N acquisition and the recovery of the N cycle. We investigated the role of BNF as an N input seven years after experimental burns in the southeastern Amazon. At Tanguro Ranch, a large-scale experiment (150-ha) compares control forests with forests that were burned three years apart in 2004, 2007 and 2010, and annually from 2004 to 2010 (except 2008). We measured the changes in aboveground biomass using tree inventories and monthly leaf-area index measurements as a proxy for the net N demand. We quantified free-living BNF in the dry and wet seasons and symbiotic BNF in the wet season to test the hypothesis that BNF is elevated after a fire to support forest regrowth.
Results/Conclusions
Leaf N pools increased by an average of 5.76 kg N ha-1 yr-1 in the forests burned three years apart, 4.76 kg N ha-1 yr-1 in the forests burned annually, and decreased by 0.97 kg N ha-1 yr-1 in the control forests. However, free-living BNF rates to support this leaf demand were low in all forests (less than 1 kg ha-1 yr-1). Contrary to our hypothesis, no significant differences in free-living BNF were observed between forests (p > 0.05). The rates were low compared to the range of free-living BNF rates from a literature review for the tropics (3.7-7.8 kg N ha-1 yr-1). Although we did not find symbiotic BNF in the control forests or the forests burned annually, we did find symbiotic BNF in the forests burned twice three years apart, averaging 0.48 kg N ha-1 yr-1. The symbiotic BNF rates were also low compared to a mean tropical forest estimate of 5.7 kg N ha-1 yr-1. We conclude that BNF in the burned forests in the southeastern Amazon behave more like mature forests than secondary forests found elsewhere. Because we observed the high presence of nitrate concentrations in the surface and deep (8m) soils, we hypothesize that higher available N pools at this site are supporting regrowth.