Free-living nitrogen fixation rates in lowland tropical rainforests in Southern Belize with contrasting parent material

Background/Question/Methods

Biological nitrogen (N) fixation – the conversion of atmospheric di-nitrogen into bioavailable forms of N – is the principal N input to natural ecosystems.  Nitrogen limits plant productivity and carbon exchange in many ecosystems, yet our understanding of the rates and controls on biological N fixation, both symbiotic and free-living rates, are limited.  This is especially important in tropical lowland forests, where N fixation rates are thought to be highest.  Prior work has identified phosphorus and molybdenum, two bedrock-derived nutrients, as constraints to free-living N fixation. This suggests variation in state factors – parent material and soil age – could affect rates of N fixation at landscape scales.  Our study site in the Bladen Nature Reserve in Belize provides an opportunity to explore the influence of parent material on the process of free-living N fixation.  The reserve includes neighboring lowland tropical forests growing on rhyolitic volcanic rocks, limestone, and limestone influenced alluvium.  Our study asks: how do rates of free-living N fixation vary with parent material in lowland tropical forests? To address this question, we measured rates of free-living N fixation in litter and soil across three parent materials.  At each site, we quantified the carbon, nitrogen, and phosphorus of foliage, litter and soil. 

Results/Conclusions

The limestone sites have higher total soil phosphorus (P) pools in the top 30 cm (limestone = 250 ppm; limestone influenced alluvium = 390 ppm) compared to the volcanic site (147 ppm).  There were significantly higher rates of litter free-living N fixation in the limestone (27 ng N fixed/g/hr; 0.53 kg N/ha/yr) and limestone influenced alluvium (41ng N fixed/g/hr; 0.38 kg N/ha/yr) sites versus the volcanic sites (6.8 ng N fixed/g/hr; 0.22 kg N/ha/yr).  Soil free-living N fixation was greatest in the limestone site (7.11 ng N/g/hr; 0.14 kg N/ha/yr), and was undetectable in the volcanic soils.  Therefore, free-living N fixation is positively correlated with total soil P pools, likely due to the fact N fixation is an energy intensive process and requires significant amounts of P as ATP.  These results suggest that soil P may drive N fixation – the principal N input to natural ecosystems – and therefore may be the ultimate constraint on productivity and C sequestration in tropical ecosystems.