Biogeochemical controls on nitrogen fixation vary between rock types in a lowland tropical rainforest

Background/Question/Methods

Biological nitrogen (N) fixation – the conversion of atmospheric di-nitrogen into bioavailable forms – is the largest input of N into natural ecosystems. N limits plant productivity and carbon (C) exchange in many ecosystems.  Therefore, understanding the controls on N fixation is vital for predicting the degree to which new N inputs can stimulate additional C uptake. This energetically expensive process requires a significant amount of phosphorus (P) and a trace amount of the metal molybdenum (Mo). A few studies have simultaneously examined the effect of P and Mo on N fixation activity in terrestrial ecosystems, but results are highly variable. P and Mo are primarily rock-derived nutrients, so forests on contrasting rock type likely have different concentrations of these nutrients. Here, we ask the question: how do rock and soil chemistry control the nature of limitation on N fixation in decomposing litter?  To assess this relationship, we measured rates of litter N fixation in tropical lowland forests growing on volcanic (rhyolitic) vs. sedimentary (limestone) vs. mixture of the two substrates and performed chemical analyses of soil, litter and foliage.  To test limitation, we conducted a lab-based factorial fertilization experiment with P, Mo, and N on samples from all three sites. 

Results/Conclusions

We show that N fixation is positively correlated with soil P content, and N fertilization substantially suppresses N fixation. We found significantly higher rates of litter N fixation in the limestone (18 ng N fixed/g/hr; 0.39 kg N/ha/yr) and limestone influenced alluvium (34 ng N fixed/g/hr; 0.27 kg N/ha/yr) sites versus the volcanic sites (9.87 ng N fixed/g/hr; 0.25 kg N/ha/yr). Higher N fixation rates correspond to higher total soil phosphorus pools in limestone (limestone = 250 ppm; limestone-influenced alluvium = 390 ppm) than in the volcanic site (147 ppm).  Litter N fixation at the limestone-influenced soils responded significantly to P additions (p =0.08; p=0.01, respectively) but did not respond significantly to Mo, or Mo + P additions.  In contrast, litter N fixation rates at the volcanic site did not significantly respond to nutrient additions.  In all sites, rates of N fixation were significantly suppressed by the addition of N with or without additional nutrients (p <0.05).  These results suggest that the projected increases in N deposition in Central America may suppress litter N fixation.  Furthermore, soil P may drive N fixation, therefore may be the ultimate constraint on primary productivity and C sequestration in tropical ecosystems.