Preconceptions about Biological Nitrogen Fixation

It’s peas and beans and clover, right? That is what most of us know about biological nitrogen fixation (BNF). BNF actually encompasses a huge range of microbes living in places as diverse as the soil, mosses, plant nodules, dead wood, plant stems and leaves, lichens, and leaf litter. BNF is usually separated into a false dichotomy of being either symbiotic (with a plant) or free-living. But BNF associated with moss is usually considered as free-living, so hey, don’t take that too seriously.

So now we know some basics about BNF, let’s get onto debunking the two big preconceptions about BNF:

  1. Symbiotic BNF represents the vast majority (to the point that free-living is usually not considered in models)
  2. The global spatial pattern of BNF is strongly related to evapotranspiration and net primary productivity (NPP)

These ideas have been around for a while, and there’s no easy way to say this, they’re wrong. According to our new meta-analysis, published in GBC, free-living BNF represents at least a third of all BNF. That’s more than can be ignored.

Similarly, we looked at the relationship between BNF and various climate and soil variables and found that there was no statistically significant relationship. The complexity is not surprising given the range of organisms involved, but it really emphasises that models need to be doing more than assuming BNF is proportional to evapotranspiration. Why, I hear you ask? Because usually the point of models is to tell us something about the future, and if BNF is not represented in a process based way you are going to end up with a model with the wrong amount of terrestrial nitrogen, and thus the wrong amount of terrestrial carbon, and thus the wrong amount of permissible emissions, and thus the wrong climate target to keep everyone at a reasonable temperature and precipitation amount. It’s roundabout, but it matters.