The cycling of nitrogen in a terrestrial ecosystem determines its primary (and secondary) productivity, its diversity, and how much (and how) nitrogen is lost to the atmosphere and waters. In general, plant productivity is limited by the availability of nitrogen. Add a little more nitrogen, and not much changes. Productivity increases, but qualitatively, the ecosystem functions the same. Add a little more, and the ecosystem changes quantitatively, but not qualitatively. Productivity increases. N concentrations increase a bit, but it still is qualitatively similar to the unfertilized ecosystem.
Keep fertilizing the ecosystem with N, and eventually the ecosystem reaches a threshold. Not only does productivity increase, but a lot of other things change. Suddenly, plant N concentrations increase a lot. The plant community shifts towards plants that thrive under higher N. They have high N concentrations, they use alkaloids instead of tannins to defend themselves, their leaves are built to capture as much light as possible, rather than avoid capturing too much light. In the soil, the soil microbial community shifts and the richness of N causes N to start leaving the soils in ways it hadn't before. More NO3- comes out in the waters. More gaseous N is lost to the atmosphere.
This threshold has been repeated experimentally in individual ecosystems throughout the world. And we've seen it when we non-experimentally add a lot of N to pastures or croplands or even forests.
What we see at the plot level or even at the level of the stand or region could potentially have analogs at the planetary level. As humans fix more and more N and more and more N is added to the ecosystems, could the whole planet flip states and autocatalyze from a oligotrophic world to a eutrophic world? Could N limitation become the exception, rather than the rule.
In 2009, Rockstrom et al. published their summary of the state of the earth in respect to Planetary Boundaries (see my 2012 post on the issue here). These planetary boundaries are planet-wide environmental boundaries or ‘tipping points’. Exceed these thresholds, and humanity is at risk.
In contrast, for the global nitrogen cycle, the status is the same as in 2009. We are apparently beyond the zone of uncertainty, and humanity is currently at high risk of exceeding a planetary threshold.
That sounds pretty dire.
But are we?
The basis for this assessment is from a recent paper by de Vries et al. 2013.
Reading the paper, apparently, for the planet to have exceeded a planetary boundary for N requires that one of the following (according to the authors) has exceeded safe operating space:
1) eutrophication of terrestrial ecosystems
2) eutrophication of marine ecosystems
3) acidification of soils and fresh waters
4) NOx, a greenhouse gas
5) ozone formation
6) groundwater contamination
7) stratospheric ozone depletion
There is really no evidence of too much tropospheric ozone or too much groundwater contamination for humans to safely inhabit planet. Soils do not appear to be becoming acidified due to N deposition and fertilization globally. NOx levels are not deathly high. Stratospheric ozone levels are still recovering from CFC phase-outs.
Therefore, if humanity has exceeded a biogeochemical planetary boundary, then there must be evidence of planetary-scale eutrophication of terrestrial or marine ecosystems.
In a future post, I'll examine the intellectual tension about this idea...