Friday, May 11, 2012

Global Nitrogen Budgets

It's been said that the global nitrogen budget is currently at the stage that the carbon budget was 50 years ago. Some pools and fluxes are well constrained. Others not so well constrained.

As we continue to examine the global carbon and energy budgets, it becomes even more important to make sure the N cycle is nailed down. For example, N2O has the 300x the warming potential of CO2. And N availability is a key for understanding productivity and ecosystem C sequestration. It fuels productivity, can suppress decomposition, and influences water quality.

One of the least constrained parts of the global N budget is the terrestrial biosphere sink--how much N gets stored on land each year.

Schlesinger (2009) puts the biospheric increment of anthopogenically fixed N at 9 Tg N. His estimate is derived from the amount of N being deposited on land (46 Tg/y) * fraction deposited on forests (0.39) * fraction of deposited N stored in forested ecosystems (0.5) = 9 Tg. That's a pretty rough calculation. By his first-cut estimates, about the fate of 50 Tg y-1 is unknown.

Compare that number with a top-down estimate. Le Quéré estimate that the land sink for C averages 2.6 Pg C y-1 (1990-2000) and were as high as 4.7 Pg C y-1 in 2008.

The typical C:N of soil organic matter is 12, but can be as high as 20. If so, 4.7 Pg C would require 390 Tg of N. C:N of 150 (wood) would require 30 Tg N.

So how much N is currently accumulating in ecosystems today? 10 Tg N? 100? 400?

It's an incredibly unconstrained number, but probably one of the most pivotal for understanding how much C ecosystems. How it will become more constrained is not clear at this point. We aren't committed to the types of measurements required to narrow this number.

Until it does, a major part of the Earth's biotic metabolism goes unknown.

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