Where does all that carbon go? Part II

Last week, Tamino at Open Mind, Eli at Rabbet Run and I began an experiment in mob-blogging’ about the carbon cycle. Following on our initial posts, profilic Eli has posted a couple interesting CO2 concentrations maps that highlight forest fires and fossil fuel emissions.

For a refresher on where all the carbon goes, the graph at right shows the IPCC's breakdown of the resting place, for now, of fossil fuel emissions over the past 25 years. The atmospheric build-up is measured (see Tamino's post) and the ocean uptake in well-constrained by measurement: that allows us to back-out the land uptake.

The drawback to this logic is that the land is both a prominent anthropogenic carbon dioxide source (e.g., deforestation, biomass burning) and a prominent carbon dioxide sink (e.g., net regrowth of vegetation). The positive uptake by land means that the sink is greater than the source. That, however, could change in the future, which would mean a larger fraction of carbon emissions would remain in the atmosphere. To answer that, it helps to study where the net carbon uptake occurring on land, and why?

One culprit is carbon’s chemical sibling nitrogen, that’s #7 on your periodic table if you’re scoring at home. Like many siblings, carbon and nitrogen are quite co-dependent, and, one might argue, a bit resentful about the whole thing. Carbon fixation - photosynthesis, plant growth – is limited by the availability of nitrogen. Though only up to a point. If there’s too much nitrogen, things get saturated, and the carbon-based plants pout and refuse to grow more.

You might find it strange that nitrogen is limited, given that N2 or di-nitrogen gas makes up the majority of the atmosphere. However, N2 is unreactive. It only becomes available to plants when converted to reactive form by microbes. In the process of making fertilizer and burning fossil fuels, we not only have increased the rate at which this conversion happens, leaving more nitrogen in our soils and waterways, we've emitted nitrogen in other reactive, gaseous forms, like nitrogen oxides or NOx. (eli, thanks for the suggestion - ed)

The IPCC map to the right shows nitrogen oxide (NOx) concentrations in the lower atmosphere. Notice the high levels above and downstream of North American, Europe and China. Deposition of this nitrogen could be increasing carbon fixation in forests.

A recent paper in Nature found just that: nitrogen fertilization, not forest regrowth after logging, may explain the majority of the net carbon sink in northern forests. The authors used chronosequences – yes, that’s a real word, not some star trek science word referring to data taken from a forest with trees of varying age that can be used to represent different stages of tree growth – to estimate mean carbon uptake at sites across the northern hemisphere.

By integrating uptake over entire rotations (from planting to forest replacement), the authors were able to get a more complete representation of carbon uptake by forests. Using that data, they found a strong relationship between nitrogen deposition and carbon sequestration, implying nitrogen fertilization may be driving the land carbon sink.

Nitrogen oxide emissions and nitrogen deposition are expected to increase in the future without tougher air pollution policies here and especially in Asia (see this paper). That could increase the carbon sequestration in northern forests, presuming those forests do not become N-saturated. Of course, hopefully the world will reduce NOx emissions and improve air quality. Unfortunately, that could also reduce carbon uptake and thus allow a larger fraction of carbon emissions to stay in the atmosphere.