Not all nitrogen for forest growth comes from air – trees can tap into bedrock too

Nitrogen forests climate CO2And somehow “that stunning finding has the potential to change all projections related to climate change.”

Wow. That’s dramatic.

The quote comes from Benjamin Houlton, one of the authors of the associated publication in Nature, who was interviewed for the press release of the University of California Davis.

Sedimentary rock is nitrogen source

According to the new study forest biomass on nitrogen-rich sedimentary rock can contain 50 percent more of the important nutrient. Isotope analysis by the UC Davis researchers has revealed that this extra nitrogen is indeed of fossil origin – from a geological source, not a (recent) biological one.

Although the knowledge may be new, physical reality hasn’t changed. There has always been nitrogen in bedrock – and apparently some of that (in the upper inch) has always been available to trees growing on top of it.

So how can this ‘buffer’ climate change caused by rising atmospheric CO2 levels – the word the UC Davis press release uses to emphasise relevance?

Assuming forests grow because of climate change…

Well, that’s under the assumption that climate change would stimulate the world’s forests to grow harder, because of for instance an earlier onset of taiga spring or possible local precipitation increases [and largely ignoring a likely increase in rainforest droughts and temperate and boreal forest fires – and the possibility of climate-induced forest plagues – indeed like that Canadian pine beetle].

…they would need more soil nutrients too

But just like with kids if the trees would be triggered into having a growth spurt it would only work if they could eat more than they ate before. This has had some scientists worried about nitrogen-availability, a possible bottleneck, as we thought all biologically cycled nitrogen was deposited from the air – far away from agriculture and industry a rather scarce supply.

Now that we know some forests have [always had] a backup source – at least we know that.

You’ve sensed it. We feel there may be a small academic marketing component to the study’s climate statement. Seemingly the nitrogen availability does not itself change with climate change: there is no feedback involved.

The role of nutrient-availability in forest climate feedbacks

But how complex and contradictory was the forest story again if we did include such feedbacks?

Recently as you may remember we’ve looked into the forest CO2 fertilisation effect. That one does exist, to some extent, mainly in the tropics – and it may offset [for the time being] some 15 percent of the atmospheric CO2 rise [although of course deforestation is taking a toll on the figure]. The associated increase in litterfall could in turn increase forest soil CO2 emissions another recent study stated – and more than one would expect.

A proper forest feedback would widen a growth bottleneck

Usually however carbon is not a growth-limiting nutrient. Plants can breathe it in from free air, with a constant fresh supply driven by the wind. [And if for some reason the concentration would reach suboptimal levels, most plants apparently can quite easily adapt the size of their leaves’ breathing pores, or stromata.]

The carbon cycle importance of the availability of other nutrients could be different though. In the oceans iron is growth-limiting to plankton, and on land most plants will benefit from having extra nutrients like phosphor and nitrogen available in the soils, as the new Nature study also stresses.

But where that study does not suggest climate change could improve the availability of nitrogen in forest soils, another one, published in PNAS last May, has done so. In boreal forest soils [so that loose organic stuff above the bedrock] the bioavailability of nitrogen could improve with warming, albeit at the cost of having extra carbon oxidation too.

Turning nitrogen feedbacks upside down

Nitrogen however is not only a key ingredient for biomass growth and CO2 sequestration, it is also the key element in laughing gas, N2O, a much more potent greenhouse gas. For instance – in agriculture – adding fertiliser can result in more N2O than CO2 absorption, if expressed in warming potential, or CO2 equivalents.

And if the new Nature study would somehow imply that forests – combining air, organic soil and bedrock sources – may have more nitrogen at their disposal, to absorb more CO2 – here too an unexpected N2O feedback may marginalise the good news – and the potential to change all climate projections.

© Rolf Schuttenhelm |

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