Anthony's Science Blog

Ongoing debate over effects of N deposition on forest carbon storage

January 10, 2012 – 4:44 pm

There was an interesting note in the most recent Global Change Biology about whether N deposition might fertilize temperate forests and cause them to remove and store carbon from the atmosphere, providing some small relief from ever-climbing atmospheric CO2 levels. Author Peter Högberg writes:

It was first widely held that N deposition should increase tree growth in the northern hemisphere, in particular (e.g., Townsend et al., 1996); the Kyoto protocol even stated that this effect should be accounted for. However, based on the distribution of ¹⁵N tracer added experimentally to forests in Europe and N. America, Nadelhoffer et al. (1999) proposed that the effect of N additions on C sequestration should be minor as they found that most ¹⁵N ended up in the soil rather than in the above-ground parts of the trees. Their proposition was challenged by Magnani et al. (2007), who, based on estimates of net ecosystem production (NEP) derived from eddy-covariance studies, argued that the correlation between C sequestration and N deposition was very strong, with a slope indicating that many hundreds of kg of C were sequestered per kg of N deposited on the forest.

Högberg then goes on to provide some evidence that the correlation between NEP and N deposition may not be causal since humans tend to populate (and pollute) areas that are already more productive and likely to naturally sequester more C. It is an interesting explanation of the data and I will be curious to see any response Magnani et al. may have. Either way, it is certainly difficult to figure out what atmospherically deposited N does in these ecosystems since there is still a lot we don’t know about belowground ecosystems and tree ecophysiology.