Paper Reviewed
Frey, S.D., Ollinger, S., Nadelhoffer, K., Bowden, R., Brzostek, E., Burton, A., Caldwell, B.A., Crow, S., Goodale, C.L., Grandy, A.S., Finzi, A., Kramer, M.G., Lajtha, K., LeMoine, J., Martin, M., McDowell, W.H., Minocha, R., Sadowsky, J.J., Templer, P.H. and Wickings, K. 2014. Chronic nitrogen additions suppress decomposition and sequester soil carbon in temperate forests. Biogeochemistry 121: 305-316.

Introducing their study, Frey et al. (2014) write that "the terrestrial biosphere sequesters up to a third of annual anthropogenic carbon dioxide emissions," and they note in this regard that atmospheric nitrogen (N) deposition contributes to this worthy cause "by enhancing tree productivity and promoting carbon (C) storage in tree biomass." But what about the soil in which the trees grow? Does it sequester any carbon? And if it does, how does the magnitude of that which it sequesters compare with that which is stored within the trees themselves?

In broaching this question, the 20 researchers who are listed as authors of this Biogeochemistry paper employed "long-term tree inventory and litterfall data, coupled with measurements of an extensive suite of soil properties, to assess the relative importance of tree versus soil carbon accumulation for total ecosystem carbon storage following 20 years of experimental N additions to two temperate forest stands (mixed hardwood and red pine) in Central Massachusetts, USA."

This work revealed that the two decades of experimental N additions, as they describe it, "resulted in an 11-38% increase in ecosystem C storage in a hardwood stand, representing an accumulation of 20-30 kg C per kg of N added." And they report that this "nitrogen-induced soil carbon accumulation is of equal or greater magnitude to carbon stored in trees, with the degree of response being dependent on stand type and level of N addition." Last of all, they also conclude that "soil carbon accumulation in response to N enrichment was largely due to a suppression of organic matter decomposition rather than enhanced carbon inputs to soil via litter fall and root production."

In closing, Frey et al. write that "soil C responses to long-term N additions have yet to be incorporated into global-scale C balance models," citing Whittinghill et al. (2012); and they write that "this is needed to accurately simulate future changes in terrestrial C storage in response to atmospheric N deposition," citing Thomas et al. (2013a,b). Perhaps now, in the light of their most recent findings, this could soon be done.

References
Thomas, R.Q., Bonan, G.B. and Goodale, C.L. 2013a. Insights into mechanisms governing forest carbon response to nitrogen deposition: a model-data comparison using observed responses to nitrogen addition. Biogeosciences 10: 3869-3887.

Thomas, R.Q., Zaehle, S., Templer, P.H. and Goodale, C.L. 2013b. Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations. Global Change Biology 19: 2986-2998.

Whittinghill, K.A., Currie, W.S., Zak, D.R., Burton, A.J. and Pregitzer, K.S. 2012. Anthropogenic N deposition increases soil C storage by decreasing the extent of litter decay: analysis of field observations with an ecosystem model. Ecosystems 15: 450-461.