The paper shows that differences between tree species in carbon allocation to roots and root dynamics are determinants of soil carbon sequestration potential. Root litter is in intimate contact with soil minerals, and the continuous delivery of root litter to the soil matrix fuels the formation of soil organic matter. However, we show the relative importance of carbon turnover in roots for soil organic matter formation differs between tree species, and it is of lesser importance for soil carbon sequestration when the turnover of the root population is slow. Because most of the models assumed similar root turnover times, root inputs have been overestimated as a general mechanism for carbon accumulation into the soil. The paper offers a mechanism for predicting the soil carbon sequestration potential of a particular forest ecosystem.

I think both. The paper describes a new discovery, because whereas fine roots have often been assumed to have life spans of about one year, our results indicate fine-root carbon turnover times can vary from 1.25 to 9 years, depending on root diameter and forest type. The paper offers a new tool to modelers enabling inclusion of differences in root carbon turnover by forest type in predictions of soil carbon sequestration potential under varying global change scenarios. The paper also describes a new approach to studying root dynamics using stable isotopes that is an alternative to current unreliable methodology.

Our study shows that fine roots of some forest tree species live years longer than the one-year lifespan estimates often used to calculate the carbon-storing capability of forests. Sweetgum and loblolly pine trees were exposed for five years to carbon dioxide labeled with a different ratio of stable isotopes than occurs naturally in the atmosphere. This label allowed us to determine that the fine roots of sweetgum trees have shorter lives than the fine roots of loblolly pines. This finding suggests that trees that regenerate fine roots more often deposit more atmospheric carbon into the soil than trees with longer-lived fine roots. We suggest that, by using a uniform, one-year turnover time for fine roots, past studies could have over-estimated the amount of carbon dioxide the forests of the world can sequester each year.

Very early in my career, I was fortunate to be involved in work at a suite of experimental sites for research on global change, first as a student, then as a postdoctoral fellow, and now as a staff scientist. These sites are funded by the Department of Energy’s Office of Science. I was also fortunate to have collaborations and discussions with Miquel Gonzalez-Meler, Julie Jastrow, Richard Norby, William Schlesinger, and others that established the basis for this study and related investigations.