New study, conducted at the Duke Forest FACE site and co-authored by the Nicholas School’s Ram Oren, addresses problems associated with assessing fine roots’ role in forest carbon cycling.

DURHAM, N.C. – The longevity of fine-root carbon in forest soil is one of the least understood aspects of global carbon cycling, and fine-root dynamics, in general, are among the least understood processes of plant function.

A new study published in the Jan. 25 issue of the journal Science eliminates some of that confusion and may help scientists finally get to the root of those problems.

“In a nutshell, our study focused on the issues associated with trying to reconcile the differences between two of the most common methods used for estimating the amount of fine roots in soil, how much carbon they contain and how quickly they are replaced by new roots after they die,” said Ram Oren, professor of ecology at the Nicholas School of the Environment and Earth Sciences at 91.

“Our analysis demonstrated that comparing the results from these two methods is like comparing apples to oranges,” he explained. “Both methods are worthwhile, but each is best suited for answering different questions.”

Oren was one of five authors of the study, which was conducted at the Free-Air Carbon Enrichment (FACE) in Duke Forest. His fellow authors were Allan E. Strand and Seth G. Pritchard of the College of Charleston, M. Luke McCormack of The Pennsylvania State University, and Michael A. Davis of the University of Southern Mississippi.

“It is interesting that we can send remote control vehicles to analyze the surface of Mars, and we have identified buckets of subatomic particles, yet we still don't fully understand one of the most basic and important aspects of tree biology - how long their smallest roots live,” Pritchard said. “This study helps move us one step closer to an answer.”

To more fully understand the study’s implications, Oren says you must first understand a bit about the intricacies of the life and death cycle of fine roots themselves, and how scientists measure the effects of the cycle.

Fine roots help to determine a plant’s ability to take up water and nutrients from the soil. For the most part, the more roots a plant has, the greater its intake can be. When fine roots die, the plant must allocate some of its limited store of carbohydrates to produce new roots, rather than using the carbs for other purposes such as producing stems or fruit. As dead roots decompose, they are partially transformed to carbon dioxide that is released back into the atmosphere and are partially converted to different kinds of organic carbon material.  Some of this organic matter is retained in the soil, breaking down very slowly and, thus, contributing to carbon sequestration.

Most recent studies have used one of two methods to quantify these complex below-ground dynamics. In one approach, the persistence of carbon is inferred by sieving fine roots from cores that have been extracted from the soil, and then by analyzing the amount of different carbon isotopes those roots contain. In the other approach, the longevity of individual roots is measured by direct observation from minirhizotron camera images of roots growing on the outer surface of clear tubes that have been inserted permanently in the soil.

Oren and his colleagues conducted studies at the FACE site using both methods. Cores and images were taken at periodic intervals over many years.

“We found that the minirhizotron approach is more appropriate for quantifying rates of births and deaths of the smallest roots. Since these tiniest roots harvest the majority of water and nutrients taken up by trees, they are of most importance for forest biology,” Oren said.

On the other hand, the isotopic approach is a coarser technique better suited for quantifying the amount of time carbon is retained in somewhat larger fine roots, he explained. Since these roots contain more carbon than the smallest roots, they probably play a larger role in directly transferring carbon into the soil from the atmosphere.

“Our analysis shows that the results obtained from these methods cannot be confused as interchangeable but they are complementary, and can be combined to learn more about both tree nutrition and forest carbon cycling,” Oren said.