Old carbon in young fine roots in boreal forests

Abstract

A large proportion of the soil carbon (C) in boreal forests originates from roots and ectomycorrhizal fungi, and accurate quantification of fine-root litter production is needed. Methods for determination of root turnover have been under debate in recent years. Two recently used methods—radiocarbon (14C) dating and use of minirhizotrons (MR)—have yielded different results. This has been attributed to analysis of different roots by use of these methods. At Flakaliden, northern Sweden, in a long-term soil warming and irrigation experiment, we compared MR lifespan with the 14C-derived age of fine roots from soil cores of the same root diameter class. We also determined the 14C-derived age of ingrowth core roots of Norway spruce. The median lifespan of fine roots around MR tubes installed 15 years previously was shorter than 2.5 years whereas the 14C-derived age of the fine roots from soil cores varied from recently grown to 14 years. Correspondingly, the age of 14C in fine roots harvested from ingrowth cores installed in soil 3 months previously was between 1 and 20 years. Thus, cellulose in these roots contained older 14C than is possible from photosynthesis during the time of cellulose formation. By investigating whether the age of Norway spruce and Scots pine seedlings was less than their root 14C-derived age, we tested the possibility of root C originating from soil uptake. This was found to be unlikely, because fine roots of four and eight-year-old seedlings had 14C that was dated to be as old as or younger than the seedlings. We propose that further effort is required to identify the ecological conditions leading to root growth utilization of stored or recycled C.