The response of coarse root biomass to long-term CO2 enrichment and nitrogen application in a maturing Pinus taeda stand with a large broadleaved component

Elevated atmospheric CO2 (eCO(2)) typically increases aboveground growth in both growth chamber and free-air carbon enrichment (FACE) studies. Here we report on the impacts of eCO(2) and nitrogen amendment on coarse root biomass and net primary productivity (NPP) at the Duke FACE study, where half of the eight plots in a 30-year-old loblolly pine (Pinus taeda, L.) plantation, including competing naturally regenerated broadleaved species, were subjected to eCO(2) (ambient, aCO(2) plus 200 ppm) for 15-17 years, combined with annual nitrogen amendments (11.2 g N m(-2)) for 6 years. Allometric equations were developed following harvest to estimate coarse root (>2 mm diameter) biomass. Pine root biomass under eCO(2) increased 32%, 1.80 kg m(-2) above the 5.66 kg m(-2) observed in aCO(2), largely accumulating in the top 30 cm of soil. In contrast, eCO(2) increased broadleaved root biomass more than twofold (aCO(2): 0.81, eCO(2): 2.07 kg m(-2)), primarily accumulating in the 30-60 cm soil depth. Combined, pine and broadleaved root biomass increased 3.08 kg m(-2) over aCO(2) of 6.46 kg m(-2), a 48% increase. Elevated CO2 did not increase pine root:shoot ratio (average 0.24) but increased the ratio from 0.57 to 1.12 in broadleaved species. Averaged over the study (1997-2010), eCO(2) increased pine, broadleaved and total coarse root NPP by 49%, 373% and 86% respectively. Nitrogen amendment had smaller effects on any component, singly or interacting with eCO(2). A sustained increase in root NPP under eCO(2) over the study period indicates that soil nutrients were sufficient to maintain root growth response to eCO(2). These responses must be considered in computing coarse root carbon sequestration of the extensive southern pine and similar forests, and in modelling the responses of coarse root biomass of pine-broadleaved forests to CO2 concentration over a range of soil N availability.