The response of coarse root biomass to long-term CO₂ enrichment and nitrogen application in a maturing Pinus taeda...

Elevated atmospheric CO₂ (eCO₂ ) typically increases aboveground growth in both growth chamber and free-air carbon enrichment (FACE) studies. Here we report on the impacts of eCO₂ 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 eCO₂ (ambient, aCO₂ 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 eCO₂ increased 32%, 1.80 kg m^-2 above the 5.66 kg m^-2 observed in aCO₂ , largely accumulating in the top 30 cm of soil. In contrast, eCO₂ increased broadleaved root biomass more than twofold (aCO₂ : 0.81, eCO₂ : 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 aCO₂ 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), eCO₂ 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 eCO₂ . A sustained increase in root NPP under eCO₂ over the study period indicates that soil nutrients were sufficient to maintain root growth response to eCO₂ . 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.