In situ measurements of root exudation in three hardwood species in southern Indiana

Root exudation - the release of soluble organic compounds to soil - has long been considered a black box in ecology owing to methodological difficulties associated with measuring this flux in situ. This knowledge gap is significant given recent findings that suggest exudate inputs are appreciable in magnitude (2-5% of net primary production) and are coupled to microbial activities, nutrient release and soil organic matter decomposition. We developed a novel experimental system for collecting exudates from intact roots of field-grown trees using cuvettes filled with sterile glass beads. We measured root exudation for three tree species in ~80 year old mixed hardwood forest in south central Indiana, USA in the summer of 2012. Exudation rates varied from 0 to 1413 ug C/g root/day, and differed by sampling date and among trees species. Overall, rates were greater in early relative to late July, and greater in sugar maple (Acer saccharum) and white oak (Quercus alba) relative to tulip poplar (Liriodendron tulipifera). Across all species, exudation rates were correlated with root mass, indicating that greater allocation to roots likely increases the amount of C available to fuel soil microbial activity. Collectively, the results of this study should enable us to develop improved model parameterizations of the C costs associated with nutrient acquisition, an important feedback for predicting the role of vegetation in mediating climate change.