New approach for capturing soluble root exudates in forest soils

1 Soluble root exudates are notoriously difficult to collect in non-hydroponic systems because they are released in a narrow zone around roots and are rapidly assimilated by rhizosphere microbes. This has substantially limited our understanding of their rates of release and chemical composition in situ, and by extension, their ecological significance. 2 Here we describe the advantages and limitations of several commonly employed methods for measuring exudation with respect to their potential adaptability for field use in forest ecosystems. Then, we introduce a novel in situ method for measuring exudation in forest soils, and present preliminary results of the spatial and temporal dynamics of loblolly pine (Pinus taeda L.) exudation at the Duke Forest FACTS-1 site, North Carolina, USA from April 2007 to July 2008. 3 Exudation rates varied by an order of magnitude, with the highest rates occurring in late-June 2007 and mid-July 2008, and the lowest rates occurring during late-August 2007. On an annual basis, we estimate pine roots in the upper 15 cm of soil release c. 9 g C m−2 year−1 via this flux, which represents 1–2% of net primary productivity at the site. 4 The magnitude of exudation rates did not differ across an N availability gradient but did track general patterns of below-ground C allocation at the site. Exudation was well-predicted by root morphological characteristics such as surface area and the number of root and mycorrhizal tips, further supporting a possible link between root C allocation and exudation. 5 Because all methods for estimating exudates introduce experimental artefacts, we suggest that only a limited amount of ecologically relevant information is probably gleaned from a single method. Thus, a complementary suite of experimental approaches will best enable researchers to understand consequences of changing patterns of exudation in the wake of global environmental change.