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Steven A. Kannenberg, Richard P. Phillips, Plant responses to stress impacts: the C we do not see, Tree Physiology, Volume 37, Issue 2, February 2017, Pages 151–153, https://doi.org/10.1093/treephys/tpw108
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The flux of soluble organic carbon (C) from roots to soil (hereafter ‘root exudation’) is believed to have profound effects on C and nutrient cycling in ecosystems. Root exudates can comprise up to 10% of net primary productivity (NPP) in forests (Grayston et al. 1997, Phillips et al. 2011) and may accelerate C and nutrient cycling by up to 30% (Brzostek et al. 2013, Yin et al. 2014, Finzi et al. 2015) through their effects on microbial activity and mineral dissolution (Sulman et al. 2014, Keiluweit et al. 2015). However, despite their importance in altering decomposition and NPP, the primary drivers of exudation remain poorly understood. A primary reason for this is that root exudates are notoriously difficult to collect, especially in field studies. Most exudates are low molecular weight organic compounds that are rapidly assimilated by root-associated soil microbes upon their release into the root apoplast or soil solution. Thus, most of what is known about exudation comes from short-term studies of roots suspended in sterile liquid culture or artificial soil matrices (Phillips et al. 2008). A second impediment to understanding the drivers of exudation is that there are dozens of compounds released by roots (Bais et al. 2006), and the flux of each may depend on whether the exudates are released passively (i.e., via diffusion) or actively (i.e., as an energy dependent process). Given that exudation rates can be driven by environmental conditions, resource availability and plant physiology—factors that may operate alone or in conjunction with one another in nature—it is perhaps unsurprising that we lack a unifying framework for understanding the factors that control this important process.
