Effects of fine-root senescence upon soil communities and nutrient flux into soil pools (Final Report)

Abstract

Fine roots represent an important flux of carbon into terrestrial soils. Much of this flux occurs through the exudation of polysaccharides and other carbon compounds during the lifetime of fine-roots, but little is known about their contributions to flux as roots senesce, die and decompose. This project was designed 1) to test approaches for inducing senescence in fine roots and 2) assess the effects of the treatments on fine roots and the associated rhizosphere community. We implemented two fine-root treatments, a steam girdling approach and full severing of fine root modules from the tree, and we contrasted them with a control where roots were handled but not girdled. Fine roots and their associated community subjected to each of these three treatments were subsequently analyzed for changes in physical structure, nutrient content, community structure and gene function. Both girdling manipulations resulted in tissue disruption in fine roots and changes in the soil community relative to controls. In particular, microbial diversity for most taxa declined in girdled treatments whereas fungal diversity increased. We also observed changes in overall plant and fungal gene expression associated with treatment with plant gene expression declining over time in girdled roots with a simultaneous increase in fungal gene expression in these same treatments. Finally, in girdled fine root modules, we observed changes in gene function associated with senescence and plant stress in pine-associated genes while at the same time observing upregulation of genes associated with growth and proliferation in fungi. In conclusion, we were able to successfully girdle fine-roots in a field setting and resolve plant-level and community level changes in response to these girdling treatments.