Biogeochemical consequences of changes in root-derived carbon inputs to soil in a forest exposed to CO2 enrichment

Background/Question/Methods The release of organic compounds to soil by plant roots strongly influences soil biogeochemistry by stimulating microbial activity, acidifying soil, complexing metals and promoting redox reactions.  We sought to better understand the ecosystem consequences of changes in carbon (C) supply from roots to soil in a 28 year-old loblolly pine stand exposed to elevated atmospheric CO₂ (ambient + 200 ppm) and nitrogen fertilization (11 g m^-2 yr^-1) at the Duke Forest FACE site, NC.

Results/Conclusions

Over a two-year period, trees exposed to CO₂ enrichment increased the release of root exudates by ~50% in nutrient-poor soils, resulting in a doubling of microbial activity in the upper mineral soil (p < 0.05).  Fumigated trees that received nitrogen (N) fertilizer did not exhibit any significant increases in exudation or microbial activity.  We then sought to examine how changes in C supply may influence rhizosphere biogeochemistry by adding model root exudates to soil via artificial roots.  Increases in C supply stimulated microbial respiration in all soils (p < 0.05), with the largest changes occurring in nutrient-poor soils.  Moreover, increased C supply induced significant changes decreases in N and increases in P availability as a result of changes in microbial activity and to a lesser extent, changes in soil pH.  Collectively, our results suggest that dynamic processes occurring at fine spatial scales (e.g. in the rhizosphere) may disproportionately influence biogeochemical responses to global environmental change.