title: The rhizosphere and hyphosphere differ in their impacts on carbon and nitrogen cycling in forests exposed to elevated CO₂.
abstract: While multiple experiments have demonstrated that trees exposed to elevated CO₂ can stimulate microbes to release nutrients from soil organic matter, the importance of root- versus mycorrhizal-induced changes in soil processes are presently unknown. We analyzed the contribution of roots and mycorrhizal activities to carbon (C) and nitrogen (N) turnover in a loblolly pine (Pinus taeda) forest exposed to elevated CO₂ by measuring extracellular enzyme activities at soil microsites accessed via root windows. Specifically, we quantified enzyme activity from soil adjacent to root tips (rhizosphere), soil adjacent to hyphal tips (hyphosphere), and bulk soil. During the peak growing season, CO₂ enrichment induced a greater increase of N-releasing enzymes in the rhizosphere (215% increase) than in the hyphosphere (36% increase), but a greater increase of recalcitrant C-degrading enzymes in the hyphosphere (118%) than in the rhizosphere (19%). Nitrogen fertilization influenced the magnitude of CO₂ effects on enzyme activities in the rhizosphere only. At the ecosystem scale, the rhizosphere accounted for c. 50% and 40% of the total activity of N- and C-releasing enzymes, respectively. Collectively, our results suggest that root exudates may contribute more to accelerated N cycling under elevated CO₂ at this site, while mycorrhizal fungi may contribute more to soil C degradation.
authors.affiliation: Department of Biology, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Ina C
authors.initials: IC
authors.lastname: Meier
authors.affiliation: Department of Biology, College of Charleston, Charleston, SC, 29401, USA.
authors.forename: Seth G
authors.initials: SG
authors.lastname: Pritchard
authors.affiliation: Department of Biology, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Edward R
authors.initials: ER
authors.lastname: Brzostek
authors.affiliation: Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Science, Beijing, China.
authors.forename: M Luke
authors.initials: ML
authors.lastname: McCormack
authors.affiliation: Department of Biology, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Richard P
authors.initials: RP
authors.lastname: Phillips
mesh_terms: D055162:Biocatalysis / Q000187:drug effects; D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000494:pharmacology*; D005110:Extracellular Space / Q000201:enzymology; D005308:Fertilizers; D065928:Forests; D009584:Nitrogen / Q000378:metabolism; D058458:Nitrogen Cycle / Q000187:drug effects*; D009657:North Carolina; D028223:Pinus / Q000201:enzymology / Q000378:metabolism*; D018517:Plant Roots / Q000187:drug effects / Q000382:microbiology; D058441:Rhizosphere; D012987:Soil / Q000737:chemistry; D058438:Water Cycle
chemical_list: D005308:Fertilizers; D012987:Soil; D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen
keywords: Duke free-air carbon enrichment(FACE) experiment; exudation; hyphosphere; land carbon sink; progressive nutrient limitation; rhizosphere priming effects