Document: JOH-3-66-26

In situ dynamics of fine root populations in a native temperate calcareous grassland: Effects of elevated atmospheric CO₂.

ARNONE III, J.A.* ^1,2, J.G.ZALLER ^1,3, E.M.SPEHN ¹, P.A.NIKLAUS ¹, C.E.WELLS ³ and C.KÖRNER ¹

University of Basel, Switzerland ¹
Desert Research Institute, Reno, NV 89512 ²
Utah State University, Logan, UT 84322 ³

Abstract:
We monitored root production and mortality in a species-rich calcareous grassland community using minirhizotrons to test the hypothesis that an increase in these two measures would help explain the increase in net ecosystem CO₂ uptake (net ecosystem exchange, NEE) previously observed under elevated CO₂ (600 vs. 350 l CO₂ l^-1; 8 1.2 m² experimental plots per CO₂ level using the Screen Aided CO₂ Control method) at this site. However, results from the first two years showed no difference in overall root production or mortality in the top 18 cm of soil (where 80-90% of the roots occur). Elevated CO₂ was associated with an upward shift in root length density: a greater proportion of roots were found in the upper 0-6 cm soil layer, and a lower proportion of roots in the lower 12-18 cm, under elevated CO₂ than that seen under ambient CO₂. Elevated CO₂ was also associated with an increase in root survival probability (RSP, e.g. for roots still alive 280 d after they were produced: amb. CO₂: 0.30; elev. CO₂: 0.56) and an increase (+48%) in median root lifespan in the deepest (6-18 cm) soil layer. The factors driving changes in root distribution and longevity with depth under elevated CO₂ were not clear, but may have been related to increase in soil moisture under elevated CO₂ interacting with vertical patterns in soil temperatures. Thus, extra CO₂ taken up in this grassland ecosystem during the growing season under elevated CO₂ could not be explained by changes in root production and mortality. However, C and nutrient cycling may be shifted closer to the soil surface, which could potentially have a substantial effect on the activities of soil heterotrophic organisms as CO₂ levels rise.

Keywords: Fine root "turnover", Root production, Root mortality, CO2 enrichment, Global change, Species-rich grassland, Missing carbon, Root life span, Root demography

This abstract is being presented at: 10:30 AM in session:
Poster Session #18: Elevated CO₂.