Temporal responses of community fine root populations to long-term elevated atmospheric CO 2 and soil nutrient patches in model tropicalecosystems

Abstract

Biomass and length density of fine roots, as well as overall allocation of dry matter to root growth, of C 3 plants has been shown to increase under elevated CO 2. However, it is uncertain whether the stimulatory effect of elevated CO 2 on fine root population size in plant communities will persist, or whether fine root populations at high CO 2 simply reach their maximum sooner (or possibly later) than those produced under ambient CO 2. It is also unclear whether increased nutrient demand at the stand-level under elevated CO 2 will lead to more intense nutrient foraging via enhanced fine root proliferation into relatively nutrient-rich soil microsites. I addressed these questions in a 530 day experiment with model tropical plant communities established in four equivalent ecosystem (17 m 3) in which plants shared a common low fertility soil. Fine root (≤2 mm Ø) populations (biomass and length density) in ecosystems maintained at elevated CO 2 (610 μl l −1) increased more rapidly than those in ecosystems maintained at ambient CO 2 (340 μl l −1) during the first half of the experiment and also remained greater over the entire experiment. The data also indicate that: (1) fine root populations at both CO 2 levels eventually stabilize, (2) stabilization occurs sooner under elevated CO 2 (occupation of the soil volume), and (3) steady-state populations under elevated CO 2 may be slightly larger than those maintained under ambient CO 2. Fine root proliferation into artificially nutrient-enriched microsites was dramatic in all ecosystems (22% to 75% greater than into non-enriched soil). However, proliferation into enriched microsites was not enhanced by elevated CO 2. Thus, elevated CO 2 may not enhance exploitation of nutrient-rich microsites even in low fertility soils, suggesting that increased plant nutrient capture under elevated CO 2 also may be unlikely.