PS 15-141
Perennial forb invasions of annual grassland vary in their enhancement of soil carbon flux and carbon pools

Monday, August 5, 2013
Exhibit Hall B, Minneapolis Convention Center
Ling Zhang, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, China
Jianwen Zou, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, Jiangsu, China
Evan Siemann, Department of Ecology and Evolutionary Biology, Rice University, Houston
Background/Question/Methods

Soil CO2 flux (Rsoil) plays a major role in carbon cycling. It exceeds all other terrestrial-atmospheric carbon exchanges and is estimated to be an order of magnitude greater than the combined carbon emitted by fossil fuel combustion and deforestation. Soil organic carbon (SOC) pools are predicted to decline if carbon loss through Rsoil is not offset, possibly resulting in elevated atmospheric carbon. Plant invasions have been shown to increase ecosystem carbon cycling. Despite the importance of changing atmospheric carbon, only a few studies have addressed potential effects of invasive plants on terrestrial carbon cycling and carbon pools. To our knowledge, this has not been investigated in annual grassland invaded by perennial forbs. We asked: 1) Do invasive forbs increase Rsoil and/or reduce SOC pools? 2) Do invasive forbs produce more litter and/or have higher litter decomposition rates compared to a native graminoid? To address these questions, we set up 3-2x2 m2 plots with a 5 m buffer zone enclosing areas with invasive forbs (Alternanthera philoxeroides, Solidago canadensis) and native graminoid (Eragrostis pilosa) monocultures. We measured SOC monthly and measured Rsoil weekly. We measured litter decomposition rate in situusing litter bags.

Results/Conclusions

As we predicted, both Rsoil and SOC pools were increased following invasion by either forb species. The increases in cumulative Rsoil were smaller with goldenrod than with alligator weed (~45% vs. ~71%) but SOC increases were larger (~47% vs. ~16%). Compared to the native grass plots, each invasive forb species produced larger amounts (alligator weed ~155%, goldenrod ~360%) of more rapidly decomposing litter (kalligator weed : kEragrostis ~3.6:1, kgoldenrod : kEragrostis ~1.9:1). Increases in Rsoil and SOC with either invader suggest that there are consistent effects of high litter quantity and quality of perennial forbs relative to the native grass. However, the relative magnitudes of Rsoil and SOC increases were sensitive to differences in litter quantity and quality that were negatively related for these two invasive forbs. The larger quantity of litter with goldenrod invasion was associated with relatively higher SOC pools, while higher litter quality associated with alligator weed invasions was associated with relatively higher Rsoil. Functional groups of invaders and natives appear to be useful predictors of directional changes in Rsoil and SOC but the magnitude of effects on soil carbon fluxes and pools seem sensitive to variation in traits of invaders.