ARS | Publication request: Belowground grassland herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 29, 2007
Publication Date: December 31, 2007
Citation: Ayers, E., Wall, D.H., Simmons, B.L., Field, C.B., Milchunas, D., Morgan, J.A., Roy, J. 2008. Belowground grassland herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems. Soil Biology and Biochemistry 40:978-985.

Interpretive Summary: Grasslands are known to be especially susceptible to climate change and rising atmospheric carbon dioxide (CO2), but little work has been conducted on the biological responses of organisms which live in the soil and have important impacts on grassland ecology. This report describes the results from a five-year field experiment conducted in the shortgrass prairie of north-eastern Colorado in which CO2 was elevated to twice the present-day concentration to observe how rising levels of atmospheric CO2 will affect this ecosystem. The focus of the belowground work reported here was on nematodes, small parasitic worms which live in the soil, consume a major portion of plant biomass, and have important consequences for nutrient cycling. Despite increased plant shoot and root production in grassland plots exposed to elevated CO2, abundances of different classes of nematodes were generally unaffected by CO2. Elevated CO2 also had no affect on total nematode abundance, numbers of nematode species, or diversity of the nematode community. The results seem counter-intuitive, in that nematodes were unresponsive under conditions which promoted root growth, which they feed on. We suspect that other factors which occurred at elevated CO2, like increased soil water content and lower quality plant tissues may have hindered potential nematode responses to the increased root biomass. The lack of nematode responses to CO2 suggests that some of the increased belowground biomass may go uneaten, and therefore tend to increase the soil’s organic matter as CO2 increases in the atmosphere.

Technical Abstract: Grasslands are considered to be one of the most sensitive ecosystems to rising atmospheric CO2 concentrations. Moreover, grasslands support large populations of belowground herbivores that consume a major portion of plant biomass. The direct trophic link between herbivores and plants suggests that primary consumers may be particularly sensitive to rising CO2 concentrations, however, the responses of belowground grassland herbivores has rarely been investigated. Here we report the response of a range of herbivorous nematode populations to elevated CO2 concentrations from three distinct grassland experiments. The three studies each involved 5-6 years of CO2 fumigation, utilized natural or representative plant and soil communities, and were sampled at the end of the growing season. In the vast majority of cases elevated CO2 did not affect the abundance of nematode families; only two nematode families were significantly influenced by CO2 enrichment. Similarly, elevated CO2 did not influence the total abundance, family richness, diversity or plant parasitic index of the herbivorous nematode community. These neutral responses to CO2 enrichment occurred despite increased root production in all three experiments, suggesting a simultaneous antagonistic mechanism may have operated, possibly decreased root quality. Whatever the mechanism, our findings suggest herbivorous nematodes in grassland ecosystems are resistant to rising atmospheric CO2 concentrations.