The potential interactive effects of elevated carbon dioxide and ozone and invasive earthworm activity in forest systems

Sara M Top, Purdue University

Abstract

Rising atmospheric CO2 and O3 levels have the potential to change both the chemistry and input rate of plant and microbial organic matter to soil, causing fundamental changes in both above and below ground biogeochemical cycles. Because forests store vast amounts of C, small shifts in the photosynthetic input and ecosystem respiration can result in large changes to the overall C and N budget in the forest ecosystem. Pairing the effects of elevated CO2 and O3 with the increasing occurrence of invasive earthworm activity in temperate forests, the outcome results in a shift in how the forest functions. In an effort to examine the shift that occurs we examined the effects of elevated levels of CO2 and O3 on plant and soil (0-25 cm) amino acids at the Rhinelander free-air CO2 enrichment forest (FACE) site in northern Wisconsin, USA. We examined soil amino acids, which are mainly derived from plant and microbial input, because they are of particular importance to soil organic matter dynamics and terrestrial productivity because they store between 35-80% of total soil N, and shifts in amino acid content of the plant litter may shift the N cycle in the Rhinelander FACE site. In addition we also examined earthworm populations and the amino acids of their fecal matter in an effort to understand how earthworms can alter the ecosystem and how they are influenced by changes in ecosystems, such as elevated CO 2 or O3 levels. Also, embedded in this objective is the role that invasive earthworms play in the incorporation of the chemically altered plant material into the soil. These objectives were accomplished through the use of molecular and isotopic analysis of the plant, soil, and earthworm samples. Our work showed that the presence of elevated CO2 resulted in a decrease in concentration of amino acids in the leaf litter, but no change in the proportion of specific amino acids compared to ambient CO2. Amino acids found in the root however, did not change. Although the concentration of amino acids in leaf litter was lower in elevated CO2, because of the increased leaf and root biomass, there was a net increase of amino acid-N added to the soil each year. The presence of elevated O3 resulted in an increase in leaf litter amino acid-C, with no change to the root amino acids. The presence of elevated atmospheric gases results in mainly a change to the leaf litter chemistry, but no significant changes to underground plant tissue. Despite changes to the plant litter, no changes were observed in the soil amino acids with either elevated CO2 or O3. In our one time sampling of earthworms, we found that the presence of elevated CO2 resulted in an increased total number of earthworms, while elevated O3 did not affect the population of earthworms. In the amino acid analysis of the fecal matter, we did not find a treatment effect on the fecal matter on the extracted amino acids, despite the fact that the epigeic earthworms consume more leaf litter and the endogeic earthworms consume more soil. We did, however, find that the endogeic earthworm fecal matter contained more amino acids than all other samples measured, making their fecal matter nutrient hotspots when deposited in the soil. Using an isotopic mixing model for multiple potential sources, we were able to determine a percentage range for the potential sources to the earthworm fecal matter. For the epigeic earthworm fecal matter, the largest contribution to the overall isotopic signature is from the leaf litter (50-80%), while the other is coming from soil and root tissue. For the endogeic earthworm fecal matter, the largest contributor is the surface soil and fine root tissue. It also illustrates that earthworms are a key driver of the incorporation and movement of soil and organic matter at this FACE site. Overall, these results reveal that elevated CO2 and O 3 paired with the change in soil invertebrates can have a large impact on the cycling of C and N in a forest ecosystem. Earthworms at this FACE site and others may be an important factor in the movement of C and N.

Degree

Ph.D.

Advisors

Filley, Purdue University.

Subject Area

Biogeochemistry|Forestry

COinS