Long-term CO2 and N enrichment impacts the structure and function of components of the soil food web

Background/Question/Methods

Enrichment of atmospheric CO₂ and soil N impacts the structure and function of plant communities. Understanding how cryptic soil communities respond to these changes in resource availability can help us understand how the structure and function of terrestrial ecosystems is impacted by global change. We investigated the responses of arbuscular mycorrhizal (AM) fungi, Collembola, mites and nematodes to 7 years of simultaneous CO₂ enrichment (368 or 560 ppm) and N addition (0 or 4g N per m²) in the BioCON field experiment at Cedar Creek, Minnesota. We addressed function by examining the relationships among soil community members and ecosystem parameters such as soil N and C, soil moisture and plant biomass. We predicted that all taxa would respond to CO₂ and N enrichment with changes in abundance, community composition and biomass. We predicted that these responses would be a direct result of changes in how plants influence movement of C, N and water belowground in response to resource changes.

Results/Conclusions

Abundance of AM hyphae and Collembola increased in response to elevated CO₂, whereas nematode predator abundance decreased.  The trophic structure of nematode communities and the community composition of Collembola (family level) also changed in response to elevated CO₂. In contrast, mites did not respond to CO₂. No taxa responded to N enrichment as predicted. This may be a result of the deep sandy soil (~96% sand), allowing available N to leach quickly through the soil.  Treatments changed the availability of C, N and water in the soil. The increase in AM hyphae was correlated with an increase in plant root biomass. There was a strong positive relationship between AM hyphae and Collembola abundance, particularly with the family Isotomidae, which likely represents fungal feeding. The increase in AM hyphae was negatively correlated with predator nematode abundance, which may indicate an associated change in the nematode trophic structure as a result of shifting food resources. This is supported by the additional responses of the nematode trophic structure to changes in soil organic matter. Our results demonstrate that soil taxa are sensitive to long-term changes in resource availability and may subsequently impact ecosystem functions.