Significant changes in soil microfauna in grazed pasture under elevated carbon dioxide

Soil microfauna in 0- to 10-cm soil under grazed pasture on a sand (Mollic Psammaquent) was assessed quarterly in free air CO2 enrichment (FACE) rings that were at either ambient CO2 or had been exposed to 475 μl l−1 CO2 for 4–5 years. There were significant increases in nematode (1.5×) and rotifer (4.1×) abundance in soils subjected to elevated CO2. Ten nematode taxa were significantly more abundant under elevated CO2. The greatest increase was 4.3× in root-feeding Longidorus; three other root-feeders showed no increase in population densities at elevated CO2. Bacterial-feeding Cervidellus was the only nematode with a significant decrease (0.4×). The abundance of all nematode feeding groups increased significantly in soils subjected to elevated CO2. The relative increases in abundance of feeding groups (bacterial-feeders 1.3×, root-feeders 1.3×, plant-associated 1.5×, fungal-feeders 1.6×, omnivores 2.0×, predators 2.1×) suggest marked increases in fluxes through microbial-feeding nematodes and a multitrophic response among the soil biota to the increase in atmospheric CO2 above ambient. Data from the site suggest soil microbial biomass C and N pools were unchanged over the sampling period. Of eight nematode indices only total maturity index increased (2.9 to 3.2), reflecting the increased proportion of the large Longidorus. Further work on microbial-microfaunal interactions and their micro-scale relation to roots is needed to better understand the impact of increasing atmospheric CO2 on soil processes.