Long-term changes in topsoil nematode populations in grazed pasture under elevated atmospheric carbon dioxide

Abstract

Increasing atmospheric CO₂ concentrations are expected to affect ecosystems processes, and while a New Zealand study reported a response in soil biological activity after 4 years of CO₂ enrichment, apparently reflecting increased populations of Longidorus elongatus, similar findings have not been reported from other sites. Soil microfauna in 0–10 cm soil under a sheep-grazed pasture on a sand was assessed quarterly in FACE rings that were either at ambient CO₂ or had been exposed to 475 μl l⁻¹ CO₂ for some 9 years. Although the area had been subject to a severe drought and microfaunal populations were lower than previously found, the effects of elevated CO₂ on microfaunal populations were broadly similar to those at 4 years. Average populations of the root-feeding L. elongatus increased from 67,000 to 233,000 m⁻² (3.48×) compared with a 4.26× increase after 4 years; microbial-feeding nematodes increased slightly, while predacious nematodes showed a 2.0× increase. A pot experiment showed an additive effect of elevated CO₂ and L. elongatus abundance in reducing specific root length. That similar effects have been found 4 and 9 years after CO₂ enrichment commenced suggests they are real, and emphasises the difference to other sites around the world where much lower responses to elevated CO₂ have been found. This, in part, reflects the unique combination of soil, plant and soil biological conditions at each site and confirms the strong effect of soil type and vegetation on soil biological processes. Just as the effects of global climate change on a given region are idiosyncratic, so it seems are the effects of elevated CO₂ on soil and ecosystem processes. In part, this reflects our limited understanding of below-ground processes.