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CO2-Induced Soil Microfauna Changes in a New Zealand Pasture
Volume 7, Number 16: 21 April 2004

Yeates et al. (2003) report a number of interesting results they obtained from a season-long FACE study of a 30-year-old grazed pasture at Bulls, Manawatu, New Zealand, where three experimental plots had been maintained at the ambient atmospheric CO2 concentration of 360 ppm and three others had been maintained at a concentration of 475 ppm (a CO2 enhancement of only 32%) for a period of four to five years.  The pasture contained about twenty species of plants, including C4 and C3 grasses (of which Lolium perenne was prominent), legumes and forbs; but the scientists' attention was focused more on what happened to the microfauna inhabiting the soil in which the plants grew than on the plants themselves.  So what did they learn?

Nematode populations increased significantly in response to the 32% increase in the air's CO2 concentration.  Of the various feeding groups studied, Yeates et al. report that the relative increase "was lowest in bacterial-feeders (27%), slightly higher in plant (root) feeders (32%), while those with delicate stylets (or narrow lumens; plant-associated, fungal-feeding) increased more (52% and 57%, respectively)."  The greatest nematode increases, however, were recorded among omnivores (97%) and predators (105%).  Most dramatic of all, root-feeding populations of the Longidorus nematode taxon rose by a whopping 330%, while rotifer populations increased by an equally astounding 310%.

Also increasing in abundance were earthworms: Aporrectodea caliginosa by 25% and Lumbricus rubellus by 58%.  Enchytraeids, on the other hand, decreased in abundance, by approximately 30%.  What are some of the ramifications of these observations?

With respect to earthworms, Yeates et al. note that the introduction of lumbricids has been demonstrated to improve soil conditions in New Zealand pastures (Stockdill, 1982), which obviously helps pasture plants grow better.  In addition, Zaller and Arnone (1999) found that pasture plants growing near earthworm surface casts are more responsive to atmospheric CO2 enrichment than are plants growing further away from them.  Hence, as elevated CO2 concentrations increase earthworm numbers, their subsequent enhanced activities tend to augment the aerial fertilization effect of atmospheric CO2 enrichment, helping plants to grow better still.  What is more, Jongmans et al. (2003) have shown that "earthworms play an important role in the intimate mixing of organic residues and fine mineral soil particles and the formation of organic matter-rich micro-aggregates and can, therefore, contribute to physical protection of organic matter, thereby slowing down organic matter turnover and increasing the soil's potential for carbon sequestration."

With respect to enchytraeid worms, Cole et al. (2000) demonstrated that the grazing pressure they exert on soil microbes enhances microbial activity and the mineralization of carbon in organic-carbon-rich soils.  Hence, the CO2-induced decrease in the abundance of these worms noted by Yeates et al. would be expected to reduce the loss of carbon from these soils, thereby enhancing their potential for carbon sequestration, as has also been demonstrated by Cole et al. (2002) to occur in response to warming-induced decreases in enchytraeid abundances.

A final phenomenon of interest is the response of soil microbes to atmospheric CO2 enrichment.  As noted in our Editorial of 10 Dec 2003, Hungate et al. 2003 suggest that the extra carbon entering soil ecosystems under CO2-enriched conditions will increase the size of soil microbial communities, which will tie up more nitrogen and therefore lead to reductions in nitrogen mineralization, which (being the main source of nitrogen for plants) will ultimately lead to reduced plant growth.  Just as was observed by Finzi and Schlesinger (2003) in the Duke Forest FACE study, however, and just as was observed by Richter et al. (2003) in the Swiss Grassland FACE study, Yeates et al. note that in their study, too, "the standing crop of soil microbial biomass in the whole soil has not responded to the elevated CO2," even though they say that such "might have been expected," as suggested by Hungate et al.

In view of these several observations, it seems safe to say that the ongoing rise in the air's CO2 content will continue to stimulate the growth of earth's higher plants and lead to more carbon being sequestered in the planet's forests and soils, without soil microbes acting as spoilers that bottle up valuable nitrogen by unduly expanding their populations.

Sherwood, Keith and Craig Idso

Cole, L., Bardgett, R.D. and Ineson, P.  2000.  Enchytraeid worms (Oligochaeta) enhance mineralization of carbon in organic upland soils.  European Journal of Soil Science 51: 185-192.

Cole, L., Bardgett, R.D., Ineson, P. and Hobbs, P.J.  2002.  Enchytraeid worm (Oligochaeta) influences on microbial community structure, nutrient dynamics and plant growth in blanket peat subjected to warming.  Soil Biology & Biochemistry 34: 83-92.

Finzi, A.C. and Schlesinger, W.H.  2003.  Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide.  Ecosystems 6: 444-456.

Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B.  2003.  Nitrogen and climate change.  Science 302: 1512-1513.

Jongmans, A.G., Pulleman, M.M., Balabane, M., van Oort, F. and Marinissen, J.C.Y.  2003.  Soil structure and characteristics of organic matter in two orchards differing in earthworm activity.  Applied Soil Ecology 24: 219-232.

Richter, M., Hartwig, U.A., Frossard, E., Nosberger, J. and Cadisch, G.  2003.  Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years.  Soil Biology & Biochemistry 35: 1325-1335.

Stockdill, S.M.J.  1982.  Effects of introduced earthworms on the productivity of New Zealand pastures.  Pedobiologia 24: 29-35.

Yeates, G.W., Newton, P.C.D. and Ross, D.J.  2003.  Significant changes in soil microfauna in grazed pasture under elevated carbon dioxide.  Biology and Fertility of Soils 38: 319-326.

Zaller, J.G. and Arnone III, J.A.  1999.  Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2Ecology 80: 873-881.