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Written by Administrator   
Monday, 02 July 2007

CO2 Science

July 1 2007  

Animals (Worms) -- Summary Perhaps the best known worm in the world is the common earthworm.  How will it be affected as the air's CO2 content continues to climb, and how will its various responses affect the biosphere?  What about other worms?  How will they fare in a CO2-enriched world of the future, and what will be the results of their responses?  Last of all -- which question we shall address first of all -- why do we even care?

"Earthworms," in the words of Edwards (1988), "play a major role in improving and maintaining the fertility, structure, aeration and drainage of agricultural soils."  As noted by Sharpley et al. (1988), for example, "by ingestion and digestion of plant residue and subsequent egestion of cast material, earthworms can redistribute nutrients in a soil and enhance enzyme activity, thereby increasing plant availability of both soil and plant residue nutrients," as others have also demonstrated (Bertsch et al., 1988; McCabe et al., 1988; Zachmann and Molina, 1988).  Likewise, Kemper (1988) describes how "burrows opened to the surface by surface-feeding worms provide drainage for water accumulating on the surface during intense rainfall," noting that "the highly compacted soil surrounding the expanded burrows has low permeability to water which often allows water to flow through these holes for a meter or so before it is sorbed into the surrounding soil."  Hall and Dudas (1988) additionally report that the presence of earthworms appears to mitigate the deleterious effects of certain soil toxins; while Logsdon and Lindon (1988) describe a number of other beneficial effects of earthworms, including (1) enhancement of soil aeration, since under wet conditions earthworm channels do not swell shut as many soil cracks do, (2) enhancement of soil water uptake, since roots can explore deeper soil layers by following earthworm channels, and (3) enhancement of nutrient uptake, since earthworm casts and channel walls have a more neutral pH and higher available nutrient level than bulk soil.  Hence, we care about what happens to earthworms as the air's CO2 content rises because of the many important services they provide for earth's plant life.

So how might rising atmospheric CO2 concentrations impact earthworms?  Edwards (1988) says that "the most important factor in maintaining good earthworm populations [our italics] in agricultural soils is that there be adequate availability of organic matter," while Hendrix et al. (1988) and Kladivko (1988) report that greater levels of plant productivity promote greater levels of earthworm activity.  Consequently, since the most ubiquitous and powerful effect of atmospheric CO2 enrichment is its stimulation of plant productivity, which leads to enhanced organic matter delivery to soils, it logically follows that this aerial fertilization effect of the ongoing rise in the air's CO2 content should significantly increase earthworm populations and amplify the many beneficial services they provide for plants.

Then there's the second most significant and common impact of atmospheric CO2 enrichment on plants: its antitranspirant effect, whereby elevated levels of atmospheric CO2 reduce leaf stomatal apertures and slow the rate of evaporative water loss from the vast bulk of earth's vegetation.  Both growth chamber studies and field experiments that have studied this phenomenon provide voluminous evidence that it often leads to increased soil water contents in many terrestrial ecosystems, which is also something earthworms seem to love, i.e., plenty of soil moisture.

In light of these many proven facts, it should not have been surprising that when Zaller and Arnone (1997) fumigated open-top and -bottom chambers they established in a calcareous grassland near Basal, Switzerland with air of either 350 or 600 ppm CO2 for an entire growing season, they found that the mean annual soil moisture content in the CO2-enriched chambers was 10% greater than that observed in the ambient-air chambers; and because rates of surface cast production by earthworms are typically positively correlated with soil moisture content, they found that cumulative surface cast production after only one year was 35% greater in the CO2-enriched chambers than in the control chambers.  In addition, because earthworm casts are rich in organic carbon and nitrogen, the cumulative amount of these important nutrients on a per-land-area basis was found to be 28% greater in the CO2-enriched chambers than it was in the ambient-air chambers.  And in a subsequent study of the same grassland, Zaller and Arnone (1999) found that plants growing in close proximity to the earthworm casts produced more biomass than similar plants growing further away from them.  What is more, they found that the CO2-induced growth stimulation experienced by the various grasses was also greater for those plants growing nearer the earthworm casts.

The upshot of these various observations is that atmospheric CO2 enrichment sets in motion a self-enhancing cycle of positive biological phenomena, whereby increases in the air's CO2 content (1) stimulate plant productivity and (2) reduce plant evaporative water loss, which results in (1) more organic matter entering the soil and (2) a longer soil moisture retention time and/or greater soil water contents, all of which factors lead to the development of larger and more active earthworm populations, which enhance many important soil properties, including fertility, structure, aeration and drainage, which improved properties further enhance the growth of the plants whose CO2-induced increase in productivity was the factor that started the whole series of processes on the road to a higher level of activity in the first place, and so on.

But the good news doesn't end there.  As Jongmans et al. (2003) point out, "the rate of organic matter decomposition can be decreased in worm casts compared to bulk soil aggregates (Martin, 1991; Haynes and Fraser, 1998)."  Hence, on the basis of these studies and their own micro-morphological investigation of structural development and organic matter distribution in two calcareous marine loam soils on which pear trees had been grown for 45 years (one of which soils exhibited little to no earthworm activity and one of which exhibited high earthworm activity, due to different levels of heavy metal contamination of the soils as a consequence of the prior use of different amounts of fungicides), they concluded 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."  Put more simply, atmospheric CO2 enrichment that stimulates the activity of earthworms also leads to more -- and more secure -- sequestration of carbon in earth's soils, thereby reducing the potential for CO2-induced global warming.

But there's still more to the story of CO2 and worms.  In an intriguing research paper published in Soil Biology & Biochemsitry, Cole et al. (2002) report that "in the peatlands of northern England, which are classified as blanket peat, it has been suggested that the potential effects of global warming on carbon and nutrient dynamics will be related to the activities of dominant soil fauna, and especially enchytraeid worms."  In harmony with these ideas, Cole et al. say they "hypothesized" that warming would lead to increased enchytraeid worm activity, which would lead to higher grazing pressure on microbes in the soil; and since enchytraeid grazing has been observed to enhance microbial activity (Cole et al., 2000), they further hypothesized that more carbon would be liberated in dissolved organic form, "supporting the view that global warming will increase carbon loss from blanket peat ecosystems."

The scientists next describe how they constructed small microcosms from soil and litter they collected near the summit of Great Dun Fell, Cumbria, England.  Subsequent to "defaunating" this material by reducing its temperature to -80°C for 24 hours, they thawed and inoculated it with native soil microbes, after which half of the microcosms were incubated in the dark at 12°C and half at 18°C, the former of which temperatures was approximately equal to mean August soil temperature at a depth of 10 cm at the site of soil collection, while the latter was said by them to be "close to model predictions for soil warming that might result from a doubling of CO2 in blanket peat environments."

Ten seedlings of an indigenous grass of blanket peat were then transplanted into each of the microcosms, while 100 enchytraeid worms were added to each of half of the mini-ecosystems.  These procedures resulted in the creation of four experimental treatments: ambient temperature, ambient temperature + enchytraeid worms, elevated temperature, and elevated temperature + enchytraeid worms.  The resulting 48 microcosms - sufficient to destructively harvest three replicates of each treatment four different times throughout the course of the 64-day experiment - were arranged in a fully randomized design and maintained at either 12 or 18°C with alternating 12-hour light and dark periods.  In addition, throughout the entire course of the study, the microcosms were given distilled water every two days to maintain their original weights.

So what did the researchers find?  First of all, and contrary to their hypothesis, elevated temperature reduced the ability of the enchytraeid worms to enhance the loss of carbon from the microcosms.  At the normal ambient temperature, for example, the presence of the worms enhanced dissolved organic carbon (DOC) loss by 16%, while at the elevated temperature expected for a doubling of the air's CO2 content, the worms had no effect at all on DOC.  In addition, Cole et al. note that "warming may cause drying at the soil surface, forcing enchytraeids to burrow to deeper subsurface horizons."  Hence, since the worms are known to have little influence on soil carbon dynamics below a depth of 4 cm (Cole et al., 2000), they concluded that this additional consequence of warming would further reduce the ability of enchytraeids to enhance carbon loss from blanket peatlands.

In summarizing their findings, Cole et al. say that "the soil biotic response to warming in this study was negative."  That is, it was of such a nature that it resulted in a reduced loss of carbon to the atmosphere, which would tend to slow the rate of rise of the air's CO2 content, just as was suggested by the results of the study of Jongmans et al.

In concluding this summary of research results related to the direct and indirect effects of atmospheric CO2 enrichment on various types of worms and their subsequent activities that (1) amplify the positive effects of CO2 on plants and (2) reduce the negative effects of CO2 on climate, we briefly recount the findings of Yeates et al. (2003), who report a number of interesting results they obtained from a season-long FACE study of a 30-year-old New Zealand pasture, where three experimental plots had been maintained at the ambient atmospheric CO2 concentration of 360 ppm and three others 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 C3 and C4 grasses, 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.

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%.  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 the ramifications of these observations?  With respect to earthworms, Yeates et al. note that just as was found in the studies cited in the first part of this review, the introduction of lumbricids has been demonstrated to improve soil conditions in New Zealand pastures (Stockdill, 1982), which obviously helps pasture plants to grow better.  Hence, the CO2-induced increase in earthworm numbers observed in Yeates et al.'s study would be expected to do more of the same, while the reduced abundance of enchytraeids they documented in the CO2-enriched pasture would supposedly lead to less carbon being released to the air from the soil, as per the known ability of enchytraeids to promote carbon loss from British peat lands under current temperatures.

In summary, it would appear that the lowly earthworm and still lowlier soil nematodes respond to increases in the air's CO2 content, via a number of plant-mediated phenomena, in ways that further enhance the positive effects of atmospheric CO2 enrichment on plant growth and development, while at the same time helping to sequester more carbon more securely in the soil and thereby reducing the potential for CO2-induced global warming.

Not a bad day's work for something some of us only use as bait for catching fish!

References

Bertsch, P.M., Peters, R.A., Luce, H.D. and Claude, D.  1988.  Comparison of earthworm activity in long-term no-tillage and conventionally tilled corn systems.  Agronomy Abstracts 80: 271.

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. 

Edwards, C.A.  1988.  Earthworms and agriculture.  Agronomy Abstracts 80: 274.

Hall, R.B. and Dudas, M.J.  1988.  Effects of chromium loading on earthworms in an amended soil.  Agronomy Abstracts 80: 275.

Haynes, R.J. and Fraser, P.M.  1998.  A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat and lucerne straw.  European Journal of Soil Science 49: 629-636.

Hendrix, P.F., Mueller, B.R., van Vliet, P., Bruce, R.R. and Langdale, G.W.  1988.  Earthworm abundance and distribution in agricultural landscapes of the Georgia piedmont.  Agronomy Abstracts 80: 276.

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.

Kemper, W.D.  1988.  Earthworm burrowing and effects on soil structure and transmissivity.  Agronomy Abstracts 80: 278.

Kladivko, E.J.  1988.  Soil management effects on earthworm populations and activity.  Agronomy Abstracts 80: 278.

Logsdon, S.D. and Linden, D.L.  1988.  Earthworm effects on root growth and function, and on crop growth.  Agronomy Abstracts 80: 280.

Martin, A.  1991.  Short- and long-term effects of the endogenic earthworm Millsonia anomala (Omodeo) (Megascolecidae, Oligochaeta) of tropical savannas on soil organic matter.  Biology and Fertility of Soils 11: 234-238.

McCabe, D., Protz, R. and Tomlin, A.D.  1988.  Earthworm influence on soil quality in native sites of southern Ontario.  Agronomy Abstracts 80: 281.

Sharpley, A.N., Syers, J.K. and Springett, J.  1988.  Earthworm effects on the cycling of organic matter and nutrients.  Agronomy Abstracts 80: 285.

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.

Zachmann, J.E. and Molina, J.A.  1988.  Earthworm-microbe interactions in soil.  Agronomy Abstracts 80: 289.

Zaller, J.G. and Arnone III, J.A.  1997.  Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2.  Oecologia 111: 249-254.

Zaller, J.G. and Arnone III, J.A.  1999.  Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2.  Ecology 80: 873-881.    
 
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