How might the roots of the world's many species of grass respond to continued atmospheric CO2 enrichment and/or global warming? Most people probably give the question little thought, since we never see the roots of plants unless we dig them up; and "out of sight, out of mind" is a natural characteristic of non-inquiring minds. Nevertheless, it is the roots of most plants that acquire the water and nutrients that sustain them; and the question is consequently of much more than purely academic interest. Therefore, we here review what several experimental studies have learned about how the roots of grasses respond to increases in the air's CO2 concentration and/or soil temperature.
In the area of warming, Fitter et al. (1999) studied a site on the Great Dun Fell in the United Kingdom, determining root birth and death rates from biweekly mini-rhizotron video images obtained over one experiment of six month's duration and another of 18 month's duration in an upland grassland, where the soils of half of the treatment replications were artificially maintained 2.8°C above ambient at a depth of 2 cm. This warming was found by them to increase both root production and root death by approximately equivalent amounts. Therefore, they concluded that "the effect of a warmer climate will be a similar acceleration in both [root] birth and death processes and these will have no direct effect on the soil carbon store."
In the area of atmospheric CO2 enrichment, Milchunas et al. (2005) conducted a five-year open-top chamber study (ambient CO2 = 360 ppm, enriched CO2 = 720 ppm) in semiarid shortgrass steppe grassland at the USDA-ARS Central Plains Experimental Range in north central Colorado (USA), where 88% of the ecosystem's biomass was provided by three co-dominant species -- Bouteloua gracilis (H.B.K.) Lag., Stipa comata (Trin and Rupr.) and Pascopyrum smithii (Rybd.) -- and where a subfrutescent shrub (Artemesia frigida Willd.) was also abundant, obtaining video-image data relative to root growth and decay in each treatment-replicate from 18 minirhizotron tubes that acquired video images to a depth of 40 cm four to five times a year, while root biomass data were obtained from two 20-cm-diameter cylinders driven into the ground within each chamber and collected yearly in the autumn.
At the conclusion of the study, it was determined that root-length growth had been 52% greater in the CO2-enriched chambers than in the ambient-air chambers, while root-length losses had been 37% greater in the elevated-CO2 air. The difference between the CO2-induced growth and decay stimulations was largely attributable to the 41% longer life span of the CO2-enriched roots, which resulted in a CO2-induced root-length pool size increase of 41%. In addition, in the upper part of the soil profile, root diameters were typically observed to be larger in the CO2-enriched chambers, leading to an ultimate CO2-induced root biomass increase of 59%. With respect to the implications of their findings, therefore, the four researchers stated that the "slower turnover of new soil carbon, and increased life span of roots suggest an increased storage of carbon under elevated CO2."
Three years later, Ayres et al. (2008) reported the responses of belowground nematode herbivores to atmospheric CO2 enrichment to approximately 350 ppm above ambient in experiments conducted on three grassland ecosystems in Colorado and California (USA) and Montpellier, France. With respect to the soils involved, they stated that soil moisture increased in response to elevated CO2 in all three experiments, citing the work of Hungate et al. (1997), Nijs et al. (2000) and Morgan et al. (2004); while with respect to the plants involved, they said "elevated CO2 increased root biomass by approximately 3-32% in the first five years of the Coloradoan study (Pendall et al., 2004), by 23% after six years in the Californian study (Rillig et al., 1999), and by 31% after six months in the French study (Dhillion et al., 1996)." But with respect to the nematodes involved, they stated that "CO2 enrichment did not significantly affect the family richness, diversity, or plant parasitic nematode index of herbivorous nematodes in the Colorado, California, or French study," noting that "in each experiment, neutral effects were the most frequent response to CO2 enrichment." Hence,the seven researchers concluded that "one consequence of increased root production, without changes in belowground herbivore populations, might be greater plant inputs to soil," which "may lead to greater soil organic matter pools in grassland ecosystems, potentially enhancing soil carbon sequestration."
One year later, Adair et al. (2009) employed mass balance calculations to quantify the effects of biodiversity, atmospheric CO2 concentration and soil nitrogen (N) content on the total amount of carbon (C) allocated belowground by plants (total belowground C allocation or TBCA), as well as ecosystem C storage, in an eight-year experiment that was part of the BioCON study of a periodically-burned Minnesota grassland. This work revealed that annual TBCA increased in response to all three treatment variables -- "elevated CO2, enriched N, and increasing diversity" -- and that it was also "positively related to standing root biomass." Upon removing the influence of root biomass, however, they found that the effects of N and diversity became neutral or even negative (depending on the year), but that "the effect of elevated CO2 remained positive." In the case of years with fire, on the other hand, they found that "greater litter production in high diversity, elevated CO2, and enhanced N treatments increased annual ecosystem C loss." Consequently, under non-fire conditions, elevated CO2, N and biodiversity would generally tend to increase ecosystem carbon gain; but if grasslands are frequently burned, they could well remain neutral in this regard.
Last of all, Anderson et al. (2010) studied various root responses of a C3-C4 grassland community at Temple, Texas (USA) over a CO2 concentration gradient stretching from 230 to 550 ppm, which they created in two CO2-gradient above-ground "tunnels" of clear polyethylene film. One of the 60-m-long and 1.5-m-wide chambers had ambient air pumped into one end of it; and by the time that air exited the chamber through its other end, its CO2 concentration was reduced by the photosynthetic activity of the plants within the chamber to a value of approximately 230 ppm. At the same time, the other chamber had air enriched to a CO2 concentration of 550 ppm pumped into one end of it; and as this air exited out the other end of that chamber, its CO2 concentration was reduced to a value approximately equivalent to that of the ambient air (~380 ppm). Concurrently, community ingrowth root biomass was assessed along the lengths of the tunnels every two to four months from May 1997 through November 1999, via the help of two ingrowth cores in each 5-meter chamber section; and root biomass response was calculated as the ratio of each measurement date's result to that prevailing at the start of the experiment in May 1997.
At the end of the study, Anderson et al. found that based on the linear relationship they derived from all twenty of the ingrowth biomass assessments they conducted, there was "a 40% increase in the ingrowth root biomass ratio from 380 to 480 ppm as compared with a 36% increase from 280 to 380 ppm," but they say that excluding one extremely variable data point, and using a power function they fit to the data, "the contrast is even greater: a 50% increase from 380 to 480 ppm vs. a 41% increase from 280 to 380 ppm." And we additionally note, in this regard, that in going from the linear relationship to the power function, (1) the r2 value of the relationship jumped from 0.10 to 0.50, and (2) P dropped from 0.095 to less than 0.001.
As a result of their observations, the six scientists concluded that "root biomass in grasslands may have changed markedly as atmospheric CO2 increased since the last glacial period, but that more substantial changes are ahead if the air's CO2 content doubles by the end of this century as predicted." And, of course, those anticipated "changes" should all be positive, implying ever greater grassland root biomass -- as well as all of the good things that phenomenon implies -- as the air's CO2 content continues to climb ever higher. And that conclusion would appear to be the major message of most all of the other studies that have addressed the subject as well.
References
Adair, E.C., Reich, P.B., Hobbie, S.E. and Knops, J.M.H. 2009. Interactive effects of time, CO2, N, and diversity on total belowground carbon allocation and ecosystem carbon storage in a grassland community. Ecosystems 12: 1037-1052.
Anderson, L.J., Derner, J.D., Polley, H.W., Gordon, W.S., Eissenstat, D.M. and Jackson, R.B. 2010. Root responses along a subambient to elevated CO2 gradient in a C3-C4 grassland. Global Change Biology 16: 454-468.
Ayres, E., Wall, D.H., Simmons, B.L., Field, C.B., Milchunas, D.G., Morgan, J.A. and Roy, J. 2008. Belowground nematode herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems. Soil Biology & Biochemistry 40: 978-985.
Dhillion, S.D., Roy, J. and Abrams, M. 1996. Assessing the impact of elevated CO2 on soil microbial activity in a Mediterranean model ecosystem. Plant & Soil 187: 333-342.
Fitter, A.H., Self, G.K., Brown, T.K., Bogie, D.S., Graves, J.D., Benham, D. and Ineson, P. 1999. Root production and turnover in an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature. Oecologia 120: 575-581.
Hungate, B.A., Holland, E.A., Jackson, R.B., Chapin, F.S., Mooney, H.A. and Field, C.B. 1997. The fate of carbon in grasslands under carbon dioxide enrichment. Nature 388: 576-579.
Milchunas, D.G., Morgan, J.A., Mosier, A.R. and LeCain, D.R. 2005. Root dynamics and demography in shortgrass steppe under elevated CO2, and comments on minirhizotron methodology. Global Change Biology 11: 1837-1855.
Morgan, J.A., Mosier, A.R., Milchunas, D.G., LeCain, D.R., Nelson, J.A. and Parton, W.J. 2004. CO2 enhances productivity, alters species composition, and reduces digestibility of shortgrass steppe vegetation. Ecological Applications 14: 208-219.
Nijs, I., Roy, J., Salager, J.-L. and Fabreguettes, J. 2000. Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems. Global Change Biology 6: 981-994.
Pendall, E., Mosier, A.R. and Morgan, J.A. 2004. Rhizodeposition stimulated by elevated CO2 in a semiarid grassland. New Phytologist 162: 447-458.
Rillig, M.C., Field, C.B. and Allen, M.F. 1999. Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands. Oecologia 119: 572-577.
Last updated 19 May 2010