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Effects of CO2 on Forage Quality
Volume 6, Number 40: 1 October 2003

It is a well-established fact that atmospheric CO2 enrichment significantly boosts the productivity of both crops and natural vegetation.  Much less, however, is known about the impact of elevated CO2 on the quality of plant tissues, particularly with respect to their suitability as food for animals and humans.  Hence, the subject is often a topic of lively debate; and we have engaged in a fair amount of it (see, for example, our Editorials of 16 Oct 2002, 30 Oct 2002, 20 Nov 2002 and 27 Nov 2002a and 2002b).

Recently, a paper has appeared that merits special attention in this regard.  Although some of its conclusions may appear to be negative -- and its authors clearly interpret them in that light -- closer inspection suggests that such is not the case, and that the big-picture view of the situation is actually positive.  Before we get too far ahead of ourselves, however, we must first review some of the details of the study.

Newman et al. (2003) investigated the effects of two levels of nitrogen (N) fertilization and an approximate doubling of the air's CO2 concentration on the growth and chemical composition of an important forage crop -- tall fescue (Festuca arundinacea Schreber cv. KY-31) -- both when infected and uninfected with a mutualistic fungal endophyte (Neotyphodium coenophialum Morgan-Jones and Gams).

The plants were initially grown from seed in greenhouse flats, but after sixteen weeks they were transplanted into 19-L pots filled with potting media that received periodic applications of a slow-release fertilizer.  Then, over the next two years of outdoor growth, they were periodically clipped, divided and repotted to ensure they did not become root-bound; and at the end of that time, they were placed within 20 1.3-m-diameter open-top chambers, half of which were maintained at the ambient atmospheric CO2 concentration and half of which were maintained at an approximately doubled CO2 concentration of 700 ppm.  In addition, half of the pots in each CO2 treatment received 0.0673 kg N m-2 applied over a period of three consecutive days, while half of them received one-tenth that amount, with this procedure being repeated three times during the course of the 12-week experiment.

So what was learned?  In the words of the authors, "plants grown in twice ambient CO2 concentrations: photosynthesized 15% more; produced tillers at a faster rate; produced 53% more dry matter (DM) yield under low N conditions and 61% more DM under high N conditions; the % organic matter (OM) was little changed except under elevated CO2 and high N when %OM [as %DM] increased by 3%; lignin decreased by 14%; crude protein (CP) concentrations (as %DM) declined by 21% ? and in vitro neutral detergent fiber digestibility declined by 5% under high N conditions but not under low N."

In reviewing these results, it is clear that the doubled atmospheric CO2 concentration had a large positive effect on total forage productivity, inducing a 53% increase in dry-weight biomass production in the low N treatment and a 61% increase in the high N treatment.  The extra CO2 also slightly increased the organic matter content of the biomass, by 1.4% in the low N treatment and by 3.4% in the high N treatment.  In terms of the overall effect of elevated CO2 on plant quantity, therefore, it can be concluded that atmospheric CO2 enrichment was a huge positive factor.

With respect to plant quality, the 14% decrease in lignin content was also a plus, as Newman et al. report that reduced lignin favors grazing mammals and herbivorous insects by reducing the indigestible parts of plants.

On the seemingly negative side of the ledger, elevated CO2 reduced the crude protein content of the forage by an average of 21% in three of the four situations studied: non-endophyte-infected plants in both the low and high N treatments, and endophyte-infected plants in the high N treatment.  However, there was no protein reduction for endophyte-infected plants grown in low nitrogen conditions.  This latter point is very important; for as noted by Newman et al., "the endophyte is present in many native and naturalized populations and the most widely sown cultivars of F. arundinacea," so that the first two situations in which the CO2-induced protein reduction occurred (those involving non-endophyte-infected plants) are not typical of the real world.  In addition, since the dry-weight biomass yield of the forage was increased by fully 53% under the low N regime, and since the ten-times-greater high N regime only boosted yields by a paltry additional 8%, there would appear to be no need to apply any extra N to F. arundinacea in a CO2-enriched environment.  Consequently, under best management practices in a doubled-CO2 world of the future, little to no N would be added to the soil and there would be little to no reduction in the crude protein content of F. arundinacea, but there would be more than 50% more of it produced on the same amount of land.

With respect to the final plant quality issue studied, i.e., forage digestibility, increasing soil N lowered in vitro neutral detergent fiber digestibility in both ambient and CO2-enriched air; and this phenomenon was most pronounced in the elevated CO2 treatment.  Again, however, under low N conditions there was no decline in plant digestibility.  Hence, there is a second good reason to not apply extra nitrogen to F. arundinacea in a high CO2 world of the future and, of course, little to no need to do so.

In conclusion, under best management practices in a future CO2-enriched atmosphere, the results of this study suggest that much greater quantities of good quality forage should be able to be produced without the addition of any -- or very little -- extra nitrogen to the soil.  We also note that a likely environmental bonus resulting from this situation would be a significant reduction in nitrogen pollution of groundwater and rivers.

Sherwood, Keith and Craig Idso

Newman, J.A., Abner, M.L., Dado, R.G., Gibson, D.J., Brookings, A. and Parsons, A.J.  2003.  Effects of elevated CO2, nitrogen and fungal endophyte-infection on tall fescue: growth, photosynthesis, chemical composition and digestibility.  Global Change Biology 9: 425-437.