Grasses with the C 3 photosynthetic pathway are commonly considered to be more nutritious host plants than C 4 grasses, but the nutritional quality of C 3 grasses is also more greatly impacted by elevated atmospheric CO 2 than is that of C 4 grasses; C 3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C 4 grasses under elevated CO 2 . Will C 3 grasses remain nutritionally superior to C 4 grasses under elevated CO 2 levels? We addressed this question by determining whether levels of protein in C 3 grasses decline to similar levels as in C 4 grasses, and whether total carbohydrate : protein ratios become similar in C 3 and C 4 grasses under elevated CO 2 . In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C 3 grasses, levels of fructan respond most strongly to elevated CO 2 . Five C 3 and five C 4 grass species were grown from seed in outdoor opentop chambers at ambient (370 ppm) or elevated (740 ppm) CO 2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C 3 grasses under elevated CO 2 was associated with a significant reduction in their protein levels, while protein levels in most C 4 grasses were little affected by elevated CO 2 . However, this differential response of the two types of grasses was insufficient to reduce protein in C 3 grasses to the levels in C 4 grasses. Although levels of fructan in the C 3 grasses tripled under elevated CO 2 , the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C 3 grasses. We conclude that C 3 grasses will generally remain more nutritious than C 4 grasses at elevated CO 2 concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C 4 grasses.

It is plausible that the nutritional quality of C3 plants will decline more under elevated atmospheric CO2 than will the nutritional quality of C4 plants, causing herbivorous insects to increase their feeding on C3 plants relative to C4 plants. We tested this hypothesis with a C3 and C4 grass and two caterpillar species with different diet breadths. Lolium multiflorum (C3) and Bouteloua curtipendula (C4) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO2. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changed to a greater extent than did that of B. curtipendula when grown in elevated CO2; levels of protein (considered growth limiting) declined in the C3 grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C3 grass to compensate for its lower nutritional quality when grown in an elevated CO2 atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C3 grass than the C4 grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C4 grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO2 levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C4 grasses would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C4 grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.

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The predicted increase in liquid water availability in the McMurdo Dry Valleys (MDV), Antarctica, may have profound consequences for nutrient cycling in soil and aquatic ecosystems. Our ability to predict future changes relies on our understanding of current nutrient cycling processes. Multiple hypotheses exist to explain the variability in soil phosphorus content and availability found throughout the MDV region. We analysed 146 surface soil samples from the MDV to determine the relative importance of parent material, landscape age, soil chemistry and texture, and topography on two biologically relevant phosphorus pools, HCl- and NaHCO3-extractable phosphorus. While HCl-extractable phosphorus is highly predicted by parent material, NaHCO3-extractable phosphorus is unrelated to parent material but is significantly correlated with soil conductivity, soil texture and topography. Neither measure of soil phosphorus was related to landscape age across a gradient of ~20 000 to 1 500 000 years. Glacial history has played an important role in the availability of soil phosphorus by shaping patterns of soil texture and parent material. With a predicted increase in water availability, the rate of mineral weathering may increase, releasing more HCl-extractable phosphorus into soil and aquatic ecosystems.