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Title: CO2 enhancement effects in container- versus ground-grown soybeans at equal planting densities

item Fiscus, Edwin
item Booker, Fitzgerald
item Dubois, Jean Jacques
item Burton, Joseph
item Pursley, Walter

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2007
Publication Date: 11/8/2007
Citation: Fiscus, E.L., Booker, F.L., Dubois, J.B., Rufty, T., Burton, J.W., Pursley, W.A. 2007. Co2 enhancement effects in container- versus ground-grown soybeans at equal planting densities. Crop Science. 47:2486-2494.

Interpretive Summary: The CO2 enhancement ratio for yield (CER) is the yield at elevated CO2 concentrations divided by the yield at current atmospheric concentrations and is used to estimate crop yields in the future when the atmospheric CO2 concentrations are predicted to be much higher than currently. The question often arises in climate change research as to whether it is legitimate to estimate CER from potted plants. Previous studies have indicated that this would seem to be the case. However, even though the CERs were not different in those studies, there were substantial differences between the yield per plant because the plants grown in the ground (G) were planted at much higher densities, akin to normal agricultural practices, than those in the pots (P). The present study reinforces the overall conclusion of those studies by demonstrating that P and G plants grown at equal planting densities also exhibit similar CERs (average of 1.2 times for CO2 concentrations of 370 ppm and 700 ppm) even though the yields per plant were higher in G than P in this instance. There were some differences in biomass allocation between the treatments that probably resulted from the larger and more stable resource base in the G treatment.

Technical Abstract: Experiments were conducted to test the hypothesis that the response to elevated carbon dioxide will be the same regardless of whether plants are grown in 21l pots (P) or in the ground (G) in open-top chambers (OTCs) at equal planting density. Previous studies (Booker et al., 2005) have shown that the CO2 enhancement ratio for yield (CER) for soybeans in OTCs was not different between G and P plants averaging 1.23 and 1.25 respectively. However, those grown in the ground were planted at standard planting densities while the pot grown plant density was limited by the size of the pot resulting in much lower densities compared to those in the ground. The resultant density differences led to much lower yields on a per plant basis in the G plants because of the competition for resources. In the present study however, the G treatment was planted at the same density as the P and the CERs, for were found to be 1.22 and 1.18 respectively for [CO2]s similar to the aforementioned studies (370ppm and 700ppm). The stem biomass was substantially lower in P than G but increased with elevated CO2 by 40% and 41% respectively. Although the CERs were close to each other and similar to previous studies, the seed yields in both [CO2]s were significantly lower in the P treatment traceable primarily to a reduction in the number of pods. Seed analysis revealed a small but significant increase in oil but not protein content in elevated CO2. Photosynthetic rates were unaffected by rooting medium but increased substantially in elevated CO2. Root zone temperature measurements showed that the daily maximum soil temperature was consistently higher and (Tmax - Tmin) was consistenty larger in the P treatment than the G. The phase lag of the daily temperatures resulted in higher nighttime root temperatures in P than G which might have resulted in greater respiratory losses leading to some of the yield and stem biomass reductions noted in the P treatment. Also, the G rooting environment was much less restricted than for P, resulting in more stability and perhaps greater and more even distribution of resources which would be less subject to rapid cycling than in the pots. Even given the observed differences in yield and biomass parameters between the P and G treatments, CER was consistent with previous studies and not different between P and G. Therefore, for the purposes of determining the response of soybean yield to elevated atmospheric CO2 levels it seems acceptable to grow plants in large pots.