Effects of elevated atmospheric CO2 on root dynamics and productivity of sorghum grown under conventional and conservation agricultural management practices

Although it is widely acknowledged that rising atmospheric CO2 concentrations will increase crop root growth, no study has considered how this response could be influenced by agricultural management practices. Therefore, we examined the influence of elevated atmospheric CO2 (ambient + 360 mmol mol ) on root dynamics of sorghum (Sorghum bicolor) produced under conventional (tillage following winter fallow) and conservation (no-till following clover (Trifolium repens) winter cover crop) agricultural management practices. Crops were grown in an outdoor soil bin facility and CO2 treatments were administered using open-top field chambers (OTC). Root dynamics were analyzed using minirhizotrons. In conventional tillage plots, CO2-enrichment increased sorghum seasonal root production and mortality by 58 and 59%, respectively. Root growth, however, was unaffected by [CO2] in conservation plots. Growth in CO2-enriched atmospheres increased residue production by 15 and 11% in conventional and conservation plots, respectively. Grain production was 6% greater in the conservation than conventional plots but was unaffected by atmospheric CO2. Neither carbon dioxide nor management practices had any impact on the proportion of roots that died by physiological maturity (i.e., root turnover). The fraction of roots that had died by physiological maturity decreased in a linear fashion from shallow to greater soil depths. Although management did not affect cumulative seasonal root production or mortality it did influence vertical root distribution; conservation management favored shallow root systems whereas conventional management favored deeper rooting. Data emphasize the importance of quantifying production and mortality as separate processes. This study suggests that conversion from conventional to conservation management practices might diminish stimulation of rooting by rising atmospheric CO2, at least in the C4 crop sorghum. # 2005 Elsevier B.V. All rights reserved.