Agricultural crops grown at elevated atmospheric CO2 concentrations nearly always exhibit increased photosynthetic rates and biomass production. Due to this productivity enhancement, more plant material is typically added to soils from root growth, turnover and exudation, as well as from leaves and stems following their abscission during senescence. This plant material often has a greater carbon-to-nitrogen ratio than plant material produced in ambient air. A question thus arises as to whether or not such changes in plant litter quality affect its decomposition rate. Moreover, if any changes do occur, how might they impact biological carbon sequestration? In searching for answers to these important questions, we turn to the scientific literature to see what has recently been published in this field.
In the study of Booker et al. (2000), leaves from defoliated cotton plants grown at an atmospheric CO2 concentration of 720 ppm displayed significantly greater amounts of starch and soluble sugars and significantly lower concentrations of nitrogen than the leaves of plants grown in ambient air. These changes in the quality of the leaf litter produced under high CO2 likely affected its subsequent decomposition rate, which was 10 to 14% slower than that observed for leaf litter collected from plants grown in air of normal CO2 concentration. Likewise, when crop residues from soybean and sorghum plants that were raised in twice-ambient CO2 environments were mixed with soils to study their decomposition rates, Torbert et al. (1998) noted that they lost significantly less carbon – up to 40% less – than similarly treated crop residues from ambiently-grown crops.
In contrast to the aforementioned studies, where residues from CO2-enriched agricultural crops decomposed at slower rates than residues from crops grown at ambient CO2 concentrations, neither Van Vuuren et al. (2000), for spring wheat, nor Henning et al. (1996), for soybean and sorghum, found any significant differences in the decomposition rates of the residues of crops grown under these two conditions of high or normal atmospheric CO2 concentration.
As the air’s CO2 content continues to rise, therefore, and agricultural crops grow more robustly and return greater amounts of litter to the soil, it is likely that greater amounts of carbon will be sequestered in the soil in which they grew, as crop residue decomposition rates are significantly decreased or remain unchanged.
References
Booker, F.L., Shafer, S.R., Wei, C.-M. and Horton, S.J. 2000. Carbon dioxide enrichment and nitrogen fertilization effects on cotton (Gossypium hirsutum L.) plant residue chemistry and decomposition. Plant and Soil 220: 89-98.
Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A. 1996. Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827.
Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B. 1998. Crop residue decomposition as affected by growth under elevated atmospheric CO2. Soil Science 163: 412-419.
Van Vuuren, M.M.I., Robinson, D., Scrimgeour, C.M., Raven, J.A. and Fitter, A.H. 2000. Decomposition of 13C-labelled wheat root systems following growth at different CO2 concentrations. Soil Biology & Biochemistry 32: 403-413.