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Title: DECOMPOSITION OF SOYBEAN GROWN UNDER ELEVATED CONCENTRATIONS OF CO2 AND O3

Author
item Booker, Fitzgerald
item Prior, Stephen - Steve
item Torbert, Henry - Allen
item Fiscus, Edwin
item PURSLEY, WALTER - NC STATE UNIVERSITY
item HU, SHUIJIN - NC STATE UNIVERSITY

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2004
Publication Date: 4/1/2005
Citation: Booker, F.L., Prior, S.A., Torbert Iii, H.A., Fiscus, E.L., Pursley, W.A., Hu, S. 2005. Decomposition of soybean grown under elevated concentrations of CO2 and O3. Global Change Biology. 11:685-698.

Interpretive Summary: Atmospheric carbon dioxide and air pollutant ozone concentrations continue to increase globally. A critical issue is how these changes will affect agricultural productivity. This includes effects on the decomposition of crop residues left in the field and the availability of mineral nutrients to subsequent crops. To address questions about effects on decomposition processes, a two-year experiment was conducted to determine the chemistry and decomposition rate of residues of soybean plants grown under elevated carbon dioxide and air pollutant ozone in open-top field chambers. Treatment effects on belowground soil activities were indicated by soil microbial biomass. A variety of measurements showed that elevated carbon dioxide increased residue production, soil nitrogen immobilization, and microbial biomass, all of which might increase soil carbon accumulation. Ozone reduced plant residue production but increased nitrogen immobilization. Ozone-induced changes in plant chemistry also slowed decomposition of leaf residue. Soil microbial biomass was not significantly affected by ozone. In combination, the effect of elevated ozone was largely negated by twice-ambient carbon dioxide. Increased nitrogen immobilization with both gases could affect soil carbon accumulation but it is unlikely to influence future crop responses to elevated carbon dioxide and ozone.

Technical Abstract: Increasing concentrations of atmospheric CO2 usually increase growth and yield of many agricultural crops while current levels of tropospheric O3 tend to suppress them. Both gases can alter crop chemistry. Changes in biomass input and plant chemistry affect decomposition of crop residues, depending on interactions with belowground systems. The primary objective of this experiment was to determine the chemistry and decomposition rate of residues of soybean grown under mixtures of CO2 and O3. A second objective was to determine treatment effects on soil microbial biomass. In a two-year experiment, soybean (Glycine max) residues were obtained from plants treated in open-top field chambers to reciprocal combinations of two CO2 (ambient and twice ambient) and two O3 (charcoal-filtered air and 1.5 times ambient) concentrations. Microbial biomass was determined from soil samples collected from each chamber in the latter part of each growing season. Residue biomass was increased 41% by elevated CO2 and lowered 32% by added O3. In combination, elevated CO2 ameliorated up to 90% of the suppressive effect of O3 on residue biomass. Residue chemistry was generally unaffected by elevated CO2. Leaflet residues from the O3 treatments had lower concentrations of starch and soluble sugars, but higher N, fiber and lignin contents. A microcosm experiment indicated that C mineralization was not significantly different among treatments, although immobilization of N was increased by residues from the elevated gas treatments. In a litter bag experiment, remaining leaflet residue mass from the O3 treatments was 50% greater than that in the control treatment whereas other treatment effects were relatively minor. Microbial biomass C was 20% higher in soil samples from the elevated CO2 treatment. This study suggested that twice-ambient CO2 concentration would increase soybean residue input, microbial biomass and N immobilization, possibly resulting in soil C accumulation and lower N availability. Soybean residue input to the soil was curtailed by O3 but changes in residue chemistry could slow decomposition. In combination, the effect of elevated O3 on was largely negated by twice-ambient CO2. Increased N immobilization with both gases could affect soil C accumulation but it is unlikely to influence future crop responses to elevated CO2 and O3.