Review of elevated atmospheric CO2 effects on agro-ecosystems: residue decomposition processes and soil C storage

A series of studies using major crops (cotton [Gossypium hirsutum L.], whea t [Triticum aestivum L.], grain sorghum [Sorghum bicolor (L.) Moench.] and soybean [Glycine max (L.) Merr.]) were reviewed to examine the impact of el evated atmospheric CO2 on crop residue decomposition within agro-ecosystems . Experiments evaluated utilized plant and soil material collected from CO2 study sites using Free Air CO2 Enrichment (FACE) and open top chambers (OT C). A incubation study of FACE residue revealed that CO2-induced changes in cotton residue composition could alter decomposition processes, with a dec rease in N mineralization observed with FACE, which was dependent on plant organ and soil series. Incubation studies utilizing plant material grown in OTC considered CO2-induced changes in relation to quantity and quality of crop residue for two species, soybean and grain sorghum. As with cotton, N mineralization was reduced with elevated CO2 in both species, however, diff erence in both quantity and quality of residue impacted patterns of C miner alization. Over the short-term (14 d), little difference was observed for C O2 treatments in soybean, but C mineralization was reduced with elevated CO 2 in grain sorghum. For longer incubation periods (60 d), a significant red uction in CO2-C mineralized per g of residue added was observed with the el evated atmospheric CO2 treatment in both crop species. Results from incubat ion studies agreed with those from the OTC field observations for both meas urements of short-term CO2 efflux following spring tillage and the cumulati ve effect of elevated CO2 (> 2 years) in this study. Observations from fiel d and laboratory studies indicate that with elevated atmospheric CO2, the r ate of plant residue decomposition may be limited by N and the release of N from decomposing plant material may be slowed. This indicates that underst anding N cycling as affected by elevated CO2 is fundamental to understandin g the potential for soil C storage on a global scale.