The impacts of elevated atmospheric carbon dioxide and ozone on litter decomposition in corn-soybean rotations

Background/Question/Methods

Studies in natural ecosystems have shown that elevated atmospheric carbon dioxide and ozone can alter litter mass loss and nutrient dynamics. An understanding of these changes can be particularly important in agroecosystems, where they can inform management of nutrient supply and loss. In this project, we studied the effects of elevated atmospheric CO₂ and O₃ on litter mass and N loss in a corn/soybean rotation. Using a free-air gas concentration enrichment facility, replicated 20-m diameter soybean plots were exposed to elevated CO₂ (550 ppm), elevated O₃ (80 ppb), simultaneous elevated CO₂ and O₃, and ambient conditions (4 replicates of each treatment). Corn plots were exposed to elevated and ambient CO₂. In order to assess the relative impacts of litter chemistry and decomposition environment, litter bags were incubated in their home treatments, and control litter was incubated in all treatments. Separate litter bags were used for leaves, stover and roots, in order to assess the impacts of treatments on decomposition of different tissue types.  

Results/Conclusions

When litter was incubated under the conditions from which it was derived (e.g. soybean grown under elevated CO₂ decomposing in soybean under elevated CO₂), soybean leaf mass loss after 6 months increased under elevated O₃ and elevated CO₂ (but not in elevated CO₂ and O₃). Soybean leaf litter chemistry was not impacted by treatment, suggesting that litter mass loss rates were affected by plot conditions. In contrast, soybean stover mass loss did not differ across treatments, even though its litter chemistry was impacted by treatments.

 When control litter was incubated in all treatments, the impacts of crop rotation were stronger than elevated O₃ and/or CO₂. Most tissue types decomposed faster in corn fields than in soybean fields. Within soybean treatments, control leaves had faster mass loss when incubated under elevated CO₂. In contrast, control soybean stover decomposed faster when exposed to elevated O₃, and especially when exposed to elevated CO₂ and O₃ . Treatments had no impact on soybean or corn root litter mass loss. Corn leaf and stover litter decomposed most slowly in soybean plots exposed to elevated O₃.

 Overall, CO₂ and O₃ treatments impacted litter mass loss, but due to changes in the decomposing environment, rather than through changes on litter chemistry.