Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/1996
Publication Date: N/A
Interpretive Summary: Increasing levels of atmospheric carbon dioxide may affect both the quantity and quality of crop residue returned to the soil. These changes may also affect soil carbon storage in agricultural systems. Our goal was to determine short-term CO2 evolution from the soil (over 8 days) following a tillage event and to compare these readings to no-till conditions. Study plots that were used had been previously grown under no-till conditions for two growing seasons using two different crop species (i.e., grain sorghum ('Savanna 5') and soybean ('Stonewall') grown under a present day CO2 level (359 ppm) and a projected future level of CO2 (705 ppm). The soil type was a Blanton loamy sand. During the two growing seasons CO2 enrichment lead to increased yields, residue and root biomass; increased residue returned to the soil surface resulted in higher percent ground coverage under CO2-enriched conditions. The carbon to nitrogen ratio for sorghum tissue was higher than soybean and CO2 enrichment clearly increased this ratio for sorghum. Differences in this ratio between plant species affected soil CO2 evolution. Flushes of CO2 were observed following initial tillage and after two rainfall events. Our data indicated that short-term CO2 fluxes will be higher for tilled soybean, but, tillage will have a small affect with areas cropped with sorghum. Furthermore, short-term CO2 evolution will be higher for areas with soybean grown under CO2-enriched conditions, however CO2 evolution for areas cropped to sorghum will be similar despite increases in atmospheric CO2 levels.
Technical Abstract: Increasing atmospheric CO2 concentration can increase biomass production which may influence C storage in terrestrial ecosystems. Future C dynamics in terms of soil CO2 fluxes as affected by crop residues from high CO2 environments managed under different tillage systems has not been explored. This study examined the effects of tillage systems in a legume (soybean [Glycine max (L.) Merr.] and non-legume (grain sorghum [Sorghum bicolor] (L.) Moench.] CO2-enriched agro-ecosystem on the rates of short-term CO2 evolution from a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). In the spring of 1994, CO2 flux observations initiated within 5 seconds after a tillage event were compared to no-tillage conditions for 8 days in plots where both crop species had been grown in open top field chambers under ambient (360 ppm) and CO2-enriched (720 ppm) conditions for two seasons (1992 and 1993). Added CO2 increased yields, residue and root biomass; higher percent ground coverage in CO2-enriched plots were also observed prior to the tillage treatment. C:N ratio influenced CO2 flux rates; C:N ratio was highest for sorghum and elevated CO2 clearly increased the sorghum C:N ratio. Flux patterns were characterized by flushes of CO2 following initial tillage and after two rainfall events. Species by tillage and CO2 by species interactions were noted on several days and for total CO2 flux values. Our results suggest that short-term CO2 fluxes will be greater for tilled soybean, but, tillage practice will have little affect with a sorghum crop. In addition, short-term higher flux can be expected for areas with soybean under CO2 enrichment, whereas flux patterns for areas cropped to sorghum will be similar regardless of CO2 level.