Multi-year effects of feral Sorghum spp. grown under ambient and global change conditions in sunlit mesocoms

Background/Question/Methods

Biofuel crops, proposed as a means to reduce dependence on fossil fuels, raise concerns regarding ecological risks of their escape from cultivation. We report here second year results of our study on potential effects of feral biofuel crops on native plant habitats constructed in tubs in outdoor sunlit mesocosm chambers. Constructed plant communities representative of oak savannah (12 chambers) and wet prairie habitats (9 chambers) were established in 2010: one tub in each chamber contained the control community of five native grass and one forb species; a second tub contained natives plus the annual biofuel crop Sorghum bicolor; and the third, the native species plus Johnsongrass (Sorghum halepense), a weedy perennial that can hybridize with crop sorghum (Sorghum bicolor).  The communities were exposed to one of two temperature treatments (ambient or elevated + 2C during summer months), two moisture levels: ambient or drought-stressed (20% less then controls during summer dry period), or a combination of the elevated temperature and drought treatments. Control soil moistures were based on seasonal moisture patterns at nearby native oak savannah and wet prairie sites.  Experimental endpoints included seed production, aboveground biomass, soil chemistry, active soil bacteria, active soil fungi, and nematode populations. 

Results/Conclusions

In the oak savannah habitats in 2010 and 2011, both Sorghum species produced significantly less biomass than the native grass species (P< 0.05).  At the community-level the S. bicolor community had greater aboveground biomass than the native-only community (P<0.05).  Neither of the Sorghum species had a significant effect on active bacterial biomass, but active fungal biomass was highest in the S. bicolor tubs.  The climate change (elevated temperature and/or drought) treatments had significant effects on aboveground biomass, active bacterial biomass and soil carbon in the oak savannah constructed communities.  Climate change treatments did not affect total aboveground biomass in the wet prairie communities.  Total aboveground biomass of the community was greatest in the control and drought treatments, lowest with elevated temperature + drought, and intermediate in high temperature treatments (P<0.0001) in 2011. Similarly, S. bicolor seed biomass was greatest in the elevated temperature mesocosms and lowest in the elevated temperature + drought mesocosms (P<0.05).  Active bacterial biomass was lowest in the drought and elevated temperature + drought mesocosms (P<0.0011). Percent total carbon in the soil increased between 2010 and 2011; it was lowest in the elevated temperature/drought mesocosms (P<0.05).