ARS | Publication request: Carbon and nitrogen co-dependence of soil microbial responses to elevated carbon dioxide and ozone in a wheat-soybean agroecosystem

Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: August 1, 2010
Publication Date: August 1, 2010
Citation: Cheng, L., Booker, F.L., Burkey, K.O., Tu, C., Shew, H.D., Rufty, T., Fiscus, E.L., Deforest, J.L., Hu, S. 2010. Carbon and nitrogen co-dependence of soil microbial responses to elevated carbon dioxide and ozone in a wheat-soybean agroecosystem. Ecological Society of America Abstracts.

Technical Abstract: Climate change factors such as elevated atmospheric CO2 and ozone can exert significant impacts on soil microbes and microbially-mediated ecosystem processes. However, the underlying mechanisms through which soil microbes respond to these environmental changes remain poorly understood. The current hypothesis states that CO2- or ozone-induced changes in C availability drive alterations in soil microbes and the processes they regulate. This hypothesis successfully predicts outcomes in some cases, but fails in others, and has not been evaluated in long-term crop studies. We conducted a four-year field study using open-top chambers to test the hypothesis that alterations in C and N availability induced by these climate change factors exert interactive controls over microbial biomass and activities. The experiment was established in May 2005 to assess responses of a no-till wheat-soybean system. The experiment was a 2 × 2 factorial design with four treatments (charcoal-filtered air, elevated ozone, elevated CO2, elevated CO2 + ozone). Upon senescence of the plants, all aboveground biomass in each chamber was harvested, dried and quantified. Residues other than seeds were uniformly returned to the soil surface of their corresponding treatment plots. Soil samples were taken in June and November of each year. An array of plant, soil and microbial parameters was measured. Linear mixed models were used to test the main effects of CO2, ozone and their interactions and determine whether these effects significantly changed over time. Soybean plants fixed about 20.1 and 28.6 g N m2 per year under ambient and elevated CO2, respectively. Elevated CO2 increased the total amount of N in soybean plant residues by 23% while ozone reduced it by 13%. Neither treatment had a significant effect on wheat residue N. Elevated ozone had no significant effect on any microbial parameter. Elevated CO2 significantly enhanced microbial biomass C, N, respiration, net N mineralization and metabolic quotient. All the CO2-stimulation of microbial properties occurred largely in the third and fourth years of the experiment. The fungal to bacterial biomass ratio decreased over time, but it was higher under elevated than ambient CO2. CO2-enhancement of microbial activities coincided with increased N availability in soil and higher microbial biomass N. High N availability stimulated microbial responses likely through increasing bacterial growth, microbial metabolic activities and microbial biomass turnover. Ozone effects on soil C and N availability were insufficient in magnitude to produce detectable changes in the soil microbial parameters measured. These findings suggest that under future CO2 scenarios, high N availability in many agricultural soils may accelerate organic C turnover, constraining the potential of C sequestration in agroecosystems.