The carbon and nitrogen cycle collide in soil: An examination of the effects of switchgrass root exudates on soil denitrification

Background/Question/Methods

Greenhouse gas emissions are one of the main contributors to global climate change. Consequently, biofuels are being assessed as potential alternative energy sources to fossil fuels. Switchgrass is an ideal candidate as a sustainable biofuel because it is a perennial crop, which thrives in marginal land soil and acts as a potential carbon sink. Although much is known about how to maximize switchgrass yields and its viability as a biofuel, there is limited evidence linking carbon quantity and quality to microbially mediated denitrification and the ratio of nitrous oxide to nitrogen gas (N₂O:N₂) emissions from the microbial community. Since N₂O is a potent greenhouse gas, it is imperative that we understand more about the influence of switchgrass root exudates on soil denitrification rates in order to comprehensively understand the potential of switchgrass as a biofuel crop and factor its influence on the greenhouse gas budget. In this experiment four carbon-based switchgrass root exudates (mannitol, glutamate, lignin, and butyrate) were added to marginal land soil along with a high (0.16% N- NO₃^- (w/w)) and low (0.0017 % N- NO₃^- (w/w)) nitrate (NO₃^-) concentration. Denitrification enzyme assays were run and the ratio of N₂O:N₂was estimated.

Results/Conclusions

It was found that overall denitrification resulted in mainly N₂O production, not N₂ production. Additionally, it was found that the addition of NO₃^- increased denitrification rates, specifically N₂O production. Butyrate and glutamate appeared to inhibit N₂O production as the amount of total organic carbon added was increased. This suggests that both compounds likely inhibit processes upstream of N₂O production, causing little to no N₂O to be produced at all. On the other hand, mannitol and lignin did not appear to have an affect on denitrification rates or N₂O production. This indicates that the microbes involved in these processes could not utilize either compound. We speculate that the nitrous oxide reductase (NosZ) gene, which converts N₂O into N₂, occurs in limited numbers in this microbial community due to the reaction being unfavorable when organic carbon quantity and quality are low. Microbes can therefore maximize energy yields by producing N₂O only, not N₂.