Switchgrass root detritusphere inherited from rhizosphere: A hotspot of nitrous oxide and microbial activity

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Kyungmin Kim, Maxwell Oerther, Andrey Guber and Alexandra N. Kravchenko, Department of Plant Soil and Microbial Sciences, Michigan State University, East Lansing, MI, Jenie Gil, Nathaniel E. Ostrom and Hasand Gandhi, Department of Integrative Biology, Michigan State University, East Lansing, MI, Yakov Kuzyakov, Soil Science of Temperate Ecosystems, University of Goettingen, Göttingen, Germany

Background/Question/Methods
Switchgrass, a North American native prairie grass, is a promising bioenergy feedstock. Switchgrass root residues remaining in the field after harvest potentially increases greenhouse gas emissions such as nitrous oxide. Such detritusphere differs from the input of fresh organic residues such as manure and residue incorporation, in that it is inherited by the rhizosphere formed from the growth stage. In this study, we used in-situ grown roots for a more comprehensive assessment of N2O emission from the root detritusphere. A combination of stable isotope labeling (13C and 15N) and zymography enabled us to track the source of C/N compounds and spatially separate the enzyme activities in roots and bulk soil. We aimed to compare the magnitude and dynamics of microbial activity and N2O emission in the detritusphere as opposed to those in the bulk soil, under two contrasting moisture contents (40% and 70% water-filled pore space) and dominant pore sizes (> 30μm and < 10μm).
Results/Conclusions
In the soil of > 30μm pores, the root-driven N2O emission consistently accounted for ~70% of the total emission during 21-day incubation, while it accounted for up to ~60% and decreased after day 3 in the soil of < 10μm pores. Chitinase activity in the root was consistent with the N2O emission results, showing higher activity when surrounded by the soil with a prevalent pore size of > 30μm. Ammonium and dissolved organic C/N indicated that the fast decomposition of switchgrass in larger pores (> 30μm) leads to the stimulation of denitrification activity in the root detritusphere. Additionally, we found that the chitinase activity on the root area is positively correlated to the root-driven N2O emission, supporting that the strong N2O hotspot indeed is driven from decomposing roots. Different enzyme activity and temporal dynamics in hotspots and bulk soil, and the closer relationship of hotspot enzyme activity with the N2O emission emphasize the importance of studying biochemical processes and dynamics in hotspots rather than the whole soil.