Compared to annual crops, perennial bioenergy feedstocks require lower exogenous nitrogen (N) input via fertilization, and thus exhibit lower rates of nitrate leaching and N gaseous losses. Despite the lack of fertilization, Miscanthus x giganteus (MxG) maintains high biomass yield through time, making it an environmentally desirable perennial feedstock for bioenergy production. However, we have a limited understanding of where MxG obtains sufficient N to support its high productivity. Soil N mineralization of N could account for a significant portion of MxG N nutrition, and as a result, soil N content would decline over the development of MxG stands. Such a decline could reduce the longevity of MxG as a sustainable bioenergy feedstock with low N input and high yield. We leveraged soil N and isotopic data collected over the life of 10-year-old MxG plots at the University of Illinois Energy Farm in Urbana, IL, to explore evidence for high N mineralization in developing MxG stands. We also measured contemporary gross N mineralization across four fertilization treatments in a 10-year-old MxG fertility trial in Illinois, and across three fertilization treatments in 1- and 3-year-old MxG stands at the Long-Term Assessment of Miscanthus Productivity and Sustainability (LAMPS) trial in Ames, Iowa.
Results/Conclusions
Across all depths (0-100cm), soil δ15N increased an average of 1.09‰ (from 6.05‰ to 7.14‰) from 2008, before MxG stand establishment, through 2015, with the strongest increase (1.9‰) in the 50-100cm depth. Throughout the same period, soil percent N declined an average of 0.01%, with the strongest decline in the 30-50cm depth. Gross N mineralization in the surface soil was high across both sites in 2018, with no effect of stand age nor fertilizer addition. Changes in soil profile δ15N and N content suggest that MxG mines N throughout the soil profile by stimulating high mineralization rates. The lack of mineralization response to stand age and fertilizer indicate that fertilization of MxG stands could lead to leaching of nitrate rather than higher fertilizer N use by the crop. Our results suggest that soil N mineralization serves as an important source of N in MxG but likely cannot account for the amount of N removed in MxG in biomass harvest. Instead, biological nitrogen fixation may meet the balance of the nitrogen demand by MxG.