## 1 Introduction
[2] applied AD digestate as a plant fertilizer to _Sida hermaphrodita_ (Virginia fam- petal), maize, and _Medicago sativa_ L. (Alfalfa) and found the effect on biomass production was similar to synthetic NPK (Nitrogen, Phosphorus, and Potassium) fertilizer. Similarly, [PERSON] and [PERSON] [31] compared the application of digestate and synthetic NPK fertilizer in alfalfa and found that macronutrients increased in leaves of the alfalfa fertilized with digestate. Recently, [PERSON] et al. [32] applied digestate from wetland biomass to soils and confirmed that digestate resulted in an increase in GHG fluxes to the atmosphere due to the mineralization of its easily biodegradable organic compounds. [PERSON] et al. [2] and [PERSON] et al. [33] did not find a yield difference in maize treated with digestate and urea fertilizer, but noted that digestate released 23% more N\({}_{2}\)O gases while urea emitted 66% more NH\({}_{3}\) gas. Hydrothermal carbonization (HTC) is a thermochemical pretreatment process where biomass is treated under hot pressurized water to produce hydrochar, a carbon-rich product [3]. Although there is a wealth of research on the application of biochar (the product of pyrolysis of raw biomass feedstocks)to soil, little is known about soil amendment with hydrochar and its impact on soil microorganisms that cycle nutrients [3]. The HTC method has the advantage of being able to utilize wet feedstocks [34], and thus additional energy will not be needed to dry the feedstock before being fed into the HTC reactor. As an organic fertilizer, the application of hydrochar in agriculture is encouraged as a replacement for chemical fertilizers because its application has been linked to a reduction in GHG emissions [35]. Despite the potential benefits, some studies find contradictory responses of soils to hydrochar. [PERSON] et al. [36] discovered that hydrocarbons only reduced N\({}_{2}\)O emissions initially but significantly increased N\({}_{2}\)O emissions up to 302% compared to a control treatment when N fertilizer was added. A recent soil column experiment by [PERSON] et al. [37] investigated the effect of sawdust-derived hydrochar on yield and N\({}_{2}\)O fluxes in rice paddies and found an increase in yield with a decrease in N\({}_{2}\)O emissions. [PERSON] et al. [38] observed a 13.5-26.8% increase in rice yield when hydrochar derived from _Chlorella vulgaris_ was applied in a soil column experiment, but they report a 2.17-2.82-fold increase in N\({}_{2}\)O emissions from amended treatments compared to a control treatment. The few studies investigating hydrochar effects on GHG are mainly conducted in soil columns in a laboratory setting [36; 37; 38] which may not fully explain the soil responses in field conditions. Field studies with hydrochar are rare, and the effects of hydrochar application are still uncertain. While several studies have investigated the effects of synthetic N fertilization on the yield and GHG emissions in miscanthus plots, at the time of this writing no studies have determined the relative response of miscanthus to digestate and hydrochar soil amendments compared to a synthetic N fertilizer. Therefore, the objective of this study was to evaluate the effects of three different fertilization treatments (urea, digestate from an anaerobic digester, and hydrochar) on biomass yield and soil GHG fluxes in an established miscanthus stand grown on abandoned land. We hypothesized that the urea would result in the greatest biomass yield compared to the other treatments and the control. With respect to soil GHG fluxes, it was expected that urea would result in greater CH\({}_{4}\), CO\({}_{2}\), and N\({}_{2}\)O fluxes compared to the other treatments. We also hypothesized that raw digestate would result in lower biomass yield compared to the urea, greater biomass than the hydrochar treatment, and greater soil fluxes of CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O than hydrochar-amended soils.