## 1 Introduction  
Nutrient management in crops can be the largest factor affecting life cycle greenhouse gas (GHG) emissions of advanced bioenergy systems [1]. Synthetic fertilizers are typical in industrial agricultural systems but require substantial energy to produce and sometimes have unintended negative consequences (e.g., volatilization to atmosphere, reduced soil health, and nutrient runoff), especially when applied in excess of crop nutrient demands. Alternative soil amendments, including digestate and hydrochars, have the potential to reduce these undesirable impacts [2; 3]. Digestate is a renewable form of fertilizer that is produced as a coproduct of biogas production from organic matter (e.g., food waste, manure, or crop residue) [4; 5], while hydrochar is a stable, hydrophobic, high carbonaceous product formed when organic material is processed through hydrothermal carbonization (a thermochemical pretreatment process whereby biomass is treated under hot pressurized water) [3; 6]. One of the main challenges for growing commercial bioenergy crops is to produce high yields with minimal fertilizer application and other inputs while also avoiding competition with food crops [7]. Perennial grasses have been proposed as feasible bioenergy crops due to their high yield capacity, ability to grow on marginal lands without displacing food crops, and belowground carbon (C) allocation that enhances soil C sequestration [8; 9; 10; 11]. One such perennial crop is _Miscanthus x giganteus_ (miscanthus), which is a widely recognized advanced biofuel feedstock [11; 12; 13; 14; 15; 16; 17]. Research over the past decade indicates that advanced lignocellulosic biofuel feedstocks like miscanthus have greater GHG mitigation potential than _Zea mays_ (maize) [10; 18] but the sustainability and financial profitability of any energy crop depend in part on the application and management of nitrogen (N) fertilizers [19]. There is no consensus about the effects of N fertilizer application on miscanthus biomass yield, with various long-term studies around the world reporting variable productivity with fertilizer [20; 21; 22; 23]. Some studies have found no correlation between N fertilization and the biomass productivity of miscanthus [7; 24], while others observed a small response in biomass yield [25]. Initial site conditions prior to the application of N fertilizer could affect biomass responses to N fertilizer. The inconsistent effect of N fertilizer application on miscanthus yields, in combination with the evidence that miscanthus crops do not always require N inputs, underscores the importance of evaluating alternative soil amendments. Several studies have measured greenhouse gas (GHG) emissions from miscanthus production [26; 27; 28; 29] and established a link between the application of N fertilizer and soil GHG emissions. In order to protect soil N reserves and decrease the potential of soil GHG emissions while also maximizing biomass yield, a more targeted fertilization approach might be required. One strategy is to determine nutrient requirements using a method based on the amounts of N removed at harvest, which led [PERSON] et al. [30] to recommend a maximum nutrient fertilization of 98 kg N ha\({}^{-1}\) y\({}^{-1}\), 4.7 kg P ha\({}^{-1}\) y\({}^{-1}\), and 140 kg K ha\({}^{-1}\) y\({}^{-1}\) for a dry matter yield of 20 Mg miscanthus ha\({}^{-1}\) at harvest (except when soil nutrient contents are already high). Due to the potential adverse impacts of applying fertilizer on soil and water quality, research is still needed to test alternative forms of fertilization that could potentially increase the yield of perennial crops while decreasing the net GHG emissions. Digestate from anaerobic digestion (AD) provides an alternative to synthetic fertilizers. It is the nutrient rich solid waste left after the production of biogas via AD. The application of AD digestate as an organic fertilizer can increase the amount of N available to crops, biomass yield, and also improve soil physical properties [2]. [PERSON] et al.