The fertilizer requirements, and in particular nitrogen requirements, of biomass crops have significant implications for the carbon footprint and environmental impact of biomass production systems.

Cadoux et al. [90] reviewed 27 studies dealing with the nitrogen, phosphorus and potas­sium (NPK) requirements of Miscanthus x giganteus. While significant amounts of nitro­gen, phosphorus and potassium are taken up by the crop, only a fraction of this peak nutrient content is removed during the winter harvest due to translocation of nutrients from shoots to rhizomes during winter (Table 4.3). In addition, leaf fall and nutrient leaching from the stems returns some of the absorbed nitrogen, phosphorus and potassium to the soil [37, 38], a proportion of which is used by the crop in subsequent years, along with remobilized nutrients from the rhizomes [28, 37, 38, 91].

The yield response of M. x giganteus to nitrogen fertilization is limited and varies between sites [90, 92, 93]. Just over half of eleven studies reviewed by Cadoux et al. [90] concluded a positive response of M. x giganteus to nitrogen, while five showed an absence of a response. In the studies showing a positive response to nitrogen, the response was generally moderate except under irrigation, where the response was higher. The absence of response in the remaining studies can be explained by the plants having sourced their nitrogen via rhizome remobilization and from available soil mineral nitrogen. Crop age also impacts on the nitrogen response with young plants requiring little to no nitrogen in the first few years. For example, using a meta-analysis, Miguez et al. [63] found Miscanthus did

not respond to nitrogen fertilizer during the first two growing seasons and then responded only slightly to a nitrogen rate of 100 kg ha-1.

The environmental impacts of over fertilization must be taken into account when consid­ering the nitrogen requirements of Miscanthus. Christian et al. [46] compared the nitrogen balance of Miscanthus crops over 10 years. For crops not fertilized, the nitrogen balance after a decade was negative (-254 kg ha-1), implying an overall decline in soil nitrogen reserves. In crops treated with nitrogen at a rate of 120 kg ha-1 yr-1, biomass yield was not significantly different from unfertilized crops while the nitrogen balance was positive (790 kg ha-1 after 10 years) with 280% more nitrogen having leached from the system than the unfertilized treatment.

As stated by Cadoux et al. [90], fertilizer recommendations for Miscanthus will be a compromise between the needs of the crop and the need to maintain soil nutrient reserves while limiting nutrient losses. As the exact nutrient needs of Miscanthus are not yet known, Cadoux et al. [90] proposed that nutrient recommendations should be based on the amount of nutrients removed or likely to be removed at harvest (using expected yield and median concentrations given in Table 4.3). For nitrogen, they recommended no fertilization during the first two years of cultivation because nitrogen requirements are low at this stage while the risk of nitrate leaching is high [94].

While M. x giganteus shows only a small yield response to nitrogen input, some vari­ability in nitrogen response is likely among Miscanthus species, especially in relation to rhizome development and internal nutrient cycling. It would be valuable for breeding pur­poses to determine which progenitor species has contributed M. x giganteus’s efficient yield response to nitrogen.