Nutrient Concentrations and Uptakes in Giant Reed (Arundo donax L.) as Affected by Harvest Time and Frequency

The underlying aim of biomass crops is to combine high yields and low nutrient contents. Delayed harvests of perennial grasses can reduce nutrient concentrations, while higher levels are generally observed at early harvests. However, depending on the supply chain and the conversion technology, harvesting before senescence could be viable, leading to multiple harvesting, improved feedstock digestibility, and wet biomass storage. In this study, the influence of harvest time and frequency of giant reed (Arundo donax L.) was assessed on aboveground nitrogen, phosphorus, and potassium concentrations, removal, and nutrient use efficiency. In order to evaluate the effects of different cutting regimes, three single harvest (SH) and six double harvest systems (DH) were compared. Nutrient concentrations declined over the season from 10.3 to 2.5 gN kg−1, from 1.8 to 0.8 gP kg−1, and from 30.0 to 8.2 gK kg−1. Overall, DH led to higher nutrient concentrations than SH. Biomass at second cut tended to be richer in nutrients when harvested in autumn compared with winter, and when first cuts were delayed. Nutrient removal was markedly higher in DH for all the elements considered (on average, 196 kgN, 43 kgP, 530 kgK ha−1 in DH, 111 kgN, 29 kgP, 297 kg Kha−1 in SH). In DH systems, nitrogen and potassium use efficiencies were nearly halved compared with single late cuts, while phosphorus use efficiency decreased by about 30 %. The high nutrient removal rates of double-cut management suggest that it may be not sustainable, unless nutrient cycles are closed and nitrogen losses are evaluated.