**The Influence of Three Years of Supplemental Nitrogen on Above- and Belowground Biomass Partitioning in a Decade-Old** *Miscanthus* × *giganteus* **in the Lower Silesian Voivodeship (Poland)**

**Izabela Goł ˛ab-Bogacz 1, Waldemar Helios 2, Andrzej Kotecki 2, Marcin Kozak 2and Anna Jama-Rodze ´nska 2,\***


Received: 3 July 2020; Accepted: 23 September 2020; Published: 13 October 2020

**Abstract:** Because of the different opinions regarding nitrogen (N) requirements for *Miscanthus* × *giganteus* biomass production, we conducted an experiment with a set dose of nitrogen. The objective of this study was to examine the effects of nitrogen fertilization on the biomass yield, water content, and morphological features of rhizomes and aboveground plant parts in various terms during a growing season over the course of three years (2014–2016) in Lower Silesia (Wroclaw, Poland). The nitrogen fertilization (dose 60 kg/ha and control) significantly affected the number of shoots (*p* = 0.0018), the water concentration of rhizomes (*p* = 0.0004) and stems (*p* = 0.0218), the dry matter yield of leaves (*p* = 0.0000), and the nitrogen uptake (*p* = 0.0000). Nitrogen fertilization significantly affected the nitrogen uptake in all plant parts (*p* = 0.0000). Although low levels of nitrogen appeared to be important in maintaining the maximum growth potentials of mature *Miscanthus* × *giganteus*, the small reductions in the above- and belowground biomass production are unlikely to outweigh the environmental costs of applying nitrogen. More studies should use the protocols for the above- and belowground yield determination described in this paper in order to create site- and year-specific fertilizer regimes that are optimized for quality and yield for autumn (green) and spring (delayed) harvests.

**Keywords:** *Miscanthus*; nitrogen fertilization; rhizomes; stem; leaves

#### **1. Introduction**

New technologies, excessive fossil fuel combustion, and future fossil fuel depletion will contribute to permanent changes in the natural environment. One of the most pivotal environmental problems is climate change, which is caused by the anthropogenic heating of the atmosphere as a result of rising greenhouse gas concentrations [1–5]. To overcome this difficulty, we must increase the use of renewable energy sources. Renewable energy sources play an increasingly essential role in the energy policy of European countries [6]. Among all renewable energy sources, plant biomass deserves special attention. Fast growing bioenergy crops are characterized by a great potential to provide raw material for renewable energy. *Miscanthus* has been proposed as a biomass energy crop in Europe [7,8], and its use could increase in the near future, as it is one of the most productive plants among bioenergy crops [9–13]. Additionally, biomass combustion is regarded to be more beneficial for the environment than fossil fuel combustion [14–16].

The success of this bioenergy crop is also determined by its low environmental requirements—for instance, its low nitrogen and water requirements, the mechanization of its planting and harvesting, and the resistance of the plants to diseases and pests [13,14,16,17]. Because of its low nutrient requirements, *Miscanthus* can be successfully cultivated on sandy and high organic matter soils with a wide pH range. Additionally, it is being successfully grown in unused marginal areas and has a tolerance to various abiotic stresses, including excessive salinity, low humidity, or the presence of heavy metals [7,18,19]. According to Galatsidas et al. (2018) [20], the total area of marginal land that is appropriate for *Miscanthus* cultivation in Europe is thought to be as high as 11.11 million ha.

For the successful development of *Miscanthus* production, it is necessary to consider the end specific uses and precise information on the effective management of nitrogen fertilization for different soil types under various climatic and growth conditions [14,21]. Although nitrogen is the main element that determines the efficiency of biomass production, it can have negative environmental effects such as water eutrophication and increased carbon dioxide emissions [22,23].

The literature varies regarding the nitrogen requirements for *Miscanthus* × *giganteus* biomass production [14,24–27], because the nitrogen applications of *Miscanthus* × *giganteus* are characterized by variable productivity results. The N requirements of *Miscanthus* × *giganteus* are low compared to those of other bioenergy crops [16,28,29]. According to Cadoux et al. (2011, 2012) [30,31], these low nutrient requirements are caused by various factors, including a high nutrient use efficiency and the nutrient recycling accumulated in the rhizomes. However, there is a serious debate about the exact need for N fertilizer in a given crop and whether N fertilizer should be required at all. The translocation of nitrogen to rhizomes during the late vegetation period is a major factor in the high efficiency of nitrogen utilization [17]. There are divergent results regarding the requirements of Mischanthus ×*giganteus* for N fertilization. The findings are divided on this matter; some studies have shown that the yield increases after the application of N fertilizer [14,25–27,32], while some state the contrary [13,33–38].

There are many European studies that provide estimates of the belowground biomass for *M. giganteus* at a single point in time [29,36–39]; however, there have been few previous studies that determined the dynamics of the rhizome yield which were not based on regular sampling through the growing season [40,41].

The organ of wintering in the *Mischanthus* is the rhizome, an underground part that grows horizontally that is important for nutrient storage and accumulation. Most research on the yield and biometric traits of *Mischanthus* is concentrated on the aboveground parts of the plants [9,14,29,42]. The main aspects of experimental research are mainly focused on the environmental impact of *Mischanthus*, the different terms of harvesting, the different genotypes of *Mischanthus,* and its chemical composition during multiannual study periods. Thus far, the elemental composition and resistance to frost and salinity have been examined in the rhizomes; however, there is a lack of information on the water content in the rhizomes during the whole growing season [43–46]. A new aspect of our research is the determination of the changes in the rhizome water content during the entire vegetation period (May–December) on a 10-year-old plant.

The objective of this study was to examine the effects of nitrogen fertilization on the number, height, and diameter of leaves on a shoot, as well as the water concentration, dry mass yield, and nitrogen uptake of *Miscanthus* × *giganteus*. The growth rate of the aboveground and belowground biomass of *Miscanthus* × *giganteus* (Greef et Deu) was evaluated in the conditions of southwest Poland, with and without nitrogen fertilization. Additionally, research was undertaken to determine the influence of nitrogen fertilization on the dynamics of the water content changes in the rhizomes during the whole vegetation period.

#### **2. Materials and Methods**
