Effect of irrigation and nitrogen fertilization on biomass yield and efficiency of energy use in crop production of Miscanthus

Abstract

The perennial C₄ grass Miscanthus has been proposed as a biomass energy crop in Europe. Effects of crop age, irrigation and nitrogen fertilization on biomass and energy yields and N content of Miscanthus were investigated and the energy costs of production determined. After an establishment period of 1 year, cultivation of Miscanthus resulted in a dry matter production of over 37 t ha⁻¹ year⁻¹ over a period of 4 years. Irrigation and nitrogen level greatly affected Miscanthus biomass yield. In absence of N fertilization, irrigation did not modify biomass yield and the effect of irrigation increased with the increase in N level. The average N response ranged from 37 to 50 kg biomass kg⁻¹ N applied. Because the calorific value of Miscanthus biomass (16.5 MJ kg⁻¹) was not affected by irrigation and N fertilization, energy production depended exclusively on biomass yield. Maximum energy yield was 564 GJ ha⁻¹ year⁻¹. Without N supply and irrigation, energy yield was 291 GJ h⁻¹. Net energy yield, calculated as the difference between energy output and input, but without inclusion of drying costs, was 543 GJ ha⁻¹ with N fertilization and irrigation and 284 GJ ha⁻¹ without; the ratios of energy output to input in crop production were 22 and 47, respectively.

Introduction

Several plant species have been investigated for their potential as biomass crops. As biomass conversion techniques have been advanced, interest has focused on the cultivation of Miscanthus species, which have potential for high yields of biomass in European conditions with yields of 25 to 35 t ha⁻¹ year⁻¹ once the crop has been established (Schwarz, 1993; van der Werf et al., 1993; Hotz, 1996; Himken et al., 1997; Venendaal et al., 1997).

Miscanthus is a C₄ rhizomatous perennial grass originating from Asia. In temperate climates, its growth is limited by low temperature. In Europe it begins growth in April and continues until halted by frost in November. In winter the above-ground parts are killed by frost. Regrowth occurs from crowns in spring. Nutrients and carbohydrates stored in rhizomes during fall are mobilized to shoots in late April/early May, supporting rapid growth. The plant has an extensive root system that responds quickly to a rapid demand for nutrients during spring growth, thus reducing the risk of nitrate leaching (Himken et al., 1997).

Nursery plants are planted in spring and the crop can be harvested annually for up to 15 years after establishment (Schwarz et al., 1994). Although maximum above-ground dry matter yield is attained in late summer, harvest can be delayed until February/March when the crop has its highest dry matter concentration (Petrini et al., 1996; Himken et al., 1997). This is important for net energy production, as energy requirements for drying of the plant material prior to combustion is less for standing dead material than for green material. At harvest, shoots are mechanically cut near ground level.

The C₄ photosynthetic pathway contributes to high water-use efficiency and to a low nitrogen content of the biomass. Long (1983)and Beale and Long (1997)calculated a requirement of 450 mm water and 92 kg N ha⁻¹ for a crop producing an above-ground harvest of 15 t ha⁻¹ dry matter. It has been reported that the effect of N fertilization on biomass yield is not high: Schwarz et al. (1994), reported an increase of only 1.1 t ha⁻¹ in biomass dry weight (from 20.6 to 21.7 t ha⁻¹) with an increase in N level from 0 to 180 kg ha⁻¹ in a 3-year-old crop. Himken et al. (1997)calculated an annual removal of N from the field (equivalent to the fertilizer demand of an established Miscanthus crop) around 50 to 70 kg N ha⁻¹ in a 4-year-old crop. Christian et al. (1997)found, in 1-year-old plants, that of the 117 kg ha⁻¹ N taken up by the crop, only 23 kg ha⁻¹ (38%) was derived from the fertilizer.

Long-term experience on Miscanthus is still lacking, especially about how to establish a good crop, about its nutritional demands and how yield will vary in the course of an estimated 10–20 years of usage.

The objective of this research was to determine the effects of irrigation and nitrogen fertilizer on yield and nitrogen content of Miscanthus sinensis cv. Giganteus. In addition, the use of energy resources was analyzed to determine the energy cost of management techniques and the energy balance of the crop.