Growth, yield and mineral content of Miscanthus × giganteus grown as a biofuel for 14 successive harvests

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Abstract

Miscanthus × giganteus, a perennial rhizomatous grass commercially used as a biofuel crop was grown in a field experiment on a silty clay loam soil for 14 years. There were 3 rates of fertilizer nitrogen (N), none (control), 60 kg N ha−1 yr−1 and 120 kg N ha−1 yr−1 as cumulative applications. The crop was harvested in winter and dry matter yield measured. N did not influence yield. Yield, which increased for the first 6 years, decreased in years 7 and 8, but then increased again and was highest in the 10th year averaging 17.7 t ha−1 across all treatments. Differences in total production over the14 years were only 5% between the highest and lowest yielding treatments and averaged 178.9 t ha−1 equivalent to 12.8 t ha−1 yr−1. In the first 10 harvests, 92% of dry matter was stem. Although the study showed N fertilizer was not required, it is considered that an application of 7 kg P ha−1 yr−1 and 100 kg K ha−1 yr−1 would avoid soil reserve depletion. Pesticides were not required every year and the crop can be considered as low input with a high level of sustainability for at least 14 years.

Introduction

Recent EU and UK governmental reports predict a more prominent future for biomass crops as a renewable fuel for heat, electrical and motive power production (Anon., 2003, CEC, 2000, European Commission, 1996). The main policy driver is the consequent net reduction of atmospheric emissions of carbon dioxide (CO2), a greenhouse gas emitted mainly by burning fossil fuels. Plant biomass is often termed a CO2-neutral fuel, releasing on conversion only CO2 previously removed from the atmosphere during photosynthesis. However this is not strictly true as some fossil derived CO2 evolves as a result of its cultivation, planting and harvesting and the manufacture of any agrochemicals used. However, for electrical generation, using plant biomass results in a large decrease in net fossil CO2 emission when substituted for a fossil fuel (DTI, 1999).

Nonhebel (2002) considered that for a plant species to have potential as an energy crop, the bioenergy yield must be produced with a low level of inputs that themselves require minimal energy for their production and use so that a positive energy balance is achieved. One species with potential as an energy crop is Miscanthus × giganteus (subsequently referred to as Miscanthus) which is a tall growing C-4 perennial rhizomatous grass from Asia (Greef and Deuter, 1993). There is an estimated 10,000 ha of Miscanthus in the UK (Nix, 2007). The majority of the crop is being used for heat generation in power stations. The energy output of Miscanthus in comparison with energy input has been reported to be circa 15–20:1 (Lewandowski and Kicherer, 1997). In recent years there have been several studies of growth and yield of Miscanthus in Europe (Swartz, 1993, Lewandowski et al., 2000, Jones and Walsh, 2001, Lewandowski and Schmidt, 2006). Miscanthus is sterile requiring vegetative propagation which is expensive; therefore the crop must remain productive for several years so that establishment costs can be recovered (Christian and Riche, 1999). It is believed that Miscanthus remains productive for 15–20 years (Lewandowski et al., 2000), however we are aware of only one other experiment that has been monitored for long-term productivity (15 years) (Clifton-Brown et al., 2007).

The study reported in this paper initially formed part of the EU Miscanthus Productivity Network between 1993 and 1995 (AIR-CT-92-0294) where the objective was to obtain information on the growth and yield of Miscanthus in different regions of Europe (McCarthy and Walsh, 1996). When the network study finished the experiment was continued in order to obtain data on its growth and yield in the longer term.

Miscanthus is harvested annually when stems are dead, which is normally in late winter or spring of the following year. At this time mineral nutrient content has been reduced by re-mobilization to rhizomes and natural weathering (which causes leaching from leaves and stems). A low mineral content at harvest is desirable in biomass intended for thermal conversion because it minimises the impact on combustion efficiency and lowers stack emissions. Also it reduces mineral removal in the harvested biomass (offtake) which in turn may lower future input costs and therefore improve production sustainability.

The objective of the experiment which has continued for 14 years was (a) to study the effect of different rates of N fertilizer on growth (1993–2002) and (b) yield (1993–2006) and (c) to obtain information on the mineral content of the biomass at harvest (1993–2002).

Section snippets

Methods

The experiment was conducted on Rothamsted Research Farm, Harpenden, Hertfordshire (latitude 51°48′N, longitude 0°21′W, altitude 128 m OD). The soil is a silty clay loam over clay-with-flints of the Batcombe Series (Avery and Catt, 1995). The topsoil, 0–23 cm contains approximately 20% clay and has a pH of 7 (in water). The USDA classification of the soil is aquic paleudalf (Soil Survey Staff, 1992), and the FAO classification is chromic luvisol (FAO, 1990). Rainfall was measured at a weather

Results and discussion

Miscanthus growth normally started in April or early May, which is the same time as reported in North-East Japan (JIBP Synthesis, 1975) and continued until about the end of October, although, in some years, greenness was present in the stem and upper leaves later into the year. The warmest months of the year coincide with the April–October growth period when mean temperature is 13.58 °C (range 12.51–14.89) (Table 2). The long-term average rainfall for the growth season was 396 mm whilst the

Conclusions

Miscanthus × giganteus can be successfully cultivated over several years on a silty clay loam soil in Southern England and the reliability of the crop in a supply chain is enhanced by the absence of pests and low levels of disease present.

Dry matter yield, measured on dead stems standing in winter, increased for the first 6 years of growth, was lower for 2 years then increased again. Heaviest yield was measured in the 10th year and it is not clear whether maximum yield has been determined.

There

Acknowledgements

We would like to thank A. Todd for the statistical analysis. This research was funded under the Agro-Industry Research (AIR) programme of the European Union's Directorate General for Agriculture (DG VI) and the UK Department of Trade and Industry (now Department for Business Enterprise and Regulatory Reform), through ETSU (now Future Energy Solutions), AEA Technology Environment, Harwell, Oxfordshire, UK. Rothamsted Research receives grant-aided support from the Biotechnology and Biological

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