by Christina Whalen
Growing concern about greenhouse gas (GHG) emissions and climate change due to fossil fuel dependency has led to the consideration of more attractive energy sources, especially bioenergy sources. In Northern Europe, the two crops that have worked the most effectively are Miscanthus and willow, two perennial energy grasses that have proven to be sustainable energy crops due to high yields of biomass from low inputs. Farmers are fairly attracted to cultivating these crops because of the declining farming market and the future promise of a biomass energy market. Farmers use break crops to control for disease and weeds, a technique that also increases wheat production. Currently sugar beet and oilseed rape are used in Northern Europe, but because of reduction in the sugar beet industry, hemp has been predicted to be an effective break crop because its root system aids soil structure. Various studies have demonstrated that it produces high yields of biomass with no agrochemical input and very little fertilizer use. It offers the potential of being an effective break crop as well as an energy crop. Finnan et al. compare hemp with other annual and perennial energy crops, economically and as a way to mitigate GHG emissions.
Hemp was compared with sugar beet and oilseed rape using Life Cycle Assessment (LCA) and Net Present Value (NPV) economic assessment on collected Irish data. The environmental and economic comparison requires accurate yield estimates, which vary as a result of meteorological conditions, agronomic practices, and soil type. Because of this the crops were considered across four different levels: a low yield, two mid-level yields, and a high yield. Hemp, sugar beet, and oilseed rape were all grown on tillage farms as break crops. The ground was prepared for Miscanthus cultivation by applying herbicide and then subsoiling and ploughing. Average farm models were built using previous models as examples. Inputs and sinks of GHG’s were considered. Energy use was divided into two categories: those that used diesel and those which used electricity.
Carbon is stored under roots and stays in the soil for long periods of time, thus increasing carbon levels in soils correlates to long-term removal from the atmosphere. Other studies have shown that crop rotations increase soil carbon. In this study two scenarios were considered for below ground carbon storage from perennial crops, both grassland and arable. The assumption that there would be no soil carbon increase when grasslands were converted to perennial energy crops was made, but that soil carbon would increase if perennial energy crops were sown on arable land. Net cultivation emissions were calculated by subtracting carbon sequestration from cultivation emissions. There are large land areas in the European Union (EU) used for liquid biofuel production from oilseed rape and sugar beet. Hemp could be grown on parts of this land to produce feedstock for heat and electricity. An economic analysis was then performed on the yield levels for each crop, assessing the life cycle of each crop.
The results of the study demonstrate that hemp’s GHG mitigation potential is comparable to that of perennial energy crops such as Miscanthus and SRC grown on grassland. However, the results depend on the various assumptions, particularly the comparative yields and end use of biomass. Biomass production depends on local conditions such as climate and soil type. Productive lifetimes of perennial energy crops are unknown because it is hard to predict stresses that these crops will face over 15-20 year plantation lifetimes. Perennial energy crops are characterized by low cultivation inputs and by their potential for soil carbon sequestration, which is an effect that is most significant in tillage soils because of the low carbon content. Converting grassland to perennial crop energy has an initial expected loss of stored carbon following ploughing and soil preparation, but after the initial loss, soil carbon reserves return to greater levels than those of grassland soils. Therefore, low cultivation emissions and carbon sequestration ability are the advantages that perennial energy crops have over annual energy crops. Cultivation emissions are small compared to GHG mitigation through fuel substitution, thus hemp is favorable compared to perennial crops with regard to GHG reduction, assuming that all the crops studied have similar yield potential.
Compared to perennial energy crops, hemp has higher annual costs because of soil preparation, seed purchasing, and higher fertilizer requirements. However, hemp is more appealing to risk-averse farmers. Farmers would receive full returns within the year of planting and can continue or discontinue using hemp cultivation the next year based on the experience. In comparison, growing perennial energy crops requires a high initial investment, a long waiting period before cash flows are positive, and a commitment of land for over 20 years.
This study assumed that all of the compared crops would substitute for oil, but Miscanthus and hemp biomass would more likely be used for electricity generation through co-firing in coal and peat power stations in Ireland, which would require less processing and lead to more GHG abatement. Hemp could also substitute for a greater amount of oil than biodiesel and bioethanol from sugar beet. One major criticism of using annual energy crops for biofuel production is the detrimental impact on food supply. Thus, hemp is particularly attractive because it is not a food crop and acts as a low input break crop that improves soil quality and yields of subsequent crops. Hemp production can complement food production rather than compete with it. Another advantage of hemp over perennial energy crops is that it can be supplied immediately because it produces high biomass without having to wait until the end of the yield-building phase. Hemp may be an important crop to farmers because of the diminishing of the EU sugar sector in the last few years meaning that much of the tillage land in Europe does not have efficient break crops. Hemp is a better alternative to sugar beet as a break crop that can be used for bioenergy production and GHG mitigation.
Finnan, J., Styles, D., 2013. Hemp: A more sustainable annual energy crop for climate and energy policy. Energy Policy 58, 152-162.