A decision support tool for modifications in crop cultivation method based on life cycle assessment: a case study on greenhouse gas emission reduction in Taiwanese sugarcane cultivation

Background, aim, and scope Nowadays, various crops are cultivated to supply emerging needs in sustainable fuels and materials. In addition to the development of crop processing technologies, cultivation processes in a cropping system could be modified to meet the emerging needs, along with the conventional needs in food supply. This study provides a decision tool for modifications in cultivation of crops based on life cycle assessment. Sugarcane cultivation in Taiwan is chosen as a case study to present such a decision tool, because it is an important potential indigenous resource for energy (for example, bio-ethanol) and materials (for example, bio-plastics). First, this study presents the amount of greenhouse gas (GHG) emissions associated with the production of 1 ton of sugarcane in Taiwan, which makes it possible to understand how it is consistent to develop this bio-resource in terms of both objectives: i.e., resource security and reduction of global warming impact. Next, sensitivity of the parameters in cropping systems, such as amount of irrigation, fertilization and tillage are assessed from a viewpoint of GHG emissions, using the LCI model constructed in the first step. Finally, equivalent impact level (EIL) lines are presented for some important parameters in the cropping system to support considerations in modification of agricultural methods. Because the objective is to discuss parameters in cultivation processes, the scope of study is limited to cradle-to-gate of raw sugarcane transported to the cane processing plant. Materials and methods In addition to GHG emissions from cultivation processes, such as soil preparation, growing, harvesting and transportation, auxiliary processes such as agrochemicals production, power generation and fossil diesel fuel refining was accounted for. To quantify the nitrous oxide emission from the soil ecosystem, a denitrification-decomposition (DNDC) model was used with the localized data obtained in this study. EIL lines were developed aiming at supporting decisions about modifying agricultural methods that should be made considering consequential changes in environmental impacts. For example, if the envisioned improvement in yield as a result of modification of parameters such as irrigation, tillage and fertilization is likely to achieve a value above the EIL line, GHG emission per ton of sugarcane would be decreased. An iterative procedure is applied to draw nonlinear EIL lines using Intergovernmental Panel on Climate Change (IPCC) Tier 3 method for nitrous oxide emission from soil. Results A 3 year crop cycle was assumed, and the net GHG emissions associated with the sugarcane produced was 280 kg-CO2-equiv. per ton of raw sugarcane. The emission of nitrous oxide from soil during cultivation accounted for 50.4% of the total emissions. One-at-a-time sensitivity analysis indicates that this result is most sensitive to yield and amount of nitrogen fertilizer applied, which are correlated to each other. The EIL lines were drawn for yield over five parameters in cultivation including amount of nitrogen fertilizer applied with respect to GHG emission. For example, if an additional 5% of nitrogen fertilizer application realizes enhancement of yield by 5 tons per hectare, such modification simultaneously reduces total GHGs associated with sugarcane production. Discussion The EIL lines drawn with respect to various environmental impacts aims to provide a simple-to-use guidance in reconsideration of agricultural options. The cradle-to-gate LCA of sugarcane provides information useful in development of bioethanol and bioplastics derived from sucrose in the cane juice. In doing so, it must be noted, if extension of farm area is among the options to be assessed, that release of the carbon stock from the original state of use should be accounted for in addition to the result presented in this study. Absorbed CO2 should be quantified separately based on the content of the sugarcane, which could be varied for different cultivars. Conclusions This study presents a decision tool to allow introduction of an environmental life cycle perspective in modification of cultivation methods, which has already been undergoing as driven by economic reasons. The data and inventory method presented in this study can be applied for varied end products derived from sugarcane. The algorithm to develop an EIL line for correlated parameters in the inventory model was presented. Recommendations and perspectives Results from this study require appropriate modification if they are to be used for studies for other regions. Such modifications would be straightforward, because all inventory data and assumptions are presented in this paper. Essentially, the functional unit used in this study is unit amount (by weight) of sugarcane. If some specific content (for example, sucrose or cellulose) is important in a study which attempts to use results from this study, variations in the content over cultivation methods and cultivar of sugarcane must be investigated further.