Assessing the environmental impacts of contrasting farming systems more

Aspects of Applied Biology 93, 2009 Integrated Agricultural Systems: Methodologies, Modelling and Measuring Assessing the environmental impacts of contrasting farming systems By H L TUOMISTO1, I D HODGE2, P RIORDAN1 and D W MACDONALD 1 1 Wildlife Conservation Research Unit, University of Oxford, Tubney, Oxon OX13 5QL, UK 2 Deparment of Land Economy, University of Cambridge, Cambridge CB3 9EP, UK Summary This paper examines how opportunity costs of land use can be taken into account when life cycle assessment (LCA) is used to compare environmental impacts of contrasting farming systems. Energy and greenhouse gas (GHG) balances of organic, conventional and integrated farm models are assessed. It is assumed that the farm size and food product output are equivalent in all farm models, and the remaining land that is not needed for food crops is used for Miscanthus energy crop production. The impacts of integrating biogas production into the farming systems are also explored. The results illustrate the significance of taking into account the opportunity costs of land use and suggest that integrated farming systems have potential to reduce negative environmental impacts compared to organic and conventional systems. Key words: Organic farming, integrated farming, greenhouse gas emissions, land use, energy balance, bioenergy, biogas Introduction Life cycle assessment (LCA) is commonly used to compare environmental impacts of different farming systems (e.g. Williams et al., 2006; Thomassen et al., 2008). However, the results of LCA studies may be misinterpreted if opportunity costs of land use are not taken into account when intensive and extensive farming systems are compared (Berlin & Uhlin, 2004). Organic farming has often been found to have environmental benefits compared to conventional farming (e.g. Pimentel et al., 2005; Norton et al., 2009), but requires more land per unit of product output. In order to reduce energy use and greenhouse gas (GHG) emissions, and enhance the level of biodiversity it may be reasonable to use less land for intensive agriculture and more land for bioenergy production or wildlife conservation, as opposed to using more land for extensive agriculture. Integrated farming systems that aim at reducing negative environmental impacts while producing high yields may offer a sustainable solution. This paper examines how opportunity costs of land use can be taken into account when LCA is used for assessing environmental impacts of contrasting farming systems. In this study the method has been applied to assessing the energy and GHG balances of organic, conventional and integrated farming systems. Material and Methods Farm models LCA was used to assess the energy and GHG balances of organic, conventional and integrated farm models. In order to assess the opportunity costs of land use, farm size and food product output 167 of the farms were standardised. It was assumed that all farms used 100 ha land and produced 500 t potatoes, 180 t wheat and 60 t field beans. Land that was not needed for food crops was assumed to be used for Miscanthus energy grass production. The system boundaries included the production of farming inputs and machinery, farming operations, and crop cooling and drying. The production of farm buildings was excluded from the study. The model organic crop rotation was designed to be nitrogen self sufficient, consisting of: 1. grassclover (GC); 2. potatoes (Solanium tuberosum); 3. winter wheat (Triticum aestivum) + undersown overwinter cover crop (CC); 4. spring beans (Vicia faba) + CC; and 5. spring wheat (T. aestivum) +undersown GC. The model farming systems compared were: 1. Organic farm without biogas production (O). The GC, CC and crop residues (CR) were incorporated into the soil. Ploughing was used. 2. Organic with biogas production (OB). The GC, CC and CR (straw of wheat and pea crops) were harvested for biogas production. 3. Conventional farm (C). Used mineral fertilisers and non-organic pesticides. No CC or biogas production. 4. Integrated farm (IF). The crop rotation and biogas production was similar to the OB system, but non-organic pesticides were used. 5. Integrated special (IFS). As IF but instead of GC municipal biowaste was used as a fertiliser. Non-organic pesticides and direct drilling were used. In the C and IF models three different tillage methods were assessed: ploughing (p), reduced tillage (r) and direct drilling (d). The practices included in the p method were ploughing, rolling and seedbed preparation with disc & pack and power harrow. The r method included power harrowing, discing, rolling, subsoiling and seedbed preparation with spring tine harrows. Method d did not include any tillage, but the seed were assumed to be drilled directly to the stubble of the previous crop. In the O, OB and IF models the biogas digestate was applied to potatoes, winter wheat and spring wheat. Inventory data and impact assessment The crop yields for the O and C models were based on published averages for organic (Lampkin et al., 2006) and conventional (Nix, 2007) farming in the UK (Table 1). In the OB model it was assumed that the yields increased due to the enhanced nutrient management, by factors based on experimental results from similar systems studied in Germany (Stinner et al., 2008). It was assumed that in the IF systems the yields were increased from the OB yields due to the use of conventional pesticides, by factors adjusted according to published field experiments comparing yields with and without use of pesticides (Cooper, 2008; Deike et al., 2008; Delin et al., 2008). It was assumed that in the IFS model, the yields were equal to those in the C models because of the high nutrient inputs and use of pesticides. Table 1. Crop yields assumed in this model t (wet weight) ha-1 O 46 25 5.0 3.0 4.0 OB 46 25 5.5 3.0 5.0 IF 46 36 7.6 3.5 5.5 C, IFS 45 8.3 3.7 5.8 Grass-clover Potatoes Winter wheat Spring beans Spring wheat O=organic, OB=organic + biogas, C=conventional, IF=Integrated, IFS=integrated special Straw yields from winter wheat, spring beans and spring wheat in the OB model were 3.0, 2.0 and 168 2.5 tDM ha-1, respectively, and 3.5, 2.5 and 3.0 tDM ha-1, respectively for IF and IFS models. The CC yields were 2.8 tDM ha-1 in the all models where CC was used. The Miscanthus yield, taken from a 14 year field experiment in the UK (Christian et al., 2008) was assumed to be 13 tDM ha-1. It was assumed that biogas was produced in a farm-scale biogas plant using one-stage continuous digestion technology operating at mesophilic temperatures. A dry process was assumed and heat exchangers were used. The energy input needed for heating the reactor was 240 MJ t-1, and for pumping and mixing 92 MJ t-1raw material (Börjesson & Berglund, 2006). The methane yields of GC and CC were assumed to be 10.6 GJ tDM-1 and for CR 7.1 GJ tDM-1, respectively (Berglund & Börjesson, 2006). The energy yield of Miscanthus was calculated by using the lower heating value of 18 MJ kgDM-1 (Styles & Jones, 2008). The energy input required for production of Miscanthus has been estimated to be 5.0–6.7% of the output (Christian et al., 2008), and an average value of 5.7% was used in this study. The primary energy used for the field operations, production of mineral fertilisers and pesticides, and crop cooling and storage were based on the data from Williams et al. (2006). The energy used for spreading and transportation of biogas digestate and the energy use of the biogas reactor was based on (Berglund & Börjesson, 2006). It was assumed that the digestate was transported 1 km by tractor with an empty return, which requires energy 3.5 MJ t-1 km. The transportation of municipal biowaste by truck requires energy 1.6 MJ t-1 km with an empty return transport. It was assumed that 35 t wet weight municipal biowaste per hectare was applied in the IFS model and an average transport distance was 10 km. The GHG emission factors were based on the data from Williams et al. (2006). The soil carbon emissions and sequestration were not taken into account due to the lack of available scientific data. The fertilisation scheme for conventional crops was derived from Williams et al. (2006): 208-1836 (N-P-K kg ha-1) for wheat, 150-17-195 for potatoes and 0-12-32 for spring beans. In the organic and integrated models P and K were assumed to be used in ratios 10-41 (P-K kg ha-1), 9-129 and 13-28 for wheat, potatoes and spring beans, respectively. Results In the O model the entire land area (100 ha) was needed for production of the food crops and green manure, whereas, in the OB model, 6 ha was available for the energy crop due to the improved food crop yields (Fig. 1). The C and IFS models required only 53% of the land area for production of food crops. 120 100 80 Energy crop ha 60 40 20 0 O OB C IF IFS Green manure Food crops Fig. 1. Land use in the different models (O=organic, OB=organic + biogas, C=conventional, IF=Integrated, IFS=integrated special). 169 The IFS model had the lowest energy input in food crop production, the highest net bioenergy output (energy output minus energy inputs needed for bioenergy production) and the highest energy balance (ouputs-inputs: 11.3 TJ/Farm) of the all the models compared (Fig. 2). C models had the highest energy input for food crops due to the high energy requirement for production of fertilisers. However, the energy balance of C models (9.0 TJ/Farm) was higher than in O (-1.0 TJ/ Farm), OB (4.5 TJ/Farm) and IF (7.6 TJ/Farm) models. The energy inputs in O, OB and IF models were approximately equal. The energy balance of the IF models, however, was approximately 70% higher compared to the OB model. The GHG emissions from production of food crops in the C models were approximately 52% higher compared to the all other models (Fig. 3). The IFS model had the highest GHG emission net mitigation potential (the emissions from production of bioenergy are taken into account), with bioenergy produced in the farming systems assumed to replace oil. While C models had higher energy balance compared to IF models, the IF models had a higher GHG mitigation potential than C models. 14 12 10 8 TJ Farm -1 Energy crop Biogas Fertilising Pesticides Cooling/drying Tillage+harvest 6 4 2 0 -2 O OB Cp Cr Cd IFp IFr IFd IFS Fig. 2. The energy input (negative values) in production of the food crops and net energy output from biogas and the Miscanthus energy crop (positive values). (O=organic, OB=organic + biogas, C=conventional, IF=Integrated, IFS=integrated special, TJ=Tera Joule). Discussion The results clearly illustrate the importance of taking into account the opportunity costs of land use in LCA. Conventional systems had the highest energy inputs and GHG emissions per food product output, but the whole farm energy and GHG balances were far more favourable for the conventional systems compared to the organic systems. The integrated systems had both the lowest impacts per product unit and the most favourable whole farm energy and GHG balances. Therefore the results suggest that integrating farming systems that produce high yields while using environmentally beneficial farming practices may lead to the lowest negative environmental impacts. The results presented here are from indicative models, inevitably based on many assumptions. Therefore a sensitivity analysis would be needed for assessing the impacts of changes in the input values. Crop yields, for example, can be considerably variable across different farms. By 170 400 200 0 t CO2-eq Farm -1 -200 -400 -600 -800 -1000 O OB Cp Cr Cd IFp IFr IFd IFS Energy crop Biogas Nitrous oxide Fertilising Pesticides Cooling/drying Tillage+harvest Fig. 3. Greenhouse gas (GHG) emissions from the production of food crops (positive values) and net GHG emission mitigation when energy produced from biogas and Miscanthus energy crop is assumed to replace oil. (O = Organic, OB = Organic + Biogas, C = Conventional, I =IF= Integrated, IS = Integrated Special p = ploughing, r = reduced tillage, d = direct drilling). looking at energy use, land use and GHG emissions this study provides insights into the sustainability of the farming systems, though clearly not the full picture. 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