Energy use and greenhouse gas emissions in organic and conventional grain crop production: Accounting for nutrient inflows

Agriculture is a large source of greenhouse gas (GHG) emissions and has large energy requirements. Previous research has shown that organic farming and conservation tillage practices can reduce environmental impacts from agriculture. We used the Farm Energy Analysis Tool (FEAT) to quantify the energy use and GHG emissions on area (ha) and crop yield (kg crop) bases for five cropping systems that comprise the Farming Systems Project (FSP) at the USDA-Agricultural Research Service (ARS), Beltsville Agricultural Research Center in Maryland, US. The FSP consists of five grain cropping systems that mimic those used in the mid-Atlantic region of the US: 1) a 3-year conventional no-till corn (Zea mays L.)–soybean (Glycine max (L.) Merr)–wheat (Triticum aestivum L.)/soybean rotation (NT), 2) a 3-year conventional chisel-till corn–soybean–wheat/soybean rotation (CT), 3) a 2-year organic corn–soybean rotation (Org2), 4) a 3-year organic corn–soybean–wheat rotation (Org3), and 5) a 6-year organic corn–soybean–wheat–alfalfa (Medicago sativa L.) rotation (Org6). We accounted for nutrient inflows into organic systems by using a mass-energy allocation method, which accounts for the total energy and GHG emissions from the original production of nutrients found in poultry litter through synthetic fertilizer production (N) and nutrient mining (P and K). We believe this is the first attempt to quantify energy use and GHG emissions from nutrients applied in organic systems that originated through industrial processes used in conventional agriculture. Energy use was greatest in the conventional systems when expressed on a per area basis, with energy costs of producing synthetic N fertilizer accounting for 45 to 46% of total energy use. When expressed per unit of crop yield, energy use was greatest in Org2, lowest in Org6, and similar in Org3, NT and CT. Energy use decreased with increasing crop rotation length and complexity among organic systems whether expressed on an area or yield basis. Greenhouse gas emissions were higher in the Org2 and Org3 systems than in the conventional systems and were lowest in Org6 whether expressed on an area or yield basis. Our results indicate that organic management consistently had lower energy use than conventional management on an area basis, but not when expressed on a crop yield basis. Of particular interest is that diversifying grain cropping systems to include perennials was a more effective management strategy than organic management per se to reduce energy use and GHG emissions in agriculture.