Abstract
Changes in cropland production and management influence energy consumption and emissions of CO2 from fossil-fuel combustion. A method was developed to calculate on-site and off-site energy and CO2 emissions for cropping practices in the US at the county scale. Energy consumption and emissions occur on-site from the operation of farm machinery and occur off-site from the manufacture and transport of cropland production inputs, such as fertilizers, pesticides, and agricultural lime. Estimates of fossil-fuel consumption and associated CO2 emissions for cropping practices enable (a) the monitoring of energy and emissions with changes in land management, and (b) the calculation and balancing of regional and national carbon budgets. Results indicate on-site energy use and total energy use (i.e., the sum of on-site and off-site) on US croplands in 2004 ranged from 1.6-7.9 GJ ha-1 yr-1 and from 5.5-20.5 GJ ha-1 yr-1, respectively. On-site and total CO2 emissions in 2004 ranged from 23-176 kg C ha-1 yr-1 and from 91-365 kg C ha-1 yr-1, respectively. During the period of this analysis (1990-2004), national total energy consumption for crop production ranged from 1204-1297 PJ yr-1 (Petajoule = 1�1015 Joule) with associated total fossil CO2 emissions ranging from 22.0-23.2 Tg C yr-1 (Teragram = 1�1012 gram). The annual proportion of on-site CO2 to total CO2 emissions changed depending on the diversity of crops planted. Adoption of reduced tillage practices in the US from 1990 to 2004 resulted in a net emissions reduction of 2.4 Tg C.
