Summary
1.Exchange of carbon dioxide (CO 2 ) from soils can contribute significantly to the global warming potential (GWP) of agro-ecosystems. Due to variations in soil type, climatic conditions and land management practices, exchange of CO 2 can differ markedly in different geographical locations. The food industry is developing carbon footprints for their products necessitating integration of CO 2 exchange from soils with other CO 2 emissions along the food chain. It may be advantageous to grow certain crops in different geographical locations to minimize CO 2 emissions from the soil, and this may provide potential to offset other emissions in the food chain, such as transport.
2.Values are derived for the C balance of soils growing horticultural crops in the UK, Spain and Uganda. Net ecosystem production ( NEP ) is firstly calculated from the difference in net primary production ( NPP ) and heterotrophic soil respiration ( R h ). Both NPP and R h were estimated from intensive direct field measurements. Secondly, net biome production ( NBP ) is calculated by subtracting the crop biomass from NEP to give an indication of C balance. The importance of soil exchange is discussed in the light of recent discussions on carbon footprints and within the context of food life-cycle assessment (LCA). 3. The amount of crop relative to the biomass and the R h prevailing in the different countries were the dominant factors influencing the magnitude of NEP and NBP . The majority of the biomass for lettuce Lactuca sativa and vining peas Pisum sativum , was removed from the field as crop; therefore, NEP and NBP were mainly negative. This was amplified for lettuces grown in Uganda ( − 16·5 and − 17 t C ha − 1 year − 1 compared to UK and Spain − 4·8 to 7·4 and − 5·1 to 6·3 t C ha − 1 year − 1 for NEP and NBP , respectively) where the climate elevated R h . 4. Synthesis and applications. This study demonstrates the importance of soil emissions in the overall life cycle of vegetables. Variability in such emissions suggests that assigning a single value to food carbon footprints may not be adequate, even within a country. Locations with high heterotrophic soil respiration, such as Spain and Uganda (21·9 and 21·6 t C ha − 1 year − 1 , respectively), could mitigate the negative effects of climate on the C costs of crop production by growth of crops with greater returns of residue to the soil. This would minimize net CO 2 emissions from these agricultural ecosystems.
