Soil Carbon Dioxide Flux and Organic Carbon Content: Effects of Tillage and Nitrogen Fertilization

The response of soil CO 2 flux to long‐term tillage practices (no‐tillage, NT; minimum tillage, MT; conventional tillage, CT) and N fertilization level (zero; medium, 60 kg N ha −1 ; high, 120 kg N ha −1 ) was studied during three growing season in a rainfed Mediterranean agroecosystem. Soil CO 2 flux was related to the crop growth, with the highest flux during spring (i.e., March–May). Tillage and N fertilization effects on soil CO 2 flux during growing seasons depended on weather conditions: greater soil CO 2 flux under MT and NT on dry years, greater under CT and MT on a wet year. Nitrogen fertilization affected soil CO 2 flux during this wet growing season: flux with N fertilizer additions was higher than flux on the unfertilized plots. Thirteen years after establishment of the experiment, the soil organic carbon (SOC) stock under long‐term NT was 3.9 Mg C ha −1 greater than under CT and 4.3 Mg C ha −1 greater than under MT. The SOC stocks with N fertilizer additions were 4 Mg C ha −1 greater than the stock on unfertilized plots. The increase of C inputs with N fertilization was more pronounced under NT than under MT or CT. For this reason the increased response of SOC stock to N fertilization is expected under NT in a longer period of time. The product between soil water content and soil temperature explained between 75 and 94% of the seasonal variability of soil CO 2 flux. However, soil CO 2 flux and SOC stock were hardly related.