Effect of tillage systems and different rates of nitrogen fertilisation on the carbon footprint of irrigated maize in a semiarid area of Castile and Leon, Spain

In recent decades, the agricultural sector contributed to the increase of the greenhouse gases (GHGs) concentrations into the atmosphere. Soil management and the use of high amounts of energy (direct and indirect) contribute significantly to these emissions. This work aims to estimate the carbon footprint (C footprint) in two different soil managements in continuous irrigated maize, conventional (CT) and no tillage (NT), using different nitrogen fertilisation rates. The study was carried out from 2011 to 2017, in Zamaduellas experimental field in the Spanish province of Valladolid. The design included blocks randomly chosen, where the main factor studied was tillage system and the second was the nitrogen fertilisation. Under CT, the seedbed was prepared with a mouldboard plough (at a depth of 30 cm) followed by a spring cultivator, while only an herbicide was applied under NT system. In 2012/2014, N-fertilisation included the application of 700 (FC) and 600 (FR) kg ha -1 of Calcium Ammonium Nitrate 27%. In 2015/2017, it consisted of 700 kg ha(-1) (FC) of NAC 27%, 700 (FE) and 600 (FER) kg ha(-1) of Ammonium Sulfate Nitrate 26%. The assessment of CO2 emissions depended on the analysis of the energy consumed and produced in the maize production process and on the amount of soil organic carbon (SOC) obtained by collecting soil samples at three depths 0-10, 10-20, and 20-30 cm in 2011 and were after re-sampled at 2 years intervals. N2O emissions estimation was based on the methodology suggested by IPCC (2006). The results obtained showed that maize C footprint was mainly due to direct and indirect N2O emissions produced by the application of synthetic fertilisers and ranged from 3.3 to 4.2 tCO(2eq) ha(-1) and from 3.4 to 4.4 tCO(2eq) ha(-l) under CT and NT management respectively. The highest N2O emissions were produced by the use of FC and FE fertilisation while FR and FER resulted in the lowest N2O emissions under both tillage treatments. The emissions resulting from the energy inputs of electricity, fuel combustion and agricultural machinery contributed to the C footprint with means ranging from 0.25 to 0.27 tCO(2eq) ha(-1) and from 0.23 to 2.25 tCO(2eq) ha(-1) under CT and NT system respectively during the 6-year study. SOC changes played a very important part in the quantification of maize C footprint as they reversed the results from high and positive when the calculation excluded the SOC to low and negative when the SOC was included.