Modelling tillage and nitrogen fertilization effects on soil organic carbon

1 Agricultural management plays an important role in global warming mitigation due to 2 its effects on soil organic carbon (SOC) dynamics. In Mediterranean agroecosystems, 3 the interactive effects of tillage and N fertilization on SOC storage have scarcely been 4 studied. Hence, we here present a modelling study in which the effects of both tillage 5 and N fertilization on SOC dynamics are investigated. We used SOC and C input data 6 from a long-term (13 years) field study located in northeast Spain, firstly to validate 7 both the Century model and the Rothamsted Carbon (RothC) model and secondly to 8 predict future SOC dynamics until the year 2030. Tillage and N fertilization affected 9 SOC stocks in the 0-30 cm soil layer. However, the interaction of the two factors was 10 not significant. Averaged over the three N fertilization rates, the observed mean SOC 11 stocks in conventional tillage (CT) and no-tillage (NT) were 29.8 and 36.8 Mg C ha, 12 respectively. In addition, the observed SOC stocks, averaged for both tillage systems, 13 increased with increasing N rates, with 30.6, 33.5 and 35.8 Mg C ha for the 0, 60 14 and 120 kg N ha rates, respectively. In general, both the Century model and the 15 RothC model performed well in predicting SOC dynamics. Model predictions showed 16 that in Mediterranean dryland agroecosystems SOC dynamics in the next 20 years 17 would be variable according to the tillage and N fertilization applied. According to 18 these predictions, scenarios with NT and high fertilization rates (e.g., 60-120 kg N ha 19 ) could lead to significant SOC sequestration and associated CO2 emission offsetting. 20 However, these scenarios with high SOC sequestration rates also showed high mineral 21 N accumulation in the soil profile with its associated environmental side effects. 22