Soil Organic Carbon sequestration in agricultural soils: the combined impact of no tillage continuous cropping and no tillage crop-pasture rotation on the level of soil organic carbon in the unfractionated soil and three size fractions and on the C:N ratio in an Uruguayan Agriudoll.

Sequestration of atmospheric carbon in agricultural soils in the form of soil organic carbon (SOC) can help fight climate change and improve soil fertility. This can be achieved by changing current agricultural management practices to new practices that store more carbon in the soil. Two promising practices are no-tillage and crop pasture rotation. This thesis evaluated the combined effect of these two practices by comparing no-tillage continuous cropping, no-tillage crop pasture rotation with a short (two year) pasture phase and no-tillage crop pasture rotation with a long (four year) pasture phase and their on the level of soil organic carbon in the unfractionated soil and the soil fractions coarse particulate organic matter (coarse POM), fine particulate organic matter (fine POM) and mineral associated organic matter (MAOM). Additionally it tried to measure the C:N (carbon to nitrogen) ratios to confirm and explain possible enhanced humification under no-tillage short and long crop pasture rotation (from here on all mentioned treatments were performed without tillage unless stated otherwise). The samples were taken at 0-5 cm, 5-10 cm and 10-20 cm depth at an eighteen year old experimental site at Paysandu (Uruguay) on a Typic Agriudoll. The carbon and nitrogen measurements were performed with dry combustion and factorial mixed ANOVA was used for the statistical analysis.\n\nSignificant differences between the three agricultural practices were only found in the 5-10 cm layer in the coarse and total POM-C (‘-C’ refers to the carbon in the fraction) fractions. This confirms that total POM-C and especially coarse POM-C are sensitive to changes in agricultural management. Crop pasture rotation with a four year pasture phase had the highest coarse POM-C concentration (0.34 g/kg), followed by the short crop pasture rotation with a two year pasture phase (0.26 g/kg) and continuous cropping with the lowest carbon level (0.20 g/kg). For total POM-C the concentrations were 0.90 g/kg for long crop pasture rotation, 0.62 g/kg for short crop pasture rotation and 0.58 g/kg for continuous cropping. So in the sensitive SOM fractions in the 5-10 cm layer long crop pasture rotation had the highest SOC level, followed by short crop pasture rotation and continuous cropping with the lowest SOC level.\n\nThere were differences between the total SOC concentrations of the three rotation systems, but they were not significant. The experiment needs more time for the effects to accumulate. That total SOC concentrations will keep accumulating in the coming years is indicated by the significant changes in the sensitive coarse and total POM-C fractions. Based on comparable scientific literature, it is estimated that sequestration at the experimental site will take place for about 22 more years and that the current changes will double.\nThe pasture phases of short and long crop pasture rotation had several significantly higher SOC concentrations in the soil fractions than the cropping phases. This supports that under no-tillage pastures have a positive effect on the carbon level. Furthermore long crop pasture rotation had higher coarse and total POM-C concentrations than short crop pasture rotation and the fourth year of pasture also had significantly higher concentrations than the second year of pasture of long crop pasture rotation. So a long (four year) pasture phase included in a crop rotation system is thus better regarding coarse and total POM-C concentrations than a short (two year) pasture phase.\nIt was not possible to confirm the occurrence of enhanced humification under short and long crop pasture rotation because the nitrogen measurement failed.\n\nThe recommendations for future research are measuring nitrogen to confirm enhanced humification under the crop pasture rotation systems, measuring the carbon saturation deficit to know more about the future development of the SOC level, using the fractionation method again, because it yielded significant results and using at least three replications to get significant result from the 0-5 cm layer and fine POM-C.