Effect of sugar beet (Beta vulgaris L.) cultivation on soil organic carbon stocks in Germany

Sugar beet is generally seen as detrimental to soil organic carbon (SOC) stocks for multiple reasons although actual data verifying this claim are scarce. In this study, two approaches were combined to examine the effect of sugar beet on SOC from field data in Germany. First, SOC data of the German Agricultural Soil Inventory were used to compare sugar beet sites with similar sites without sugar beet cultivation. Second, a long-term crop rotation trial in Central Germany was evaluated for differences in SOC among crop rotations with and without sugar beet. Further, carbon input into soil from sugar beet residues was compared with wheat as a reference. In the nationwide dataset, lower SOC stocks (-4.6%) were found for sugar beet sites compared with those without. However, a re-sampling of the sites 10 years later showed no (further) SOC loss. In the long-term trial, no negative impact of sugar beet cultivation on SOC was found. From both databases, carbon input from sugar beet crop residues (2 and 2.7 Mg ha-1 year-1, respectively) was much lower than from wheat (3.6 and 5.8 Mg ha-1 year-1, respectively) because of evident differences in the amount of belowground residues. However, this may be counteracted by growing cover crops before sugar beet, as done in the long-term field trial studied. We conclude that sugar beet might have had a negative impact on SOC stocks in the past, yet that this does not necessarily continue in the present on long-term sugar beet fields, possibly because of a current steady SOC state. When growing cover crops, sugar beet cultivation might have no negative effect on SOC at all. In any case, a general loss of SOC because of sugar beet cultivation cannot be assumed.