Response of a two-year sugar beet-sweet sorghum rotation to an agronomic management approach diversified by soil tillage and nitrogen fertilisation

Conservative agriculture and nitrogen fertilisation have been evaluated for the purpose of assessing their impact on the sustainability of a cropping system based on a two-year rotation with two crops considered for the bio-ethanol supply chain: sugar beet (Beta vulgaris L. subsp. vulgaris) and sweet sorghum (Sorghum bicolor L. Moench). The experimental activity started in 2009 in Foggia (Apulia, southern Italy). We discuss the results obtained in the 2010-2011 period. Soil minimum tillage (MT) vs no tillage (NT) combined with two doses of nitrogen fertilisation (75 and 150 kg ha^–1 of mineral nitrogen as ammonium nitrate) were compared. The experimental system, which is still operational (soil tillage plus nitrogen fertilisation), was arranged with a split-plot design with three replicates. Treatments were applied on the same plots every year with both crops present at the same time. At the first harvest in 2010, no difference was observed. As to the second year, the comparison between NT vs MT treatments showed that sugar beet had lower total yield (35 vs 42 t ha^–1), dry biomass (10 vs 14 t ha^–1), and sucrose yield (6.7 vs 8.2 t ha^–1). Total soluble solids, on average 19%, were not influenced by the experimental treatments. Nitrogen (N) control was less productive than the fertilised treatments (average between N75 and N150) in terms of total fresh root yield (32 vs 42 t ha^–1), dry biomass (10 vs 14 t ha^–1), and sucrose yield (6.0 vs 8.1 t ha^–1). As with sugar beet, during the second year, also sweet sorghum sown in NT vs MT plots had a reduced yield, although the difference was more marked for fresh biomass (–35%) than for dry biomass (–20%). No interaction in terms of soil tillage nitrogen fertilisation occurred. In summary, in the first two-year period (2010-2011) of the experimental trial, no tillage soil management showed decreased yields of both crops. Sugar beet displayed a higher sensitivity to the lack of nitrogen supply than sweet sorghum.