Sweet and fibre sorghum (Sorghum bicolor (L.) Moench), energy crops in the frame of environmental protection from excessive nitrogen loads

Sweet and fibre sorghum (Sorghum bicolor (L.) Moench) are multipurpose cereals of potential interest for several non-food uses. In order to assess the effects of nitrogen (N) fertilization on crop growth, yield and N budget during crop cycle, field trials were carried out in Northern Italy (44°33′N, 11°21′E) in the years 1997–1999. Sweet and fibre sorghum were grown in combination with three N rates (0, 60, 120 kg ha−1). Both crops depicted a sigmoidal growth-pattern, but fibre sorghum showed an earlier and steeper growth. Sweet sorghum recovered due to a longer cycle, 118 versus 105 days in the 3 years’ average, and attained a similar final yield. Nitrogen fertilization did not affect growth pattern, nor yield partitioning among plant organs: the sweet type allocated more photosynthates to stems (about 75% of total dry weight) compared to the fibre one (55–60%), due to a limited partitioning to panicles. Total dry weight at harvest showed an interaction with years: fibre sorghum yielded significantly more than sweet sorghum in 1997 (23.8 versus 17.7 Mg ha−1), but the opposite happened in 1998 and 1999 (20 versus 24.2 Mg ha−1 as average), when a different sweet hybrid was grown, involving a longer season. When only the stem was considered of potential interest, such as in the case of ethanol production or fibre extraction, fibre sorghum showed a slight advantage in the 1st year (14.4 versus 12 Mg ha−1), whereas the sweet type prevailed in the following 2 years (18.7 versus 11 Mg ha−1). Fertilization did not significantly influence total yield, although interacted with sorghum type in plant N concentration and uptake: fibre sorghum rose in both parameters, due to bulkier panicles acting as a late-season sink for the nutrient, while the sweet type was not affected. Nitrogen budget at harvest was clearly influenced by applied fertilizer and plant uptake, whereas nutrient losses as ammonia volatilization and nitrate leaching and natural supplies along with precipitation played a less relevant role. The expected variations in soil reserves ranged between −216 and +77 kg ha−1 of N, depending also on the plant portion removed from the field (whole plant or stem). A slight decrease in soil reserves, more favourable in environmental terms, is achieved when the whole biomass is removed from the field and when fertilizer rates are tight-suited to crop needs in specific growth conditions.