The Impact of Soil Water Content on Yield, Composition, Energy, and Water Indicators of the Bioenergy Grass <i>Saccharum spontaneum</i> ssp. <i>aegyptiacum</i> under Three-Growing Seasons

Raising water and energy productivity in agriculture can contribute to reducing the pressure on the limited freshwater availability and non-renewable energy sources. Bioenergy perennial grasses are efficient from a water perspective and can afford a low-energy cultivation system; however, crop selection and cultivation practices for minimizing land use change and maximizing resource use efficiencies remain a challenging task in view of sustainable bioeconomy development. The present work investigated the soil water effect on a long-term plantation of <i>Saccharum</i> (<i>Saccharum spontaneum</i> ssp. <i>aegyptiacum</i>), a bioenergy perennial grass holding great promise for semiarid Mediterranean areas. The plantation was in its 13th year following establishment and was subjected to three levels of irrigation for three successive growing seasons. Regression models between crop water use (CWU) and productivity, biomass composition, energy, and water indicators showed different prediction curves. Raising CWU (from 230 to 920 mm) enhanced the dry biomass yield (from 14.8 to 30.1 Mg ha⁻¹) and the net energy value (from 257.6 to 511 GJ ha⁻¹). On the same CWU range, unirrigated crops improved the energy efficiency (from 99.8 to 58.5 GJ ha⁻¹), the energy productivity (from 5.6 to 3.4 Mg GJ⁻¹) and the water productivity (from 114.5 to 56.1 MJ m⁻³) by reducing the water footprint (from 8.7 to 17.8 m³ GJ⁻¹). Biomass composition was also superior in unirrigated crops, as the lower heating value, structural polysaccharides, and the acid detergent lignin were higher, while ash and soluble compounds were lower. Present findings demonstrated the good yield levels and persistence of <i>Saccharum</i>, improving our knowledge of plant responses to changing soil water availability to maximize energy and conserve natural resources, paving the way for sustainable bioeconomy development in the Mediterranean area.