Is the salinity stress mitigated by fertilization treatment on perennial energy grasses?

Salinity is one of the most important challenge facing the supply of food to the world population in the future. The soil electrical conductivity threshold set by the Joint Research Center (JRC) to define marginal lands due to salinity is 3.2 dS/m in the top soil. The aim of the present study was to evaluate the biomass yield, morphology and physiology of three different perennial energy grasses, namely Arundo donax, Miscanthus x giganteus and Saccharum spontaneum ssp. aegypticum under increasing soil salinity levels (0, 9.0 and 18.0 dS/m) and NPK fertilization differentiation (0, 60 and 120 kg ha-1). The leaf area index (LAI) of investigated species under salinity and fertilization treatments showed an upward trend followed by a decline as the growing seasons progressed. Across treatments, Arundo showed the highest LAI. In Miscanthus this trait was much more affected by salinity levels than Sacchaum. LAI reduced proportionally with increasing salinity and decreasing fertilization levels. The instantaneous water use efficiency (iWUE), which represents the net photosynthesis over the transpiration rate, showed a fluctuating trend around the mean throughout the growing season. It was the highest in the C4 crops, however, across the average of treatments, the C3 Arundo had an iWUE as high as the other crops in comparison. Surprisingly, the higher the salinity levels the higher the iWUE, across species and fertilization. Fertilization, across species and salinity levels, led to a higher iWUE in unfertilized and intermediated than the highest fertilization level. As expected, the biomass weight at harvest showed a significant effect of species, salinity and fertilization. Across the average of salinity and fertilization, Arundo and Saccharum showed the highest yields (21.8 and 19.4 g plant-1), and Miscanthus the lowest (7.2 g plant-1). Across the average of species and fertilization, there was a significantly higher yield on the lowest salinity level (24.9 g plant-1), followed by the intermediate (13.3 g plant-1) and the highest salinity condition (10.1 g plant-1). The highest level of fertilization significantly improved the biomass weight as compared with the control and the medium fertilization level. Increasing the knowledge on energy crops under soils affected by salinity is strategic to reduce land abandonment, recommending bioenergy crops for marginal lands where food crops may face difficulties to be grown, in turn decreasing the food vs fuel competition and the effect of land use changes.