Biomass Crops in Marginal Lands.

In recent years, a debate has emerged regarding food security and land use for bioenergy/industrial non-food crops. Cultivating biomass crops on marginal land unsuitable for food production is consistently proposed as an alternative to minimize iLUC effects about land-use competition for food production, and its adverse effects (direct or indirect) on food security, land based GHG emissions and biodiversity loss. Several studies agree on the existence of a considerable extension of land in Europe less suitable for conventional agriculture. This land has been either abandoned for its low productivity, or it is used as grassland. No-food biomass crops can provide abundant renewable feedstocks for the production of high added-value bio-based commodities and bioenergy, thus feeding the bio-based economy. Nowadays, the cultivation of biomass crops on marginal land to avoid land-use competition with food production is a central debate; therefore, this study aims to promoting the sustainable development of resource-efficient and economically profitable lignocellulosic biomass crops grown on soil affected by salinity, which is considered a biophysical constraint leading to cultivation of bioenergy crops on marginal lands. The present experiment focused on lignocellulosic crops that grow naturally in the Mediterranean environment (Arundo donax, and Saccharum spontaneum), and the leading perennial bioenergy grass (Miscanthus x giganteus) in order to provide information on their tolerance to salinity stress. Furthermore, the use of fertilization as a further experimental factor was employed, with the aim to ascertain a possible sensitivity mitigation to salinity stress. Two independent experiments were carried out, named Experiment 1 (throughout 2019 growing season) and Experiment 2 (throughout 2020 growing season), both involving perennial bioenergy grasses under soil salinity and fertilization levels in pots amounting to 10 kg of soil each. In Experiment 1, rhizomes were used to propagate perennial grasses, which were grown in a clay soil under open air. In Experiment 2, stem node cuttings were used to propagate perennial grasses, the soil was of volcanic origin and crops were grown under semi-open air in a glasshouse which was open in the four sides. Both substrates were differentiated since representatives of two different types of marginal soil in south-eastern Sicily (Italy). In both Experiment 1 and Experiment 2 the three different species, three levels of NaCl (S0, S1 and S2, respectively 0, 9 and 18 dS m-1) and three levels of NPK fertilizer (F0, F1 and F2, respectively 0, 60 and 120 kg NPK ha-1), were replicated three times in a completely randomized experimental design. During the crop cycle and at harvest, the soil electrical conductivity (EC), physiological (net photosynthesis, stomatal conductance, transpiration rate and instantaneous water use efficiency, chlorophyll fluorescence), morphological (height of main stem, leaf area index, number of green and dry leaves), productive (above and belowground dry biomass) and quality traits (protein, hemicellulose, cellulose, ADL and ash) were measured. The soil salinity concentration, on average for the three species per single treatment, increased during the growth cycle. In all treatments the initial EC increased approximately to 5 dS m-1 in S0, 20 dS m-1 in S1 and 25 dS m-1 in S2 after 1 month of treatment with NaCl. Soil EC also increased in the untreated treatment due to the effect of the salt concentration in the irrigation, medium brackish, water. In Experiment 1, main findings highlighted the high tolerance to salinity of the investigated perennial grasses. While morphological and physiological traits throughout the experimental period were someway affected, there was a not clear trend with the treatments imposed to the crops, although main effects were statistically significant. On the other hand, biomass productivity at harvest showed a higher biomass production for Arundo and Saccharum than for Miscanthus. Salinity followed a gradient, decreasing as the salinity level increased: the biomass production between the salinity control (S0) and the medium salinity level (S1), across fertilizations, reduced by 21% in Arundo, 59% in Saccharum and 63% in Miscanthus. The reduction between S0 and the highest salinity level (S2) was 38% in Arundo, 60% in Saccharum and 70% in Miscanthus. Fertilization had a beneficial effect in the different salinity levels and crops. As expected, the highest fertilization clearly improved the biomass production in all crops under not saline environments. The biomass quality in Experiment 1 was carried out for hemicellulose and cellulose content on stems and leaves. The soil salinity increase lead to a linear decrease of hemicellulose content both on leaves and stems, while increasing the fertilization increased hemicellulose on leaves but not in stems. Among species, Miscanthus and Saccharum had a significantly higher hemicellulose content than Arundo in leaves, while Saccharum and Arundo on stems. Arundo showed also the significantly highest cellulose content on stems, followed by Miscanthus and by Saccharum. Also, in this case, increasing the salinity level decreased significantly the cellulose content on stems. The fertilization decreased the cellulose content on stems but was ineffective on leaves. In Experiment 2 an inverse response of morphological (stem height and leaf area index) and productive traits (stems, leaves and root dry weight) to increasing salinity levels was also observed. Increasing fertilization levels with NPK increased the main morphological and productive traits mitigating, in part, the salinity stress. Among species, Saccharum was the tallest, Arundo had the highest LAI, Miscanthus the highest leaf and root dry weight, while Arundo the highest stem dry weight. As expected, the C3 Arundo showed the significantly highest stomatal conductance, which lead to a higher transpiration rate among species. The two C4, Saccharum and Miscanthus, although did not differ their stomatal conductance showed a significantly different instantaneous water use efficiency, with Saccharum having the highest overall, however, both were more efficient than Arundo. The salinity stress reduced both stomatal conductance and the instantaneous water use efficiency, while the fertilization increased both parameters, but the significant effect was observed only for the stomatal conductance. On the other hand, the maximum efficiency of photosystem II was quite similar among the C3 and the C4 crops. The fertilization increased slightly this trait, while the salinity stress had a strong, depressing effect on the maximum efficiency of photosystem II. It is worth to mention that Arundo seems to be less affected to harsher conditions, reducing this trait to a lesser extent as compared to the other species. The biomass quality was affected to a different extent in relation to the treatments, species and plant part. Generally, leaves had a higher amount of protein and ash than stems, while stems had a higher amount of cellulose and ADL. The fertilization increased both protein and ash in leaves and stems, but the structural compounds (i.e., hemicellulose, cellulose and ADL) were less affected. The increase in salinity decreased the protein and increased the hemicellulose on leaves, while increased the ash on stems. The other biomass compounds were not significantly modified by salinity. Among species, Arundo had the highest protein and ash, both on stems and leaves, the hemicellulose was the highest in Saccharum leaves and in Miscanthus stems, the cellulose was the highest in Saccharum and Miscanthus leaves, while the three species had a similar cellulose amount on stems. The ADL was the highest in Miscanthus and Arundo and the lowest in Saccharum, both stems and leaves.