# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 1 Introduction  
The increase in global population and living standard has caused an increase in the demand for food, energy, soil and water, among other resources, and has led to an overexploitation of natural resources. Crop production globally suffers severe constraints as arable lands are diminishing because of soil salimization as well as freshwater scarcity due to the lack of precipitation and improper water resource management practices by the users. Salt-affected soils occur on all continents and under almost all climatic conditions. Approximately 43% of the land mass of earth is arid or semi-arid and 98% of its water is saline. Worldwide, over 800 million hectares are affected by salinity, 20% of the 230 million hectares of prime irrigated agricultural land has become saline and the salinity threat is inching into arable lands [1].  
Most salt-affected soils are found in arid and semiarid regions compared to humid regions. Salimization of agricultural land affects a considerable area of irrigation projects. Currently, salt is degrading 20% of the world's irrigated land and causing economic losses of around US$ 27.3 billion per year, according to the assessment from the UnitedNations University [2]. This leads to investigation and promotion of environmentally sound practices and techniques to avoid further salinization, as well as research and use of salt-tolerant plants (halophytes) to produce food, feed and other marketable products, including renewable energy from biomass.  
Halophytes have a substantial potential to restore salt stressed lands and in phytosanitation or phytoremediation of contaminated soils [3]. Saline land areas or marginal zones are rich in halophytic vegetation, which can thrive in high salinity environments. Dissimilar to glycopytes that are unable to ideally grow even in low salinity levels, halophytes can grow and complete their life-cycle in a salt concentration of at least 200 mM NaCl [4], and some of them need salt for optimal growth (true halophytes). Therefore, they are potentially ideal candidates for bioenergy crops from saline soils. Halophytes can also be supplied with seawater without jeopardizing their biomass or seed productions, ultimately ensuring food security in several saline affected land areas, and the residues can be used for biogas production. Consequently, cultivation and valorization of halophytes is gaining developing recognition, more specifically in arid areas in which short supply of freshwater drives people to use marginal resources like brackish water. Hence, the cultivation of halophytes may be beneficial due to them being more economical to cultivate as well as being generally copious on saline soils. Furthermore, the sustainable processing of halophytic biomass to attain a wide range of marketable food and feed ingredients, high-value bio-based/bioactive products and bioenergy (biofuels, power and/or heat) allows for the optimum potential utilization of all viable feedstock components considering the availability of efficient methods of conversion and valorization [5].  
In this context, biogas production from halophyte plants or their residues after processing of other valuable products offers an environmentally sound process combining production of renewable energy from biomass with recycling of plant nutrients and humus. Furthermore, biogas technology is a versatile technology that can be applied to a variety of biomass resources, in mono- or co-digestion and at different scales, from single households and farms to industry scale. Through this review paper, we would like to give an overview on the potential of using halophytes for biogas production--either directly or after pre-processing--and to identify further routes to establish widespread application of this technology to treat biomass from saline soils.