## 1 Introduction  
The transition towards a more sustainable economy, both from an economic and an environmental point of view, can be achieved through the replacement of fossil fuels (e.g., coal or petroleum) with renewable alternatives. Reliance on traditional fuels could be decreased through the development of efficient strategies for biomass-derived biofuels, such as biogas, which is the main product of anaerobic digestion (AD). Furthermore, biofuels are expected to play an important role in the reduction in carbon emissions, which is one of the most important causes of global warming. Biogas is a renewable gas, mainly comprising methane (CH\({}_{4}\)) and carbon dioxide (CO\({}_{2}\)), and can be used to produce heat and/or electricity. Biogas can also be utilised directly as a fuel for vehicles or, following upgrade to biomethane, can be injected into the natural gas grid. Energy crops are commonly used today for the production of biofuels, such as biogas and bioethanol [1; 2; 3]. Alternative materials that can be used as feedstock for AD systems include lignocellulosic residues, such as wheat straw (WS). WS is considered the most abundant source of biomass in Europe [4; 5] and the second worldwide after rice straw [6]. Unfortunately, a high percentage of the annually produced straw worldwide still remains unexploited. As an example, only China produces between 180 and 280 million tons of rice straw annually and more than half of it is left unused [7]. WS is primarily used as feed and bedding material for ruminants [8]. Along with its high availability, its relatively low price is another factor which renders WS an attractive substrate for AD [9]. For example, in the UK, the market price for a premium-quality WS variety was within the range of GBP 10-15/t in 2020 [10]. However, the anaerobic biodegradability of WS is usually low due to its rigid structure and its chemical composition. Firstly, WS contains high amounts of non-anaerobically degradable lignin [11]. Lignin is a cross-linked polymer that creates bonds with cellulose and hemicellulose generating a structure that is not easily accessible by the microorganisms of AD. Additionally, its usually low protein content renders WS not favourable as a sole feedstock for AD systems where no supplementation of nitrogen takes place [12]. An important parameter that limits the anaerobic biodegradation of WS is the ratio of carbon to nitrogen content (C/N), which is sometimes higher than 90 (_w/w_) [13; 14]. Previous studies have already stated that the microorganisms of AD consume carbon faster than nitrogen and the optimal C/N ratio for an AD system is usually within the range of 20 to 30 (_w/w_) [15]. C/N values lower than the optimal range can cause an increase in total ammonia nitrogen levels and/or high accumulation of volatile fatty acids (VFAs), which can, in turn, inhibit the methanogenesis stage and lead to failure of the whole AD process [16]. On the other hand, higher C/N values may inhibit the microbial growth and decrease the biodegradability of the feedstock. As a consequence, when the C/N ratio is not close to the optimal range for AD, biogas production and substrate biodegradability are negatively impacted. A commonly used strategy for AD efficiency improvement when utilising feedstocks with C/N values outside the optimal range is co-digestion. Different organic materials with high nitrogen content have been tested as co-substrates along with WS, including food processing by-products [17] and various animal wastes, such as chicken manure [18]. Furthermore, the addition of inorganic nitrogen (e.g., NH\({}_{4}\)Cl) is another potential way to balance the C/N ratio in AD systems operating with WS as a sole substrate. Examples of agri-industrial by-products with high nitrogen content that can be used as co-substrate in AD systems are dried distillers' grains with solubles (DDGS) and rapeseed meal (RM). DDGS is the main by-product of the dry-grind distillation process for the production of alcoholic drinks (whisky) or biofuels (bioethanol) [19].