**3. Results and Discussion**

### *3.1. Marginal Agro-Ecological Zones in Europe*

As illustrated in Figure 2, there are various biophysical constraints and socio-economic challenges which need to be considered for MALLIS development. Table 2 shows the relevance of the numerous biophysical constraint combinations within each of the three AEZ. According to category 1 ('*natural constraints*'), the total marginal area across European land surface amounts to 646,833 km<sup>2</sup> (Table 2)—an area as large as France. However, this marginal agricultural land is widely scattered across Europe (Figure 4). Furthermore, there were 38 combinations of ≥ 2 constraints identified (Table 2). Across Europe, the most prevailing constraints are adverse rooting conditions, (155,519 km2), adverse climatic conditions (112,096 km2) and excessive soil wetness (108,081 km2). The total marginal arable land characterized by soil constraints accounts for about 535,000 km2. This is about 155,000 km<sup>2</sup> more than reported by Gerwin et al. (2018) (380,000 km2) [56,61]. It is likely that this difference results from the use of different thresholds for determining what is marginal and what is not. However, both values are within the same range.

**Figure 4.** Marginal agricultural lands based on biophysical constraints in Europe (ANC = agricultural natural constraint) (Source: This study).

### *3.2. The Growth Suitability of the Pre-Selected Industrial Crops in the Prevailing M-AEZ*

Potentially suitable industrial crops were identified for virtually all types of marginal agricultural land across Europe (Table A3). Each AEZ appears to have its own best-adapted industrial crops. A closer look at the type of biomass reveals that, for instance, oil crops are more suitable for Mediterranean regions than for the Atlantic region (Table A3). Among the woody lignocellulosic crops, Siberian elm outperforms poplar in the Mediterranean region (Table A3). The dominating lignocellulosic crops are tall wheatgrass, followed by reed canary grass and miscanthus (Table A3).

### *3.3. Marginal Agricultural Land Low-Input Systems (MALLIS) for Industrial Crop Cultivation*

Sections 3.1 and 3.2 revealed both the major M-AEZ in Europe and the growth suitability of the pre-selected industrial crops. This section explains how MALLIS could be developed using the information on M-AEZ and the crops' growth suitabilities. Furthermore, it discusses which other aspects need to be taken into account for MALLIS development in order to improve both the economic and social-ecological sustainability of the MALLIS in the long term.

### 3.3.1. Agricultural Measures for MALLIS Development

The potential e ffects of structured and systematic agricultural measures on agriculture facing biophysical constraints are provided in Tables A4 and A5. Furthermore, the literature review revealed that there are several ways to overcome each of the biophysical constraints. Tables A4 and A5 provide an overview of the suitability of agricultural managemen<sup>t</sup> options for dealing with the prevailing biophysical constraints on marginal agricultural lands. For example, the use of mulch helps to increase the soil thermal time, and thus increase the yield level in regions a ffected by water limitations and low temperatures [62].

### 3.3.2. Environmental Threats and Social Requirements

MALLIS implementations at a regional scale should also take both environmental threats and social requirements into consideration. Marginal agricultural lands could be characterized as fragile environments being highly susceptible to any types of external disturbance and input [6,12,63]. Key measures that can be highly recommended for the improvement of resilience include (i) the selection of low-demanding industrial crops (reduces the amount of fertilizers, and thus the risk of nutrient-leaching) [27], (ii) the development of heterogeneous landscape concepts (many small fields rather than only a few large fields) [64–67], and (iii) the implementation of agricultural diversification measures (intercropping, crop rotations, wildflower strips) [35,68,69]. Consequently, the assessment of the environmental performance of MALLIS should not be exclusively based on the global warming potential, but also on a number of other environmental impact categories, such as human toxicity threats, marine ecotoxicity, freshwater eutrophication and freshwater ecotoxicity, biodiversity and soil quality, pollution [70], and use of resources, e.g., water resources [71]. However, to enable a long-term sustainable implementation of MALLIS, besides the environmental impact categories, the social demands and the economic and market aspects must also be taken into account. The potential and viability of agricultural investments have to take into account land and labor costs, inputs, such as mechanical equipment costs, and income (which is linked with the market opportunities) [72]. The socioeconomic impacts can be measured via quantitative and qualitative parameters [73]. Moreover, aspects related to technological viability should also be taken into consideration. The yield loss associated with cultivation on marginal agricultural land may lead to higher contents of nutrients, such as nitrogen and potassium in the biomass, which may complicate further processing of the biomass [74]. Generally, this means that the prevailing structures of the existing agricultural systems [75], the farm typology [76], and the behavior patterns of the rural communities [24,77] require specific bottom-up research structures, such as the Integrated Renewable Energy Potential Assessment (IREPA) [78]. This would enable a better adaptation of MALLIS to the farm diversity [76,77] and the local community. Finally, this could potentially have a positive influence on the overall public acceptance of the MALLIS [79].
