**4. Discussion**

The comparison between specific types of production systems in two diverse environmental zones is challenging and cumbersome due to the differences in climate, soil, and management. Hence, a simple way to gain an initial overview of the environmental impacts of the studied production systems are presented in Table 5, with GHG emissions calculated per hectare. The data showed that the conventional wheat production system had the largest environmental impact ha−<sup>1</sup> compared to other studied production systems in Denmark and Italy.

The environmental impacts of the six productions systems were calculated, based on three indicators of GWP, acidification, and eutrophication. Machinery use contributed the highest GHG contribution in the silvopastoral and organic production systems whereas fertilizer contributed the largest GHG in the traditional and conventional production systems in Italy.

The comparison of results with other studies was not straightforward as the farming systems and system boundaries varied between studies [10,11,14]. For Italy, the highest GWP calculated for the traditional farming system (Table 2) was mainly attributed to fertilizer (0.15 kg CO2-eq.) and irrigation (0.13 kg CO2-eq.). Romero-Gamez et al. [14] related this with CO2 and NO2 emissions to air caused by the manufacture and application of fertilizers to the cropping systems. In the present study, CO2 and N2O from fertilizer and machinery use were significant contributors to GHG emissions in the production systems. Romero-Gamez et al. [14] found that acidification was dominated by NH3 emissions to the air and those emissions were allocated to fertilizer production, in similarity to the present study, finding that fertilization and machinery use related to NH3 and NO had the highest impact on acidification.

Due to the diversity of products from the combined food and energy system, the environmental impacts were calculated based on the income from the two types of production systems in Denmark. Thus, acidification was found to be more than seven times higher for the conventional wheat production system in comparison to the combined food and energy system (Table 2) with the main impact from fertilizer related to NH3 and NO (Table 1). Likewise, the eutrophication and GWP in the conventional wheat system was 11.0 and 11.4 times higher, respectively, compared to the combined food and energy system, mainly caused by fertilizer use (Table 4). The study by Nemecek et al. [15] found increased environmental impacts by conventional production practice compared to organic agricultural practices by 1.5 times for GWP (4474 vs. 2920 kg CO2-eq. ha−<sup>1</sup> yr<sup>−</sup>1), 1.4 times for acidification (88 vs. 61 kg SO2-eq. ha−<sup>1</sup> yr<sup>−</sup>1), and 1.4 times for eutrophication (123 vs. 88 kg N-eq. ha−<sup>1</sup> yr<sup>−</sup>1). This supports the present study's findings of less environmental impacts from practices with reduced application of fertilizers and pesticides. While the GWP for the combined food and energy system is of similar magnitude to the organic system in Nemecek et al. [15], the GWP of the Swiss conventional wheat production system was much higher than the conventional wheat production system in the present study. Likewise, Knudsen et al. [29] found low GWP values of 2032-2599 kg CO2-eq. ha−<sup>1</sup> yr<sup>−</sup><sup>1</sup> for conventional wheat production system in Denmark in a four-year barley, potatoes, and winter wheat crop rotation including one year of either faba beans or grass-clover.
