*2.1. Field Experiments*

Field trials were carried out in accordance with the general guidelines of field trials methodology [24] in the time period between 2017 to 2020, in the surroundings of Halle/Saale located in the rain shadow of the Harz Mountains range ("Mitteldeutsches Trockengebiet", Central German Dry Area). This consists of a dry but fertile landscape in the center of Germany close to Halle/Saale (Saxony-Anhalt, Germany). The altitude there is approximately 100 m above sea level. The mean for the long-term annual precipitation in this area is below 500 L/m2, and the mean annual temperature is 9.0 ◦C [25].

However, comparing the vegetation periods between 2018 and 2020, 2018 was considerably drier (<400 L/m2), whereas other locations showed even lower measurements (<300 L/m2). All three periods were significantly warmer following the general trend of the climate changes that took place there (annual mean values of up to >11 ◦C during the trial periods).

All soils of the experiments had a medium to high credit rating (diluvial and loess sites, soil value indices, "Ackerzahl" [26]) of 54–98 and had a medium to elevated level of macro and micronutrient supply. All field experiments and the control plots were managed ecologically during these experiments and were in line with good agricultural practice methodology, while weed control was carried out exclusively by mechanical means. For the cultivation of *L. squarrosa* only the variety "Laira" was used. This is a biennial genotype, which is cultivated during autumn sowing of the previous year. The crops grown after *L. squarrosa* cultivations were chosen based on their importance for a rational crop rotation scheme for this region and for organic farming, constituting the idea that different possible plant families should be represented in these experiments. Regionally common varieties were used for these subsequent cultivations. Several scenarios of field experiments were conducted and the details are described below. In addition, Figure 2 in Results and Discussion illustrates and summarizes the chronology and rotation of field and crops.

**Figure 2.** Experimental design to investigate possible horizontal PA-transfer in the context of PA-plant (*L. squarrosa*) cultivation.

#### *2.2. Soil Improvement Measures*

#### 2.2.1. Composts

Composts were prepared in 2018 and 2019 (July through September) to yield four different model composts which were used to test the impact of PA-plant compost on PAtransfer to subsequent crops: (a) control-compost contained only compost stock; (b) Seneciocompost contained compost stock of which 32% (*w*/*w*) of fresh cut *S. jacobaea* material were included; (c) Lappula-compost contained compost stock of which 14% (*w*/*w*) *L. squarrosa* press-cake powder was included; (d) Senecio/Lappula-compost contained both 15% (*w*/*w*) fresh cut *S. jacobaea* and 9% (*w*/*w*) *L. squarrosa* press-cake powder in the composite.

Details on the compost making, PA-degradation and final PA-levels of the composts are given in Chmit et al. [27]. These four composts were analyzed for value-determining soil nutrients [27].

#### 2.2.2. *L. squarrosa* Press-Cake

Dried *L. squarrosa* press-cake (residues of the pressed *L. squarrosa* seeds for oil production; harvest 2017) was used in homogenous milled form (3 mm) provided by Exsemine GmbH (Salzatal, Germany). Samples were taken to analyze the PA-content of the presscake before it was used to set up the composts (2018 and 2019) and before the material was added directly to the soil for field plot preparation in 2019 and 2020.
