*2.1. Sampling Area*

Field experiments were performed in the area of the Iławski Lake District (north-eastern Poland) at the Production and Experimental Station 'Bałcyny' (53◦35 49" N, 19◦51 20" E) of the University of Warmia and Mazury in Olsztyn (Poland). The Lake District stretches over 4230 km2 and is one of the least polluted regions of Poland, called the Green Lungs of Poland. It is characterized by young glacial relief resulting from Pleistocene glaciations, and has a temperate warm transition climate due to the clash of continental and oceanic climates. According to the Institute of Meteorology and Water Management State Research Institute (IMGW), in this area in 2018, the average annual air temperature was +9.0 ◦C, with August being the warmest month (+20.4 ◦C), and February the coldest (−4.1 ◦C). The sunshine duration ranged from 240 h in July to 40 h in January. The growing season spanned approximately 206 days, and snow cover persisted for 70 days. The annual sum of precipitation was 550 mm, with the highest amount of precipitation being recorded in July (140.7 mm) and the lowest in February (2.0 mm). The average air humidity was approximately 81%.

The rhizosphere of the three following plant species was chosen for the study: winter wheat (*Triticum aestivum*) of Julius cultivar, winter rape (*Brassica napus*) of Garou cultivar, and field pea (*Pisum sativum* subsp. *arvense*) of Milwa cultivar. The plants were grown simultaneously on three plots, each with a surface area of 20,000 m2. The fraction size distribution of the soil used in the experiment was established and is presented in Table 1.


**Table 1.** The granulometric composition of the soil used in the experiment.

Homogeneous groups denoted with letters (a, b) were calculated separately for each of the properties.

Winter rape was used as the previous crop of winter wheat, whereas winter wheat was used as the previous crop of winter rape and field pea. Winter wheat was sown on October 2, 2017, and the crop was harvested on August 2, 2018; winter rape was sown on August 19, 2017, and the crop was harvested on July 20, 2018, while pea was sown on April 7, 2018, and the crop was harvested on July 15, 2018. All plants were grown in accordance with the recommended technology for cultivating these plant species. All soil samples were collected the next day after the last crop harvest. In the case of winter wheat, fertilization was as follows: 66 kg N ha<sup>−</sup>1, 20 kg P ha−1, and 36 kg K ha−1. The nitrogen dose was divided into three portions that were administered at stage 23 of plant development according

to the BBCH scale, in the form of an aqueous solution of NH4NO3 and CO(NH2)2 + H2O, and at stages 32 and 52 according to the BBCH scale, in the form of NH4NO3. Potassium and phosphorus were applied once before sowing. Potassium was used in the form of KCl and phosphorus in the form of Ca(H2PO4)2. In the case of winter rape, fertilization was as follows: 75.8 kg N ha<sup>−</sup>1, 40 kg P ha−1, and 60 kg K ha−1. Nitrogen dose was divided into three portions that were administered before sowing in the form of CO(NH2)2 and at stages 30 and 50 of plant development according to the BBCH scale, in the form of NH4NO3. Phosphorus and potassium were applied once before sowing in the form of Ca(H2PO4)2 and KCl, respectively. No fertilization was applied in the case of field pea. The plant density of winter wheat per 1 m<sup>2</sup> was 400 plants; that of winter rape was 45 plants, and that of field pea, 90 plants. Soil was cultivated in a conventional (ploughing) manner in the case of all of the studied plant species.

The area of 20,000 m<sup>2</sup> was divided into five plots, each measuring 4000 m2. After the harvest, 15 soil samples were collected at random from a depth of 0–20 cm from each plot using the zigzag sampling method. All soil samples collected from each plot were homogenized and combined into one collective sample. Hence, a total of five samples of soil from the cultivation of each plant species studied were used for microbiological and biochemical analyses. Soil samples were collected using an Enger-Riehm probe.
