## 2 Materials and Methods  
### Site Description and Experimental Design  
The field study was conducted at an experimental area of vegetable crops on a farm belonging to the Polytechnic Institute of Castelo Branco (39.823655, \(-\)7.451606 Decimal Degrees, WGS 84 coordinates). The soil is a Dystric Cambisol [14] developed from granitic rocks. The region has a temperate climate with dry summer (Csa, Koppen classification) with a mean annual temperature of 15.0 \({}^{\circ}\)C, a mean maximum and minimum temperatures of 21.5 and 9.4 \({}^{\circ}\)C, respectively, and mean annual rainfall of 735 mm [15]. This trial was conducted during 2020-2021 with two successive vegetable crops, lettuce (_Lactuca sativa_ L.) and kale (_Brassica oleracea_ var. Winterborn). Lettuce is a salinity sensitive crop with a high ability to uptake heavy metals accumulated in its leaves. So, it is an appropriate crop to assess the entry of toxic elements in the food chain and changes in soil fertility. The experimental design for the two vegetable crops was completed randomised with 9 treatments, each one with 4 replicates with 36 plots. Each plot (replicate) had 1.08 m\({}^{2}\) (1.20 \(\times\) 0.90 m) with 12 lettuce. The lettuces had a row spacing of 0.30 m, and in the row, the distance between two lettuces was 0.30 m. The lettuce seedlings were planted on 6 March 2020 and harvested on 7 May 2020 with a crop cycle of 62 days. In February, before planting, the soil was harvested and milled, then the digestate was incorporated manually into the soil, and the plots were watered. For 11 days, the soil with the digestate was left to rest. The mineral nitrogen fertiliser was applied two days before seedling and incorporated into the soil manually. The top-dressing nitrogen fertilisation using the mineral fertiliser was done twice after 22 and 49 days after planting. During the lettuce crop cycle, the average temperature was 13 \({}^{\circ}\)C and the rainfall 250 mm, and for the kale crop cycle was 10 \({}^{\circ}\)C and 500 mm, respectively. Concerning the kale experiment, the agricultural practices were the same as the lettuce. However, the plants had a row spacing of 0.35 m and a distance between two kale plants of 0.35 m. The kale seedlings were planted on 15 October 2020 and harvested on 16 March 2021 with a crop cycle of 152 days. The top-dressing nitrogen fertilisation using the mineral fertiliser was done twice, 61 and 120 days after planting. In the lettuce trial, ammonium nitrate (27% Ni) was used for the Ni basal fertilisation, and for the first top-dressing and calcium nitrate (15.5% Ni) was used, in the second top-dressing Ni fertilisation. In the kale trial, ammonium sulphate (20.5% Ni) was used for the Ni basal fertilisation and for the first top-dressing and for the second top-dressing ammonium nitrate (27% Ni) was used. The fertilisers with the ammoniacal form of N were used to avoid N leaching whenever the probability of rain was high. The treatments were the same for the two crops. Since the soil had high levels of phosphorus (149 mg P kg\({}^{-1}\) evaluated by the method of [PERSON] et al. [16]) and of potassium (530 mg K kg\({}^{-1}\) evaluated by the method of [PERSON] et al. [17]), only the nitrogen mineral fertilisation was done (Table 1). The nine treatments done in the experiments were: a control without any fertilisation (control); mineral N fertilisation with an N application rate of 35 kg N ha\({}^{-1}\) before seedling planting, and 25 kg N ha\({}^{-1}\) in each top-dressing with a total application of 85 kg ha\({}^{-1}\) of inorganic N (Ni85).