minerals Article Fractions of Ni, Pb, Cr, and Their Impact on Enzyme Activities of Arable Land Cultivated by the Simpliﬁed Method Adam Lukowski 1, * and Dorota Dec 2   Citation: Lukowski, A.; Dec, D. Fractions of Ni, Pb, Cr, and Their Impact on Enzyme Activities of Arable Land Cultivated by the Simpliﬁed Method. Minerals 2021, 11, 584. https://doi.org/10.3390/ min11060584 Academic Editor: Maria Economou-Eliopoulos Received: 20 April 2021 Accepted: 28 May 2021 Published: 31 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Technology in Environmental Engineering, Institute of Environmental Engineering and Energy Production, Faculty of Civil Engineering and Environmental Sciences, Bialystok University of Technology, Wiejska 45A, 15-351 Bialystok, Poland 2 Department of Agri-Food Engineering and Environmental Management, Institute of Environmental Engineering and Energy Production, Faculty of Civil Engineering and Environmental Sciences, Bialystok University of Technology, Wiejska 45A, 15-351 Bialystok, Poland; d.dec@pb.edu.pl * Correspondence: a.lukowski@pb.edu.pl Abstract: Total metal content is not representing its availability and thus does not provide the details about potential environmental hazards, including the impact on soil enzyme activities. To understand metal availability, chemical fractions must be considered. The goal of this study was to evaluate the inﬂuence of Ni, Cr, and Pb fractions on the enzymatic activity of soils cultivated by the simpliﬁed method, which is rare not only in Poland. The percentage of studied metals in fractions was determined according to the Community Bureau of Reference (BCR) method. Four fractions were extracted: acid soluble and exchangeable (F1), reducible (F2), oxidizable (F3), and residual (F4). The highest Ni and Cr percentages were noted in fraction F4, and for Pb, they were noted in fraction F2. The smallest Ni and Pb percentages were observed in fraction F1 (most mobile) and for Cr, they were observed in fraction F2. In soil samples collected in spring, the signiﬁcant relationship was stated between F1/Ni/dehydrogenase, F2/Pb/dehydrogenase, and F2/Pb/urease. Such dependence occurred between F1/Ni/phosphatase and F4/Ni/urease during summer as well as between F1/Ni/phosphatase and F4/Ni/dehydrogenase in autumn. F1/Pb caused a drop in phosphatase activity, whereas F4/Cr inﬂuenced its increase. The study results indicated that metal fractions inﬂuenced phosphatase activity the most, while protease activity in the soil was not affected. Keywords: metal fraction; enzyme activity; heavy metal; arable soil; BCR method 1. Introduction Soil is a very important part of the ecosystem. Heavy metals in soil represent de- structive and potent environmental hazards [1]. They are a source of concern because of their potential reactivity, toxicity, and mobility [2,3]. High concentrations of metals in soil can cause the limitation of vegetation [4]. According to Xia et al. [5], nickel toxicity toward microorganisms in soil appears at concentrations higher than 100 mg kg −1 , causing the decrease of microbial biomass carbon and soil basal respiration. As to lead, the toxic level is 150 mg kg −1 for microbial biomass carbon and 100 mg kg −1 for microbial biomass nitrogen [6]. Chromium belongs to the metals that are extremely harmful to microbes. In the environment, only trivalent and hexavalent forms of chromium are prevalent. They are differentiated in terms of biological reactivity and physicochemical properties. Hexavalent ions are more toxic due to the high mobility in biological systems [7]. As stated by ul Hassan et al. [8], the 50% reduction in overall microbial soil activity is observed at a total chromium concentration of 263 g kg −1 . The toxicity of heavy metals to plants is dependent on soil composition, plant species, metal concentration, soil pH, and the chemical form of the metal [9]. Critical concentrations in soil are within the range of 10–100, 30–300, and 5–30 mg·kg −1 for Ni, Pb, and Cr, respectively [10]. The persistence of enzymes in the soil is inﬂuenced by parameters such as soil temperature, depth, organic matter, acidity, and Minerals 2021, 11, 584. https://doi.org/10.3390/min11060584 https://www.mdpi.com/journal/minerals