Unlocking the environmental impact of rail traffic: insight and remediation potential of black locust linear systems

Railway networks constitute a vast and intricate infrastructure system that connects various environments, exerting a significant impact on them. Nonetheless, our comprehension of railway ecology remains limited, encompassing issues such as habitat loss, surrogates and connectivity, and the effects of pollution (Barrientos et al. 2019). Studies have drawn attention to elevated soil contamination levels along railway infrastructure, demonstrating a wide spatial reach of influence, extending up to 50 meters from the tracks (Burkhardt et al. 2008; Wilkomirski et al. 2012; Mętrak et al. 2015; Brtnický et al. 2022). As time progresses, this pollution accumulates, presenting a plausible long-term ecological threat (Jiasheng et al. 2020). To address this concern, buffers, as permanent vegetation strips, assume a pivotal role in mitigating the mobilization and transportation of pollutants towards fields through natural processes, filtering and reducing water runoff, and reducing bank erosion (Dosskey 2001). Regrettably, the potential and actual environmental pressures stemming from rail traffic is overlooked in landscape planning. Agroforestry, a practice that reintegrates tree planting or spontaneous tree growth into agricultural systems, offers a balanced approach to addressing environmental challenges while considering both productivity and protection(Smith et al. 2013). The black locust (Robinia pseudoacacia L.), a widespread non-native invasive tree in Europe found alongside railway tracks (Cierjacks et al. 2013; Pfeiffenschneider and Ries 2014; Sitzia et al. 2016), has been widely planted along railways and gained attention in agroforestry systems for biomass and honey production. Consequently, there is a pressing need for further research into its remediation capabilities. The project seeks to accomplish two primary goals: first, to advance our understanding of soil contamination resulting from rail traffic; second, to investigate the remediation potential of black locust linear systems as farmland or riparian buffer strips by examining their functional and structural characteristics in relation to soil contamination levels alongside railway infrastructures. Exploratory analyses were conducted in the north-eastern Po plain to investigate the influence of rail traffic on the quality and quantity of heavy metals. We selected two railway lines characterized by differing traffic intensities for our study. Each line had two transects perpendicular to the tracks, and we collected soil samples at various distances from the tracks (at 3, 8, 13, 18, 25, and 40 m) at two different depths (0-10 cm and 10-20 cm). The selection criteria for the study areas were designed to minimize variables, except for track distance, ensuring homogeneity in topsoil and the absence of recent earthmoving activities, among other factors. The concentrations of total Cd, Cr, Cu, Ni, Pb, Tl, V, Zn, As, Hg, Sb, and Sn were determined using inductively coupled plasma spectroscopy. Subsequently, a characterization of black locust linear systems was conducted alongside electrified railway infrastructures. Forty-two sampling units were chosen along various railway lines with differing traffic intensities and functional segments. The sampling units were defined with a fixed length of 10 meters along the tree linear system, running parallel to the rail track. The width of each area varied according to the width of the tree system's crown projections. Within each unit, data on general attributes, tree size, and understory composition were collected. Soil and black locust leaves were sampled for the analysis of heavy metal concentrations using the same method as mentioned earlier. The analyses conducted along the distance gradient from the railway infrastructure revealed a significant increase in heavy metal content near the tracks, followed by a rapid decline in metal content beyond distances of 8 to 13 meters from the tracks. When comparing data collected from different train lines characterized by varying levels of rail traffic, it becomes evident that there is a correlation between contaminant levels and the intensity of traffic. The investigation concerning the black locust linear systems and their relative contamination levels highlighted a wide range of heavy metal content across various sampling units. The metal with the highest concentration was copper often exceeding the Italian environmental regulatory threshold for industrial sites, and lead, zinc, nickel, and tin, which surpass the regulatory thresholds for green areas. Preliminary analyses confirmed existing evidences that black locust tree systems do not appear to be negatively influenced by the level of soil contamination (Sitzia et al. 2016). The preliminary findings of the study underscore a significant level of contamination along the railway infrastructure, with high concentrations of heavy metals in the soil frequently exceeding regulatory threshold values. Interestingly, the analysis of the black locust linear systems appears to support the notion that these systems exhibit a high degree of tolerance to soil contamination. Further research is warranted to compare the distribution gradient of heavy metals at increasing distances from the tracks under various conditions, including: i) with and without the presence of tree linear systems, ii) with different types of tree linear systems, and iii) across different land-use types and farmland areas.