Phytoscreening-based assessment of mercury in soil

The what, how, why, and when of dendrochemistry: (paleo)environmental information from the chemical analysis of tree rings

The chemical analysis of tree rings has attracted the interest of researchers in the past five decades in view of the possibility of exploiting this biological indicator as a widely available, high-resolution environmental archive. Information regarding the surrounding environment can be derived either by directly measuring environmental variables (nutrient availability, presence of pollutants, etc.) or by exploiting proxies (e.g. paleoclimatic and paleoenvironmental reconstructions). This review systematically covers the topic and provides a critical view on the reliability of dendrochemical information. First, we introduce the determinable chemical species, such as major elements, trace metals, isotopic ratios, and organic compounds, together with a brief description of their uptake mechanisms and functions in trees. Subsequently, we present the possibilities offered by analytical techniques in the field of tree ring analysis, focusing on direct methods and recent developments. The latter strongly improved the details of the accessible information, enabling the investigation of complex phenomena associated with plant life and encouraging the direct analysis of new analytes, particularly minor organic compounds. With regard to their applications, dendrochemical proxies have been used to trace several processes, such as environmental contamination, paleoclimate reconstruction, global environmental changes, tree physiology, extreme events, ecological trends, and dendroprovenance. Several case studies are discussed for each proposed application, with special emphasis on the reliability of tracing each process. Starting from the reviewed literature data, the second part of the paper is devoted to the critical assessment of the reliability of tree ring proxies. We provide an overview of the current knowledge, discuss the limitations of the inferences that may be drawn from the dendrochemical data, and provide recommendations for the best practices to be used for their validation. Finally, we present the future perspectives related to the advancements in analytical instrumentation and further extension of application fields.

Evaluation of mercury phytoavailability in Oxisols

Mercury is a metal which is potentially toxic for the environment. Many factors control its retention in the soil, such as cation exchange capacity, pH, clay content, organic matter, and redox potential. It is important to know the phytotoxic effects of soil Hg to prevent environmental contamination and its entry into the food chain. Several analytical methods are used to measure metal phytoavailability in soils, but none has been reported for Hg in Oxisols, the most common soil class in Brazil and a very important soil class throughout the tropics. The aim of this study was to select the chemical extractor that best correlated the Hg levels in plants and the Oxisols. The soils used were classified as Dystrophic Red-Yellow Oxisol (LVAd) and Dystroferric Red Oxisol (LVdf), which were collected in the 0-0.2-m soil layer. The species selected for cultivation were a monocotyledon, oat (Avena sativa L. cv. São Carlos) and a eudicotyledon, common bean (Phaseolus vulgaris L. cv. Madrepérola). Each test plot was composed of a 500 cm³ pot filled with soil samples contaminated with HgCl₂. Treatments were arranged in a completely randomized design, with four replications. The experiment was conducted for 30 days. Mercury contents were separately extracted with the following extractors: USEPA 3051A, Mehlich-1, Mehlich-3, DTPA, and water. Mercury was determined by hydride generation atomic absorption spectroscopy. The extracted contents were correlated with the contents in the tissues of the plants' aerial part by the Pearson correlation. Although it is not considered a standard procedure to evaluate metal phytoavailable contents, the method that presented the best correlations between soil Hg and plant Hg was USEPA 3051A (r = 0.75*). As expected, the worst correlation was with water (r = 0.57* for common bean and r = 0,05^ns for oat).

Dendrochemical assessment of mercury releases from a pond and dredged-sediment landfill impacted by a chlor-alkali plant

Although current Hg emissions from industrial activities may be accurately monitored, evidence of past releases to the atmosphere must rely on one or more environmental proxies. We used Hg concentrations in tree cores collected from poplars and willows to investigate the historical changes of Hg emissions from a dredged sediment landfill and compared them to a nearby control location. Our results demonstrated the potential value of using dendrochemistry to record historical Hg emissions from past industrial activities.