**1. Introduction**

Human activities have gradually transferred many toxic metals from the earth's crust to the environment, resulting in the spread and contamination of toxic metals in the ecosystem [1–4]. The metals originate from many sources including mining activities, industrial waste disposals, paints, and gasoline additives that lead to physical and chemical processes such as leaching and oxidation thus causing the accumulation of metals in the soil. Toxic metal pollution has become a serious problem worldwide in the last decade. Cadmium (Cd) is considered the most toxic element among toxic metals for living organisms and has been detected in some agricultural lands [1]. Cadmium accumulates in plants and animals and threatens their health when entering through the food chain [5,6]. Thus, it is important but challenging to remediate Cd-contaminated soils worldwide.

The Tar Creek area is designated as a superfund site located in the tri-state regions of Oklahoma, Kansas, and Missouri by US Environmental Protection Agency (EPA), and is one of the most polluted sites in the world [7]. A Superfund site is an abandoned hazardous waste site in the United States subject to the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), which allows federal funding to be spent on investigating and remediating the environmental contamination at the designated site. The Tar Creek area was mined for approximately 70 years starting from the beginning of the 1900s. Large quantities of coarse material contaminated with Cd were left on the ground surface in piles due to the mining and milling operations. Such a coarse material is gravellike waste well known as "chat pile." The large size and the considerable amount of chat piles resulted in Cd contamination in the surrounding areas due to wind blowing and the chat materials used as gravel. Reclamations of contaminated lands have been attempted by

**Citation:** Antonangelo, J.; Zhang, H. Influence of Biochar Derived Nitrogen on Cadmium Removal by Ryegrass in a Contaminated Soil. *Environments* **2021**, *8*, 11. https:// doi.org/10.3390/environments8020011

Academic Editor: Dionisios Gasparatos Received: 7 January 2021 Accepted: 2 February 2021 Published: 8 February 2021

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the US Government and the Quapaw Tribe. Although the status of Cd contamination in the Tar Creek has been addressed in numerous studies [8–10], with some of them elucidating the use of biochars for Cd remediation and phytoavailability reduction [11], there is still a lack of research evaluating the effect of nitrogen (N) application via biochars on the accumulation of Cd by crops.

Nitrogen is the most important essential nutrient for plants, and N application is the biggest contributor to biomass production [12]. Although some studies suggested that Cd concentration in plants decreased as their biomass increased by N application [13,14], other works concluded that N fertilizer not only improved plants' biomass but also increased their Cd concentration [12,15–17]. In an experiment conducted by Symanowicz et al. [18], N fertilization significantly contributed to increase Cd accumulation in the aboveground portion (shoots) of eastern galega. Thus, the effect of N application on Cd accumulation remains controversial [12]. In general, N fertilization often decreases soil pH, which increases the solubility and mobility of Cd in the soil [19,20]. Ji et al. [12] also highlighted a more direct effect of N fertilization in Cd uptake by suggesting a mechanism in which the active uptake and influx rate of Cd2+ into the roots of Italian ryegrass (*Lolium multifolorum* Lam.) were enhanced by the greater affinity of the membrane transporter to Cd2+ as a consequence of the urea application.

Although the studies mentioned above have evaluated the effect of N on Cd bioavailability and plant uptake, all of them were performed through the addition of chemical fertilizers. Therefore, the evaluation of N addition using biochar aiming the same goal makes our research unique since there might be a compound effect of biochar properties, such as alkalinity and surface functional groups, also affecting Cd uptake, not just the N addition itself.

Perennial ryegrasses (*Lolium perenne*) are widely used in grazed pastures due to its potential to reduce nitrate (NO3 −) leaching [21], which makes such grass a potential scavenger of soil N. Ryegrass is also known for its efficient phytoaccumulation of toxic metals such as Cd [11]. These characteristics make ryegrass a promising tool to study the N and Cd accumulation relationship in plant tissues. It is a gramineous grass with fast growth, high yield potential, strong resistance to toxic metals, and able to normally grow in tailing areas where toxic metal pollution is severe and the environment is harsh [12]. Moreover, Mongkhonsin et al. [22] pointed out that ryegrass shoots had the highest Cd enrichment of up to fourfold of what is commonly found in other plant species that are hyperaccumulators (*Solanum nigrum* and *Indian mustard*); and Italian ryegrass had the strongest tolerance and accumulation capacity of Cd among eight C3 herbage grass species. Therefore, the perennial ryegrass with high biomass yields is also an appropriate species to use for the remediation of Cd-contaminated soils [23].

Our objective was to determine the influence of increased N addition via biochar derived from two feedstocks on Cd removal by ryegrass. This study is derived from the work of Antonangelo and Zhang [11] and data were reanalyzed to evaluate the N and Cd removal instead of their simple uptake. Since current literature is undoubtedly deficient in such information, we believe that our research has the potential to expand from small-scale to field-scale studies. This will serve as a multi-purpose biochar application to increase forage biomass and Cd uptake for phytoremediation purposes, at the same time immobilizing a certain amount of bioavailable Cd in the soil, as observed with the previous study of Antonangelo and Zhang [11], thus avoiding its leaching to the groundwater.

This would be a pioneering work for other studies focusing on N rates from biochars and/or the combination of biochar + N fertilizers to increase toxic metal phytoaccumulation while immobilizing the metal in the soil-plant system. This multi-purpose aspect contributes to the sustainable use of resources, since only immobilization is not enough if there is a need for disposal, land application, or landfill. Hence, if animal grazing is needed in such contaminated areas, the risk of intoxication could be still avoided depending on the metal accumulation in shoots. A sustainable attitude (multi-purpose) like this might be useful for contaminated lands, such as the Tar Creek superfund site.
