**1. Introduction**

Soil heavy metal pollution is a worldwide problem, with accumulation leading to toxicity and environmental persistence; this is especially true for cadmium (Cd) and lead (Pb), both of which pose serious human and ecological health threats [1,2]. In China, over 16% of all agricultural soils are contaminated with heavy metals, with soil Cd and Pb concentrations exceeding China's environmental standards in 7% and 1.5% of all arable lands (9.8 <sup>×</sup> 104 and 2.1 <sup>×</sup> <sup>10</sup><sup>4</sup> km2, respectively [3]. It is obvious that in order to protect human health and the environment, reducing heavy metal bioavailability is of paramount importance.

Reducing heavy metal bioavailability has followed several pathways, including amendment additions for heavy metal stabilization [4]. Heavy metal stabilization is typically dominated by chemisorption mechanisms, with other sorption mechanisms such as ion exchange, electrostatic

attraction and complexation playing roles [1]. Stabilization/immobilization has been considered practical with respect to short-term effectiveness when using different stabilizers, yet longer-term effectiveness does require more research, especially for the biochar [5]. For example, Senneca et al., found that a cement-based stabilization/solidification treatment positively affected a chromium (Cr) contaminated soil, with the treatment increasing the amount of Cr in the soil residual fraction and thus reducing its mobility [6]. Cui et al., found that the biochar could stabilize Cd and Pb in the paddy soil and reduce the rice uptake in short term [7]. Finding a heavy metal stabilizing material that is effective both in the short- and long-term may be as simple as utilizing biochar [8].

Biochars are created by pyrolyzing various carbonaceous materials, such as agricultural crops wastes, at moderate temperature under anoxic conditions [9]. Biochar is widely accepted as an effective agent for reducing heavy metal bioavailability via biochar organic-heavy metal complexation and biochar oxide, hydroxide, and carbonate phase-heavy metal precipitation [10]. As examples, Khan et al., showed that corn straw biochar could effectively sorb Cd via chemisorption, electrostatic interactions, and inner-sphere complexation reactions [11]. Golden shower tree (*Cassia fistula*) biochar has been shown to sorb and remove up to 303.5 mg Cu g−<sup>1</sup> from wastewater [12]. In addition, barley grass biochar has been proven to sorb ~90 to 95% of soil borne Cu and Pb from a contaminated soil, leading to enhanced plant growth [13].

Increasing heavy metal sorption via biochar use can lead to alterations in heavy metal phases present in contaminated soils. Essentially, relatively high bioavailable heavy metal concentrations can be altered to less bioavailable forms via biochar application to metal-contaminated soils. Qin et al., showed that the addition of pig manure biochar to contaminated soil sorbed both Cd and Pb, reducing their leaching losses (i.e., bioavailable forms) by 38% and 71%, respectively, as compared to a control [14]. Ippolito et al., observed up to an 88% and 100% decrease in bioavailable Cd and Pb, respectively, with the use of either lodgepole pine or tamarisk biochar in metal contaminated soils; decreases were driven by precipitation reactions [15]. Cadmium sorption onto wheat straw biochar was driven by precipitation reactions [e.g., Cd(OH)2 and CdCO3] and interaction with carbonyl and carboxyl groups, leading to reduced bioavailable soil Cd concentrations [16]. Water hyacinth biochar has been shown to decrease rice paddy soil exchangeable Cd content by ~25%, while increasing Cd in less-bioavailable forms such as those associated with carbonate- and Fe/Mn oxide phases [17].

Altering heavy metal phases present, in favor of lower bioavailability, has been linked to alterations in plant and human metal uptake. For example, corn straw biochar has been shown to significantly reduce the proportion of Cd in the soil exchangeable and carbonate phases (i.e., relatively highly bioavailable) and increase the proportion of Cd in the residual fraction (i.e., highly unavailable), leading to a decrease in plant and human bioavailability [18]. Bian et al., reported that wheat straw biochar reduced Cd and Pb bioavailability by ~60% via sorption onto biochar (hydr)oxide phases present (i.e., unavailable), with Cd and Pb rice uptake reduced by between 27% and 69%, suggesting lower heavy metal consumption by humans [19].

The above studies indicate that biochars may be used to not only sorb, but to alter heavy metals forms to those less bioavailable. However, the additional benefit of biochar land application lies in the fact that biochars can also positively alter soil physicochemical attributes. Zhang et al., showed that rice straw biochar sorbed and immobilized heavy metals, while improving soil water and nutrient dynamics [20]. Bamboo biochar has been shown to increase acidic soil pH, available K, Fe, Mg, and Mn content, and SOM, while decreasing bioavailable heavy metal mobility via surface adsorption and precipitation reactions [21]. Cui et al., reported that biochar increased soil pH and SOM, and significantly reduced metal bioavailability, leading to a decrease in rice and wheat grain metal concentrations over the short-term (i.e., 2 years) [7,22]. Cui et al., showed a similar long-term (i.e., 5 years) trend with wheat straw biochar-Cd/Pb sorption in an amended paddy soil [23].

Based on the information presented above, continued research is needed to verify biochar alterations to soil properties, heavy metal bioavailability, and plant metal uptake over short- and longer-term timeframes. Desperately required is a focus on in-field research. Thus, the objective of the present study was to evaluate the mechanisms by which biochar alters soil physicochemical as well as biological properties, soil Cd and Pb bioavailability, and the corresponding metal uptake in rice from a contaminated paddy soil under the short- and long-term. Our hypothesis was that biochar would bind Cd and Pb and reduce the soil bioavailable fraction/increase the recalcitrant fraction, leading to less Cd and Pb uptake by rice over both a relatively short and longer timeframe. This hypothesis should be supported by correlations between chemical, physical, and biological changes in soil due to biochar application.
