The potential benefits of biochar to improving soil health through nutrient addition, and consequent im­provements in fertilizer use efficiency have been well recognized through glasshouse and field trials (Sohi et al., 2010; Verheijen et al., 2010). These studies have shown that biochar and biochar-amended soil help retain plant nutrients and this is one of the means by which biochar application to soils is known to improve soil quality and increase crop yields in some cases. How­ever, it is important to recognize that if the nutrients are retained by biochar particles in soil to a degree that plants are unable to take up the nutrients, this could impact productivity (Kookana et al., 2011).

Studies have shown that retention of nitrogen (N) (Hollister et al., 2013; Liu et al., 2013), as well as phos­phorus (P) and potassium (K) (Schnell et al., 2012), is achievable through incorporation of biochar, demon­strating potential agronomic and wider environmental benefits. Within the context of an agroecosystem, plant nutrients are essential and often the limiting factor

determining the extent of crop growth. However, once N or P leave an agroecosystem, either through overland flow or leaching, they could potentially pose a threat to surface waters, with P being the limiting nutrient govern­ing eutrophication of fresh water and N being the limiting nutrient governing the eutrophication of estuarine and ocean systems (Brady and Weil, 2008). Similar to gaseous C emissions, aqueous N and P losses from agricultural soil have global effects. While the majority of biochar research focuses on short-term impacts of its application, more long-term field research focused on net C sequestra­tion, net GHG emissions, microbial community dy­namics, nutrient use efficiency, and water use efficiency is needed (Ippolito et al., 2012). Furthermore, an increased fundamental understanding of the mechanisms underly­ing the interactions between biochar and soil in order to optimize agricultural systems and protect the environ­ment should be a further focus (Spokas et al., 2012b).

While applied plant nutrients outside of an agricul­tural context represent a threat to the environment that biochar has demonstrated potential to address, it is possible to retain other pollutants (e. g. heavy metals and pesticides) using biochar, as well. When biochar is applied to cadmium (Cd)-, copper (Cu)-, and lead (Pb)-contaminated soils, these metals have been observed to become immobilized, decreasing phytotox­icity and bioavailability, and vastly improved crop pro­duction (Park et al., 2011). Other studies have shown biochar to exhibit strong sorption and degradation
inhibition of pesticide residues, leading to potential con­cerns regarding long-term accumulation in biochar amended soils treated with pesticides (Kookana, 2010). Some biochars have also been shown to retain estrogenic steroid hormones on dairy farm soils (Sarmah et al., 2010). While the potential for soil organic and inorganic contaminants (e. g. metal remediation, pesticide accumu­lation, and hormone retention) remediation are valid, it is important to consider that the enormous heterogene­ity of biochars with respect to their chemical qualities and resultant effects on soil pollutants remain largely uninvestigated (Kookana et al., 2011). Research on potential agronomic and environmental applications of biochar is currently in its infancy and it is through the establishment and monitoring of additional long-term field trials that its full potential could be realized (Sarmah, 2009).