Introduction

Applying biochar to soil has been proposed to improve soil fertility (Chan & Xu, 2009; Atkinson et al., 2010) while sequestering carbon (Lehmann, 2007; Sohi et al., 2010; Ippolito et al., 2012; Manyà, 2012) and reducing or suppressing the release of greenhouse gases such as CO 2 , N 2 O and CH 4 (Spokas & Reicosky, 2009; Zhang et al., 2010; Bruun et al., 2011). Due to the large variety of biomass potentially available for conversion into biochar, as well as different pyrolysis technologies (thermal, microwave etc.) and possible processing conditions (temperature, heating rate, vapour residence time etc.), an infinite range of biochar types could be created. These will differ in their physicochemical properties and functional performance (Verheijen et al., 2009; Enders et al., 2012; Ronsse et al., 2013). While the influence of production conditions on the physiochemical properties of biochar has been widely covered (Williams & Besler, 1996; Antal & Grønli, 2003; Demirbas, 2006; Shackley & Sohi, 2010; Enders et al., 2012; Angin, 2013) little has been reported on the corresponding effects on biochar functional properties (Atkinson et al., 2010; Rajkovich et al., 2011; Crombie et al., 2013; Mašek et al., 2013). Functional properties are those which could contribute to soil water holding capacity, crop nutrient availability, carbon storage, cation exchange capacity, favourable pH, etc.

Biochar has been consistently shown to be recalcitrant (Spokas, 2010; Enders et al., 2012; Crombie et al., 2013) when applied to soil which is its most important property in terms of C sequestration potential. Although having high levels of resistance, biochar is still gradually mineralized to CO 2 ; otherwise, soil organic matter (SOM) would be dominated by biochar accumulated over long time scales (Masiello, 2004; Cheng et al., 2006; Lehmann et al., 2008). Therefore the absolute longevity of biochar in soil cannot be quantified by one number as biochar is not one consistent homogeneous state (Hedges et al., 2000). Different fractions and pools of biochar will decompose at different rates under different conditions determined by method of production, feedstock material, as well as climate and soil properties. This makes the quantification of stability and degradation rates extremely important to the environmental and economic feasibility of biochar production. Direct measurements of stability on the timescale of decades or even a century is not possible leading to the development of laboratory based assessment tools for the rapid screening of fresh biochar (Hammes et al., 2007; Cross & Sohi, 2011, 2013; Harvey et al., 2012; Crombie et al., 2013).

After low temperature pyrolysis, biochar may contain an unconverted or partially converted biomass fraction, known as labile‐C, which is rapidly mineralized on addition to soil. The mineralization of labile‐C results in a small short‐term CO 2 flux (Zimmerman, 2010; Bruun et al., 2011; Calvelo Pereira et al., 2011; Cross & Sohi, 2011; Jones et al., 2011) and could be responsible for mineralization of other soil C, i.e. priming (Hamer et al., 2004; Cross & Sohi, 2011; Jones et al., 2011; Lehmann et al., 2011; Zimmerman et al., 2011) however labile‐C can also provide a readily available food source for soil microorganisms (Smith et al., 2010). However, this stimulated microbial activity occurs over a short time period (Cheng et al., 2006) with long incubation tests actually showing decreased or no mineralization of other soil C following biochar application (Kuzyakov et al., 2009; Spokas & Reicosky, 2009; Zimmerman, 2010; Cross & Sohi, 2011; Zimmerman et al., 2011). In many cases, the observed release of CO 2 from biochar takes place over a relatively short period of weeks or months before dissipating (Smith et al., 2010; Jones et al., 2011). However, the inconsistency in CO 2 evolution following the addition of biochar to soil could be a result of large variability in the nature of applied biochar (feedstock, temperature, heating rate, pre/posttreatment) as well as the conditions used during incubation studies (temperature, soil type, incubation time, atmosphere, pH) (Jones et al., 2011; Zimmerman et al., 2011) making conclusions on the positive or negative aspects of labile‐C difficult.

Many studies have reported the effectiveness of biochar in improving soil quality and crop production (Lehmann et al., 2006; Liang et al., 2006; Laird, 2008; Atkinson et al., 2010; Van Zwieten et al., 2010; Rajkovich et al., 2011; Ippolito et al., 2012; Spokas et al., 2012; Liu et al., 2013). The positive impact of biochar could be due to a range of potential reactions that remove soil‐related constraints otherwise limiting plant growth: soil nutrient status and soil pH, toxins, improved soil physical properties and improved N‐fertilizer use efficiency (Chan & Xu, 2009; Van Zwieten et al., 2010). As biochar is produced by thermal carbonization of biomass (virgin and nonvirgin), it often contains a high concentration of C, as well as varying amounts of plant macro nutrients [phosphorous (P), potassium (K), magnesium (Mg), calcium (Ca) etc.] and micro nutrients [iron (Fe), copper (Cu), sodium (Na), zinc (Zn), chlorine (Cl) etc.] (Chan & Xu, 2009; Lehmann et al., 2011). However the total concentration of nutrients within biochar is not necessarily an appropriate indicator of the content of bioavailable nutrients, as many can be bound in stable forms not readily available to plants (Chan & Xu, 2009; Spokas et al., 2012). Cation exchange capacity is the capacity of biochar to retain cations in a plant‐available and exchangeable form (e.g. nitrogen in the form of ammonium, NH 4 +). The CEC is relatively low at low (acidic) pH but increases at higher pH as well as generally being very low at low HTT with substantial improvement as temperature is increased (Lehmann, 2007). While freshly produced biochar demonstrates minimal CEC compared to SOM, biochar has shown the ability to increase its CEC upon addition to soil through abiotic and biotic oxidation and the adsorption of SOM onto its surface (Cheng et al., 2006; Liang et al., 2006; Lehmann, 2007). Increasing the CEC of biochar can result in reducing the leaching of nutrients (e.g. P, ammonium, nitrate, Mg and Ca) from soil, manure, slurry etc. thus increasing the potential availability of nutrients in the root zone for plant uptake and improved soil fertility (Glaser et al., 2001; Chan & Xu, 2009; Major et al., 2009; Clough & Condron, 2010; Angst et al., 2013). Furthermore by improving the sorption ability of biochar, the efficiency of fertilizer can be increased by absorbing it to the biochar thus improving its retention in the root zone for uptake by plants (Chan & Xu, 2009; Xu et al., 2013). Increasing the N‐fertilizer use efficiency can then lead to a reduction in fertilizer application rates, thus decreasing GHG emissions associated with fertilizer production, transport etc. (Major et al., 2009) as well as the direct release of GHG (Zhang et al., 2010). However, adding biochar to soil does not necessarily guarantee a related increase in the CEC of the soil. While some studies have shown a positive increase in soil pH and CEC following the incorporation of biochar into soil other studies have shown the opposite effect (Van Zwieten et al., 2010). There are relatively few studies on the nutrient composition of biochar and its importance to soil amendment (Atkinson et al., 2010; Rajkovich et al., 2011; Angst & Sohi, 2013; Xu et al., 2013; Zheng et al., 2013) and less concerning how production conditions can influence the nutrient content of biochar and their availability (Zheng et al., 2013).

This study therefore aims to establish relationships between production conditions and biochar functional properties related to its soil performance such as long‐term biochar stability, labile‐C concentration, pH, CEC as well as the nutrient retention. This should then improve the understanding of how selected production conditions impact the effectiveness of biochar for soil amendment while also identifying possible or impossible combinations of functional properties which ultimately determine any potential to maximize the environmental benefits of biochar while considering possible trade‐offs with other biochar benefits.