1.5 Biochar addition changes nitrogen processes in cultivated system
1.5.1 Effects of biochar addition on soil nitrogen content and transformations
Biochar’s porosity and the presence of both polar and nonpolar surface sites suggest it is possible to adsorb nutrients ions (Cao and Harris, 2010; Laird et al., 2010; Hale et al., 2013).
In addition, biochar with a high TC/TN ratio has a greater potential for adsorption of nitrogen ions from soil to biochar inducing sustained higher nitrogen fertility in surface soils (Steiner et al., 2007). Many studies using biochar as a soil amendment to control nitrogen leaching loss showed that biochar can improve soil nitrogen retention by adsorbing ammonium, ammonia and nitrate nitrogen (Cao and Harris, 2010; Ding et al., 2010; Laird et al., 2010; Spokas et al., 2012; Yao et al., 2012) and then still maintain bioavailability until subsequently utilized by the plant (Taghizadeh-Toosi et al., 2012). However, direct adsorption of ammonium and nitrate nitrogen biochar has not been extensively studied. Most data referred to here are calculated with the reductions of ammonium and nitrate nitrogen in leachate or solution.
Some studies estimate biochar’s maximum adsorptive rate of ammonium nitrogen by kinetic models resulting in 0.85-3.24 mg N/g (Ding et al., 2010; Zhu et al., 2012). The maximum ammonium nitrogen adsorption from slurry at 1400 mg N/L can reach to 44.64 mg/g and 39.8 mg/g for wood and rice husk biochar (Kizito et al., 2015), which was attributed to the
(a) Regular biochar (b) SEM micrograph of wheat
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adsorptive capacity of biochar’s superior BET surface area and presence of dissolved matter in the slurry (Saltalı et al., 2007). These results are much higher than those calculated in leaching experiments. Yao et al. (2012) examined biochar produced using different raw materials and pyrolysis temperatures and found biochar could remove ammonium nitrogen from solution with adsorptive rates ranging from 0.05 to 0.19 mg N/g and nitrate nitrogen with adsorptive rate ranging from 0.02 to 0.64 mg N/g. Dempster et al. (2012b) found that biochar would adsorb both ammonium nitrogen and nitrate nitrogen when added into an ammonium nitrate solution, with the adsorptive rate of ammonium nitrogen ranging from 0.02 to 0.25 mg N/g and the adsorptive rate of nitrate nitrogen ranging from 0.02 to 0.19 mg N/g. Compared to ammonium adsorption, nitrate adsorption of biochar has been reported to be insignificant in most studies(Lehmann et al., 2003; Jones et al., 2010; Hale et al., 2013).
Adsorptive rates of ammonium nitrogen and nitrate nitrogen vary widely with biochar’s characteristics and the nitrogen contents of solutions, but there is a significant relationship that adsorptive rates of both ammonium nitrogen and nitrate nitrogen increase with increased nitrogen content of solution. Biochar produced using a high pyrolysis temperature has the potential for higher nitrate nitrogen absorption, because absorption of nitrate nitrogen is a result of base functional groups on biochar (Kameyama et al., 2012). Different from nitrate nitrogen, the reason generally given for the adsorption of ammonium nitrogen is cation exchange capacity (Cheng et al., 2008) and pore structures (Amonette and Joseph, 2009; Saleh et al., 2012) of biochar.
1.5.1.2 Mineralization, immobilization and nitrification
Nitrogen mineralization and immobilization are transformation processes occurring at the same time and are affected by TC/TN ratios in both the soil and biochar. The result of these two transformation processes determines the mineral nitrogen content in soil. Typically as TC/TN ratio increases, nitrogen immobilization would be enhanced. With respect to the high TC/TN ratio in biochar, it may be speculated that nitrogen immobilization would be enhanced after biochar was added to soil. However, the effects of biochar addition on soil nitrogen are quite complicated. Various results have been reported that biochar addition increased nitrogen
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mineralization (Castaldi et al., 2011; Bruun et al., 2012), decreased nitrogen mineralization (Dempster et al., 2012a), or had no effect on mineralization (Streubel et al., 2011). Two potential mechanisms presented below may be helpful to explain the effects of biochar on soil nitrogen mineralization. (1)The amount of labile carbon content in biochar may result in microbial-available soil nitrogen becoming immobilized (Bruun et al., 2012) and (2) the addition of biochar will stimulate soil nitrogen mineralization by mineralizing the nitrogen coming from a more recalcitrant fraction (Nelissen et al., 2012).
Nitrification represents the oxidation of organic nitrogen (via heterotrophic organisms) or ammonium nitrogen to nitrate nitrogen by autotrophic bacteria and archaea (Leininger et al., 2006; Jia and Conrad, 2009). Nitrification is the first and rate-limiting step in nitrogen cycling.
Biochar addition to forest soils has been found to increase nitrification, whereas the effects in agricultural systems are inconsistent. Biochar was added to forest soils and it was found that promotion of soil nitrification was attributed to the alleviation of factors limiting nitrification by biochar addition instead of a result of limited substrate increasing (DeLuca et al., 2002;
Berglund et al., 2004). A short-term increase in nitrification suggests that biochar can adsorb some organic compounds which inhibit nitrification or cause immobilization of ammonium nitrogen (DeLuca et al., 2006). The long-term effect of enhancing nitrification is found due to biochar addition after a forest fire, which has been ascribed to the promotion of soil conditions resulting in an increase in the abundance of ammonia-oxidizing bacteria (Ball et al., 2010).
Different from forest soils, most soil in the agricultural system already contains high nitrate nitrogen and exhibits inherently high rates of nitrification before biochar addition. Some studies suggest that the content of ammonium nitrogen may be reduced by the absorption of biochar and thus the nitrification process may be affected because of limited substrate (Steiner et al., 2007; Spokas et al., 2012). However, in contrast to this result it has been reported that a significant increase in nitrification rates with the addition of biochar and the increase in nitrification is attributed to a greater substrate for autotrophic nitrifying bacteria (Nelissen et al., 2012). Similarly, experiments using coastal alkaline soils show that biochar addition can enhance the abundance of both nitrifiers and alter the composition of ammonia oxidizers by
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increasing the diversity of ammonia-oxidizing bacteria (Song et al., 2014). Improvement of nitrification may be partly explained by the change in soil chemical properties induced by biochar addition. Ulyett et al. (2014) found that biochar addition enhances nitrification with pH increase in the condition with nitrogen fertilizer application.