BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 107

exchangeable ammonium (using 2 M KCl extractant) when a sandy soil was incubated with
biochar and fertilizer for 6 weeks.
Sorption of nitrate is expected to be of lesser extent than ammonium since the biochar's
anion exchange capacity is smaller than the cation exchange capacity (Figure 4). Studies had
demonstrated that nitrate sorption occurs on biochars produced at ≥450 ºC, e.g. (1) pine wood
produced at 450 ºC (Sika & Hardie, 2013), (2) Brazilian pepperwood, (3) peanut hull
produced at 600 ºC (Yao, et al., 2012), and (4) bagasse sugarcane produced at ≥700 ºC
(Kameyama, et al., 2012). However, nitrate did not sorb on biochars from bagasse sugarcane
and bamboo produced at 600 ºC (Yao, et al., 2012), cacao shell and corn cobs produced from
300 to 550 ºC (Hale, et al., 2013). The above discrepancy on nitrate sorption by biochar
bagasse sugarcane might be due to the different conditions that the sorption isotherms were
conducted. Biochars were rinsed with DI water before use to remove impurities and the
sorption equilibrium was set for 20 h (Yao, et al., 2012) or biochars used without rinsing and
the sorption equilibrium was set at 120 h (Kameyama, et al., 2012). It is hypothesized that
base functional groups of biochar produced at high temperatures play an important role on
nitrate sorption on biochars (Kameyama, et al., 2012). Although nitrate is weakly sorbed on
biochar, nitrate residence's time in soils might increase in biochar amended soils as compared
to soils without biochar; thus, nitrate might be available for plant uptake (Kameyama, et al.,
2012; Sika & Hardie, 2013) while reducing nitrate leaching (Kameyama, et al., 2012; Sika &
Hardie, 2013; Singh, et al., 2010; Yao, et al., 2012).
Soil organic N transformations and microbial community are influenced by biochar
additions to soils. Biochar produced at lower temperatures contains more labile C (Figures 2
and 4) and N than one produced in higher temperatures, hence, acting in part as substrate.
Similarly, biochar C/N ratio and liming capacity (Figure 4) are expected to increase with
increase in biochar peak production temperature and to affect soil microbial community
structure and function. Ameloot et al. (2013) observed that the addition of poultry litter or
pine chips biochars produced at 400 and 500 ºC to acid soils (pH KCl 4.6, 5.3) increased the
soil bacteria:fungi ratio (B:F ratio), with biochars produced at 500 ºC shifting soil B:F to
higher values than those produced at 400 ºC. The additions of biochar from silage corn
induced mineralization of the soil recalcitrant N to ammonium (350 ºC biochar >550 ºC
biochar >control), but no differences were observed on the mineralization of the labile
organic N, suggesting that biochar additions promote SOM turnover, a priming effect induced
by biochar (Nelissen et al., 2012). Furthermore, the addition of biochar of lower C/N ratio
(poultry litter) resulted in net N mineralization while addition of the higher C/N ratio pine
chips biochar resulted in net N immobilization (Ameloot et al., 2013). While filamentous
fungi dominate aerobic mesophilic flora in soils of pH below 5.5, bacterial population
increases with increase in soil pH and available N, with optimal activity of decomposer
community occurring at natural pH, i.e. from 6.5 to 7.2 values (Alexander, 1964). The effects
of pH, substrate and N availability on microbial community and B:F ratio are also well
documented in non-biochar related studies conducted at different ecosystems and land uses
(Bardgett et al., 1996; Blagodatskaya & Anderson, 1998; Baath & Anderson, 2003; De Vries
et al., 2006; Rousk et al., 2009). Hence, reduction in fungi and increase in bacteria and an
increase in overall soil microbial activity with increase in N availability as shown by Ameloot
et al. (2013) is an expected outcome (Alexander, 1964), especially in acid soils where biochar
application results in increase in soil pH towards, but not exceeding, natural pH values.