BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 106

soil effective CEC, %C, C/N ratio, water retention, and plant available water (Laird et al.,
2010a, 2010b). At high application rates (>5 g kg -1 soil), biochar also increased soil surface
area and Mehlich-3 extractable P (Laird et al., 2010a). Similarly, positive effects of biochar
application on soil quality indicators were reported by Novak et al. (2009b) for a Typic
Kandiudults loamy sand coastal plain soil from Eastern US. Application of pecan [ Carya
illinoinensis (Wangenh.) K. Koch] shell biochar (up to 20 g kg -1 ) increased soil pH and
reduced exchangeable acidity, but had no effect on soil CEC. In addition, the biochar
application reduced S and Zn availability, while increasing availability of other plant essential
nutrients, such as Ca, K, Mn, and P as determined by the Mehlich-1 extraction. The reduction
in soil exchangeable acidity was attributed to dissolution of alkaline earth metal oxides in the
biochar ash (e.g. CaO), leading to speciation of soluble Al into nontoxic species for plants or
precipitation as aluminum oxide (Novak et al., 2009a, 2009b).
Biochar affects soil nitrogen dynamics through sorption and chemical transformations of
both inorganic and organic forms of N; these processes are biochar- and N species-dependent.
Sorption of the plant nutrients ammonium (cationic form) and nitrate (anionic form) will
greatly depend on the biochar chemical and physical properties, e.g. cation exchange
capacity, anion exchange capacity, surface area, and pH, which all depend on the pyrolysis
temperature (Figure 4). Ammonium likely sorbs on biochars by electrostatic interactions since
most biochars possess negative charges as suggested by the biochar cation exchange capacity.
In general, ammonium is sorbed in biochars produced at temperatures ≥300 ºC, as
demonstrated by sorption isotherms of ammonium on biochars from cacao ( Theobroma
cacao ) shell and corn cobs at 300-350 ºC (Hale, et al., 2013), pine wood at 450 ºC (Sika &
Hardie, 2013), Brazilian pepperwood ( Schinus terebinthifolius ) at 300, 450, and 600 ºC,
sugarcane bagasse ( Saccharum officinarum ), peanut hull, and bamboo ( Bambuseae spp )
produced at 300 and 600 ºC (Yao, et al., 2012). Oddly, biochars produced at 450 ºC from
sugarcane bagasse, peanut hull, and bamboo did not sorb ammonium (Yao, et al., 2012). As
biochar properties change with time due to biogeochemical processes, e.g. redox reactions,
and microbial colonization; sorption of ammonium is also affected, e.g. Singh et al. (2010)
observed that 4 months after applying biochar to soils, the leaching of ammonium decreased,
attributed to the increase of biochar sorptivity as biochar surface oxidizes with aging. Another
factor affecting ammonium sorption on biochar is pH since aqueous NH 4 + and gas NH 3 exist
in equilibrium (pK a of NH 3 is 9.2 at 20 ºC); as pH increases from neutral to alkaline, gas NH 3
starts forming while aqueous NH 4 + concentration decreases (Bates & Pinching, 1950). Zhang
et al. (2013) demonstrated that the efficacy of corn biochar (produced at 600 ºC) for
ammonium sorption decreased about 35% as the pH solution increased from 4 to 10; perhaps
due to the conversion of aqueous NH 4 + to the gas NH 3 . Taghizadeh-Toosi et al. (2012)
observed that 15 N labeled NH 3 sorbed on Monterey pine ( Pinus radiata ) biochars (300 to 500
ºC, pH H2O 5.2 to 7.8 ) and that the sorbed NH 3 was available for plant uptake as demonstrated
when perennial ryegrass was grown in a Temuka silt loam soil (Endoaquept) amended with
enriched 15 N labeled NH 3 biochars; NH 3 sorption is attributed to the carboxyl acid functional
groups (Asada, et al., 2002; Kastner, et al., 2009) and low pH of biochars (Kastner, et al.,
2009). Acidified biochar from pine chips (slow pyrolysis at 400 ºC), when applied to an
amended soil with poultry litter, reduced NH 3 losses by 59% (Doydora, et al., 2011).
However, Sika and Hardie (2013) observed that the sorbed ammonium on pine wood biochar
was not readily available for plant uptake. The authors found only small amounts of