BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 103

soluble P and NH 4 + with increase in pyrolysis temperature from 300 to 600 °C; yet the
authors reported increase in soluble K with increase in peak pyrolysis temperature.
The fact that nutrient availability decreased with increase in peak pyrolysis temperature
implies that pyrolysis can favorably be used to mitigate elevated excess nutrient content in
organic waste (e.g. chicken manure). Waste volume and pathogen reduction accompanied by
entrapment of the manure-borne excess nutrients into less mobile phases should be viewed
favorably as it lessen the adverse environmental impact associated with manure application
while still maintaining high levels of available nutrients (Lima & Marshall, 2005; Gaskin et
al., 2008; Lima et al., 2009; Hass et al., 2012). Gaskin et al. (2008) evaluated nutrient content
and availability in biochar produced from chicken litter, peanut hulls, and pine chips
feedstock at two different temperatures, 400 and 500 °C. The increase in temperature
increased the nutrient content; however, it decreased their availability for plant uptake, as
measured by the Mehlich-1 extraction. Nevertheless, while Mehlich-1 available P and K
decreased with temperature in all biochars, their concentration in poultry litter biochars (5.33
and 38.1 g kg -1 , respectively) were much higher than biochar from peanut hulls (0.57 and 5.91
g kg -1 , respectively) or pine chips (0.04 and 0.41 g kg -1 , respectively). Hass et al. (2012)
showed a reduction in soil Mehlich-3 available K, P, and, S and increase in Cu and Zn with
increase in production temperature of chicken manure biochar (350 vs. 700 °C) when
incubated for eight weeks in a Typic Hapludult soil (pH 4.8). Yet, nutrient availability
increased with biochar application, and to alarming levels at high application rates (equivalent
to > 20 Mg ha -1 ), as high levels of total P and PO 4 2- appeared in the leachate (above 4 and 2
mg L -1 , respectively). Moreover, Hass et al. (2012) showed that at application rate of 10 Mg
biochar ha -1 , Mehlich-3 extractable P reached levels (>200 mg kg -1 ) considered critical
threshold value above which excess P can be expected in runoff (>1 mg L -1 ; Sharpley et al.,
1996).
Biochar liming capacity is an influential property affecting soil fertility (Verheijen et al.,
2009). While most biochar have high pH (7 - 11), their pH value do not reflect their acid
neutralization capacity or ‗liming capacity' - the ability to increase soil pH, usually expressed
as percent calcium carbonate equivalent (CCE). Biochar CCE is limited mostly to the mineral
or ash content and composition. For a given feedstock, biochar CCE increases with pyrolysis
temperature due to the increase in ash and metal content and due to transformation of alkali
(K and Na) and alkaline earth (Ca and Mg) metals into oxide and carbonate minerals such as
K 2 O, CaO, Ca/MgCO 3 and /or MgO. Yuan et al. (2011a) showed an increase in biochar pH
and alkalinity with pyrolysis temperature (300 to 700 °C) in biochars from canola ( Brassica
campestris L. ), corn, soybean ( Glycine max L. ), and peanut straws. The authors suggested that
the CaCO 3 generated at the higher temperatures (500 and 700 °C) is the dominant component
contributing to biochar alkalinity at these temperatures, while biochar acidic functional
groups, such as hydroxyl and carboxyl, contribute to biochar alkalinity at lower temperature
(300 °C). Conversely, Wu et al. (2012) showed an increase in CaCO 3 content in rice straw
biochar produced at low temperatures (300 to 500 °C) and a decrease at higher pyrolysis
temperatures (600 to 700 °C). However, biochar ash content and alkalinity tend to increase
with pyrolysis temperature (Yuan et al., 2011a; Yuan et al., 2011b; Wu et al., 2012).
While ash content might be an indicator of biochar CCE, the actual contribution of the
ash depend on its elemental and mineral composition, as different plants result in different
nutrient composition. For example, woody biomass generally has very low Si content (0.05 to
0.11); oak ( Quercus spp ) accumulates more Ca and K than pine ( Pinus spp ) (Ragland et al.,