BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 102

(Wang et al., 2010; Reprinted with permission from Energy and Fuels. Copyright 2013, American
Chemical Society).
Figure 3. Emission of nitrogen species during pyrolysis of wheat straw.
These changes in N dynamics occur as consequence of both (1) the relative increases in
their proportion as decomposition of hydrolysable N progress with temperature and (2) the
synthesis of aromatic and heterocyclic N from aliphatic or low-molecular weight heterocyclic
structures during the pyrolysis process (Almendros et al., 2003). Overall, increase in pyrolysis
temperature decreases feedstock N recovery in biochar and reduces the availability of the
remaining nitrogen. Noteworthy, the content of N and other elements in biochar increases in
the process at lower temperature due to preferential loss of O and C. Overall, decrease in N
due to volatilization occurs at higher temperatures (ca. above 500 °C) as reported by others
(Tsai et al 2012; Wu et al., 2012). Inasmuch as heterocyclic N is suggested to be plant
available, being trapped in a not-readily available carbonaceous structure is likely to limit its
availability for plant uptake (Wang et al., 2012).
Sulfur is released as carbonyl sulfide (COS) and hydrogen sulfide (H 2 S) at 200 - 400 °C,
which is attributed to the decomposition of organically-bound sulfur, while release of SO 2 at
temperatures above 950 °C is attributed to evaporation and/or transformation of inorganic
sulfate (Wang et al., 2010). Negligible potassium loss at temperatures below 600 °C was
reported in pyrolysis of wheat straw (Wang et al., 2010). Yu et al. (2005; in Chan & Xu,
2009) reported a sharp loss of nearly 50% in K during pyrolysis of rice ( Oryza sativa ) straw
at temperature between 473 to 673 °C, attributed mostly to the loss of the water soluble K
fraction. Inasmuch as other nutrients such as P tend to concentrate in the biochar with
increase in peak pyrolysis temperature, they also tend to shift into chemical forms that are less
available for plant uptake, e.g. apatite-type minerals (Chan & Xu, 2009; Gaskin et al., 2008).
Using giant reed ( Arundo donax L. ), Zheng et al. (2013) also showed a decrease in water