BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 112

could further lead to reduction in plant available nutrients. Hart and Mellbye (2010) for
example, showed that annual ryegrass growth and yield indicators did not improve by raising
soil pH above 5.3. Conversely, increase in productivity of legumes when biochar is applied in
acid soils (Jeffery et al., 2011) is likely to the fact that nodulation and N fixation by legumes
are very pH-sensitive (Zahran, 1999).
Evans et al. (1980) showed that N fixation does not occur at pH <4.8 and that nodulation
was about 10 times more sensitive to acidity than did rhizobia or root growth. As available
Al 3+ in acid soils inhibits both nod gene expression and rhizobia activity (Zahran, 1999 and
references therein), a marked increase in legume crop productivity should be expected in
response to biochar application due to its liming effect. This can likely explain the lack of
adequate response of peanut to NPK, lime and/or biochars as soil pH was below 5.5
throughout the study reported by Slovich et al. (2013). The positive relationship between
aboveground growth and pyrolysis temperature were hypothesized to relate to reduction in
volatile and other organic compounds at high pyrolysis temperatures that can otherwise limit
plant growth (Biederman & Harpole, 2012). Yet, the positive effect of biochar pH and
pyrolysis temperature on plant growth is likely to occur due to the positive trajectory and
correlation of these characteristics with biochar ash, alkalinity, and liming content (neither of
which was included in the meta-analysis), especially as increase in pyrolysis temperature
adversely affect biochar nutrient availability.
The effect of biochar on yield in soils of near or above natural pH is minor and
inconsistent (Van Zwieten et al., 2010; Lentz & Ippolito, 2012; Schnell et al., 2012; Quilliam
et al., 2012; Alburquerque et al., 2013; O'Toole et al., 2013). While improving wheat grain
yield and aboveground biomass, use of wheat straw or olive-tree pruning biochar at rates of
up to 2.5% (wt/wt) to a loamy sand soil (pH 6.5) did not improved wheat yield parameters as
compared to the addition of mineral fertilizer alone (Alburquerque et al., 2013). Moreover,
while improving soil NH 4 + and resin PO 4 3- , and plant P content, biochar application
negatively affected plant N and micronutrient (Fe, Mn, Cu) content, likely due to a low N
mineralization rate and reduction in nutrient availability as soil pH increased to 8.2 and 7.6 in
the high application levels of olive-tree pruning and wheat straw biochar, respectively
(Alburquerque et al., 2013). Similarly, wheat straw biochar applied to a sandy loam soil (pH
6.8) resulted in no or negative effect on perennial ryegrass biomass and led to reduction in
foliar N, Ca, and Mg content (O'Toole et al., 2013).While increasing nitrogenase activity,
wood biochar (450 °C for 48 h) applied at rates of up to 50 Mg ha -1 to a soil with neutral pH
showed no significant positive effect on white clover ( Trifolium repens ) root or shoot biomass
and nodulation (Quilliam et al., 2013b). In the above study by Van Zwieten et al. (2010),
application of biochar in combination with fertilizer had mixed effects, negatively effecting
wheat and radish yield grown in calcareous soil (pH 7.7). In absence of fertilizer application,
biochar had no effect on soybean and wheat yield, while a positive effect on radish yield was
noted in response to application of biochar with a higher K content (0.2 vs 1.0 cmol K kg -1 )
and slightly lower pH and CCE (9.4 vs. 8.2, and 33 vs. 29; Van Zwieten et al., 2010).
It is apparent that different feedstock and pyrolysis conditions result in biochar of
different properties and characteristics. Consistent and detailed information about the product,
its feedstock, and production process will benefit the end users in their decision making
process of the proper biochar for amending soils deficiencies and meeting crop requirements.