09. The fate of biochar in soil

What will happen to the biochar that is added to soils?

Once biochar has been added to the soil, it will change over time. Some biochars will be more stable than others, depending on the production process and feedstock. Environmental conditions vary (for example temperature) which also alters how stable the biochar will be in the soil.

Biochar in the soil may:

Move away from the site: over time, physical processes may lead to biochar moving for example flood events and changing water regimes. Erosion of char vertically has been measured in a Kenyan experiment as slow – 30cm per 1000 years. Char which moves from the site is ultimately likely to end up in marine sediments and be stored there in the long term.

Physically change: There is thought to be little risk that biochar in the soil be combusted during fire events. Some types of biochar are at risk from accidental ignition prior to incorporation, so care needs to be taken during transportation. Biochar particle size is likely to be reduced by the action of ploughing in agricultural areas, or by natural processes, including temperature changes (especially freeze thaw cycles). The soil type will also influence the stability of biochar.

Chemically degrade: Chemical reactions may be influenced by the presence of inorganic (abiotic) and organic (biotic) catalysts – for example the presence of degraders. Mineralisation and oxidation will occur, which will lead to decreased carbon content, increasing oxygen content and the evolution of functional groups. A labile fraction of carbon in the biochar will be lost in the soil, as will any ash which is also formed along with the biochar. Agronomic benefits related to these fractions are therefore likely to be short lived (see agribenefits from biochar).

Age: The effectiveness as a soil amendment for example may be compromised by pore blocking. the micro / macro pores can become fouled (this is also important where biochar is used as a remediation device), by the presence of organic matter.

Where carbon finance is an objective, the stability of carbon can be measured by the following methods (adapted from UKBRC http://www.biochar.org.uk):

Carbon Stability Factor (CSF): The proportion of the total carbon in freshly produced biochar which remains fixed as recalcitrant carbon over a defined time period (10 years, 100 years, etc. as defined). A CSF of 0.75 means that 75% of the carbon in the fresh biochar remains as fixed carbon over the defined time horizon, and that 25% of the carbon has been converted into CO2.

Mean residence time (MRT): is the inverse of the decay rate (1/k) and is the average time that biochar is present

Half life: is the time that elapses before half of the biochar decomposes and can be obtained by multiplying the mean residence time by the natural logarithm of 2.

Presentations by Sarah Hale & Gerard Cornielleson – Workshop: Biochar Malaysia. University of Kuala Lumpur. 25 March 2010

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