There is great research inter­est in carbon © bound by pyrol­y­sis (car­bon­i­sa­tion), due pri­mar­i­ly to the poten­tial role of biochar (charred biomass) as a long-term carbon sink in soils and sed­i­ments. This is because biochar resists micro­bial decom­po­si­tion and chem­i­cal trans­for­ma­tions in the soil for a com­par­a­tive­ly long time. In their ground-break­ing study, the authors Kuzyakov, Bogo­molo­va and Glaser dealt with this sta­bil­i­ty of biochar in soils. For this purpose, 14C-labelled (radioac­tive­ly labelled) biochar was used to monitor its decom­po­si­tion to CO2 over 8.5 years and the con­ver­sion of its chem­i­cal com­pounds: neutral lipids, gly­col­ipids, phos­pho­lipids, poly­sac­cha­rides and benzene-poly­car­boxylic acids (BPCA, aro­mat­ic com­pound).

Extreme­ly slow decom­po­si­tion of biochar

1414C-labelled biochar was pro­duced by char­ring 14C-labelled rye­grass (Lolium) residues. The result­ing biochar was then mixed with soil or loess. After 3.2 years of incu­ba­tion, the first mea­sure­ments showed an extreme­ly slow 14CO2 release of about 0.5% C per year. The authors there­fore esti­mate the average res­i­dence time of biochar to be around 2,000 years, and even 4,000 years for soils in tem­per­ate cli­mates. Accord­ing to Kuzyakov, Bogo­molo­va and Glaser, this was the first exper­i­men­tal proof of very slow degra­da­tion rates of biochar.
In view of the very slow degra­da­tion rate of biochar and its con­tin­u­ous decline, incu­ba­tion was extend­ed by a further 5 years. In total, only about 6% of the orig­i­nal­ly added biochar was min­er­al­ized (released) to CO2 in the 8.5 years. Most of this was released in the first 2 years. Accord­ing to the authors of the study, this is prob­a­bly the slowest decom­po­si­tion exper­i­men­tal­ly obtained for any natural organic com­pound. The average degra­da­tion rate for biochar was thus less than 0.3% per year and is thus about 2.5 times slower, as report­ed in a shorter study (max. 3.5 years).
The highest pro­por­tion of 14C in the orig­i­nal biochar was bound in the BPCA (87%), which decreased by only 7% over 3.5 years. Con­densed aro­mat­ic units were thus the most stable frac­tion in com­par­i­son to all other plant carbon com­pounds. Accord­ing to the authors, the high pro­por­tion of BPCA in plant coal explains its very high sta­bil­i­ty and thus its impor­tant con­tri­bu­tion to long-term C seques­tra­tion in soil.

Eco­nom­ic benefit from stable CO2-binding

The use of 14C-labelled biochar opens up a new way to track not only the plant char­coal itself, but also its trans­for­ma­tion prod­ucts. The C derived from biochar can be detect­ed in microor­gan­isms, dis­solved organic sub­stances and organic sub­stance frac­tions – even after very slow con­ver­sion and degra­da­tion process­es, since 14C analy­sis is highly sen­si­tive and spe­cif­ic.
The results also showed a low micro­bial avail­abil­i­ty of plant coal and con­firmed the assump­tion that plant coal is of neg­li­gi­ble impor­tance as a C source for microor­gan­isms. The researchers were able to demon­strate that carbon from biochar is used less inten­sive­ly by microor­gan­isms than carbon from organic soil sub­stances (dead plant and animal sub­stances). Accord­ing to the authors, this is also an indi­rect con­fir­ma­tion of a very low micro­bial avail­abil­i­ty of biochar. They assume that mainly lipids and poly­sac­cha­rides from biochar were incor­po­rat­ed into microor­gan­isms. More than 80% of gly­col­ipids and phos­pho­lipids were decom­posed within the first 3.5 years.

Orig­i­nal article: Biochar sta­bil­i­ty in soil. Decom­po­si­tion during eight years and trans­for­ma­tion as assessed by com­pound-spe­cif­ic 14C analy­sis

Author: Yakov Kuzyakov, Irina Bogo­molo­va, Bruno Glaser

Pub­lished in: Soil Biology & Bio­chem­istry 70 (2014), Else­vi­er, p. 229–236