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

Stability and transformation products of incomplete combustion of vegetation or fossil fuel, frequently called pyrogenic or black carbon and of biochar in soil, remains unknown mainly because of their high recalcitrance compared to other natural substances. Therefore, direct estimations of biochar decomposition and transformations are difficult because 1) changes are too small for any relevant experimental period and 2) due to methodological constraints (ambiguity of the origin of investigated compounds). We used ¹⁴C-labeled biochar to trace its decomposition to CO₂ during 8.5 years and transformation of its chemical compounds: neutral lipids, glycolipids, phospholipids, polysaccharides and benzenepolycarboxylic acids (BPCA). ¹⁴C-labeled biochar was produced by charring ¹⁴C-labeled Lolium residues. We incubated the ¹⁴C-labeled biochar in a Haplic Luvisol and in loess for 8.5 years under controlled conditions. In total only about 6% of initially added biochar were mineralized to CO₂ during the 8.5 years. This is probably the slowest decomposition obtained experimentally for any natural organic compound. The biochar decomposition rates estimated by ¹⁴CO₂ efflux between the 5th and 8th years were of 7 × 10^‑4 % per day. This corresponds to less than 0.3% per year under optimal conditions and is about 2.5 times slower as reported from the previous shorter study (3.5 years). After 3.5 years of incubation, we analyzed ¹⁴C in dissolved organic matter, microbial biomass, and sequentially extracted neutral lipids, glycolipids, phospholipids, polysaccharides and BPCA. Biochar-derived C (¹⁴C) in microbial biomass ranged between 0.3 and 0.95% of the ¹⁴C input. Biochar-derived C in all lipid fractions was less than 1%. Over 3.5 years, glycolipids and phospholipids were decomposed 1.6 times faster (23% of their initial content per year) compared to neutral lipids (15% year^‑1). Polysaccharides contributed ca. 17% of the ¹⁴C activity in biochar. The highest portion of ¹⁴C in the initial biochar (87%) was in BPCA decreasing only 7% over 3.5 years. Condensed aromatic moieties were the most stable fraction compared to all other biochar compounds and the high portion of BPCA in biochar explains its very high stability and its contribution to long-term C sequestration in soil. Our new approach for analysis of biochar stability combines ¹⁴C-labeled biochar with ¹⁴C determination in chemical fractions allowed tracing of transformation products not only in released CO₂ and in microbial biomass, but also evaluation of decomposition of various biochar compounds with different chemical properties.