<scp> ¹³ C </scp> methodologies for quantifying biochar stability in soil: A critique

Methodologies based on 13C-enrichment (E), 13C-depletion (D) and 13C-natural abundance (NA) to estimate the stability of biochar in soil were critically examined. The stability of 13C-enriched biochar can be estimated by the quantitative recovery of excess 13C, either in the soil or in evolved CO2. Both approaches have advantages and disadvantages. Recovery in the soil is a measure of both residual biochar 13C + 13C immobilised in soil organic matter during biochar decomposition. Variable proportions of organic- and inorganic-C are present in alkaline biochars, and few data exist on the uniformity of labelling, which is a basic requirement of the respired 13CO2 and E methodology. The E technique has had limited application due to the cost and difficulty of obtaining a uniformly-enriched feedstock through continuous labelling of plants with 13CO2 at a constant 13C enrichment. In contrast, the NA technique has been widely applied. The NA and D techniques are in situ methods that involve the addition of C4-derived biochar to a C3-soil or vice versa. Stability is estimated by a two-end-member mixing model that allows the proportion of evolved CO2 derived from the biochar (Cdfb) to be estimated. The mixing model has recently been misused to estimate the Cdfb of 13C-enriched biochar, with 13C-abundance expressed as erroneously large δ values. 13C-based methods provide a yardstick against which rapid stability tests should be evaluated. While numerous laboratory incubation comparisons have been conducted, very few field-based data have been published. Highlights 13C methods for estimating biochar stability are based on uniform isotopic labelling. Organic and inorganic constituents of biochar may not be uniformly labelled. Expression of 13C enriched biochar as large δ values (>500 units) rather than atom fraction excess led to larger errors in stability estimation. Few 13C field-based estimates of biochar stability exist.