Acid hydrolysis to define a biologically-resistant pool is compromised by carbon loss and transformation

Biologically-resistant carbon (C) comprises the bulk of C in most soils and is often estimated from chemical separation of a soil organic matter (SOM) fraction that is not hydrolysed by strong acid, and exhibits an older radiocarbon age and slower turnover than the whole SOM. Here we examine the effects of acid hydrolysis - the method used to separate the nonhydrolysable C - on the quantity, structure and isotopic makeup of C in pure model carbohydrates and those contained in senescent maize that resemble C inputs to SOM. We demonstrate that significant alterations occur during hydrolysis resulting in C mass losses (up to 75% preferential loss of ¹³C) and de novo synthesis of nonhydrolysable, ¹³C-depleted material dominated by aromatic>alkyl>carbonyl moieties. We infer that similar losses, and transformations of ¹⁴C, would partly explain the greater ages attributed to the chemically resistant C, seriously impairing the use of acid hydrolysis to identify this pool of SOM and that de novo synthesis during hydrolysis has obfuscated the true chemical nature of the nonhydrolysable fraction of SOM. •Biologically-resistant C in soil is usually estimated by acid hydrolysis of SOM.•We examined the effect of acid hydrolysis on C in simple and complex carbohydrates.•Alterations occur during hydrolysis resulting in significant C loss and transformation.•These transformations impair the use of acid hydrolysis to identify resistant SOM.