Sources of Variability that Compromise Mineralizable Carbon as a Soil Health Indicator

Core Ideas Inter‐laboratory variability for mineralizable C is greater than for other commercial soil tests. Water content and direction of rewetting both affect values of mineralizable C. As a soil health indicator, mineralizable C should have a standardized protocol. Analytical variability of mineralizable C is highly affected by soil type. Mineralizable C, or C that is respired upon the rewetting of dried soil, is a common metric of soil health, but the metric still lacks a widely accepted and standardized protocol. A standardized protocol is an essential first step in quality control needed for a robust soil test. Here we examined numerous sources of laboratory variability associated with mineralizable C, with the overall goal of understanding the influence of each source on final values. Mineralizable C had twofold to 20‐fold greater inter‐laboratory variability than other commonly used soil tests, leading to a high degree of uncertainty associated with the interpretation of results. Procedural differences—such as sieve size and the method of rewetting—significantly influenced measurements of mineralizable C and underscore the need for the development of a standardized and universally adopted protocol. Capillary rewetting consistently suppressed mineralizable C relative to rewetting with a specific amount of water and is therefore not a recommended approach. However, the sensitivity of mineralizable C to changes in management did not differ among incubation intervals of 6, 24, and 72 h. While these procedural effects may influence inter‐laboratory variability, there was also a considerable amount of analytical variability associated with mineralizable C measurements within a laboratory that is highly dependent on soil type.