New organic matter decomposition test method as an indicator for soil health

Soil microbial processes are critical to soil health and provide essential ecosystem services, such as nutrient cycling. For carbon cycling, microorganisms are constantly active in organic matter decomposition (OMD) and disruption to this process can indicate an impairment to the ecosystem. Therefore, methods to assess changes to OMD are critical. However, no standardized laboratory methods have been available to measure OMD in a consistent manner. We adapted a field OMD test to a laboratory environment, where OMD is measured by evaluating the percentage mass loss of cellulose filter paper, which acts as an organic matter substrate within test soil, based on the in situ microbial community activity. The test can be used with various soils and is adaptable to both chemically-spiked soils for dose-response analysis but also contaminated and reference field-collected soils. Analyzing 8 different datasets enabled us to establish validity criterion such that filter papers in the control soil must be ≥30 % degraded. To ensure reproducibility, a ring test was conducted using a series of tests; the coefficient of variation across laboratories ranged from 18.9 to 29.7 %. When evaluating a dose-response design, the mean effective concentration (EC50) of AgNO₃ was 65.4 ± 12.0 (n = 11) mg/kg in the ring test, with a CV of 18.4 %. Here, we present the development and validation procedure for the laboratory-based OMD test method, providing the basis for a standardized method.