A comparison of methods to determine the biodegradable dissolved organic carbon from different terrestrial sources

The importance of dissolved organic matter (DOM) in many soil processes is determined in large part by its availability to microbial uptake and decomposition, as this biodegradation can yield both energy and limiting nutrients. Despite its importance in soil ecology, there are no standard approaches to measuring the biodegradable fraction of DOC (BDOC) in soils. Here, we evaluate the comparability and reproducibility of methods employed in six laboratories including batch, kinetic, and bioreactor methods. Solutions from a variety of sources (throughfall, soil solution and soil extracts) were analysed using methods typically employed in each of the six participating laboratories. Our results show that the precision of various BDOC methods was similar (5–15%) across a broad range of BDOC (from 12% to 56% of total DOC). Differences in mean BDOC for the various test solutions were statistically significant when results were pooled across all the methods, and only a 90-day incubation resulted in consistently higher values for BDOC than the other methods. For 4 of 6 test solutions, measured BDOC increased by 6–13% with added nutrients. Current methods produce largely comparable results, providing the justification for comparisons among existing data sets collected with different methodologies. We recommend two standard methods for future studies: (1) a rapid determination of relatively labile DOC (measurement of DOC removal after 7 days of incubation with added nutrients) and (2) a 42-day incubation with repeated analysis of CO2 production when determination of decomposition rate constants and a labile and relatively refractory component of DOC is desired.