Carbon dynamics during a long-term incubation of separate and recombined density fractions from seven forest soils

Density fractions in soils differ in their turnover rates, but direct measurement of the C dynamics in the individual density fractions is limited. In 300-day incubations of mineral soils from forests in Washington and Oregon, USA, light fractions (LF), heavy fractions (HF), whole soils (WS), and physically recombined light and heavy fractions (RF), were measured for respiration and shifts in microbial biomass. A combined fraction was calculated from the incubation results of the light and heavy fractions, and called the summed fraction (SF). Carbon concentration followed the pattern: LF>RF>HF. In accordance with this pattern, when cumulative respiration was considered per gram of substrate, the physical fractions exhibited a predictable response: LF>RF>HF. However, when expressed per gram of initial C, the respiration of the LF was not significantly different from that of the HF. These findings suggest the recalcitrance of HF is similar to that of LF and, consequently, differences in their turnover rates in WS may be due to microbial accessibility or physical protection. Whether expressed per gram of substrate or per gram of initial C, the respiration of the SF was not different from that of the WS. Within the SF, the HF was responsible for 35% of the total respiration. Lower respiration in the RF compared with WS and SF might be explained by an antagonistic interaction between the varied microbial communities that degrade LF and HF; in the heterogeneous WS, these communities may be spatially separated to a greater extent than in the laboratory substrate. Unfortunately, the microbial data were highly variable and provided little evidence to either support or refute this idea. The density separation technique appears to be a viable method for isolating different soil organic matter fractions. However, the function of these fractions should be considered more carefully in the context of accessibility and C content.