Particle+Density Six

Description and working steps

This method was designed to evaluate if aggregate occlusion, and association to silt- and clay-sized minerals increase the turnover time of light fraction (LF) and SOM, respectively. This was achieved by a combination of density floatation in SPT before and after aggregate dispersion, and a size separation at 53mm, to isolate the fine mineral associated organic matter. In brief, 1.85 g cm-3 SPT was added to 10 g of 2 mm sieved oven dried soil sample, and the suspension shaken gently up and down 10 times. Air entrapped within the aggregates was evacuated. The free LF was floated off after centrifugation at 1094 g for 60 minutes. The heavy fraction was then rinsed 3 times to remove SPT, and shaken in dilute (0.5 %) sodium hexametaphosphate and beads for 18 hours to completely disperse sample. The dispersed sample was then rinsed onto 53 µm sieve. The coarse fraction remaining on sieve was dried at 60°, and then shaken for 10 minutes with 1.85 g cm-3 SPT. The occluded LF was float off after centrifugation at 1094 g for 45 minutes. The heavy fraction was rinsed 3 times to remove SPT and represented the sand-sized fraction. The sample rinsed through the 53 mm sieve represented the silt & clay fractions. All fractions were oven dried at 60°C and weighed. Density floatation, aggregate dispersion and wet sieving procedures were performed as described in Six et al. (1998). Results suggest that separation of occluded and not occluded LF, at least for these soils, does not partition pools with a distinct 13C enrichment, while it adds two intense rinses that reduce overall C recovery. The separation by wet sieving at 53mm showed, instead, to be an effective method to separate a mineral associated organic matter fraction with a distinct 13C signal.

Initial Aim

This method was designed to evaluate if aggregate occlusion, and association to silt- and clay-sized minerals increase the turnover time of light fraction (LF) and SOM, respectively.

Advantage

The initial density fractionation before dispersion allows for the isolation of free light fraction. The inclusion of a second density fractionation after dispersion allows for the separation of light fraction bound in aggregates.

Disadvantage

Time intensive, low C recovery, not all fractions are 13C distinct.

References

Six, J., E.T. Elliott, K. Paustian and J.W. Doran. 1998. Aggregation and soil organic matter accumulation in cultivated and native grassland soils. Soil Science Society of America Journal 62: 1367-1377 (Adapted from).