Aggregates+Density Six

Description and working steps

Air-dried soil 30 g was taken and wet-sieved manually through 2000 µm sieve (in a case of our study, the soil was already sieved through the 2000 µm sieve before fractionation, and so, the fraction >2000 µm is absent) by moving sieves up and down 3 cm with 50 repetitions during 2 min. Aggregates which were left on the sieve were collected into the jar. The rest of soil was sieved through the 250 µm sieve by the method described above and stable aggregates which were left on the sieve were collected. The rest of the soil was sieved through the 53 µm sieve by the method described above, which gave another two fractions: >53 µm and <53 µm. All fractions were dried at 105 ºC.

After aggregates dried, 5g of subsample was put into the 50 ml falcon flask, 35 mL of 1.85 g cm^-3 sodium polytungstate (SPT) were added and gently mixed with the soil by hand shaking. After that samples were centrifuged at 1250 g for 1 h. The floating material (free light fraction) was collected and washed with deionized water (at least with 200 ml) on the glass fiber to remove SPT. The heavy fraction was rinsed with deionized water (50 ml, 4 times). Then heavy fraction was dispersed in 0.5% hexametaphosphate by shaking for 18 h on a reciprocal shaker. The dispersed fraction was passed through a 2000-, 250-, and/or 53 sieves depending on the aggregate size (see scheme). Mineral fractions were dried at 105 ºC and free POM was dried at 60 ºC before the measurements. For total description, please, see Six et al., (1998)

Initial Aim

Initially, the method was intended to isolate SOM fractions that relate to soil structure.

Advantages

The advantage of the method is the separation of intra-aggregate POM and mineral fraction after the separation of water stable aggregates. This help to avoid mixing of intra-aggregate POM (and mineral associated C) from various aggregate size classes like in simple density fractionation.

Disadvantages

High time and space requirements.

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)