A novel method for soil aggregate stability measurement by laser granulometry with sonication

摘要

Regulatory authorities need to establish rapid, cost-effective methods to measure soil physical indicators – suchudas aggregate stability – which can be applied to large numbers of soil samples to detect changes of soil qualityudthrough monitoring. Limitations of sieve-based methods to measure the stability of soil macro-aggregates include:udi) the mass of stable aggregates is measured, only for a few, discrete sieve/size fractions, ii) no account is taken ofudthe fundamental particle size distribution of the sub-sampled material, and iii) they are labour intensive. These limitationsudcould be overcome by measurements with a Laser Granulometer (LG) instrument, but this technology hasudnot been widely applied to the quantification of aggregate stability of soils. We present a novel method to quantifyudmacro-aggregate (1–2 mm) stability. We measure the difference between the mean weight diameter (MWD; m)udof aggregates that are stable in circulating water of low ionic strength, and the MWD of the fundamental particlesudof the soil to which these aggregates are reduced by sonication. The suspension is circulated rapidly through a LGudanalytical cell from a connected vessel for ten seconds; during this period hydrodynamic forces associated with theudcirculating water lead to the destruction of unstable aggregates. The MWD of stable aggregates is then measuredudby LG. In the next step, the aggregates – which are kept in the vessel at a minimal water circulation speed – areudsubject to sonication (18W for ten minutes) so the vast majority of the sample is broken down into its fundamentaludparticles. The suspension is then recirculated rapidly through the LG and the MWD measured again. We refer toudthe difference between these two measurements as disaggregation reduction (DR) – the reduction in MWD on disaggregationudby sonication. Soil types with more stable aggregates have larger values of DR. The stable aggregatesud– which are resistant to both slaking and mechanical breakdown by the hydrodynamic forces during circulation –udare disrupted only by sonication.udWe used this method to compare macro-aggregate (1–2 mm) stability of air-dried agricultural topsoils under conventionaludtillage developed from two contrasting parent material types and compared the results with an alternativeudsieve-based technique. The first soil from the Midlands of England (developed from sedimentary mudstone; meanudsoil organic carbon (SOC) 2.5%) contained a substantially larger amount of illite/smectite (I/S) minerals comparedudto the second from the Wensum catchment in eastern England (developed from sands and glacial deposits; meanudSOC=1.7%). The latter soils are prone to large erosive losses of fine sediment. Both sets of samples had been storedudair-dried for 6 months prior to aggregate analyses. The mean values of DR (n=10 repeated subsample analyses) forudthe Midlands soil was 178m; mean DR (n=10 repeat subsample analyses) for the Wensum soil was 30m. Theudlarge difference in DR is most likely due to differences in soil mineralogy. The coefficient of variation of mean DRudfor duplicate analyses of sub-samples from the two topsoil types is around 10%. The majority of this variation isudlikely to be related to the difference in composition of the sub-samples. A standard, aggregated material could beudincluded in further analyses to determine the relative magnitude of sub-sampling and analytical variance for thisudmeasurement technique.udWe then used the technique to investigate whether – as previously observed – variations (range 1000 – 4000 mgudkg