Soil aggregate size distribution and structure stability are important soil physical quality indices. The objective of this study was to quantify soil structure stability as a result of land-use conversion from forest to tea farm within Guilan Province of northern Iran over the past 50 years. Soil aggregate or structure stability metrics, along with more common soil chemical and physical properties, were determined on triplicate soil samples (0-30 cm depth), randomly collected from six sites. Dexter’s S-index (slope of soil water retention curve at its inflection point), tensile strength (TS), aggregate mean weight diameter (MWD), and water stable aggregate (WSA) were also used to assess stability. Lastly, the high energy moisture characteristic (HEMC) method measured the destructive force used to break the aggregates through controlled wetting. Structure stability using HEMC method and S-index was determined based on the soil water retention curve but using different ranges of matric potentials. The results showed that despite an insignificant decrease in TS, WSA and MWD between land use types there was significant correlation (P< 0.05) between tensile strength with HEMC method’s, stability ratio (SR) and Dexter’s S-index. The volume of drainable pores after fast wetting of aggregates (fast-VDP) in HEMC method reflected a significant (P<0.01) decrease in aggregate size between forest and tea farms with means of 0.347 and 0.297 g g-1, respectively. In only one of six locations, the soil water retention curves and the aggregate stability indexes (SI and SR) of HEMC method decreased significantly after land use change. Our results indicated that macro aggregates and structure stability in HEMC method could distinguish differences between two land uses and better than previous methods like MWD, WSA, and TS. The results of S-index and stability ratio of HEMC method were different. This difference can be related to: 1) the soil initial aggregate size that is between 0.5 to 1 mm in HEMC method and about 4 mm in Dexter’s S-index and 2) the range of matric potential in each method that is 0 to 50 and 0 to 15000 hPa in HEMC and S-index, respectively. Although both methods are based on soil water retention curve to estimate soil structure stability and physical quality, but HEMC results show macrostructure stability (macro aggregates) whereas S-index is inflection point of soil water retention curve between textural and structural porosity and estimate microstructure stability.

Keywords