Statistical and Geostatistical Appraisal of Spatial Variability of Aggregate Stability and Aggregate-Associated Organic Carbon Content on a Catchment Scale in a Semi-arid Region, Central Iran

In a semiarid region of central Iran, effects of parent materials, physiography and landscape position, land use, and
management practices on association of organic carbon with secondary (aggregates) particles and aggregate stability can
have important consequences in terms of carbon sequestration and budgeting, deciding on the proper land use strategy and
suitable soil conservation practices. It was used wet sieved aggregates to evaluate the effects of different factors on soil
aggregate stability and organic carbon concentration within three aggregate size fractions (>2 mm, 1-2 mm, <1 mm). 111
soil samples were collected to measure water stable aggregates (WSA), aggregate organic carbon concentration (OC), and
mean weight diameter (MWD). Some other related soil and terrain properties including bulk density, infiltration rate,
saturated hydraulic conductivity and erodibility index were also measured. Analyses of variance indicated that water
stability of aggregates was influenced by aggregate size. Higher percentage of water stable aggregates was found for
microaggregates (< 1 mm), followed by mesoaggregates (1 to 2 mm). Aggregate organic carbon content was highest in
mesoaggregates (9 g kg-1), followed by microaggregates (7 g kg-1), while the least OC concentration was found in
macroaggregates (3 g kg-1). Both aggregate size fraction and slope aspect significantly impacted aggregate organic carbon
concentration. Although a significant effect of aggregate size on aggregate organic carbon content was found, however,
our findings did not support the model of aggregate hierarchy. Both MWD and GMD were significantly impacted by
aggregate size fractions. Geostatistical analysis showed that the measured soil attributes exhibited differences in their
spatial patterns in both magnitude and space at each aggregate size fractions. The relative nugget variance for most
aggregate-associated properties was lower than 45%. The range for water stable aggregates was almost similar (3 km)
for the three studied aggregate size classes. The range for aggregate-associated organic carbon contents ranged from about
3 km for macroaggregates to about 6.5 km for mesoaggregates. Kriged maps of predicted WSA, OC and MWD for the
three studied aggregate size fractions showed clear spatial patterns. However, a close spatial similarity (coregionalization)
was observed between WSA and MWD.