Abstract. In the natural environment, soils undergo wetting and drying (WD) cycles due
to precipitation and evapotranspiration. The WD cycles have a profound
impact on soil physical, chemical, and biological properties and drive the
development of structure in soils. Degraded soils are often lacking
structure, and the effect of organic amendments and WD cycles on structure
formation of these soils is poorly understood. The aim of this study was to
evaluate the role of biotic and abiotic factors on aggregate formation and
stabilization of sodic soils after the addition of gypsum and organic
amendments (feedlot manure, chicken manure, lucerne pallets, and anionic
poly acrylamide). Amended soils were incubated at 25 ∘C over four
WD cycles, with assessment of soil microbial respiration, electrical
conductivity, pH, sodium adsorption ratio (SAR), aggregate stability in
water (ASWAT), aggregate size distribution, and mean weight diameter. Our
results demonstrate that WD cycles can improve aggregate stability after the
addition of amendments in sodic Vertisols, but this process depends on the
type of organic amendment. Lucerne pellets resulted in highest soil
microbial respiration, proportions of large macroaggregates (>2000 µm), and mean weight diameter. In contrast, dispersion was
significantly reduced when soils were treated with chicken manure, whilst
anionic polyacrylamide only had a transient effect on aggregate stability.
When these organic amendments were applied together with gypsum, the
stability of aggregates was further enhanced, and dispersion became
negligible after the second WD cycle. The formation and stability of small
macroaggregates (2000–250 µm) was less dependent on the type of
organic amendments and more dependent on WD cycles as the proportion of
small macroaggregates also increased in control soils after four WD cycles,
highlighting the role of WD cycles as one of the key factors that improves
aggregation and stability of sodic Vertisols.
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