Organic amendments and gypsum reduce dispersion and increase aggregation of two sodic Vertisols

•Organic amendments and gypsum effected soil dispersion and aggregation.•Gypsum can reduce dispersion but has no effect on mean weight diameter.•Lucerne increases large macro aggregates but cannot control dispersion.•Aryl C and O-aryl C functional groups can control dispersion in the sodic soils.•Not all organic amendments can control soil dispersion and aggregation equally. Sodic soils are widespread in semi-arid and arid regions of the world, with agricultural production on these soils constrained due to structural instability resulting from high concentration of exchangeable sodium (Na). The structural instability results in clay dispersion, reduced infiltration, crust formation and poor seed germination. One way to decrease undesirable effects of Na on soil structure is to apply gypsum (G) and organic amendments. We evaluated the effect of G and four organic amendments [polyacrylamide (PAM), feedlot manure (FLM), chicken manure (CM), and lucerne pellets (LP)] on two sodic soils. These amendments differed in their chemical nature as assessed by 13C cross-polarisation magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy after amended soils were incubated for 60 d at field water capacity. Gypsum reduced dispersion but had no effect on macroaggregates or aggregate mean weight diameter (MWD). Using principal component analysis, we found that aryl C and O-aryl C functional groups were positively correlated with microaggregates and the silt + clay fraction, and O-alkyl C and di-O alkyl C were positively correlated with soil dispersion after 1 d of soil incubation. The CM reduced dispersion more than FLM, LP, or PAM, although LP increased the proportion of large macroaggregates and MWD. We also found that large macroaggregates and MWD were positively correlated with microbial respiration and dissolved organic carbon. Except for the G + LP amendment, G had no synergistic effect with other organic amendments on macroaggregates and MWD of the two sodic soils. This study demonstrates that the functional groups of organic amendments and microbial activity play important roles in aggregation and dispersion of sodic soils. This study also demonstrates that a combination of gypsum and organic amendments will be a better option to ameliorate sodic soils by decreasing dispersion and increasing aggregate formation.