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Effects of organic matter-bound multivalent cations on the internal and external formation of soil aggregates under Pinus massoniana forest

Organic matter-bound multivalent cations are important substances for the formation of soil aggregates. However, there is limited knowledge about the content and cementing ability of organic matter-bound multivalent cations in the process of driving the formation and stability of aggregates of various particle sizes. This study took a Masson pine plantation as the research object. Sodium pyrophosphate (Na4P2O7) was used to separate and determine the internal and external organic matter and multivalent cation binding states of water-stable aggregates; the macro/microaggregates that dissipated after the separation of the organic matter and multivalent cations were determined. The results show that (1) of internally formed aggregates, 1 to 0.5 mm and 0.5 to 0.25 mm particle size aggregates have the highest calcium (Ca) ion content after organic matter-bound multivalent cations are separated, followed by iron (Fe) and magnesium (Mg) ions; the aluminum (Al) ion content is the lowest. At the same time, 1 to 0.5 mm particle size aggregates dissipate into 0.5 to 0.25 mm particle size aggregates (10% to 37%) and microaggregates (11% to 61%), while 0.5 to 0.25 mm particle size aggregates mainly dissipate into microaggregates (30% to 100%). The aggregates formed internally separated organic matter-bound Ca, Mg, and Al ions, and the dissipated macroaggregates showed a significant positive correlation. (2) Of the externally formed aggregates, after the organic matter-bound multivalent cations of each particle size aggregate are separated, the organic matter-bound state has the highest Fe ion content, followed by the Mg and Al ion contents, while Ca ions are not detected. At the same time, the aggregates of each particle size dissipate into a small amount of macroaggregates (6.50% to 33.00%) and a large number of microaggregates (26% to 76%). Aggregates formed from externally split, organic matter-bound state Mg, Fe, and Al ions have a very significant negative correlation with dissipated macroaggregates, and organic matter-bound Mg and Al ions are extremely significantly and positively correlated with the dissipated microaggregates. This study concluded that the agglomeration forms internal organic matter-bound Ca, Mg, and Al ions mainly to cement macroaggregates; the agglomeration forms external organic matter-bound Mg and Al ions mainly to cement microaggregates, while organic matter-bound Ca ions hardly participate in the formation of the exterior of the aggregates. During the formation of aggregates from inside to outside, the organic matter-bound Ca and Mg ions increased, while Fe and Al ions decreased.