Speciation of Iron and Aluminum in Relation to Phosphorus Sorption and Supply Characteristics of Soil Aggregates in Subtropical Forests

Abstract

Phosphorus (P) is one of the main limiting nutrients in subtropical forest soils. Both soil type and aggregate structure affect the P sorption capacity of soil; thus, determining soil P supply and leaching characteristics. However, the mechanism of their interactions on soil P sorption and leaching at an aggregate level remains unclear. We classified soil aggregates from red soils and limestone soils in a subtropical forest via wet-sieving and carried out P isothermal sorption experiments. The P sorption maximum (Qm), P sorption strength (KL, KF), P sorption index (PSI) and maximum buffer capacity (MBC) were obtained by fitting to Langmuir and Freundlich isotherm equations. Moreover, different P fractions were determined to estimate the degree of P sorption saturation (DPS) of aggregates. The results showed that the Qm of the two soils were similar, but the sorption strength (KL, KF) and MBC of the limestone soil were higher than those of the red soil. Higher contents of free iron (Fe) oxide and amorphous aluminum (Al) oxide in the limestone soil may enhance the P sorption capacity and, thus, reduce P availability, resulting in a higher total P retention capacity than in the red soil. A higher content of complex Fe in red soil may reduce P sorption and, therefore, play a role in maintaining the supply capacity of soil-available P. The 0.25–0.5 mm aggregates of the two soils had the largest MBC among all aggregate sizes, and their P sorption and buffering capacity were stronger than other aggregates. The DPS of different aggregate sizes were all low, indicating that the soils of subtropical forests were in a state of P deficiency; thus, the risk of P leaching was low. The <0.1 mm aggregate in red soil had relatively high DPS and significantly lower PSI than the other aggregate sizes, indicating that it was more prone to P leaching. The results provide further insight into forest management to improve P availability and reduce P leaching in subtropical forest soils.