Phosphorus Sorption-Desorption Kinetics in Aggregates of Ultisol: Effects of Combined Application of Rice Straw and Pig Manure over 28 Years

Soil aggregates play a crucial role as carriers for storing and cycling soil phosphorus (P). Changes in the composition of soil aggregates (2–0.25, 0.25–0.05, 0.05–0.01, 0.01–0.005 mm, <0.005 mm) due to the long-term fertilization directly impact the P adsorption-desorption behavior of soil aggregate—associated P. This study aimed to investigate the effects of applying rice straw (RS) or pig manure (PM) in combination with NPK fertilizers on soil total P (TP) and Bray-P across all soil aggregates, as well as the adsorption capacity and release potential of aggregate-associated P. PM application significantly increased TP content by 90.7 to 175.1% and Bray-P content by 480.7 to 1009.9% in all aggregates, while RS application had no significant impact compared to NPK fertilizers alone application (CK). As the aggregate-associated P content increased, the P sorption capacity of all aggregates initially increased rapidly and then reached a plateau. PM application resulted in the lowest P adsorption capacity and the highest P release potential of all soil aggregates; among all soil aggregates, <0.005 mm aggregates exhibited the highest P retention capacity and the lowest P desorption ratio. Furthermore, regardless of aggregate size, correlation analyses indicated that free iron-aluminum oxides were the primary factors influencing both P sorption capacity and P release potential of soil aggregates in Ultisol.