Aggregate size effects on the sorption and release of phosphorus in an ultisol

Despite extensive research on soil-P reactions, little is known of the effect of soil aggregation on these processes. This study was conducted to determine the role of aggregate size on the sorption and release of P in a Haiku clay (Typic Palehumult). A range of aggregate sizes up to 4 mm in diameter were used in this study. Autoradiography of sorbed 32P and P sorption studies indicated that added P was initially sorbed to a 0.188-mm layer around aggregates and remained in this peripheral layer for up to 28 d. We defined this layer as the reactive mass. Analysis of P in aggregates from a 4-yr P management field experiment on the same soil indicated that diffusion of P from this peripheral layer into the aggregate was slow, requiring >2 yr for aggregate P to reach equilibrium. When P was added to a combined mix of aggregate size fractions, P sorption increased from 55 to 245 mg P kg(-1) as mean aggregate diameter decreased from 3.4 to 0.375 mm, and was linearly related to the aggregates' reactive mass (r(2)=0.96). Similarly, the reactive mass of aggregates was linearly correlated to P release from aggregates (r(2)=0.99) and to the linear buffer coefficients derived from P-sorption isotherms (r(2)=0.98). Since aggregate size is related to P sorption, soil buffering capacity, and P release from soil, it is likely that aggregation affects short- and long-term plant P availability. The effect of aggregate size, however, needs to be investigated for other soils.