Background Colloid-facilitated phosphorus (P) transport is recognized as an important pathway for the loss of soil P in agricultural systems; however, information regarding soil aggregate-associated colloidal P (P coll ) is lacking. To elucidate the effects of aggregate size on the potential loss of P coll in agricultural systems, soils (0–20?cm depth) from six land-use types were sampled in the Zhejiang Province in the Yangtze River Delta region, China. The aggregate size fractions (2–8?mm, 0.26–2?mm, 0.053–0.26?mm and??0.053?mm) were separated using the wet sieving method. Colloidal P and other soil parameters in aggregates were analyzed. Results Our study demonstrated that 0.26–2?mm small macroaggregates had the highest total P (TP) content. In acidic soils, the highest P coll content was observed in the 0.26- to 2-mm-sized aggregates, while the lowest was reported in the??0.053?mm (silt? ?clay)-sized particles, the opposite of that revealed in alkaline and neutral soils. Paddy soils contained less P coll than other land-use types. The proportion of P coll in total dissolved P (TDP) was dominated by??0.053?mm (silt? ?clay)-sized particles. Aggregate size strongly influenced the loss potential of P coll in paddy soils, where P coll contributed up to 83% TDP in the silt? ?clay-sized particles. The P coll content was positively correlated with TP, Al, Fe, and the mean weight diameter. Aggregate-associated total carbon (TC), total nitrogen (TN), C/P, and C/N had significant negative effects on the contribution of P coll to potential soil P loss. The P coll content of the aggregates was controlled by the aggregate-associated TP and Al content, as well as the soil pH value. The potential loss of P coll from aggregates was controlled by its organic matter content. Conclusion We concluded that management practices that increase soil aggregate stability or its organic carbon content will limit P coll loss in agricultural systems.
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