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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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The long-term application of organic amendments like manure, biochar and biogas slurry can increase phosphorus (P) levels in agricultural soils; however, at present, it's not clear how this affects the P association with different mobile water-dispersible colloidal particles (Pcoll). Thus, this study aimed to assess the effects of the long-term application of different organic amendments on the abundance, size and compositional characteristics of Pcoll. For this purpose, a total of 12 soils amended with the above three organic amendments were sampled from the Zhejiang Province, China, and Pcoll were fractionated into nano-sized (NC; 1–20 nm), fine-sized (FC; 20–220 nm), and medium-sized (MC; 220–450 nm) by a combination of differential centrifugation and ultrafiltration steps. These three Pcoll forms together accounted for 74 ± 14% of the total soil solution dissolved P content, indicating that Pcoll release was a key process in the overland P transport from these soils. Soils treated with biochar showed lower Pcoll contents than those treated with manure or slurry alone; this effect should be further explored in a controlled inductive research approach. Compositional analysis showed that inorganic P was the predominant Pcoll form in the NC (54 ± 20%) and FC (63 ± 28%) fractions, but not in the MC (42 ± 26%) fraction. Among the three fractions, the organic carbon (OC)–calcium (Ca) complex was the major carrier of NC-bound Pcoll, MC-bound Pcoll was better correlated with OC–manganese/iron/aluminium colloids than with OC–Ca colloids, and both of these phenomena co-occurred in the FC fraction. The current study provides novel insights into the impact of various carbon amendments on the propensity for P loss associated with different soil mobile colloidal fractions, and will therefore, inform future agronomic and environmental-related policies and studies.

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