Size and composition of colloidal phosphorus across agricultural soils amended with biochar, manure and biogas slurry

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 (P_coll). 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 P_coll. For this purpose, a total of 12 soils amended with the above three organic amendments were sampled from the Zhejiang Province, China, and P_coll 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 P_coll forms together accounted for 74 ± 14% of the total soil solution dissolved P content, indicating that P_coll release was a key process in the overland P transport from these soils. Soils treated with biochar showed lower P_coll 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 P_coll 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 P_coll, MC-bound P_coll 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.