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Soil colloids have been shown to play a critical role in soil phosphorus (P) mobility and transport. However, identifying the potential mechanisms behind colloidal P (P coll) release and the key influencing factors remains a blind spot. Herein, a machine learning approach (random forest (RF) coupled with partial dependence plot analyses) was applied to determine the effects of different soil physicochemical parameters on P coll content in three colloidal subfractions (i.e., nano- (NC): 1–20 nm, fine- (FC): 20–220 nm and medium-sized colloids (MC): 220–450 nm) based on a regional dataset of 12 farmlands in Zhejiang Province, China. RF successfully predicted P coll content (R2 = 0.98). Results showed that colloidal- organic carbon (OC coll) and minerals were the major determinants of total P coll content (1–450 nm); their critical values for increasing P coll release were 87.0 mg L−1 for OC coll , 11.0 mg L−1 for iron (Fe coll) or aluminium (Al coll), 2.6 mg L−1 for calcium (Ca coll), 9.0 mg L−1 for magnesium (Mg coll), 2.5 mg L−1 for silicon (Si coll), and 1.4 mg L−1 for manganese (Mn coll). Among three colloidal subfractions, the major factors determining P coll were soil Olsen-P (P Olsen ; 125.0 mg kg−1), Ca coll (2.5 mg L−1), and colloidal P saturation (21.0%) in NC; Mn coll (1.5 mg L−1), Mg coll (6.8 mg L−1), and P Olsen (135.0 mg kg−1) in FC; while Mn coll (1.5 mg L−1), Al coll (2.5 mg L−1), and Fe coll (3.8 mg L−1) in MC, respectively. OC coll had a considerable effect in the three fractions, with critical values of 80.0 mg L−1 in NC or FC, and 50.0 mg L−1 in MC. Our study concluded that the information gleaned using the RF model can be used as crucial evidence to identify the key determinants of different size fractionated P coll contents. However, we still need to discover one or more easy-to-measure parameters that can help us better predict P coll. [Display omitted] • Effects of different soil physicochemical factors on colloidal P content were investigated. • Random Forest showed good performance to predict colloidal P content in agricultural soils. • Colloidal organic carbon and minerals were the key determinants of colloidal P content. [ABSTRACT FROM AUTHOR]

• Colloidal P was closely related to Olsen-P in paddy soils on a regional scale. • Cattle manure biochar amendment decreased colloidal P in paddy soils. • Cattle manure biochar amendment increased available P in paddy soils. Colloidal phosphorus (P coll) and available phosphorus (AP) are of fundamental importance in the nutrient cycle in soil solutions and subsurface drainage of ecosystems. Although many studies have been conducted on soil P loss and P availability, there remains limited focus on the relationship between P coll and AP. In this regional-scale study, soils with loam texture were collected from 63 paddy fields in southern China to explore the relationships between P coll and AP as well as other soil parameters. Subsequently, three typical paddy soils (sandy loam, silt loam, and clay loam) were incubated to study the effect of cattle manure biochar (CMB) amendment on P coll and AP at three application rates (0, 1, and 2%, w/w) for 30 d. The results showed that P coll was closely related to AP (R 2 = 0.453, P < 0.05) and pH (R 2 = 0. 487; P < 0.05) on a regional scale. In addition, clay loam soils contained higher P coll than the other soil types. The CMB amendment increased the AP by 129.0–872.0% but reduced the P coll by 9.65–22.95%, in which colloidal molybdenum-nonreactive phosphorus (MUP coll) was reduced by 7.69–34.15% and colloidal molybdenum-reactive phosphorus (MRP coll) was reduced by 5.13–22.73%. Multiple linear regression revealed that the release potentials of P coll (R2 = 0.735), MRP coll (R2 = 0.543), and MUP coll (R2 = 0.512) could be predicted by the dissolved organic carbon, total carbon, pH, and AP. Overall, this study confirmed that the application of CMB reduced the loss of P coll from paddy soils and enhanced the soil AP content. [ABSTRACT FROM AUTHOR]

• Phosphorus (P) input and crop type cause differences in P loss via runoff. • Modified biochar reduced the runoff P concentration and loss load in all cropping systems. • Solid organic fertilizer decreased particulate P and colloidal P losses. • Biogas slurry increased the P loss in the runoff of the rice-wheat rotation system. Agricultural cropping systems involve the application of high fertilizer rates, which lead to phosphorus (P) losses via surface runoff to the aquatic environment, thereby resulting in severe eutrophication. Here, to evaluate the nutrient loss, we established three monitoring stations in Zhejiang Province, China, in each of which the crop types were double cropping rice, rice–wheat rotation, and vegetable cultivation. Field experiments were conducted with four treatments and three replications over a whole planting year. The four treatments at the same P application rate were: 1) no fertilization (control; CK), 2) chemical fertilization (CF), 3) substitution of 30 % chemical P in CF by organic P in solid sheep manure (OF) or in liquid biogas slurry (BS), and 4) substitution of 30 % chemical P in CF by organic P fertilizers (solid sheep manure or liquid biogas slurry) +1.5 t ha−1 polyacrylamide modified BC (PSB). The runoff losses of various P forms, that is, the total P, total dissolved P, particulate P, and colloidal P losses were analyzed. The total P concentration varied greatly with the crop growth stages. Compared with CF treatment, PSB treatment significantly reduced the concentration of P fractions in the runoff in all three cropping systems, while the OF treatment significantly reduced the particulate P and colloidal P concentrations by coating colloidal particles with organic carbon. However, BS treatment increased particulate P and colloidal P concentrations. Compared with CF treatment, PSB treatment reduced total P loss by 41.1 %, 29.7 %, and 37.8 % in the double cropping rice, rice–wheat, and vegetable systems, respectively. The PSB and OF treatments significantly reduced the particulate P and colloidal P losses, while BS treatment increased the loss of various P forms. Compared with the CF treatment, the PSB treatment significantly increased the available P contents by 12.7–85.4 mg kg-1, and decreased the colloidal P contents by 26.7 %–51.4 % in the soils examined. The results of the redundancy analysis confirmed that P loss via runoff was mainly caused by differences in the P inputs and crop types. The results of this study highlight the important role of PSB in reducing P loss via runoff from agricultural cropping systems. [ABSTRACT FROM AUTHOR]