Prediction of nano, fine, and medium colloidal phosphorus in agricultural soils with machine learning.

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 (R ²  = 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 .
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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