Stratified soil sampling improves predictions of P concentration in surface runoff and tile discharge

Authors: Osterholz, William - Will; King, Kevin; Williams, Mark; Hanrahan, Brittany; DUNCAN, EMILY - Los Angeles Regional Water Control Board

Submitted to: Soil Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2020
Publication Date: 11/19/2020
Citation: Osterholz, W.R., King, K.W., Williams, M.R., Hanrahan, B.R., Duncan, E.W. 2020. Stratified soil sampling improves predictions of P concentration in surface runoff and tile discharge. Soil Systems. 4(4). Article 67. https://doi.org/10.3390/soilsystems4040067.
DOI: https://doi.org/10.3390/soilsystems4040067

Interpretive Summary: Phosphorus (P) losses from croplands are a major driver of hypoxia and harmful and nuisance algal blooms in waterbodies worldwide. Stratification of P in soil, where surface layers hold increased concentrations of P, potentially increases the risk of P losses. However, understanding of the importance of measuring soil P stratification for predicting P losses remains limited. This study compared two soil sampling approaches across 39 crop fields in NW Ohio: stratified soil sampling, where samples were split into two depths, and typical soil sampling approach that only sampled a single soil depth. The soil P data were used to predict P losses from the fields, and the accuracy of the predictions were assessed. Soil P from both sampling depths was positively related to P losses, but the shallow sampling depth provided better, more accurate predictions of P losses than the typical sampling depth. These results show that soil P from the shallow sampling depth is an important factor determining P losses, and the stratified soil sampling approach holds potential for improving predictions of P losses.

Technical Abstract: Phosphorus (P) stratification in agricultural soils has been proposed to increase the risk of P loss to surface waters. Stratified soil sampling that assesses soil test P (STP) in a shallow soil horizon may improve predictions of P concentrations in surface and subsurface discharge compared to single depth agronomic soil sampling. However, the utility of stratified sampling efforts for enhancing understanding of environmental P losses remains uncertain. In this study, we examined the potential benefit of integrating stratified sampling into existing agronomic soil testing efforts for predicting P concentrations in discharge from 39 crop fields in NW Ohio, USA. Edge-of-field (EoF) dissolved reactive P (DRP) and total P (TP) concentrations in surface runoff and tile drainage was positively related to soil test P (STP) measured in both the agronomic sampling depth (0-20 cm) and shallow sampling depth (0-5 cm). Tile and surface DRP and TP were more strongly predicted by shallow depth STP than agronomic STP, as indicated by regression models with greater coefficients of determination (R2) and lesser root-mean square error (RMSE). A multiple regression model including the agronomic STP and P stratification ratio (Pstrat) provided the best model fit for DRP in surface runoff and tile drainage and TP in tile drainage. Additionally, STP often varied significantly between soil sampling events at individual sites and these differences were only partially explained by management practices, highlighting the challenge of assessing STP at the field scale. Overall, the linkages between shallow STP and P transport persisted over time across agricultural fields in northcentral Ohio, USA and incorporating stratified soil sampling approaches showed potential for improving predictions of P concentrations in surface runoff and tile drainage.