National estimates of environmental thresholds for upland soil phosphorus in China based on a meta-analysis

The environmental threshold for upland soil phosphorus (P) content (ETSP, i.e., inflection point of soil P content leading to enhanced P loss) provides an important metric for guiding agricultural nonpoint source P pollution control. This study achieved the first meta-analysis to determine ETSP values for upland soils in China. The estimated national-level ETSP based on 472 field experimental observations of Olsen-P content and P loss rate was 30.1 ± 4.0 mg P kg^-1, which was lower than the average ETSP value (52.1 ± 5.0 mg P kg^-1) but higher than the average agronomic threshold values (16.0 ± 6.4 mg P kg^-1) previously reported. Lower upland ETSP values occurred in acidic soils and soils having higher organic matter content (SOM), precipitation and slope (ETSP: 30.5 for pH < 7.0 versus 46.1 for pH ≥ 7.0; >56.4 for SOM < 2%, 49.9 for SOM = 2%-3%, and <3 for SOM > 3%; 33 for precipitation < 1000 mm yr^-1, 27.5 for precipitation = 1000-1200 mm yr^-1 and <5 for precipitation > 1200 mm yr^-1; and 39.8 for slopes < 5° versus <9 for slopes ≥ 5°). A multiple regression model that incorporates SOM, pH, precipitation and slope was developed to predict upland ETSP values (R² = 0.73, p < 0.01). The model estimated national upland ETSP values ranging from ~0 to 100 mg P kg^-1 with an areal-weighted average of 60.6 mg P kg^-1 and 15% of national upland soils having ETSP values <30 mg P kg^-1. Upland soil P contents in Guangdong, Fujian and Zhejiang provinces largely exceeded their corresponding ETSP values by 1-22 mg P kg^-1, indicating high P loss risks. Controlling upland P loss requires integrated management of soil P content, SOM, pH and erosion control. This study provides the first national estimate of upland soil ETSP, providing critical quantitative information for designing management practices to attenuate agricultural nonpoint source P pollution.