Soil pH predominantly controls the forms of organic phosphorus in topsoils under natural broadleaved forests along a 2500 km latitudinal gradient

Soil organic phosphorus (P) dynamics is proposed to vary across regions. However, it remains unclear how it is driven by regional variations in environmental conditions. This study examined the variations in the amount and forms of organic P in the top 10 cm of mineral soils in natural broadleaved forests along a latitudinal gradient (18.4-40.8 degrees N) and their environmental drivers. Soil organic P amount (total Po) was expressed in both absolute (i.e. the concentration) and relative (i.e. the proportion of total Po to total P) terms, and soil organic P forms was represented by the proportion of non-residual Po (to total Po) quantified using a modified Hedley P fractionation procedure. Soil total Po amount in both absolute and relative terms were lower at the low-latitude site (18.4 degrees N) than at the high-latitude sites (25.6-40.8 degrees N) and predominantly positively determined by soil total P. The proportion of non-residual Po significantly increased with decreasing latitude and was primarily negatively affected by soil pH, highlighting a strong pH effect on soil organic P forms. Both the amount and forms of soil organic P were also considerably affected by climate (i.e. precipitation and temperature) and some other soil properties (e.g. cations and organic carbon). Latitudinal variations in climate affected soil total Po concentration largely through their effects on soil total P, organic C, and P sorption capacity, however they affected soil organic P forms mainly through their effects on soil pH and also via soil cations, organic C, and P saturation status.