Is there any way to identify which type of phosphorus (P) is exported from agricultural watersheds?

I am developing a phosphorus (P) budget for an agricultural watershed (Haean waershed, 6175 ha, South Korea). Chemical fertilizers and organic matter (compost and manure) are the main sources of P in my study site. In addition, the soil of the watershed is characterized as sandy loam. P sorption capacity of manured sandy loam soils is lower than chemical fertilized soils. Additionally, the mean P concentration is 0.041 and 0.691 mg/l for dry and wet season (more than half of the annual precipitation is concentrated during summer monsoon season (July-August)) respectively. Is there any way to identify which part of P - Org. P or Inorg. P is mainly exported from the watershed ?