Phosphorus availability in boreal forest soils: A geochemical and nutrient uptake modeling approach

Changes in soil surface charge and phosphorus (P) sorption capacity of soils may affect the availability of P in soils. We investigated how changes in soil properties in the Bear Brook Watershed in Maine (BBWM), which is a long-term paired-watershed experimental acidification study, may affect P availability using the Cushman–Barber plant nutrient uptake simulation model. Both NH4Cl- and water-extractable P and Al in mineral soils were significantly related to soil organic matter content. The results from this study suggest that P solubility in the BBWM soils is controlled through the formation of amorphous aluminum phosphate (Al(OH)2H2PO4) with a log ion activity product (IAP) between − 28.6 and − 28.8. Surface charge titration and batch P adsorption experiments were conducted on mineral soils from BBWM. The surface charge titration curve was fit to a three-site Langmuir model with the Type 1 site having an average OH− binding strength log K1 of 8.7 and 12% of the surface charge, Type 2 site with log K2 of 6.0 and 23% of the surface charge, and the Type 3 site with a log K3 of 4.2 and 65% of the surface charge. The Cushman–Barber simulation model predicted a 15% and 31% increase in P availability in the acid-treated watershed soils for the softwood and hardwood stands, respectively. This was primarily due to increased water-soluble P content in these soils, suggesting that soil acidification leads to increased P availability to plants. The results from this study clearly demonstrate the importance of the Al–P–organic matter interaction in determining the level of soluble P in soils, a key determinant of P bioavailability to plants.