Phosphorus desorption and isotope exchange kinetics in agricultural soils

To improve phosphorus (P) fertilization and environmental assessments, a better understanding of release kinetics of solid-phase P to soil solution is needed. In this study, Fe (hydr)oxide-coated filter papers (Fh papers), isotopic exchange kinetics (IEK) and chemical extractions were used to assess the sizes of fast and slowly desorbing P pools in the soils of six long-term Swedish field experiments. The P desorption data from the Fh-paper extraction of soil (20 days of continual P removal) were fitted with the Lookman two-compartment desorption model, which estimates the pools of fast (Q) and slowly (Q) desorbing P, and their desorption rates k and k. The amounts of isotope-exchangeable P (E) were calculated (E to E) and compared with Q and Q. The strongest relationship was found between E and Q (r =.87, p <.01). There was also an inverse relationship between the IEK parameter n (the rate of exchange) and k (r =.52, p <.01) and k (r =.52, p <.01), suggesting that a soil with a high value of n desorbs less P per time unit. The relationships between these results show that they deliver similar information, but both methods are hard to implement in routine analysis. However, Olsen-extractable P was similar in magnitude to Q (P-Olsen = 1.1 × Q + 2.3, r =.96), n and k were related to P-Olsen/P-CaCl, while k was related to P-oxalate/P-Olsen. Therefore, these extractions can be used to estimate the sizes and desorption rates of the different P pools, which could be important for assessments of plant availability and leaching.