Phosphorus desorption and isotope exchange kinetics in agricultural soils

Abstract 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 1 ) and slowly (Q 2 ) desorbing P, and their desorption rates k 1 and k 2 . The amounts of isotope‐exchangeable P (E) were calculated (E 1min to E >3 months ) and compared with Q 1 and Q 2 . The strongest relationship was found between E 1 min and Q 1 ( r 2 = .87, p < .01). There was also an inverse relationship between the IEK parameter n (the rate of exchange) and k 1 ( r 2 = .52, p < .01) and k 2 ( r 2 = .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 1 (P‐Olsen = 1.1 × Q 1 + 2.3, r 2 = .96), n and k 1 were related to P‐Olsen/P‐CaCl 2 , while k 2 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.