Abstract
AbstractTo 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 (Q1) and slowly (Q2) desorbing P, and their desorption rates k1 and k2. The amounts of isotope‐exchangeable P (E) were calculated (E1min to E>3 months) and compared with Q1 and Q2. The strongest relationship was found between E1 min and Q1 (r2 = .87, p < .01). There was also an inverse relationship between the IEK parameter n (the rate of exchange) and k1 (r2 = .52, p < .01) and k2 (r2 = .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 Q1 (P‐Olsen = 1.1 × Q1 + 2.3, r2 = .96), n and k1 were related to P‐Olsen/P‐CaCl2, while k2 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.
Highlights
Crop phosphorus (P) uptake is dependent on the continuous replenishment of P from the soil solid phase to the soil solution (Pierzynski et al, 2005)
Our analysis showed that the Fh paper method and the isotopic exchange kinetics (IEK) method deliver similar information in terms of P pools
The values for k1 were not governed by the net mass P balance, but were instead inversely related to the solid–solution partitioning of the fast desorbing P as estimated by P-Olsen/P-CaCl2, and by the soil properties Feox, Al-ox + Fe-ox and the (Al-ox + Fe-ox)/organic C ratio. This suggests that the desorption rate of the fast desorbing pool is mostly governed by the partitioning of P between the dissolved and sorbed phases, which in turn is determined by soil properties related to P sorption such as the soil content of Al and Feoxides and organic C
Summary
| 516 time, P speciation and soil properties such as pH and the mineralogy of the clay fraction (Penn & Camberato, 2019; Pierzynski et al, 2005). The release of P from a soil can be described considering three factors: the intensity factor (the concentration of P in solution), the quantity factor (the amount of P that can be released from the soil solid phase and reach the solution within a given time) and the buffer capacity (the ability of a soil to maintain the concentration of P in solution when the quantity varies, e.g. because of fertilization or plant uptake) (Pierzynski et al, 2005) Some information about these factors can be obtained using a combination of chemical extractions, such as dilute CaCl2 for the intensity (van Raij, 1998; Six et al, 2012), AL or Olsen for the quantity factor within a growth season (Demaria et al, 2005; van Rotterdam et al, 2012), and the ratio of quantity to intensity to estimate the buffer capacity (Reijneveld et al, 2014; van Rotterdam-Los, 2010). Soils were chosen that represent extremes in terms of P status because of long-term P depletion or excessive P inputs
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