An anion resin membrane technique to overcome detection limits of isotopically exchanged P in P‐sorbing soils

Abstract

Summary Isotopically exchanged phosphorus is difficult to determine in soils that strongly sorb P (so that there is little P in solution) and in soils with large concentrations of colloidal P in soil suspensions. A method is proposed in which anion exchange membranes (AEM) are added to the soil suspension after an initial period of isotopic exchange with 32P‐labelled phosphate ions. Isotopically exchanged P, termed EAEM, is calculated from the ratio of labelled phosphate ions to the total phosphate ions on the membrane. The EAEM was compared with the E value measured in an aqueous soil extract (EWater extract) for 14 soils with different degrees of P sorption. The two methods gave similar results in soils with large P concentrations in an aqueous soil extract. However, EWater extract values significantly exceeded the EAEM values by up to 18‐fold when soluble P was near the determination limit (0.008 mg P l−1). In a second experiment, two Ferralsols received further P from inorganic and plant sources and were incubated for 7 days. Treatment effects on labile P were erroneous as detected by the EWater extract but were significant as detected with the AEM method. Third, EAEM values were followed in a Lixisol and a Ferralsol which received labelled phosphate ions with carrier just before the beginning of a 23‐day incubation. The approximate recovery of added inorganic P in the EAEM value suggested that this method adequately samples labile P in P‐sorbing soils. All these results showed that errors in the determination of E values for soils with very small concentrations of P in the soil solution are reduced using the proposed method.

Plant-available P for Maize and Cowpea in P-deficient Soils from the Nigerian Northern Guinea Savanna – Comparison of E- and L-values

“…Total P (mg P) in shoots of maize and two cowpea varieties grown in three soils from the NGS as affected by TSP application at rates of 12.5 and 37.5 mg P kg )1 , with and without an N application at a rate of 75 mg N kg 1 (n = 4) Numbers between brackets are amounts of P derived from the seed (mg P), calculated using Equation (4) and parameters in Table 3 Maize ( nd: not determined; small letters represent significant (P<0.05) differences in total shoot P between P treatments with N application (+N) within soil and within plant variety; capital letters represent significant (P<0.05) differences in total shoot P between P treatments without N application ()N) within soil and within plant variety; * represents a significant (P < 0.05) effect of N application on shoot P content within P treatment, soil and plant variety. Maertens et al (2004) correctly reflects treatment effects and that the isotopically exchangeable P pool correlates with the available P pool in these three soils from the NGS. In Figure 3, the ratio of the L-value for maize and cowpea in the control and TSP treatments to the average E-value during the last 3 weeks of incubation (14-35 days after labeling) was plotted vs. the total shoot P content.…”

“…P concentrations in the extract were smaller than the detection limit of the malachite green method (8 lg P L )1 ) but could be calculated using the Evalue and the K D -value (Maertens et al 2004) and equaled 1 lg P L )1 for the Danayamaka and Kasuwan Magani soil and 2 lg P L )1 for the Shika soil. As such, all three soils are P deficient as shown by the small labile P quantity (E-value) and the small P intensity (P concentration in soil solution).…”

“…Duplicate soil samples were incubated at 28°C in closed containers, which were opened daily for aeration. After 8 h, and after 6, 13, 20, 27 and 34 days, a subsample was taken and the E-value was determined using a method adapted from Maertens et al (2004). An aliquot of fresh soil equivalent to 2.5 g dry soil was weighed into 30 mL oak ridge polyethylene centrifuge tubes, 25 mL of distilled water was added (soil:solution ratio = 1:10) and samples were shaken during 16 h on a vertical shaker at 250 mot min )1 .…”

“…On the one hand, the determination of the E-value becomes unreliable if P concentrations in the soil solution are near detection limits of standard colorimetric methods and when possible interference of silica, solution color or turbidity, or presence of colloids occur (Hamon and McLaughlin, 2002;Maertens et al, 2004). This situation is typically observed in highly weathered, P-deficient soils (Amer et al, 1969;Bu¨hler et al, 2003;Dalal and Hallsworth, 1977;Wolf et al, 1986) and for which a better understanding of P availability and P acquisition by crops is particularly needed.…”

“…This situation is typically observed in highly weathered, P-deficient soils (Amer et al, 1969;Bu¨hler et al, 2003;Dalal and Hallsworth, 1977;Wolf et al, 1986) and for which a better understanding of P availability and P acquisition by crops is particularly needed. The approach of Maertens et al (2004) allows assessment of the labile P pool in these soils by combining isotopic exchange with resin extraction. An anion exchange membrane (AEM) samples P in the soil solution and a fraction of the isotopically exchanged P from the solid phase.…”