Assessment of the soil phosphorus–mobilization potential by microbial reduction using the Fe(III)‐reduction test

The mobility of soil P is greatly influenced by the redox potential (Eh), which depends on the reducing activity of soil microorganisms. Standard extraction methods for the determination of the mobile soil P disregard the P mobilization caused by the influence of microorganisms on Eh, while P test methods that include soil microbial activities are lacking. Thus, the Fe(III)‐reduction test was investigated for its suitability to determine the P fraction that is mobilized in soil under reducing conditions (P_Red). In this test, the soil‐microbial reducing activity is measured from the microbial Fe(III) reduction combining a bioassay with 7 d incubation and a chemical extraction using 1M KCl. After the incubation, Eh in 26 different soil samples ranged from –282 to –123 mV. The concentration of P_Red in the soil samples ranged from concentrations below the limit of detection to 84.9 mg kg^–1 and was on average of all soil samples by a factor of 2.4 to 18 smaller than the P fractions determined by standard soil P–extraction methods. As standard agronomic and environmental P extractants, respectively, water (P_H2O), dithionite citrate bicarbonate (P_Dith), ammonium oxalate (P_Ox), ammonium lactate (P_AL), double lactate (P_DL), and sodium bicarbonate (P_Olsen) were selected. The P_Red fraction was not correlated with P_AL, P_DL, P_olsen, and the degree of P saturation, but with P_H2O (r = 0.43*), P_Dith (r = 0.60***), and P_Ox (r = 0.61***). Furthermore, P_Red depended on the concentration of amorphous Fe oxides (Fe_Ox, r = 0.53**) and was closely correlated with the concentration of microbially reduced Fe (Fe_Red, r = 0.94***). This indicated the influence of the Fe(III)‐reducing activity of soil microorganisms on P mobilization. In subsoils, low in Fe(III)‐reducing activity, no P was released by the Fe(III)‐reduction test, which was in contrast to the results from the other chemical extraction methods. Additional alterations of the microbial activity by inhibiting and activating amendments, respectively, clearly affected the microbial Fe(III)‐reducing activity and the associated release of P_Red. Thus, P_Red, determined by the Fe(III)‐reduction test, might be termed as the fraction that is potentially released from soil by microbial reduction.