Towards a conversion factor for soil microbial phosphorus

The available methods of microbial phosphorus (P) analysis do not allow full cell lysis during fumigation and complete P extraction. Consequently, a correction of microbial P (Pmic) extraction efficiency (Kp factor) is always necessary. Here we evaluated possible under- or overestimation of microbial P by comparing the direct determination of Kp for various soils with the Kp values obtained from the literature. We determined a soil-specific Kp value for Cambisol by coupling 33P labeling, anion exchange membranes (AEM) water extraction and liquid fumigation with CHCl3. The measured Kp for the Cambisol (0.69) was much higher than the Kp commonly used in the literature over the last 35 years (0.4). Experimentally measured Kp avoided overestimation of microbial P by more than 1.7 times. Therefore, a soil-specific Kp correction of extractability is a prerequisite for microbial P analysis. To improve Pmic accuracy without direct Kp determination, we conducted stepwise regression analysis between soil-specific Kp values from the literature and soil parameters. The Kp increased linearly with decreasing total P. An exponential increase of Kp with decreasing organic C (R2 = 0.45–0.76) revealed a threshold of 10 g Corg kg−1. Combining three soil parameters in multiple regression – Kp = 0.76–0.007*Corg - 0.56*Ptot + 0.004*Clay – enables an excellent Kp prediction (R2 = 0.99). We conclude that the Kp value of 0.4 commonly used for estimating microbial biomass P cannot be accepted as a constant. Thus, in the absence of soil-specific Kp, we recommend using the regression models considering the basic soil properties.