A METHOD FOR DETERMINING MICROBIALLY AVAILABLE-N AND AVAILABLE-P IN AN ORGANIC SOIL

A bioassay of microbially available soil N and P is described. It is based on the addition of glucose together with N or P to soil, followed by monitoring of the respiration rate. The addition of glucose+N resulted in an immediate increase in the soil respiration rate followed by a short period of exponential increase, reflecting the growth of microorganisms on the added substrate. The exponential phase levelled off, when lack of P prevented further growth of the soil microorganisms. The soil respiration rate then remained constant for several hours before decreasing, when glucose became limiting. The addition of glucose+P resulted in a lower plateau of the soil respiration rate, indicating that microbial growth was more limited by N than P in this forest soil (0.28 and 0.79 Mg CO2 g-1 organic matter h-1, respectively). Additions of the limiting nutrient resulted in a proportional increase in the constant level of the soil respiration rate. This was used to calculate the increase in the soil respiration rate per mg N (0.71 mg CO2 h-1) or mg P (4.6 mg CO2 h-1) added to this particular soil. Microbially available N was then calculated in two ways from the regression equation (0.15 or 0.40 mg g-1 organic matter) and P (0.13 or 0.17 mg g-1 organic matter). A comparison with 2 M KCl extraction showed that in nutrient-poor forest soils the microbially available N was 6.3 or 18.5 times higher than the KCl extractable N.