CHANGES IN MICROBIAL BIOMASS AND NITROGEN TURNOVER IN ACIDIC ORGANIC SOILS FOLLOWING LIMING

The effect of liming on microbial activity and N transformations was e xamined in four very acidic, organic topsoils to help explain reports of poor growth of conifers on some such soils. Data were obtained from laboratory incubations and from a long-term field experiment. Liming to a pH near 6.5 caused soil respiration and microbial biomass to at l east double in the first 4 d, mainly due to the proliferation of indig enous acid intolerant bacteria. Soil respiration then declined rapidly but microbial biomass less so, and in two soils, biomass remained sig nificantly greater than the control beyond 101 d. Liming caused an ini tial increase in net N mineralization in all soils, but beyond 34 d, i t became similar to or substantially less than the control. Nitrificat ion in the limed soils increased rapidly at around 30 d. The greatest reduction in N mineralization occurred in two soils with the greatest C-to-N ratio. Immobilization of N in the microbial biomass could not a ccount for the very small N mineralization rates observed in the long term, but a relatively small increase in the N concentration of the st ableresidual soil organic matter could. Small net N mineralization rat es in limed organic soils with a large C-to-N ratio that could persist for more than 10 y may cause N stress in single species stands of tre es on plantation forest, but it is unlikely to decrease the growth of vegetation in species-rich communities. (C) 1997 Elsevier Science Ltd.