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Abstract. Previous studies of the microbial status of soils managed under ‘organic’ and ‘conventional’ regimes have produced conflicting evidence of whether there are distinct differences in the size, composition and activity of the soil microbial biomass which may be attributed to management practice. In the present study, we have compared the microbiology of organically‐ and conventionally‐managed soils at (primarily) two farms in England, over a two year period. Differences in microbial communities in soils under different management practice were subtle rather than dramatic. Many of the parameters measured, including total C and microbial biomass C, often showed no consistently significant differences in soils under different management. In soils from one farm, concentrations of ATP in Ringers solution soil extracts were mostly found to be significantly greater in organically‐managed than in comparable conventionally‐managed soils. While indirect (plate) counts showed that there were similar numbers of cultivable microorganisms present in these soils, total counts of bacteria (via DAPI‐staining) were found to parallel the trends found for readily‐extractable ATP. Numbers of metabolically‐active bacteria, determined by FISH analysis using a EUB338 probe to detect ribosome‐rich cells, indicated that the percentage of metabolically‐active bacteria present was not determined by management practice. Total and active fungi were also found to be more abundant in organically‐managed soils. It was concluded that changes in soil microbiology may occur as a consequence of switching to organic land management, but these may not be detectable by methods used frequently to assess soil biomass. In particular, increased numbers of viable but non‐culturable bacteria and fungi in organically‐managed soils points to a greater physiological diversity of microorganisms in such situations.

The effect of inorganic N additions on the biodegradation and microbial use of organic N pools in soil is poorly understood. To examine the effects of inorganic N on the mineralization rates of amino acids, four soils under contrasting management regimes were subjected to increasing loadings of NH4NO3, ranging from 0 to 120 kg N ha−1 In addition, the effect of soil sieving and storage temperature and time on amino acid mineralization was also investigated. At times ranging from 1 to 40 d after the addition of the inorganic N, the mineralization kinetics of an equimolar mixture of fifteen 14C‐labeled amino acids was followed for a subsequent 24‐h period. The rate of 14CO2 evolution was soil dependent, with half‐lives ranging from 2 h for topsoils to 25 h for subsoils. For all soils, at all times, and at all inorganic‐N loadings, the addition of inorganic N appeared to have little effect on the mineralization kinetics of the amino acids to 14CO2 In addition, the presence of inorganic N also had no major effect on the C use efficiency of the microbial biomass. It is speculated that N release from the amino acids into the soil by the microbial biomass may also be little affected by inorganic‐N additions. Sieving and storage of soil at either 4 or 18°C for up to 40 d had little impact on amino acid mineralization rate. Experiments with potential microbial disrupting agents (autoclaving, CHCl3 fumigation, HgCl2, and freeze–thaw) all indicated that the observed mineralization of amino acid C was due to microbial activity. We conclude therefore that inorganic N and soil storage has little effect on the microbial use of readily assimilatable amino acids.