Microbial diversity in soils is considered important for maintaining sustainability of agricultural production systems. However, the links between microbial diversity and ecosystem processes are not well understood. This study was designed to gain better understanding on the effect of management practices on the microbial community and how changes in the microbial community affect key soil processes. The effects of different quantities of organic matter on soil biology and nitrogen dynamics was determined in two soils with organic and conventional management histories that varied in soil microbial community structure but had the same fertility. The soils were amended with 0 (Control), 4 (L4) and 8 t ha^-1 (L8) of lupin (Lupinus angustifolius L.), respectively. Over a 81-day period, microbial biomass carbon and nitrogen, arginine deaminase activity, fluorescein diacetate hydrolysis, dehydrogenase enzyme activity, community structure of eubacteria and actinomycetes (by denaturing gradient gel electrophoresis (DGGE) following PCR amplification of 16S rDNA fragments) and N mineralisation were measured. The addition of 4 t ha^-1 of lupin was sufficient to stimulate the microbial community, resulting in microbial biomass growth and an increase in activity and N mineralisation. The response in L8 was not proportional to the extra amount added and levels of soil microbial properties were only 1.1 to 1.7-times higher than in L4. Microbial community structure was not affected by lupin amendment and did not change over time. These findings show that the addition of small amounts of green manures improved soil biology by increasing microbial bioma, enzyme ss size and activity irrespective of management history. No direct relationship existed between microbial structure, activities and N mineralisation, and that microbial community structure (by PCR-DGGE) was more strongly influenced by inherent soil and environmental factors than by short-term management practices.