The effects of four cropping systems on soil microorganisms were investigated during 3 years. The cropping systems were B0, barley without nitrogen fertilizers; B120, barley with 120 kg N ha-1 year-1; GL, grass ley receiving 200 kg N ha-1 year-1; and LL, lucerne ley without nitrogen fertilizer additions. At samplings in September during three consecutive years no differences were found between treatments. Total fungal lengths ranged between 0.7 and 2.0 × 103 m and bacterial numbers between 3.5 and 7.2 × 109 g-1 dry wt. soil.
Twenty samplings over 3 years in B120 and in GL indicated higher numbers of bacteria and protozoa during the growing season, except for periods with moisture stress. No clear seasonal trends were found for the fungi. When comparing mean values for the 20 samplings, the grass ley contained significantly (P < 0.05) higher numbers of amoebae. Means of the bacterial numbers and biomass, total and FDA-active hyphal lengths were also higher or equal (FDA-active hyphae) but not significantly so.
Seventy-nine per cent of the bacterial biomass and 73% of the total fungal lengths were found in the top soil, where also 85% of the oxygen was consumed.