IMPACT OF AGRICULTURAL PRACTICES ON THE SIZE AND ACTIVITY OF THE MICROBIAL BIOMASS IN A LONG-TERM FIELD EXPERIMENT

The Deherain long-term field experiment was initiated in 1875 to study the impact of fertilization on a wheat-sugarbeet rotation. In 1987, t he rotation was stopped to be replaced by continuous maize. Crop resid ues were soil-incorporated and the mineral fertilization was doubled i n some plots. The impact of those changes on the microbial biomass and activity are presented. In spring 1987, the soil was still in a stead y-state condition corresponding to the rotation. The microbial biomass was correlated with total organic C and decreased in the order farmya rd manure > mineral NPK > unfertilized control. Microbial specific res piratory activity was higher in the unfertilized treatments. The soil biomass was closely related to soil N plant uptake. In 1989, after 2 y ears of maize and crop residue incorporation, the steady-state conditi on corresponding to the previous agricultural practices disappeared. S o did the relationship between the biomass and total organic C, and th e soil N plant uptake. Biomass specific respiratory activity increased because of low efficiency in the use of maize residues by microbes un der N stress.