EFFECT OF CONVENTIONAL AND BIOLOGICAL FAR MING ON MICROBIAL BIOMASS AND ITS NITROGEN TURNOVER IN AGRICULTURALLY USED LUVISOLS OF THE FRIEDBERG PLAINS

The influence of conventional and biological farming on microbial biom ass-carbon (Bio-C) and nitrogen (Bio-N) of agriculturally used fields was compared. The fields were located on luvisols of the Friedberg pla ins and tilled according to each system for many years. Bio-C and Bio- N were measured using the chloroform-fumigation-extraction technique. During early spring and autumn (1990) Bio-C [% C(org)] and Bio-N [% N( t)] were slightly lower in sites of conventional farming than on field s tilled by biological farming. During the vegetation period Bio-C and Bio-N increased continuously in both systems. This increase, however was higher on conventional than on biological fields and may be ascrib ed to a higher C-input (rhizodeposition) and availability of mineral n utrients. In the soils of the biological farming system N-deficiency a pparently decreased Bio-N (but not Bio-C) in spring time. Consequently the Bio-C/N-ratio increased. It seems as if plant growth affects the quality of microbes (Bio-C/N). The average turnover-time of Bio-N was about 1 year in all soils. However, the average turnover-time of Bio-C was 1.6 years, being 2 times faster in the conventional than in the b iological farming system. It seems as if long-term management practice s may affect the amount of labile organic matter by changing the bioma ss pool and its turnover rate.