SIGNIFICANCE OF MICROBIAL BIOMASS AND MIN ERAL FIXED AMMONIUM WITH RESPECT TO THE NITROGEN TRANSFORMATIONS IN LOESS SOILS OF NIEDERSACHSEN DURING THE GROWING-SEASON OF WINTER-WHEAT .2. N-15-TURNOVER

Field experiments 1988/89 on a fallow plot of the southern Niedersachs en loess area with straw application ((L)ambda 10 t . ha(-1), homogene ously incorporated by hand) yielded an increase in microbial biomass-N (N-mic) by 60 kg N . ha(-1). 30 cm(-1) until March 1989 and further 4 0 kg N . ha(-1), till May which was almost completely remobilized unti l han est. For a cropped plot (with winter wheat and 10 t . ha(-1) str aw incorporation), N immobilization was of similar magnitude. Up to 18 % of the applied N-15- fertilizer (185 kg N . ha(-1)) were microbially immobilized. In contrast to 1988/89, no significant mass change of N- mic occurred in 1991 due to straw application ((=) over cap 10 t . ha( -1)). Variations in the amount of : N-mic were nearly independent on t he treatment (crop, with 140 kg fertilizer -N . ha(-1) or without N-fe rtilirer, respectively; fallow plot without fertilizer-N) within a ran ge of 225-400 kg N . ha(-1). 30 cm(-1), Directly after N:application ( each 70 kg N . ha(-1) in March and in May), up to 100% of the fertiliz er-N were assimilated by the microbes. Subsequently, remobilization of the immobilized nitrogen occurred within 2, (in March), or 6 weeks (i n May), respectively. Simultaneously. organic soil-N was mineralized a fter each N-application and minerally fixed for its biggest, part. Bet ween March and June, the fixed NH4+ decreased by about 112 kg N . ha(- 1) 30 cm(-1).