SIGNIFICANCE OF MICROBIAL BIOMASS AND MINERAL 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 Niedersachsen loess area with straw application ((L)ambda 10 t . ha(-1), homogeneously 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 40 kg N . ha(-1), till May which was almost completely remobilized until han est. For a cropped plot (with winter wheat and 10 t . ha(-1) straw 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 the treatment (crop, with 140 kg fertilizer -N . ha(-1) or without N-fertilirer, respectively; fallow plot without fertilizer-N) within a range 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 fertilizer-N were assimilated by the microbes. Subsequently, remobilization of the immobilized nitrogen occurred within 2, (in March), or 6 weeks (in May), respectively. Simultaneously. organic soil-N was mineralized after each N-application and minerally fixed for its biggest, part. Between March and June, the fixed NH4+ decreased by about 112 kg N . ha(-1) 30 cm(-1).