MEASUREMENT OF NET NITROGEN MINERALIZATIO N IN GRASSLAND - A COMPARISON OF METHODS

Three in situ methods to measure net nitrogen mineralization (NNM) wer e compared. The NNM (0-20 cm) was measured after plant removal as accu mulated ammonium and nitrate N according to the following methods: a) in open cores with an anion exchange resin layer (resin core method), b) in closed polyethylene bags (buried bag method) and c) in covered f allow plots. For the resin core method, the influence of exposure rime on the NNM, as well as changes in water content and microbial biomass N during the exposure were investigated. In the fallow plots, either nitrate leaching, denitrification or microbial immobilization led to u nreliable results. Compared to the resin core method, the buried bag m ethod showed higher NNM values. The resin core method gave similar res ults for exposure times of 3 and 6 weeks. Compared with the undisturbe d soil, the water content in cores after 6 weeks of exposure was highe r in dry grassland, whereas the microbial biomass N remained constant. In contrast, the irrigated plot showed no differences in water conten t, but a biomass N loss of about 26-45 %. For further in situ measurem ents of NNM the use of the resin core method is recommended with expos ure times of 6 weeks. Changes in water content and microbial biomass s hould be determined as well.