Utilization of the biological nitrogen fixation for soil evaluation

Non-symbiotic nitrogen fixation (potential nitrogenase activity - PNA) of soil samples originating from different plots of long-term field experiments (selected variants: Nil, NPK [mineral fertilisation: 64.6-100 kg N/ha/year], FYM [farmyard manure], and FYM + NPK from three blocks 111, IV and B with different crop rotation) was determined in laboratory experiments. The symbiotic nitrogen fixation (total nitrogenase activity - TNA) of the same soil samples was evaluated in hydroponic experiments with pea (2001, 2002) and lucerne (2001) in which the soil samples were used as a natural inoculum. The high values of PNA were found in the variants fertilised with FYM in all three blocks and all experiments. Simultaneously, the variants fertilised with mineral NPK reached low values of PNA. The farmyard manuring enhanced the number of free-living bacteria Azotobacter spp. that were identified in all soil samples. In the hydroponic experiments with pea, the highest nonsignificant values of TNA were found in variants B 284 (FYM + NPK) and 111 254 (FYM + NPK) in 2001, and B 214 (FYM) and 111214 (FYM) in 2002. Plants inoculated with soil from these variants formed also high amounts of nodules (significant differences in block IV in 2001) and plant biomass. In the experiments with lucerne, the nonsignificantly highest TNA values were found in variant 111 154 (NPK). Variants from block 111 (214, 254) and IV (114 and 154) showed the non significantly lowest TNA values. The rhizobia that effectuate symbiosis with pea were more active in the soil samples in 2001 than those forming nodules on lucerne.