Organic nitrogen cycle, ammonification and nitrification activity in long-term field experiment

Results concerning carbon cycle, incidence of microorganisms and respiration activity of the bare fallow field experiment in Prague-Ruzyne were presented in our previous paper (Kubat et al., 1999). This paper summarises results concerning nitrogen cycle, incidence of microorganisms participating in the nitrogen cycle and ammonification and nitrification activities. High doses of organic manure increased the soil organic nitrogen content, similarly to the organic carbon content. Nitrogen accumulation in these plots has shown the same dynamics as the organic carbon accumulation. Similar trends were also demonstrated for other variants over the whole time of the experiment. The results have shown that nitrogen cannot be accumulated in soil for a longer time period except in the soil organic matter. All mineral nitrogen applied to the mineral fertilised plots was lost. The increase of the C/N ratio in control and mineral fertilised variants indicates the fact that nitrogenous components of the soil organic matter are more vulnerable to decomposition than those not containing nitrogen in its molecule. This supports the hypothesis concerning the peripheric nitrogenous components of the humic acids molecules. Organic manuring increased the average number of proteolytic bacteria, while mineral fertilisation decreased their number to about a half of that in control. Their number dropped down in organic variants in the third period to a level at the control, while the inhibitory effect of mineral fertilisation on these bacteria was maintained. Average number of the free living N-2-fixing bacteria was lower in tilled variants as compared to non-tilled control and it decreased continuously over the whole time of the experiment. These bacteria were almost eliminated in mineral fertilised soils. Average ammonia concentrations in organic manured plots were the highest during the second (steady slate) period in which the net nitrogen mineralisation increased. Proteolytic activity correlated quite well with the number of proteolytic bacteria in soil samples. Mineral fertilisation decreased proteolytic activity in soil samples to less than a half of that in controls. Nitrate nitrogen content in soil samples has shown basically the same trends as those of the ammonia nitrogen. Nitrification activity has shown a positive effect of organic manuring and inhibitory effect of extremely high doses of mineral fertilisation on this activity.