The total nitrogen content (N_total) well demonstrates the evolutional-genetic characteristics of different soils, but doesn't represent real pool of active nitrogen transforming by microorganisms and using by plants. The active nitrogen pool includes the mineral forms and part of organic mineralizable nitrogen. The nitrogen mineralization rate in soil is controlled by hydrothermal conditions and depends on protectability of nitrogen-containing organic mater. The aim of this work is to evaluate nitrogen mineralizing ability of gray forest soil from different ecosystems (Moscow region, Russia) and leached chernozem sampled from the long-term field experiment with organic and mineral fertilizers application (Penza region, Russia). N_total amount in nonfertilized leached chernozem even after 22-years of cropping was in 2.1 and 3 times more, then in gray forest soil from forest ecosystem and nonfertilized agroecosystem, correspondently. The amount of total mineralizable nitrogen was estimated by mineral nitrogen accumulation (N_min) in soil during the 120-days incubation under 24^oC and 60% of total moisture capacity. The N_min experimental data were approximated by the first order kinetics equations. The amount of net-mineralized soil nitrogen (N_{net min}) was calculated basing on the following equations:

N_min = N_{tot min}•[1-exp(-k•t)]

N_min = N₀ + N_{net min}•[1-exp(-k•t)]

where N_{tot min} - total mineralizable nitrogen; N₀ - initial mineral N (NH₄⁺-N and NO₃^--N) in soil; k - mineralization rate constant; t - incubation time; N_{net min} - net-mineralized nitrogen amount.

The part of mineralizable nitrogen from N_total content was 2.6-5.1% in gray forest soil from different ecosystems, decreasing in a following order: forest > grassland > nonfertilized agroecosystem. The N_{tot min} part in leached chernozem was only 1.4-1.9%, and the ratio N_{tot min} / N_total was almost the same both in fallow plot and in nonfertilized variant of crop rotation. The calculated ratio N_{tot min} / N_total shows that organic matter protectability and nitrogen sequestration ability of leached chernozem are 2-3 times higher, than of gray forest soil. This fact is the main reason of nitrogen "economical" consumption and continuous nitrogen keeping in chernozemic soils. The gray forest soil from natural ecosystems has the same nitrogen mineralizable pool, as leached chernozem in the fallow plot. In nonfertilized agroecosystems the higher nitrogen mineralizing ability is peculiar to the leached chernozem. On the other hand, the manure usage once per 5 years in dozes 25 and 50 t ga^-1 separately or together with inorganic fertilizers leads to relatively small increasing of the mineralizable nitrogen in comparison with control (approximately at 1.1-1.5 time). The N_{tot min} amount in fertilized variants is less than an N_{tot min} amount in the fallow plot. The experimental and calculated N_{net min} amount shows that the real increase of mineralizable nitrogen in leached chernozem take place only under manure application. The existing practice of organic fertilizers usage once per 5 years doesn't results in increasing of carbon- and nitrogen-stabilizing potentials of leached chernozem. Increasing of the N₀ part in soil sampled from the organic-mineral sites and respective decreasing of net-mineralization are agree with modern concept. In accordance with this concept the nitrogen fertilizers reduce mineralization of humificated organic matter, and the deficient supply of leached chernozem by easily-mineralizable organic matter can be the cause of the mineralized and immobilizated nitrogen disbalance. In this case the N_min pool produced by inorganic fertilizer application is short-stored and can be lost from soil. Acknowledgements: This work was supported by Russian Foundation for Basic Researches (Grant No. 04-04-48670) and Presidium of Russian Academy of Sciences (Program No. 13, Project No. 4.1).