| Pinus koraiensis is not only the dominant forest species, but also the extremely economic value of timber species in north temperate forests, China. Currently, virgin forest of Pinus koraiensis is rare, except a few natural reserve. Natural secondary forest is instead of virgin forest because of large-scale logging. It is degradation succession by human disturbance that virgin forest of Pinus koraiensis was replaced by secondary forests. Plants frequently well adapt to their nutrition habitat during a long evolution process, and have evolved mechanisms to regulate NH4+/NO3-. Plant species differ greatly in their capacities for the utilization of special N forms and these adaptations may contribute to the unique spatial and/or temporal distributions of these species. It can be an important feature of plant community species composition. Characteristics of N quantity and form may be a main obstacle factor of Pinus koraiensis regeneration in secondary forests soil.In the current study, one-year-old Pinus koraiensis and Betula platyphylla seedlings were cultivated by nutrient solution. The experiment was carried out to investigate N uptake and utilization in nutrient solution of different N forms, N concentration and pH, and in different light intensity. Phosphorus adsorption and habitat pH change characteristics were analysed in the process of N absorption. Driving mechanisms of ecological recovery were investigated according to N status in secondary forests soil and N uptake and utilization of the two tree species. The control measures were proposed, which were beneficial to Pinus koraiensis regeneration. The main results and conclusions were as follows: 1. N uptake and utilization characteristics of Pinus koraiensis seedlings(1) Pinus koraiensis seedlings prefer ammonium over nitrate in their nutrient regimes. Absorption capacity of NH4+-N, NO3--N and total N increased with more N(NH4+-N or NO3--N) in nutrient solution. NH4+-N absorption capacity decreased because of superabundant NH4+-N in nutrient solution. Absorption capacity of NH4+-N decreased with higher light intensity, that of NO3--N increased. Absorption capacity of NH4+-N decreased with lower pH, that of NO3--N increased.(2) Nitrate reductase (NR) activity in roots was higher than in leaves, so that more nitrate may be reduced in roots. Light and nitrate generally are considered to be two major environmental factors that control NR activity. Enzyme activity responded to increased nitrate availability during light periods or increased light intensity.(3) Glutamine synthetase (GS) activity in leaves was higher than in roots, so that more ammonium may be assimilated by GS-GOGAT in leaves. GS activity of the roots and the leaves was higher in NH4+-N nutrient solution than in NO3--N nutrient solution, but GS activity decreased because of superabundant NH4+-N in nutrient solution. GS activity in the leaves of NO3--N nutrient solution increased with higher light intensity, and that of NH4+-N nutrient solution decreased.(4) Glutamate dehydrogenase (GDH) activity of the seedlings in NH4+-N nutrient solution was higher than in NO3--N nutrient solution. Enzyme activity responded to increased N (NH4+-N or/and NO3--N) availability. GDH activity of the seedlings in NH4+-N nutrient solution decreased with higher light intensity, and that in NO3--N nutrient solution increased. GDH activity of the seedlings increased with lower pH, and decreased with higher pH.(5) N concentration, N quantity and N allocation of the shoots in NH4+-N nutrient solution was more than in NO3--N nutrient solution. They responded to increased N (NH4+-N and NO3--N) availability of nutrient solution. N quantity of all the roots in NH4+-N nutrient solution increased with higher light intensity, and that of the seedlings decreased. N quantity of the seedlings in NH4+-N nutrient solution decreased with lower pH, and that in NO3--N nutrient solution decreased with higher pH.(6) Biomass in NH4+-N nutrient solution was more than in NO3--N nutrient solution. Biomass was response to increased N (NH4+-N and NO3--N) availability of nutrient solution or increased light intensity. Lower pH restrained the growth of the seedlings in NH4+-N nutrient solution, and was benefit for that in NO3--N nutrient solution.2. N uptake and utilization characteristics of Betula platyphylla seedlings(1) Betula platyphylla seedlings prefer nitrate over ammonium in their nutrient regimes. Absorption capacity of NH4+-N, NO3--N and total N increased with more N(NH4+-N or NO3--N) in nutrient solution. NH4+-N absorption capacity decreased because of superabundant NH4+-N in nutrient solution. Absorption capacity of NH4+-N, NO3--N and total N increased with higher light intensity. Absorption capacity of NH4+-N decreased with lower pH, and that of NO3--N increased.(2)NR activity in roots was higher than in leaves, so that more nitrate may be reduced in roots. Enzyme activity in the roots and the leaves responded to increased nitrate availability during light periods or increased light intensity.(3)GS activity in leaves was higher than in roots, so that more ammonium may be assimilated by GS-GOGAT in leaves. GS activity of the roots and the leaves in NO3--N was higher than in NH4+-N nutrient solution, and GS activity decreased because of superabundant NH4+-N in nutrient solution. GS activity in the roots and the leaves increased with higher light intensity.(4) GDH activity of the seedlings in NH4+-N nutrient solution was higher than in NO3--N nutrient solution. Enzyme activity responded to increased N availability. GDH activity in all the roots increased with higher light intensity, that in the leaves in NH4+-N nutrient solution increased, and that in the leaves in NO3--N nutrient solution decreased. GDH activity of roots and leaves decreased with lower pH.(5) N quantity of the seedlings and N allocation of the roots in NO3--N nutrient solution was more than in NH4+-N nutrient solution. N quantity responded to increased N (NH4+-N and NO3--N) availability of nutrient solution. N quantity of all the seedlings increased with increased light intensity. N quantity of seedlings in NH4+-N nutrient solution decreased with lower pH, and that in NO3--N nutrient solution increased.(6) Biomass in NO3--N nutrient solution was more than in NH4+-N nutrient solution. Biomass responded to increased N (NH4+-N and NO3--N) availability of nutrient solution or increased light intensity. Lower pH restrained the growth of the seedlings in NH4+-N nutrient solution, and benefit for that in NO3--N nutrient solution.3. P absorption capacity of both two tree species was decided by N uptake. P absorption capacity increased with more N(NH4+-N or NO3--N) in nutrient solution. P absorption capacity of Pinus koraiensis seedlings decreased with increased light intensity, and that of Betula platyphylla seedlings increased. Lower pH restrained P absorption capacity of the seedlings in NH4+-N nutrient solution, and was benefit for that in NO3--N nutrient solution.4. pH values of nutrient solution was affected by N forms. pH values decreased in NH4+(or more NH4+ over NO3-) nutrient solution, and increased in NO3-(or more NO3- over NH4+) nutrient solution. Habit pH values of Pinus koraiensis seedlings changed more deeply with more N (NH4+-N or NO3--N) in nutrient solution. Habit pH values of Betula platyphylla seedlings changed more deeply with more NH4+-N in nutrient solution, and that in more NO3--N nutrient solution were not decreased. pH values of Pinus koraiensis seedlings in NH4+-N nutrient solution decreased less with increased light intensity, and that of NO3--N nutrient solution increased. pH values in all the nutrient solution (NH4+-N and NO3--N) of Betula platyphylla seedlings decreased with higher light intensity. pH value raised sharply with reduction of initial pH values in nutrient solution, and that decreased with elevation of initial pH values.5. N uptake and utilization characteristics of Pinus koraiensis seedlings and Betula platyphylla seedlings was consistent with N status in their native soil. Characteristics of N quantity and form, and resources competition of pioneer species are some obstacle factors of Pinus koraiensis regeneration in secondary forests soil. |
