Regulating Mechanism Of Nitrogen Of Typical Plants Growth In Ziwuling Forest Region

Nitrogen is one of the decisive factors in controlling plant produciton. In the Loess Plateau, nitrogen is especially deficient. Therefore the regulating mechanism of nitrogen of typical plants growth will be helpful to understand the different nutrient use efficiency among plant functional types. Aimed to the conflict of vegetation construction and soil nitrogen shortage in the Loess Plateau, we study the patterns of nitrogen and the relationship between leaf nitrogen concentration and photosynthetic rate among different life forms and succession stages plants in Ziwuling forest region on the Loess Plateau, and the the mechanism of nitrogen and root restriction on plant growth and photosynthesis were revealed. At the same time, we also study nitrogen uptake and utilization by using the nitrogen isotopic (δ15N) tracing method. Base on these results, we want to explicit the control strategy of nitrogen in typical plants on the Loess Plateau, which can pose a theoretic guidance for the resume and reconstruct of vegetation in the Loess Plateau. The main results are as follows:(1): The leaf carbon concentration, nitrogen concentration, phosphorus concentration, C:N and C:P ratios varied markedly across the life forms, and there appeared significant differences of the leaf carbon concentration, nitrogen concentration, phosphorus concentration, C:N and C:P ratios, as well as statistically insignificant differences of N:P ratios among the different life forms (deciduous broad-leaved trees, evergreen needle-leaved trees, shrubs and herbage). The leaf carbon concentration appeared to vary narrowly, but the leaf nitrogen concentration and phosphorus concentration appeared to vary greatly. The C:N, C:P and N:P ratios primarily ranged within 21.9～69.1, 177～781 and 7.55～16.9, and the C:P of Pinus tabulaeformis were almost 4 times higher than that of Artemisia sacrorum.(2): The leaf nitrogen concentration per unit mass (Nm), leaf nitrogen concentration per unit area (Na), specific leaf area (SLA), photosynthetic rate (Pn) and photosynthetic nitrogen use efficiency (PNUE) significantly differed among different life forms (deciduous broad-leaved trees, evergreen needle-leaved trees, shrubs and herbage). The Nm were significantly and positively correlated with the Pn, SLA and quantum yield of PSII electron transport (ФPSII). There was positive correlation between the Pn, PNUE and SLA , while theФPSII weakly correlated with SLA. With the succession of vegetation, the leaf nitrogen concentration were increased first but decreased then, and the leaf nitrogen concentration of dominant species at shrub community stage were significantly higher than that at other succession stages. The Pn and stomatal conductance (Gs) were higher at the stages of herbage and shrub communities than at the early and climax stages of forest community, but PNUE were in the order: herbage stages>climax stages of forest community>early stages of forest community>shrub stages.(3): Fine root biomass of artifical Pinus tabulaeformis forests showed a single hump -shaped curve with soil depth increased, but fine root biomass of Betula platyphylla forests decreased with soil depth increased. Root surface area, specific leaf length and root lengthy density of artificial Pinus tabulaeformis and Betula platyphylla forests in the upper soil layer (0-20cm) were greater than these in the lower two layers (20-40 and 40-60 cm). Root surface area of Betula platyphylla in upper soil depth was 3.91 times higher than that of artifical Pinus tabulaeformis, but root surface area of Betula platyphylla in the soil layer (20-40cm) was 33% lower than that of artifical Pinus tabulaeformis. Distribution characteristics of fine root (fine root biomass, fine root surface area, specific leaf length and root lengthy density) of artificial Pinus tabulaeformis and Betula platyphylla forests more significantly related to the changes of soil total nitrogen and concentration of organic matter than ammonia (NH4+-N) and nitrate (NO3--N).(4): The shoot biomass, root biomass and root mass /shoot mass ratio of Trifolium repens L.were higher at photosynthetic photon flux density of 400±20 umol·m-2·s-1 (I400) than at photosynthetic photon flux density of 200±20 umol·m-2·s-1 (I200). Shoot biomass of Trifolium repens L. showed a single hump-shaped curve with nitrogen concentration increased, but root biomass and root mass /shoot mass ratio of Trifolium repens L decreased with nitrogen concentration increased. The Nm increased with nitrogen concentration increased, and Nm was higher at I200 than at I400. The Pn, PNUE, potential efficiency of primary conversion of light energy of PSII (Fv/Fm), the excitation capture efficiency of PSII (Fv'/Fm') andФPSII of Trifolium repens L. increased from 5 to 15 mmol·L-1 and then decreased with the nitrogen concentrations varying from 15 to 25 mmol·L-1. The Pn, PNUE andФPSII of Trifolium repens L. were higher at I400 than at I200, but Fv/Fm and Fv'/Fm'of Trifolium repens L. were lower at I400 than at I200.(5): The Pn of Forsythia suspensa (Thunb.) Vahl. were significantly different between different root zone and nitrogen concentraiton, and effects of interaction of root restriction and nitrogen concentraiton on Pn of Forsythia suspensa (Thunb.) Vahl. were significantly. Comparatively, the Fv/Fm,ФPSⅡand non-photochemical quenching coefficient (qNP) of Forsythia suspensa (Thunb.) Vahl. seedlings under low N treatment (LN) were higher than that under high N treatment (HN), and the low root zone (LR) seedlings under low or high N treatment had higher Fv/Fm and qNP but lowerФPSⅡand photochemical quenching coefficient (qP) than high root zone (HR) seedlings. The absorption and distribution of 15N of Forsythia suspensa (Thunb.) Vahl. seedlings were affected by root restriction and nitrogen concentration. The percentages of N absorbed in leaves and branches were 7%-10% and 7%-12% respectively, and 65% of the 15N absorbed by roots were allocated on aboverground of Forsythia suspensa (Thunb.) Vahl. seedlings.