Difference Of Poplar Fine Roots Longevity Between Successive Rotation Plantations And Its Regulation Mechanism

Poplar(Populus spp.) species are of considerable economic and ecological importance and they range among the most widely distributed tree species of the world, and Populus × euramericana become the main directional cultivation of poplar species with the characters of fast-growing and higher production. But, the successivereplanting poplar tree onthe same land results in the decline of soil nutrient status and forest productivity significantly. The succession management of poplar lead to the accumulation of phenolic acids around rhizosphere environment, which affects the nutrient uptake and circulation. The dynamic process of growth, senescence, death and decomposition of fine roots is along with the forest carbon distribution and nutrient cycle, and the speed of this process is decided by fine roots longevity, so it is the key to research fine roots longevity for finding the relationship between below-ground carbon consumption and forest productivity in successive poplar plantation. The growth of trees need nutrient absorbed from soil, which make trees input more photosynthetic product for fine root growing, while make the changes of fine roots morphological traits, and finally the nutrient deficient around rhizosphere lead to the senescence and death of fine roots. Therefore, the connection of fine roots growth, longevity and the rhizosphere effect of nitrogen and phenolic acids become more intimate. From this research, we can find that:1?The longevity of poplar fine roots showed significant difference between successive plantation. The number of live roots, death roots and total roots was higher in the secondary rotation forest than that of the first. The median fine roots longevity was 90 d in the secondary rotation forest, which was lower than that of the first(102d). The number and distribution of roots among each order of the first rotation forest were lower than that of the secondary, but the cumulative survival rate of lower root orders was significantly higher than that of the secondary rotation forest, while the cumulative survival rate of higher root orders was significantly lower.2?Poplar fine roots longevity(life survival time) significantly subject to the changes of soil factors. Fine roots longevity showed significantly negative correlation with soil total phenolic acids content(r =-0.707), while showed significantly positive correlation with soil available nitrogen(r =0.752), and the soil total phenol content have significant negative effect of available nitrogen. In addition, the changes of soil properties around rhizosphere envrionment caused by the accumulation of phenolic acids can indirectly affect the growth conditions of fine roots. Fine roots longevity present significantly negative correlation with soil bulk density and porosity(r value were 0.760, 0.732, respectively), and soil texture can indirectly affect fine roots longevity by influencing the status of soil physical and nutrient properties.3?The deficiency and abundance of soil nitrogen can inhibit the growth of poplar fine roots, especially for the 1–3 orders. The SRL of fine roots present gradually declined along with the increase of nitrogen concentrations, but other morphological traits of fine roots do not present a linear relationship with the increase of nitrogen concentrations. The N content of of the 3–5 orders was lower in the deficiency nitrogen treatment than that of the abundance nitrogen treatment, but fine roots C/N was present opposite tendency. Fine root C content of the 1–2 orders in the abundance nitrogen treatment was higher than that of the deficiency nitrogen treatment. Phenolic acids significantly improved fine roots diameter and volume of the 1–2 orders, while fine roots C content and C/N of higher orders were declined with the increase of phenolic acids gradients.4? There existed significant difference between the 1–3 orders, the 4th order and the 5th order of poplar fine roots in the interaction effect of phenolic acids and nitrogen concentration treatments by the analysis of PCA and correlation for the influence factors with the fine roots morphological traits and C, N content. The response of poplar fine roots to phenolic acids and nitrogen concentration treatments was closely related to the influence factor of root order. Nitrogen played an important role on the influence of poplar fine roots than that of phenolic acids gradients. Furthermore, root order was positively related to the C content and C/N of poplar fine roots, but showed negative correlation with the N content. The effect of phenolic acids on the C, N content and C/N of poplar fine roots showed opposite tendency than that of nitrogen concentration treatments, but their effects were all exactly weak.5?The dry mass of fine roots was significantly increased under the NH4+/NO3- of 1:3, while the root diameter was inhibited under the NH4+/NO3- of 1:7, while the SRL was increased of the 4–5 orders. Compare to the 0.5X phenolic acids gradient, the surface area, volume and dry mass of the 1–3 orders of poplar fine roots under the NH4+/NO3- of 1:3 and 1:7 were lower in the 0.5X phenolic acids gradient than that of 1.0X gradient, while the SRL under the NH4+/NO3- of 1:14 in the 0.5X phenolic acids gradient was lower than that of 1.0X gradient, but the dry mass and RTD of fine roots under the NH4+/NO3- of 1:14 in the 0.5X phenolic acids gradient was higher than that of 1.0X gradient. The C content and C/N of higher root orders(4–5) under the NH4+/NO3- of 1:14 were significant lower than that of 1:3 and 1:7, but the N content of higher root orders(4–5) under the NH4+/NO3- of 1:14 were significant higher than that of 1:3 and 1:7. Phenolic acids significantly improved the N content of the 4–5 orders under the NH4+/NO3- of 1:3, while inhibited the C/N of poplar fine roots under the same NH4+/NO3- treatment.6?Fine roots order was negatively related to the surface area and SRL by the analysis of PCA and correlation of the influence factors with the morphological traits and C, N content. In addition, phenolic acids present negative correlation with the fine root length, and NH4+/NO3- was significantly influenced the C, N content of poplar fine roots, but there do not present correlation between the volume and dry mass of poplar fine roots with root orders, NH4+/NO3- and phenolic acids.7?The anatomical characteristics of higher root orders were significantly increased than that of lower root orders. Root cross section diameter of the 1–2 orders under the NH4+/NO3-of 1:3 was higher than that of 1:7 and 1:14. Root vascular bundle diameter of the 1–3 orders under the NH4+/NO3- of 1:3 was higher than that of 1:14, while the root cortex thickness of the 1–2 orders under the NH4+/NO3- of 1:3 and 1:14 was higher than that of 1:7. Compared to the none phenolic acid environment(0X), the root cross section diameter and root cortex thickness of the 1–2 orders were inhibited by the 1.0X phenolic acids gradient. Fine roots order was positively related to the root cross section diameter, the root vascular bundle diameter and the ratio of vascular bundle to cross section area, while root anatomical characteristics present positive correlation with NH4+/NO3- but showed negative correlation with phenolic acids. However, the effect of NH4+/NO3- and phenolic acids all showed weak relationship with the anatomical characteristics of poplar fine roots.