Research On Coupling Effects Of Water And Nitrogen In Fast-growing And High-yield Poplar Plantations

At present, more than 50% of the timber are imported from abroad in China. At the same time as the comprehensive protection measures for natural forest in our country and the international timber trade protection consciousness gradually improve. Thus, the problem of timber security is very serious in China. One important solution to this problem is to vigorously develop fast-growing and high-yield plantations. Poplar is one of the most promising short rotation plantation species. Poplar plantation area in our country in the world first, accounting for 18.1% of the national tree plantations in China. However, the average productivity of poplar plantations in China is still below the global average level (15 m3·hm-2-yr-1), which can be mainly attributed to its inefficient intensive silvicultural practices such as water and fertilization management. Irrigation and fertilization management have great potentials to enhance yield for plantations. The objectives of this study were to evaluate the effects of water and nitrogen management on the eco-physiological characters of tree, the growth and distribution of fine root, the growth and productivities of trees, carbon storage of plant and soil in the Populus×euramericana plantations. And propose the compatible water and nitrogen combination to the benefit of the growth. Meanwhile, this study will also provide specific guidelines for water and nitrogen management in plantations of other poplar species.(1) In the three growing seasons, the average soil water potential was-13.79,-21.15 and-29.88 kPa,-14.19,-23.83 and-37.00 kPa,-14.00,-25.83 and-31.00 kPa in the three irrigation treatments of-25,-50 and-75 kPa, respectively. Additionally, our study results showed that vertically, soil organic matter, soil total nitrogen, soil available P and soil available K in all treatments were mainly at 0-20 cm depths. The combination of surface drip irrigation and fertigation management increased soil nutrients of poplar plantation, especially in the 0-20 cm and 20-40 cm depths, but did not change the vertical distribution pattern and the contents in the 40-60 cm depths. For soil available K, there was no significant difference between 20-40 cm and 40-60 cm depths. The soil organic matter, soil total nitrogen, soil available P and soil available K in the water and fertilizer treatments were significantly larger than CK. Especially, soil nutrients in the D2F3 treatment and the D3F3 treatment were significantly higher than others treatments, but no significant difference between D2F3 and D3F3 treatments. Compared with CK, the soil organic matter after first and second year of the experiment were significantly improved, respectively 1-81 and 29-134%, the soil total N after first and second year of the experiment were significantly improved, respectively 3-59% and 31-81%. According to the distribution of most soil nutrients parameters, the vertical distribution pattern of soil nutrients is not affected by the interaction between water and fertilizer. However, the surface drip irrigation and fertigation management options that significantly increase soil nutrients were identified.(2) According to the distribution of most fine root parameters, the vertical distribution pattern of fines is not affected by the interaction between water and fertilizer. Fine roots were mainly distributed in 0～10cm and 10～20cm. After the first year of the experiment, the biomass, root length density, surface area and volume of fine roots in the three irrigation treatments were significantly larger than CK under middle and high fertilizer level, but this was not found under low fertilizer level. Compared with CK, the fine roots biomass of three soil layers were significantly improved, respectively 316%,386% and 442%, and the root length density was significantly increased by 345%,176% and 176% respectively. Fine root biomass changed with the fertilization varied by a regression equation which was fitted with high a value of R2(0.702～0.891), under the same fertilization level in each soil layer but was fitted with low relationship under the same drip irrigation in each soil layer. After the second year of the experiment, fine roots in the irrigation and fertilization treatments were significantly higher than that of the CK, except the D1F1 treatment. Additionally, fine roots in the D2F3 treatment and the D3F3 treatment were significantly higher than those in other treatments, but not significantly different between D2F3 and D3F3. Finally, we found that the response of fine root growth and distribution was stronger to fertilization than to the irrigation in this poplar plantation.(3) The seasonal growth pattern and daily dynamics of LAI, photosynthesis factors and sap flow velocity in the poplar plantation appeared not affected by the interaction of irrigation and nitrogen addition. In the three growing season, the LAI in the D3F3 treatment was significantly higher than those in other treatments. The LAI in the D3F3 treatment was 21,44 and 58% higher than that in the CK-treated trees after the first, second and third years of the experiment, respectively. The Pn of poplar trees under the interaction of irrigation and nitrogen treatment were 23-63% and 26-95% higher than that in the CK-treated trees after the first and second years of the experiment, respectively. The WUE of poplar trees under the interaction of irrigation and nitrogen treatment were 30-110% and 18-82% higher than that in the CK-treated trees after the first and second years of the experiment, respectively. The sap flow velocity of poplar has a significant positive correlation with Rs, Ta and SWC, and has a significant negative correlation with RH. The Vsf of poplar trees under the interaction of irrigation and nitrogen treatment were 27-70% and 13-69% higher than that in the CK-treated trees after the first and second years of the experiment, respectively. Finally, we found that the response of eco-physiological characters were stronger to irrigation than to the fertilization in this poplar plantation.(4) The results of three-year (ages 5,6 and 7) repeated experiments showed that SDIF significantly increased biomass and carbon storage in both plant and soil compared with the CK. The treatment did not significantly affect carbon concentrations in tree biomass components. Carbon concentrations varied among biomass components, followed the pattern:trunk> branch> coarse root> fine root> leaf. Mean carbon concentrations ranged from 46.3% in the leaf component to 58.3% in trunk. The SDIF-treated plantation contained 11.49,27.68 and 38.70 t ha-1 of total biomass at age 5,6 and 7, respectively; the CK- treated plantation contained 7.54,15.77 and 24.88 t ha-1 of total biomass at age 5,6 and 7, respectively. Total biomass in the SDIF-treated trees was 52,75 and 60% higher than that in the CK-treated trees after the first, second and third years of the experiment, respectively. The SDIF-treated plantation contained 6.20,15.18 and 21.72 t ha-1 of plant carbon storage at age 5,6 and 7, respectively; the CK-treated plantation contained 4.05,8.63 and 13.46 t ha-1 of plant carbon storage at age 5,6 and 7, respectively. Carbon storage in the SDIF-treated tree was 53%,76% and 61% higher than that in the CK-treated trees after the first, second and third years of the experiment, respectively. Biomass and carbon storage in fine roots were not significantly different between the treatments, but higher percentage of root biomass increment was associated with the CK. The SDIF increased soil carbon concentration, especially in the surface soil of 0-20 cm depth. Soil organic carbon in the depth of 0-60 cm under SDIF treatment was 45.42,50.87 and 61.32 tha-1 in the three years, respectively. The corresponding soil organic carbon in the CK was 43.08,43.57 and 47.92 t ha-1 in the three years, respectively. The results confirmed the significant effect of the combined management and may be applied to the forestation practice, though it generally does not change carbon concentration in tree components.(5) Increase the fertilizer rate was significantly promote the growth of poplar under the same irrigation amount, but increase the irrigation amount was not significantly promote the growth of poplar under the same fertilizer rate. The annual increment of DBH, H and V in the poplar trees under D3F3-treated trees were 23,58 and 44% higher than that in the CK-treated trees after the first year of the experiment. After the second and third years of the experiment, the annual increment of DBH, H and V in the poplar trees under D2F3-treated trees were 33,27 and 36%, and 24,44 and 57% higher than that in the CK-treated trees, respectively. Moreover, the maximum economic benefit was 5522.37,24223.74 and 39619.07 Yuan ha-1 at the ages 5,6 and 7, respectively. The economic benefit was 4311.00,19011.50 and 32429.00 Yuan ha-1 in CK, respectively. The economic benefit in the best treatment was 28,27 and 22% higher than that in the CK-treated trees at the ages 5,6 and 7, respectively.