Study On Coupling Effects Of Water And Fertilizer On Populus Tomentosa Seedlings

With Clone S86 of Chinese white poplar (Populus tomentosa Carr.) as test materials, we conducted a pot experiment in a gas-permeable greenhouse in a nursery of Beijing Forestry University and a field experiment in the experimental base in Wei County, Hebei Province, China. The specific objectives of this study were to quantify the coupling effects of soil water and fertilizer application on the physiological indexes inducing growth parameters, photosynthetic characteristics, enzymatic activity and nutrient contents of P. tomentosa seedlings, and to achieve the result of economizing water, saving fertilizer and increasing yield. By means of research, main conclusions are elicited as follows:(1) Soil relative water content (W) had the biggest impact on the growth of P. tomentosa seedlings. All the W, nitrogen application (N) and phosphorus application (P) treatments had obvious positive effects on height growth, leaf area and biomass yield with the size sequence of W>N>P. Correlations among the growth parameters of P. tomentosa seedlings subjected to different combinations of W, N and P treatments showed that biomass yield was positively correlated with the other growth parameters and the correlation coefficient of BY and LA was the highest, next was height growth and finally was basal diameter. A significant positive correlation between leaf area and height growth or basal diameter was also found. Therefore, the biomass yield of P. tomentosa seedlings can be predicted with the values of growth parameters.There were significantly positive interaction effects of W×N on biomass yield of P. tomentosa seedlings, whereas the interaction effects of W×P and N×P were not significant. The field experiment of interactions between W and N indicted that stem had the heaviest dry matter, next was root and last was leaf, where the proportion of dry weight of stem, root and leaf in total biomass yield were 59.48%, 22.13% and 18.38%, respectively. At lower W levels, there was a higher growth and production with the lower levels of N but not with medium or higher levels. At medium W levels, the highest biomass yield could be obtained at medium N levels. At higher W levels, there was a higher growth and production at medium levels of N. Meanwhile, biomass yield of seedlings was decreased with W increasing at lower N or non-N levels and certain water deficit could improve plant growth. At medium or higher levels of N, biomass yield of seedlings was significantly increased with W increasing and the higher W levels, the greater biomass yield.(2) Net photosynthetic rate (Pn) in the leaves of P. tomentosa seedlings was sensitive to different combinations of W, N and P treatments. A progressive increase in Pn of the seedlings was found from July to September and the peak value found in September. Differing from Pn, the variation trend of transpiration rate (Tr) first increased and then dropped with time and the peak value of Tr was obtained in August. There was an obviously positive correlation between Pn and stomatal conductance (Gs),Tr or intercellular CO2 concentration (Ci) but highly negative correlation between Pn and water use efficiency (WUE) or stomatal limitation (Ls). And, there was an obviously negative correlation between WUE and Pn, Gs, Tr or Ci but highly positive correlation between WUE and Ls. Our results show that the inhibition of photosynthesis under different soil water and fertilizer treatments was mainly attributed to the stomatal factors and also due to nonstomatal factors, that is, the suppression of chloroplast capacity to fix CO2 caused by the imbalances of nutrient and water contents in leaves.At lower W levels, the value of Pn was decreased with the increase of N levels and the lowest Pn was found at the highest N levels. At medium W levels, the highest value of Pn was obtained at medium N levels and over-N applications also decreased Pn. At higher W levels, there was a higher Pn at higher N levels than medium or lower N levels and the value was lowest at non-N level. Pn kept increasing with the increase of W levels under the N at any fixed level and there was the highest values of the three parameters at the higher levels of W. When N at a fixed level, light compensate point (LCP) signifying the photosynthetic capacity in low light decreased with W increasing, whereas the value of LCP was higher at medium or lower N levels.(3) The coupling effects of soil water and fertilizer applications on WUE and nutrient contents were significant and had certain threshold. WUE in the leaves of P. tomentosa seedlings initially dropped and then improved with time and the peak value of WUE was obtained in September. From June to October, the variation trend of leaf N content was a down-up-down sequence and leaf P and K contents were up-down-up.Specifically, increasing N application could improve the nutrient absorption capacity of seedlings subjected to lower levels of W. At medium and higher W levels, plant nutrient content was initially improved with an increase of N application but actually decreased under excessively high levels of N fertilizer, suggesting that soil water was closely related to the nutrient content and sufficient water supply can significantly improved the nutrient absorption capacity. As for WUE, at medium and lower W levels, the value of WUE in the leaves of P. tomentosa seedlings was higher with the lower levels of N. And, the value of WUE of seedlings subjected to non-N level was highest when soil water was sufficient. In contrast, there was a higher WUE with the higher levels of W at higher or lower N levels.(4) In the pot experiment, a multiple target decision model synthesizing relevant parameters was established to obtain the optimized combination. We concluded that the recommended combination of W, N and P for growers was W=78.1% of the field capacity, N=4.48 g plant-1, and P=1.67 g plant-1 (i.e. coded values of W, N and P factors were 1.55,1.33 and 0.56, respectively).In the field experiment, we conducted a comprehensive evaluation on plant growth, physiological characters, water use efficiency, fertilizer absorption amount and economic benefits of Populus tomentosa seedlings grown in field coupling affected by W and N. The results showed that W3N2 treatment (i.e. W=70%～75%, N=300 kg hm-2) was the best combination whereas W1N0 treatment should be eliminated in the field management.All these recommendations should help to achieve maximum growth potential of P.tomentosa seedlings, obtain high water and fertilizer use efficiencies and reduce the risk of nitrate pollution of groundwater in arid and semi-arid regions.