| Mulberry(Morus L. Moraceae) is one of ideal ecological and economic trees that grow fast and are leafy. Artificial mulberry woodlands play important roles in environmental protection, air purification, soil and water conservation, climate modification, rural economic development and so on. The area of mulberry woodlands in China ranks the first of the world and is expected to increase in future. Previous studies have showed that natural or artificial forests can be a sink of atmosphere nitrogen(N) deposition by canopy interception as well as direct absorption of N that is retained on leaf surface. The basis of mulberry canopy interception is the water storage capacity of mulberry leaves. With increasing N deposition in China, it is expected that artificial mulberry woodlands will intercept and absorb considerable amount of active N from atmosphere, resulting in increasing N concentration in throughfall and stemflow of mulberry. However, it is little known whether increasing N deposition will change the characteristics of water holding capacity and water absorption process of mulberry leaves. If so, it will be useful to understand the ecohydrological function of mulberry woodlands. Here, the effect of varied N rates in simulated rainfall on the characteristics of water holding capacity and water absorption process of mulberry leaves with different shapes and ages were studied by indoor simulation experiments. And then, the amounts of atmosphere rainfall, throughfall and stemflow of mulberry as well as their N and phosphorus concentration during annual growth season were quantified in natural mulberry woodland. Finally, the effect of distance from mulberry woodland to vehicular road on dust retaining of mulberry leaves was evaluated. The major findings were shown as follows:(1) Shapes of heart(HS), dragon-paw(DP) and compound leaf(CL) are the most common shapes of mulberry leaves. The averaged leaf area of HS leaf was 198.5 cm2 per leaf which was 88% and 62% larger than DP and CL leaf, respectively. The values of maximum water holding capacity of mulberry leaves were varied with measuring methods(spraying or submersing). And these values were overall higher under spraying method. The maximum water holding capacity of mulberry leaves ranged from 7.5 to 10.6 mg/cm2, and these variations were mostly attributed to leaf shapes instead of N rates in simulated rainfall. On the other hand, mulberry leaves could take up water by about 0.4 g/g DW(dry weight) and then reach the statured leaf water content when submersed in simulated rainfall within two hours. This absorbed water occupied about 40% of maximum water storage capacity(water holding + water absorption) of mulberry leaves. Dynamics of leaf water content during water absorption process could be modeled well by exponential equation and saturated leaf water content was enhanced by N addition in simulated rainfall.(2) There was no significant difference in leaf areas of HS leaves with different leaf ages; whereas the averaged fresh weight of old leaves(OL) was significantly higher than that of mature leaves(ML) and new expanded leaves(NEL). Overall, the maximum water holding capacity of old leaves and mature leaves was similar but higher than that of new expanded leaves; while the maximum water holding capacity of these leaves was little affected by N rates in simulated rainfall. During annual growth season, the leaf water content of the new expanded leaves ranged from 3.8 to 4.1 g/g DW which was significantly higher than that of mature and old leaves. During water absorbing process, leaf water content of leaves with these three leaf age increased fast at the beginning, then slow down and finally reached saturated leaf water content, showing a dynamic that can be regressed by an exponential equation. The saturated leaf water content of new expanded leaves ranged from 4.4 to 4.8 g/g DW during growth season; and these leaves absorbed water as much as 0.73 g/g DW, indicating stronger capacity of water absorption. Response of mulberry leaves with different leaf age to N rates in simulated rainfall was different. High N concentration in simulated rainfall(4~8 mg/L) increased both rate and capacity of water absorption of new expanded leaves, while increased only water absorption capacity of mature and old leaves. The dynamic of water loss was similar with water absorption of leaves. Rate of water loss was fast at the beginning and slow down later, showing a dynamic that could be regressed by exponential equation. However, this process was minor affected by N rates in simulated rainfall.(3) In natural conditions, mulberry canopy as same as other economic tree species such as citrus tree had changed significantly the distribution of the rainfall and N and phosphorus(P) in rainfall within 12 samplings during the growth season in 2015. Stemflow volume of mulberry trees with heavy or light pruning ranged from 0.02 to 9.6 L/plant with an average of 4.2 and 5.5 L/plant, respectively. The averaged concentration of NH4+-N, NO3--N, total N(TN) and total P(TP) in atmosphere rainfall was 1.0, 1.1, 2.9 and 0.19 mg/L, respectively. This indicated heavy wet N deposition in this area. The NH4+-N and NO3--N concentration in throughfall and stemfall of mulberry woodlands with heavy or light pruning has no significant difference with that in atmosphere rainfall; while The averaged TN and TP concentrations in throughfall and stemfall of mulberry woodlands with heavy or light pruning were 3.8 and 0.49 mg/L, both of which were significantly higher than that in atmosphere rainfall. This indicated that the retained dust and other materials adsorbed on leaves and branches of mulberry were washed away by canopy intercepted rainfall, resulting increased N and P concentration in throughfall and stemflow.(4) Mulberry leaf has a strong ability of dust retaining. The amount of retained dust on mulberry leaves was decreased with the increasing distance from mulberry woodland to vehicular road. After 7 days without rainfall, the averaged amount of retained dust on leaves was 0.53 g/m2 for the site of 10-meters away from the vehicular road, whereas was 0.11 g/m2 for the site of 300-meters away from the vehicular road. Such trend was observed during the three samplings in 2015. The averaged content of NH4+-N, NO3--N, TN and TP in retained dust on leaves was 2.3, 1.8, 16.1 and 1.4 mg/m2, but these content had no obvious correlation with the amount of retained dust on mulberry leaves.In summary, mulberry canopy can hold and absorb considerable amount of rainfall, so as to change the ecological and hydrological processes of mulberry woodlands. At the same time, the nitrogen in rainfall increased the total water storage capacity of mulberry leaves to some extent. In the annual growth season, the new expanded leaves show the strongest capacity of water absorption; while mature and old leaves have higher water holding capacity. High N concentration in simulated rainfall increased both rate and capacity of water absorption of new expanded leaves, while increased only water absorption capacity of mature and old leaves. The distribution of atmosphere rainfall and dust as well as their N and P was changed by throughfall and stemflow of mulberry woodlands, resulting in N and P enrichment in root zone and improved nutrient content of soils under mulberry canopy. |
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