| Populus tomentosa is a widely distributed native tree species in China. There is a suitable establishment area for it in the middle and lower reaches of the yellow river.It is a kind of timber which has a long life span and the main shelterbelt tree species characterized by fast-growing,white material,fiber length.strong resistance and adaptability.The research chose four clones (BT17,B331,S86,1316) which were planted for the subject in nursery of Wei xian County as the subject,and carried on coupling effect of water and nitrogen experiment by random block design.The main results were as follows:(1)In a growing season, N, P,K content of leaf had similar seasonal variation which decre-ased along with the season.The ranges of N,P,K was 23.60-27.83g·kg-1,0.61-1.36g·kg-1,6.62-6.92 g·kg-1,respectively.The difference between leaf nutrients content was extremely significant in Apr-il andJuly,but there was no significant difference between July and October.Three layers of soil total N was less in April than October,but available P and available K content was less in Oc-tober than in April.Nutrient content in leaves increased gradually with the increase of nitrogen.(2)The range of Pn of BT17 and S86 was 6.20-18.10μmol·m-2·s-1,6.03-16.60μmol·m-2·s-1, respectively.Pn of two clones had maximum on treatment of W3N3,was 18.10μmol·m-2·s-1 and 16.15μmol·m-2·s-1,respectively.The range of Tr on two clones was 1.51-7.09mmol·m-2·s-1 and 1.38-6.76mmol·m-2·s-1.There was the highest WUE (3.76μmolCO2·mmol-1H2O) under the treatment of W1N1 on BT17 clone,and was W3N2 (5.42μmolCO2·mmol-1H2O) on S86 clone. The leaf water loss process was consistent on two clones, and BT17 was slightly faster than S86 on leaf water loss.(3)The DBH growth peak of three clones (BT17,B331,1316) occurred in July and June,and S86 clone in July and May. All of the tree height growth peak of four clones occurred in July and June.The growth of BT17 and B331 were faster than others.The effect of moisture on tree height and DBH was W3>W2>W1, and the effect of nitrogen on tree height and diameter were varied,but all the best was 75% of field capacity,and the amount of urea applied was 459.6g/tree.(4)The DBH growth of four clones (BT17,B331,S86,1316) was the fastest under the treatment of W3N3,the value was 2.83cm,3.29cm,3.59cm,2.03cm,respectively.Compared with the control, DBH increased by 52.1%,44.3%,60.3%,24.5%,respectively.The tree height growth of four clones were 3.40m (W3N3),3.57m(W3N3),3.23m(W3N3),2.43m(W3N2),increased by 54.5%,87%,38.6%,15.7% compared with the control,respectively.(5) An conclusion can be reached by the evaluation of water and fertilizer efficiency:The maximum benefit of BT17 was attained under the treatment of W3N3 or W3N2,and B331 was attained under the treatment of W3N3 or W2N3,S86 and 1316 were attained under the treatment of W3N3 or W3N2.The growth of the four clones was at the fastest rate and more economic benefits were attained when the soil moisture content was 75% of field capacity,and the amount of urea applied was 459.6g/tree,phosphoric-acid diamine was 190.6g/tree,potassium sulfate was 80g/tree,compared with the control, economic benefits increased by 76%,66%,70%,71%,respectively.
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