中文摘要：

以渭北黄土区20、30a和44a刺槐人工林生态系统为对象,研究了其内植物-枯落物-土壤的氮（N）磷（P）化学计量特征。结果表明：刺槐叶的N、P含量最高,干的N、P含量最低。土壤表层是N的富集区,而P在土层中分布均匀。随着林龄的增加,刺槐叶、根和枯落物的N、P含量呈下降趋势（p〈0.05）,刺槐枝的N、P含量呈上升趋势（p〈0.05）,刺槐干的N、P含量无显著变化;土壤中的N含量呈增加趋势,P含量则相反。刺槐叶、枝、干和枯落物不同林龄间的N∶P无显著性差异,平均值分别为18.75、21.39、15.43和17.20;30a和44a刺槐根的N∶P分别是20a的2.37和2.30倍（p〈0.05）。土壤的N∶P随着林龄的增加而增大,各层土壤20a和44a时的N∶P均达到显著性差异（p〈0.05）。土壤的N∶P随着土层深度的增大而减小,表层土壤的N∶P显著高于其下各土层（p〈0.05）。表明P是该地区刺槐生长的主要限制元素。

英文摘要：

Taking 20-,30- and 44-year-old Robinia pseudoacacia plantations growing in Weibei Loess Plat- eau region as research objects, nitrogen （N） and phosphorus （P） stoichiometry characteristics in tree-litter- soil systems were examined. The results showed that the contents of N and P were the highest in the leaves and lowest in the stems. N was mainly enriched in surface soil, and P distributed uniformly in all the soil layers. During stand development,along with the increase of stand age, the contents of N and P tended to decrease （p〈0.05） in the leaves,roots and litter layer,while increase （p〈0.05） in the branches. No sig- nificant differences were observed in the ratios of nitrogen to phosphorous among the stands with different ages in the leaves, branches, stems and litters, the average values were 18.75,1.39,15.43 and 17.20, re- spectively. The N ： P ratios of roots in 30- and 44-year-old stands were 2.37 and 2.30 times more than in 20-year-old stand （p〈0.05）. The soil N ： P ratio increased with stand age. The N ：P ratios in different soil layers were significantly different （p〈0.05） between 20- and 44-year-old stands. The N ： P ratio de- creased with soil depth. The N ： P ratio in surface soil was significantly （p〈0.05） higher than other soil layers, it was found that phosphorous was the main limited element for Robinia pseudoacacia growth in the area.