长期施氮与灌溉对毛白杨人工林土壤硝态氮分布与积累的影响

目的 研究灌溉和施氮对硝态氮在毛白杨林地土壤中积累与分布的影响，为毛白杨速生丰产林科学精准施肥和减少硝态氮淋失提供依据。 方法 在华北平原毛白杨适生地河北威县，利用"十二五"毛白杨大径材培育研究长期试验地，研究了通过灌溉保持不同土壤水分状况（田间持水量45%、60%、75%）和施氮量（0.0、101.6、203.2、304.8 kg·hm-2）对土壤硝态氮分布与积累的影响。 结果 土壤硝态氮分布具有明显的表聚性，在0200 cm土层呈先下降后上升再下降的"S"型变化趋势，且施氮量越大，硝态氮在60100 cm土层累积的趋势越明显。随着土壤水分的增加，土壤硝态氮的淋洗峰所在土层深度分别在4060、6080、80100 cm。土壤硝态氮累积量随施氮量的增加而增加，随灌水量的增加则呈先增加后减少的趋势；水氮交互作用对硝态氮的分布与积累影响显著。林地不同水氮组合土壤硝态氮积累量为86.11259.29 kg·hm-2。 结论 施氮量与土壤水分含量对毛白杨林地土壤硝态氮的分布与积累影响显著，在0200 cm土层内，随着施氮量的增加，硝态氮的积累量呈上升趋势；随着土壤含水量的增加，硝态氮积累峰所在的土层呈逐渐下移的趋势。与农田生态系统相比，试验林地0200 cm土层内硝态氮积累量较低，林地生态系统庞大林木根系对硝态氮吸收作用值得重视。同时，为了减少硝态氮的淋失，建议减少氮肥的施用，并将田间持水量控制在60% 75%。

Effects of Long-term Irrigation and Nitrogen Application on Distribution and Accumulation of Soil Nitrate in Populus tomentosa var. tomentosa Plantations

Objective To investigate the effects of irrigation and nitrogen application on soil nitrate accumulation and distribution, so as to provide visions for specific irrigation and fertilization management principles and guidelines as well as reducing nitrate leaching in intensively managed Populus tomentosa var. tomentosa industrial plantation. Method The study site locates in Weixian County of Hebei Province where a long-term irrigation and fertilization supported by "12th Five-Year Plan" research project has been initiated with aims to improve timber production. By maintaining the soil water content above different field water holding capacity (45%, 60%, 75%) and different nitrogen application rate (0.0, 101.6, 203.2, and 304.8 kg·hm-2), the distribution pattern and accumulation of soil nitrate were studied. Result Soil nitrate content had an obvious surface accumulation tendency. From 0-200 cm, the soil nitrate content followed a S-shaped pattern (decrease-increase-decrease). The higher rate the nitrogen, the more the accumulation of nitrate in the soil layer between 60-100 cm. With the increase of soil moisture level, the leaching peak of soil nitrate was going down to 40-60, 60-80, and 80-100 cm, respectively. The nitrate accumulation in soil increased with the increase of nitrogen application rate. With the increase of soil moisture level, the soil nitrate showed an up then down pattern. Meanwhile, the water and nitrogen had a significant positive interaction on the distribution and accumulation of nitrate. In this study, different combinations of water and nitrogen of the soil nitrate accumulation ranged from 86.11-259.29 kg·hm-2. Conclusion Nitrogen application and soil moisture levels have significant effects on the distribution and accumulation of soil nitrate in the P. tomentosa var. tomentosa stand. With the increase of nitrogen application rate, the soil nitrate shows a tendency to build up in the soil layer between 0-200 cm. With the increase of irrigation level, the soil nitrate accumulation peak tends to move downward gradually. Compared with the farmland ecosystem, the accumulation of nitrate between 0-200 cm in the forest ecosystem is lower. This indicates that in evaluation of soil nitrate distribution and accumulation, consideration should be given to the deep root system of the forest ecosystem. While, in order to reduce the leaching of nitrate nitrogen, it is recommended to reduce the application of nitrogen fertilizer and control the field water holding capacity between 60% and 75%.