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在通气良好土壤中,硝态氮是植物吸收氮的主要形态[1-3],但硝酸根离子水溶性极强,不易被土壤胶体吸附,很容易随土壤下渗水淋失。据统计, 全球每年施入土壤中的氮肥量高达8 500~9 000万t[4],我国每年氮肥的施用量超过270万t[5],而氮肥利用率不足50%[6]。其中的原因之一是过量施氮造成硝态氮淋失,从而导致地下水和地表水污染日益严重[7]。因此,研究长期施氮下林地土壤硝态氮淋失状况,可为农林业提高氮肥利用率和减少硝态氮淋失提供科学依据。
目前,农业土壤硝态氮淋失的报道很多,且不同土地利用类型下硝态氮积累量存在较大差异[8]。0~2 m土层内农田硝态氮的积累量可超过325 kg·hm-2,菜地、果园的土壤硝态氮积累量分别为124~3 459、93~6 791 kg·hm-2 [8-12]。大量单因素试验结果表明,施氮量直接影响硝态氮在土壤中的动态变化,而灌溉或降雨也是影响硝态氮运移的一个重要因素[13-16],特别是在通气良好的砂质土壤中,水分的渗透作用增强,可能会增加硝态氮淋失的风险[17-19]。Bergström等[20]研究表明,施用过量的氮肥会造成硝态氮在土壤剖面形成累积峰,而植物根系同样也是影响土壤硝态氮淋失的一个重要因素。王朝晖等[21]发现,根系分布较深的植物比浅根植物更能有效地吸收氮素,减少深层土壤硝态氮积累。
杨树人工林作为我国速生丰产林的重要组成部分,种植面积多达850万hm2[22]。由于速生丰产林种植面积广,而且相对于其他速生树种,杨树所需要的水分及氮肥的供应也十分巨大。长期以来,人们把提高灌溉水利用率和增加化肥用量作为提高农林业产量的重要手段,因此,有不少学者对水肥因素之间的耦合机理进行了深入研究。因地制宜地调节水分和肥料,使其处于合理的范围,使水肥产生协同作用,达到“以水促肥”和“以肥促水”的目的,是实现林业生产节水节肥和高产高效的主要途径。贺勇等[23]发现,滴灌施肥处理下107杨幼林对N、P、K的吸收能力增强,最佳施氮量为120 g·株-1。陈兰岭等[24]在研究灌水与林木生长的关系时发现,增加灌水次数,将田间供水量控制在10 t·株-1,材积年增量最大;而毛白杨(Populus tomentosa var. tomentosa)作为我国北方优良乡土树种,生长迅速、材质优良,是华北地区主要的速生丰产用材林。基于培育大径材毛白杨速生丰产用材林,施氮与灌水显著影响林木的生长,也选育出了许多优良三倍体品种,如S86、BT17等;但杨树人工林地的硝态氮淋失状况研究较少[25],而长期施氮和灌水下的毛白杨人工林硝态氮淋失状况更是鲜有报道。随着氮肥常年定期施用以及水分的定期补充,使得硝态氮在林地中淋失的风险不断增加,最终可能会导致地下水污染,所以,长期不同水氮条件下的毛白杨人工林硝态氮淋失状况值得关注。
本研究以毛白杨S86为研究对象,探讨灌水和施肥对毛白杨林地硝态氮分布及积累的影响,为毛白杨人工林地科学精准施肥,提高氮肥利用率和减少硝态氮淋失提供依据。
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连续11年不同水、氮处理后S86无性系林地0~20、20~40、40~60、60~80、80~100、100~120、120~200 cm土层的土壤硝态氮含量分别为:3.78~13.36、3.32~10.74、2.82~10.49、2.61~10.49、3.099~.53、3.54~9.01、3.0~48.45 mg·kg-1。0~200 cm各土层内硝态氮平均含量变化范围为5.59~8.82 mg·kg-1,且呈逐层递减的趋势。硝态氮含量在林地土壤中有明显的表聚现象,且随着土层深度的增加,硝态氮含量总体有下降的趋势。
表 1表明:施氮量、灌水量和水氮交互作用均对土壤硝态氮含量影响极显著(p<0.01)。由F值大小可知:三者影响程度表现为施氮量>灌水量>水氮交互作用。
表 1 不同水氮组合间土壤硝态氮含量方差分析
Table 1. Variance analysis of soil nitrate N content under different water nitrogen coupling levels
变异来源
Sources of variationⅢ类平方和
Type Ⅲ sums of squares自由度
df均方
Mean squareF 显著性
sig.施氮量×灌水量Nitrogen Application×Irrigation 109.033 6 18.172 6.179 <0.01 施氮量Nitrogen Application 499.682 3 166.561 56.639 <0.01 灌水量Irrigation 153.159 2 76.580 26.041 <0.01 氮肥对硝态氮分布的影响较明显,基本表现为N3>N2>N1>N0。施用氮肥后,硝态氮在土层中的残留量明显增加,施氮量增大,硝态氮在土壤剖面各层次中的分布也呈现升高的趋势,施氮量越大,硝态氮在剖面内的起伏变化越大,而N0处理在整个剖面未出现明显的淋洗峰;土壤中硝态氮含量与施氮量的高低成正比,施氮量越高,硝态氮在土壤剖面中出现的峰值也越高(图 1)。
图 1 不同田间持水量和施氮水平下毛白杨林地土壤剖面硝态氮含量分布
Figure 1. Nitrate nitrogen content distribution in soil profile of Populus tomentosa var. tomentosa under different field water capacity and nitrogen application levels
灌水量的不同,也造成了硝态氮分布的差异性,W1处理中,N0硝态氮平均含量为2.61~3.78 mg·kg-1; 而W2处理中,N3硝态氮平均含量为7.37~11.80 mg·kg-1。根据硝态氮累积量公式I=h×C×B/10可计算出两组合0~200 cm的积累量为86.11~252.29 kg·hm-2。随着灌水量的增加,土壤硝态氮的淋洗峰所在土层深度在逐渐下降(图 1)。在W1水平下,硝态氮主要集中在0~60 cm处,在60 cm以下,硝态氮含量有起伏,但总体呈现下降的趋势。在W2水平下,硝态氮含量大多呈现先下降后上升再下降的趋势,峰值出现在80~100 cm处,在100~200 cm处硝态氮含量有明显的下降趋势。在W3水平下,硝态氮含量大多呈现先下降后上升再下降的趋势,峰值大多出现在100 cm处,在100 cm以下,硝态氮含量呈现下降趋势。
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施氮量对土壤中硝态氮的积累有显著影响(图 2)。在同一灌水量条件下,土壤硝态氮积累量随施氮量的增加而增大。具体表现为N3(240 g·株-1)>N2(160 g·株-1)>N1(80 g·株-1)>N0(0 g·株-1),其中,施氮量在N3水平下土壤硝态氮积累量最大,达到696.90 kg·hm-2,是N0的1.87倍。
图 2 施氮量对毛白杨林地土壤硝态氮积累的影响
Figure 2. Effects of nitrogen application on soil nitrate accumulation in Populus tomentosa var. tomentosa forestland
灌水量对土壤中硝态氮的积累也有显著影响(图 3)。在同一施氮条件下,土壤硝态氮的积累量随灌水量的增加呈先上升后下降的趋势,表现为W2(60%)>W3(75%)>W1(45%),当田间持水量达到60%时,林地土壤中硝态氮积累量最大为832.88 kg·hm-2,是W1的1.40倍。
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
Effects of Long-term Irrigation and Nitrogen Application on Distribution and Accumulation of Soil Nitrate in Populus tomentosa var. tomentosa Plantations
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摘要:
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%。 Abstract: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%. -
表 1 不同水氮组合间土壤硝态氮含量方差分析
Table 1. Variance analysis of soil nitrate N content under different water nitrogen coupling levels
变异来源
Sources of variationⅢ类平方和
Type Ⅲ sums of squares自由度
df均方
Mean squareF 显著性
sig.施氮量×灌水量Nitrogen Application×Irrigation 109.033 6 18.172 6.179 <0.01 施氮量Nitrogen Application 499.682 3 166.561 56.639 <0.01 灌水量Irrigation 153.159 2 76.580 26.041 <0.01 -
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