氮水添加改变内蒙古典型草原两种优势植物的氮吸收偏好

Nitrogen and water addition alter nitrogen uptake preferences of two dominant plant species in a typical Inner Mongolian steppe

植物对不同形态氮的吸收直接影响输入到生态系统中氮的去向和氮动态的变化,但是资源添加如何影响植物氮吸收偏好仍不明确.本研究依托内蒙古典型草原连续开展了10年氮、水添加实验平台,选择克氏针茅(Stipa krylovii)和冰草(Agropyron cristatum)两个优势物种为研究对象,使用15N自然丰度法测定植物叶片和土壤各形态氮含量及其同位素特征,利用同位素混合模型量化不同氮源对植物氮的贡献,探究植物氮吸收偏好及其对资源添加的响应.结果表明,(1)自然状态下硝态氮和铵态氮对植物氮的贡献分别约为80%和20%;(2)氮添加减小了两种植物对硝态氮的利用,水分添加降低了克氏针茅而提高了冰草对硝态氮的利用,氮、水处理对植物氮吸收偏好的影响具有显著的交互作用.结果揭示,内蒙古典型草原优势植物在自然状态下主要利用较丰富的硝态氮,对较贫瘠的铵态氮利用较少;并且氮吸收特征随着资源添加而改变,水分对氮吸收的影响受到了氮素供给的调节.未来氮沉降加剧和硝态氮比例增加并非总是产生负效应,很可能促进温带草原优势植物的生长,减缓全球环境变化的生态后果.

The preferences for different nitrogen(N)forms by plants play an important role in determining the fate and dynamics of N in ecosystems.Nevertheless,it remains unclear whether and how resource addition influences N uptake preferences of plants.Here we investigated the effects of long-term(10 a)N and water addition on plant N uptake in a temperate semi-arid grassland.We used the 15N natural abundance technique to measure N content and isotope in soils and leaves of two dominant plant species,Stipa krylovii and Agropyron cristatum,and quantified the proportion of plant N derived from different N sources with an isotopic mixing model.We found that N addition,but not water addition,significantly increased soil nitrate and ammonium content,and also enhanced plant leaf N concentration.Soil nitrate accounted for 80%and ammonium for 20%contributing to plant N in non-treated controls.Compared to the controls,N addition reduced nitrate uptake of the two species,whereas water addition decreased nitrate uptake of S.krylovii but promoted the nitrate uptake of A.cristatum.Water interacted with N to significantly affect plant N uptake preference,indicating that the effect of water addition on plant N uptake varies with soil N availability.At the ambient N level,water addition enhanced ammonium utilization of S.krylovii but decreased that of A.cristatum.At the increased N level,water addition reduced ammonium uptake and enhanced nitrate uptake in both species.Our results demonstrate that the dominant plant species of the typical Inner Mongolian steppe mainly take up the relatively abundant nitrate rather than the scarce ammonium in this semiarid grassland.Plant preferences for different N forms changed with resource addition,suggesting that future N deposition and increases in nitrate availability may promote the growth of the dominant plant species in temperate grasslands,which may contribute to alleviating the global climate change.