羌塘高原降水梯度带紫花针茅叶片氮回收特征及影响因素

Leaf nitrogen resorption efficiency of Stipa purpurea and its determinants along a precipitation gradient on the Changtang Plateau

植物回收衰老叶片的氮是植物重要的养分保持和环境适应机制,在寒旱贫瘠的生境更是如此。为了理解降水梯度上植物对高寒贫瘠环境的养分适应特征,研究了羌塘高寒草原优势物种紫花针茅叶片氮回收策略及其与环境因子的关系。结果表明,降水梯度带上紫花针茅叶片具有较高的叶氮水平和氮回收能力。生长季盛期紫花针茅绿叶平均氮含量为(23.87±3.92)g/kg,高于中国草地平均水平(20.9 g/kg)及全球平均值(20.1 g/kg);绿叶氮含量与年降水量(MAP)呈显著负相关,干旱端(西部)绿叶中氮含量明显高于湿润端(东部)。枯叶养分回收后的氮水平(NRP)很低,平均为(6.76±1.42)g/kg,叶片平均氮回收效率(NRE)为(71.25±6.46)%,明显高于中国温带草原和全球的平均水平(46.9%—58.5%)。枯叶中氮回收水平对叶片氮回收效率起决定作用,是维持高养分回收效率的物质基础。NRE与MAP、土壤全氮(TN)和土壤无机氮呈显著负相关;NRP与TN相关性不显著,但与土壤无机氮显著负相关。尽管NRE与NRP呈显著负相关,但二者与绿叶氮含量均没有显著相关性。年均气温、海拔对NRE和NRP影响均不显著。因此,紫花针茅叶片极高的NRE和低NRP反映了它对极端干旱贫瘠环境的养分保持能力,通过内部氮循环来降低养分流失。土壤氮的有效性是影响紫花针茅叶片氮回收能力的关键因子,降水通过影响土壤氮的有效性以及绿叶中氮含量间接影响紫花针茅叶片氮回收效率。

Nitrogen（N） resorption from senescent leaves has long been recognized as an important mechanism by which plants can effectively use soil nutrients and adapt to their environments,particularly under arid and infertile conditions.Many studies have shown that N resorption capacity is controlled by N contents in the soil or vegetation,which is influenced by precipitation.There exists a remarkable precipitation gradient（ranging from less than 100 mm to 700 mm） from the west to east on the Changtang Plateau,where the climate is highly cold and dry and soil nutrients are poor.To better understandthe N use characteristics along a precipitation gradient in the alpine nutrient-poor environment,we investigated leaf N resorption strategies of a dominant plant species,Stipa purpurea and their correlations with environmental factors on the Changtang Plateau in Tibet.Green and senescent leaves of S.purpurea and soil samples were collected along the precipitation gradient in July and October 2014.Nitrogen resorption efficiency（NRE,relative reduction in N between green and senescent leaves） and N resorption proficiency（NRP,absolute reduction in N in senescent leaves） were calculated,and the relationships between NRE,NRP,N contents,and climate were determined.Stipa purpurea had higher leaf N concentration and NRE along the precipitation gradient.The average green leaf N concentration of S.purpurea was（23.87 ±3.92） g/kg in the growing season,which was higher than the mean levels across China（20.9 g/kg） and the world（20.1g/kg）.The leaf N concentration,which was higher in the west than in the east of the plateau,was closely correlated with mean annual precipitation（MAP）.The average NRP was（6.76 ± 1.42） g/kg.The mean NRE was（71.25 ± 6.46） %,which was considerably higher than the counterpart both in the Chinese temperate grasslands and in grasslands worldwide（46.9%—58.5%）.The N levels of senescent leaves were among the key determinants of NRE.High levels of N resorption from senescent leaves were the internal basis for high NRE.NRE was significantly correlated with MAP,soil total N（TN）,and soil inorganic N;NRP was only related with soil inorganic N,but not with TN and MAP.The NRP and NRE were not related to leaf N concentrations,although both were negatively correlated.The influence of mean annual temperature and altitude on NRE and NRP was not significant.Therefore,high NRE and low NRP of S.purpurea might be indicators of N conservation in the arid and nutrient-poor environment.Internal N cycling through N resorption might have reduced N loss.Our findings suggest that soil N availability is a key controlling factor for the N resorption capacity of S.purpurea along the precipitation gradient on the Changtang Plateau,and that precipitation has an indirect effect on NRE by influencing soil N availability and green leaf N concentrations.