氮添加对中亚热带杜鹃灌丛凋落物生产和叶分解的影响

凋落物的生产和分解是生态系统养分循环的重要过程,受到大气氮沉降的深刻影响。但目前相关研究主要集中于森林和草地生态系统,氮沉降对灌丛生态系统凋落物养分归还的影响规律尚不清楚。因此选择亚热带分布广泛的杜鹃灌丛为研究对象,进行了为期两年的模拟氮沉降试验。试验设置4个处理:对照(CK, 0 g m-2 a-1)、低氮(LN, 2 g m-2 a-1)、中氮(MN, 5 g m-2 a-1)和高氮(HN, 10 g m-2 a-1)。结果显示:CK、LN、MN和HN 4种处理下,群落年平均凋落物量分别为(1936.54±358.9)、(2541.89±112.5)、(2342.97±519.8)、(2087.22±391.8) kg/hm²,LN、MN和HN处理样地的凋落量分别比对照样地高出32.68%、21.16%和7.93%;凋落叶、花果、凋落枝和其他组分占总凋落量的比例分别为75.75%、15.09%、7.70%和1.45%,不同浓度氮处理下各组分的凋落量均高于对照样地;凋落物组分表现出明显的季节动态:凋落叶在10—11月份达到峰值,凋落枝在10月份达到峰值,花果凋落物则在5月份凋落量最高,不同氮处理下凋落物的季节动态基本一致;白檀凋落叶分解速率显著高于杜鹃,二者分解95%所需时间分别为5.08—11.11 a和7.69—17.65 a,施氮使白檀凋落叶分解周期比对照样地缩短18.18%—54.28%;凋落叶分解过程中,N元素表现为富集-释放模式,P元素表现为富集模式。研究表明,氮添加能够促进群落中白檀凋落叶分解及N、P元素的释放,说明施氮可以调节凋落叶养分释放模式,对灌丛生态系统的养分循环具有调控作用。

Effects of nitrogen addition on litter production and leaf decomposition in Rhododendron simsii shrubland in the mid-subtropical of China

Litter is an important component of ecosystem which regulates the ecosystem material cycle and elements balance. In recent years, the increasing atmospheric nitrogen deposition profoundly affects the litter production and decomposition. In order to explore the effect of nitrogen deposition on litter nutrient return in shrub ecosystem, we conducted a short-term simulated nitrogen deposition experiment on the Rhododendron simsii shrubland in Dawei Mountain (Hunan Province) from August 2012 to August 2014. In this study, we established four treatments with different levels of nitrogen addition (each level with three replicates): control (CK, no nitrogen addition), low nitrogen addition (LN, 2 g m^-2 a^-1), medium nitrogen addition (MN, 5 g m^-2 a^-1), and high nitrogen addition (HN, 10 g m^-2 a^-1). We analyzed the effects of nitrogen addition on litter production, leaf decomposition and nutrient release. The results showed that annual mean litter biomass in CK, LN, MN and HN were (1936.54±358.9), (2541.89±112.5), (2342.97±519.8), and (2087.22±391.8) kg/hm², respectively. The annual mean litter biomass of LN, MN and HN were 32.68%, 21.16%, and 7.93% higher than that of CK, respectively. Leaf litter, sexual organ litter, branch litter and other components accounted for 75.75%, 15.09%, 7.70% and 1.45% of the total litter biomass, respectively, which showed that leaf litter was the main component in litterfall. In addition, the biomass of leaf litter, sexual organ litter, branch litter and other components in LN, MN and HN treatments were all higher than that in CK. The litter components in different treatments showed obviously seasonal dynamics with the similar pattern: the peak of leaf litter, branch litter and sexual organ litter occurred in October to November, October and May, respectively. This was because the dominant species in Rhododendron simsii shrubland were deciduous species, which would produce a lot of leaf and branch litters in autumn. Additionly, the flowering period of Rhododendron simsii is in April to May, which would lead to a large number of sexual organ litters in May. The decomposition rate of Rhododendron simsii leaf litter was slower than Symplocos hunanensis: Rhododendron simsii litter required 7.69-17.65 years for 95% decomposition, while Symplocos hunanensis litter only required 5.08-11.11 years. Nitrogen addition did not show significant effects on the release of C element during decomposition process of Rhododendron simsii and Symplocos hunanensis leaf litter, but significantly promoted the release of N and P elements of Symplocos hunanensis leaf litter. This study showed that nitrogen addition promoted the decomposition and elements release of Symplocos hunanensis leaf litter in Rhododendron simsii shrubland, which indicated that nitrogen addition could affect nutrient release patterns of leaf litter and regulate the nutrient cycling of shrub ecosystem.