荒漠草原不同放牧强度背景下添加氮水对凋落物分解的影响

Effects of nitrogen and water addition on litter decomposition in desert grassland under different grazing intensities

凋落物分解是连接生态系统地上、地下过程的重要环节,决定了生态系统养分循环速率,但到目前为止对凋落物分解在荒漠草地生态系统受放牧以及外源资源补给影响的研究较少。本研究通过对不同放牧强度(对照、轻牧、中牧和重牧)短花针茅草原群落进行添加氮素(10.0 g N m-2 a-1)和增水(108 mm/a)处理,探讨群落水平凋落物分解速率的变化。研究结果显示,过去不同强度放牧历史对群落凋落物分解影响极显著(P<0.0001)。凋落物前期分解(135 d)过程中,凋落物初始C∶N比与凋落物分解速率常数呈显著负相关关系,表明凋落物可降解性在凋落物前期分解中起主要作用。轻度放牧影响下凋落物分解速度最快,这与该条件下凋落物C∶N比显著低于其他放牧强度下的有关,说明适度放牧不仅有利于群落维持,也在一定程度上有利于生态系统养分循环。当凋落物分解更长时间(870 d)后,对照区凋落物分解速率显著低于放牧处理样地,但凋落物初始C∶N比对凋落物分解速率没有显著影响。进一步分析显示,不同放牧强度背景下长期凋落物分解速率与分解环境的土壤微生物多样性成正相关关系,与群落盖度呈极显著(P<0.001)负相关关系。添加氮素显著(P<0.05)降低凋落物分解速度,但对凋落物氮含量无显著影响。生长季加水未影响凋落物质量及凋落物分解速度。研究结果表明,凋落物前期分解受凋落物质量影响,但较长时间凋落物分解则与分解过程中接受到的太阳辐射量有关。

Litter decomposition is an important ecological process which links the ground and underground components of an ecosystem,and determines the rates of nutrient cycling.However,it is still not clear that how litter decomposition is affected by grazing and potential increase in water and nitrogen(N)availability,especially in the desert grassland ecosystems where decomposition is relatively slow.In this study,we conducted an experiment on litter decomposition in a desert steppe dominated by Stipa breviflora with previous influences under different grazing intensities(no grazing,light grazing,moderate grazing,and heavy grazing)in combination with N addition(0.0 or 10.0 g N m-2 a-1)and water supplementation(0.0 or 108.0 mm/a).Our results showed that both grazing intensity(P<0.0001)and N addition(P<0.05)had significant effects on litter decomposition.After a short period decomposition(135 days),there was a significantly negative correlation between the initial litter C∶N ratio and the constant of litter decomposition rate,indicating that litter quality played an important role in this short-term decomposition.Litter decomposition was fastest under light grazing intensity as a result of significantly lower litter C∶N ratio than those of other grazing intensities.This indicated that grazing with appropriate intensity was not only beneficial to community maintenance,but also favorable to cycling of nutrients in grassland ecosystems.After a long period of decomposition(870 days),the decomposition rates in the non-grazing plots were significantly lower than those in the grazing plots regardless of grazing intensity.In addition,the decomposition was not influenced by the initial litter quality(C∶N ratio),rather it was positively correlated with the diversity of soil microbes,and negatively correlated with the community coverage.Nitrogen addition had no significant effects on litter N content,but slowed down the decomposition significantly.We also found that adding water in growing season did not affect litter quality and litter decomposition rate.Our results implicate that short-term litter decomposition was controlled by the initial litter quality while the long-term litter decomposition was influenced by solar radiation in the studied steppe ecosystem.