大气氮沉降对中亚草地生态系统净初级生产力的影响

Abstract

氮沉降作为除气候变化、CO₂浓度升高以及土地利用变化之外的第四大主要影响陆地生态系统结构和功能的因素，其对碳循环过程的影响研究相对薄弱，同时也是不确定性最大的环节之一。近年来，由于长期高强度的放牧导致草地生态系统的生产力降低，氮成为典型草地植物生长和生态系统净初级生产力的主要限制因子。据研究，亚洲的氮沉降平均增速极有可能高于全球氮沉降平均增速，成为未来氮沉降增加最快的区域。在此背景下，研究大气氮沉降对于中亚草地生态系统的影响具有重要的意义。利用反硝化-分解模型（DNDC）分析1979-2014年中亚地区草地生态系统净初级生产力（NPP）的时空分异，探讨氮沉降对草地NPP影响。结果表明：（1）1979-2014年间，中亚地区平均草地NPP约为（173.10±31.80） g C m^-2 a^-1，草地NPP时空分异明显，各草地类型的NPP从大到小依次为森林草甸、温带草原和荒漠草原，并且草地NPP以（2.67±1.30） g C m^-2 a^-1的速度逐年增长；（2）当前氮沉降情景总体上促进了中亚地区草地NPP的增长，1979-2014年氮沉降使得中亚草地NPP增加了0.42 Pg C。;Climate change, increased CO₂ concentration, nitrogen deposition and land use change are currently recognized as the main factors affecting the structure and function of ecosystems. Among them, the research on the influence of nitrogen deposition on carbon cycling process is still insufficient with great uncertainty. In recent years, as the productivity of grassland ecosystem is reduced due to long-term intensive grazing, nitrogen has become a limiting factor of typical grassland. The increasing rate of nitrogen deposition in Asia is probably higher than the global average as reported by previous research. In view of this, it is of significant importance to study the effect of atmospheric nitrogen deposition on grassland ecosystem in Central Asia. In this study, Denitrification-Decomposition (DNDC) model was utilized to analyze the temporal and spatial variation of grassland net primary productivity (NPP) in Central Asia from 1979 to 2014, to explore the effects of nitrogen deposition on grassland NPP. The results showed that:(1) During 1979-2014, the annual averaged temperature in Central Asia showed a significant increase trend((0.03±0.01)℃ a^-1, R²=0.40), while the precipitation decreased with the trend of (2.26±1.78)mm a^-1(R²=0.35). (2) from 1979 to 2014, the average NPP of grassland in Central Asia was about(173.10±31.80)g C m^-2 a^-1, and the spatial and temporal differences of grassland NPP were obvious. The highest NPP of grassland was found in forest meadow, followed by temperate grassland, and desert grassland NPP. Moreover, the grassland NPP increased at the rate of(2.67±1.30)g C m^-2 a^-1. (3) The current nitrogen deposition scenario generally benefits the growth of grassland NPP in central Asia, which increases the grassland NPP by 0.42 Pg C in the last 36 years. Based on the existing studies, we cannot determine whether the decrease of NPP in this part of grassland is caused by reaching the critical load of nitrogen, and more in-depth studies are needed.