Kübert Angelika, Götz Miriam, Kuester Emma, Piayda Arndt, Werner Christiane, Rothfuss Youri, Dubbert Maren
Ecosystem Physiology, Freiburg University, Freiburg, Germany.
Thünen Institute of Climate-Smart Agriculture, Braunschweig, Germany.
Front Plant Sci. 2019 Aug 30;10:1051. doi: 10.3389/fpls.2019.01051. eCollection 2019.
Two important threats to the sustainable functioning of seminatural grasslands in temperate zones are (1) nutrient loading due to agricultural fertilization and pollution, and (2) the increase of extreme drought events due to climate change. These threats may cause substantial shifts in species diversity and abundance and considerably affect the carbon and water balance of ecosystems. The synergistic effects between those two threats, however, can be complex and are poorly understood. Here, we experimentally investigated the effects of nitrogen addition and extreme drought (separately and in combination) on a seminatural temperate grassland, located in Freiburg (South Germany). To study the grassland response, we combined eddy-covariance techniques with open gas exchange systems. Open gas exchange chambers were connected to an infrared gas analyzer and water isotope spectrometer, which allowed the partitioning of net ecosystem exchange and evapotranspiration. Vegetation parameters were described by species richness, species abundance, and leaf area index. Our results suggest that grassland communities, strongly weakened in their stress response by nitrogen loading, can substantially lose their carbon sink function during drought. While nitrogen addition caused a significant loss in forb species (-25%), precipitation reduction promoted a strong dominance of grass species at season start. Consequently, the grass-dominated and species-poor community suffered from a strong above-ground dieback during the dry summer months, likely caused by lower water use efficiency and weaker drought adaptations of the species community. Over the growing season (April-September), the carbon sequestration of the studied grassland was reduced by more than 60% as a consequence of nitrogen addition. Nitrogen addition in combination with precipitation reduction decreased carbon sequestration by 73%. Eutrophication can severely threaten the resilient functioning of grasslands, in particular when drought periods will increase as predicted by future climate scenarios. Our findings emphasize the importance of preserving high diversity of grasslands to strengthen their resistance against extreme events such as droughts.
（1）农业施肥和污染导致的养分负荷；（2）气候变化导致的极端干旱事件增加。这些威胁可能会使物种多样性和丰度发生显著变化，并对生态系统的碳和水平衡产生重大影响。然而，这两种威胁之间的协同效应可能很复杂，目前人们对此了解甚少。在这里，我们通过实验研究了添加氮和极端干旱（单独以及联合作用）对位于德国南部弗赖堡的半天然温带草原的影响。为了研究草原的响应，我们将涡度协方差技术与开放式气体交换系统相结合。开放式气体交换室连接到红外气体分析仪和水同位素光谱仪，这使得能够对净生态系统交换和蒸散进行区分。植被参数通过物种丰富度、物种丰度和叶面积指数来描述。我们的结果表明，因氮负荷而应激反应大幅减弱的草原群落，在干旱期间可能会大幅丧失其碳汇功能。虽然添加氮导致杂类草物种显著减少（-25%），但降水减少促使草类物种在季节开始时占据强势主导地位。因此，以草为主且物种贫乏的群落在干燥的夏季遭受了严重的地上部分枯死，这可能是由于物种群落较低的水分利用效率和较弱的干旱适应性造成的。在生长季节（4月至9月），由于添加氮，所研究草原的碳固存减少了60%以上。添加氮并结合降水减少使碳固存减少了73%。富营养化会严重威胁草原的弹性功能，特别是在未来气候情景预测干旱期将会增加的情况下。我们的研究结果强调了保持草原高多样性以增强其对干旱等极端事件抵抗力的重要性。
