Data from: Shrub expansion of Alnus viridis drives former montane grassland into nitrogen saturation

The N2-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m−2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m−2 season−1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m−2 season−1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m−2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.固氮灌木绿桤木（Alnus viridis）目前因土地管理强度降低及完全弃耕，正在阿尔卑斯山区的山地草原中扩张。绿桤木会向原本氮素匮乏的草原输入大量氮（Nitrogen, N），并阻碍山地森林的自然演替。我们于瑞士中阿尔卑斯山区的三个山谷中，选取两处海拔梯度（1650米与1950米，m a.s.l.）的牧场（对照组）及相邻的绿桤木灌丛地，针对氮素库、氮素通量以及相关碳库展开研究。50年生绿桤木灌丛的总氮与碳储量，并未超过相邻牧场：两处海拔的牧场中，植物生物量加上0至30厘米深土壤的总氮储量总计约为610克氮每平方米。在绿桤木林分中，土壤氮库的缩减抵消了植物生物量氮库的增量，这大概率是土壤氮周转速率加快所导致的。绿桤木林下的土壤溶液始终富含硝酸盐，在两处海拔的50年生林分中，生长季（6月至10月）总氮淋失量为0.79克氮每平方米每季；而低海拔25年生灌丛的氮淋失通量最高，达1.76克氮每平方米每季，这清晰表明绿桤木灌丛存在有效氮过剩的现象。与之形成鲜明对比的是，所有牧场的氮淋失量在整个生长季均接近零（仅为0.08克氮每平方米）。在流域尺度上，若流域面积超过五分之一被绿桤木灌丛覆盖，溪流水体的硝酸盐浓度会升高，并在春季与秋季出现典型的冲刷峰值。我们的研究表明，无论海拔如何，绿桤木的扩张都会快速将拥有数百年历史的氮匮乏草原转变为氮饱和灌丛；且由于绿桤木的优势地位会抑制天然山地森林的自然重建，因此会降低区域景观的碳储存潜力。