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 elevatio...

受土地管理强度降低及完全弃耕影响，固氮灌木绿桤木（Alnus viridis）目前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（N），并抑制其向山地森林的自然演替。我们在瑞士中阿尔卑斯山区的三个山谷中选取了两个海拔梯度（1650 m与1950 m a.s.l.，即海拔以上）的样地，针对牧场（对照样地）与相邻绿桤木灌丛样地中的氮库、氮通量及相关碳库开展研究。在两个海拔梯度下，树龄50年的绿桤木灌丛的总氮与总碳储量均未超过相邻牧场：二者的植物量与0~30 cm土层土壤中的总氮储量合计约为610 g N m⁻²。在绿桤木灌丛样地中，土壤氮库的减少与植物量氮库的增加相互抵消，这一现象大概率源于土壤氮周转速率的提升。绿桤木灌丛下的土壤溶液始终富含硝酸盐，在两个海拔梯度下的50年生灌丛样地中，整个生长季（6月至10月）的总氮淋失量为0.79 g N m⁻²·季⁻¹...