Data from: Exacerbated nitrogen limitation ends transient stimulation of grassland productivity by increased precipitation

Given that plant growth is often water-limited in grasslands, it has been proposed that projected increases in precipitation could increase plant productivity and carbon sequestration. However, the existing evidence for this hypothesis comes primarily from observational studies along natural precipitation gradients or from short-term manipulative experiments. It remains unclear whether long-term increased precipitation persistently stimulates grassland productivity. In the world's largest remaining temperate grassland, we found that experimentally increased precipitation enhanced net primary production, soil available nitrogen and foliar nitrogen concentrations during the first six years, but it ceased to do so in the following four years, unless nitrogen was simultaneously added with water. The 15N enrichment of plant and soil nitrogen pools in later years indicates increased nitrogen losses, which exacerbated nitrogen limitation and ended the stimulation of productivity by increased precipitation. Changes in species abundance might have contributed little to the changes in water treatment effects. Our study demonstrates that the long-term response of grassland ecosystems to increased precipitation will be mediated by nitrogen availability. Our results also point to a shift from co-limitation by water and nitrogen early to perhaps limitation by nitrogen only later in this temperate grassland, highlighting significant variations in the type of resource limitation induced by climate change.

鉴于温带草原的植物生长通常受水分限制，已有研究提出，未来降水增加可提升植物生产力与碳固存（carbon sequestration）。然而，当前支撑该假说的证据主要来自沿自然降水梯度开展的观测研究，或是短期操控实验（manipulative experiments）。目前尚不清楚长期增加降水是否会持续促进草原生产力。在全球现存规模最大的温带草原中，本研究发现，实验性增加降水在最初6年内可提升净初级生产力（net primary production）、土壤有效氮（soil available nitrogen）与叶片氮浓度，但在后续4年该促进效应消失，除非同时施加水与氮素。后续年份中植物与土壤氮库的15N富集现象表明氮素损失加剧，这加重了氮素限制，终止了降水增加对生产力的促进作用。物种丰度的变化对水分处理效应的改变贡献极小。本研究证实，草原生态系统对降水增加的长期响应将由氮素有效性介导。研究结果还揭示，该温带草原的资源限制类型从早期的水氮共同限制，逐渐转变为后期可能仅受氮素限制，凸显了气候变化引发的资源限制类型的显著变化。