The role of above-ground competition and nitrogen vs. phosphorus enrichment in seedling survival of common European plant species of semi-natural grasslands

Anthropogenic activities have severely altered fluxes of nitrogen and phosphorus in ecosystems worldwide. In grasslands, subsequent negative effects are commonly attributed to competitive exclusion of plant species following increased above-ground biomass production. However, some studies have shown that this does not fully account for nutrient enrichment effects, questioning whether lowering competition by reducing grassland productivity through mowing or herbivory can mitigate the environmental impact of nutrient pollution. Furthermore, few studies so far discriminate between nitrogen and phosphorus pollution. We performed a full factorial experiment in greenhouse mesocosms combining nitrogen and phosphorus addition with two clipping regimes designed to relax above-ground competition. Next, we studied the survival and growth of seedlings of eight common European grassland species and found that five out of eight species showed higher survival under the clipping regime with the lowest above-ground competition. Phosphorus addition negatively affected seven plant species and nitrogen addition negatively affected four plant species. Importantly, the negative effects of nutrient addition and higher above-ground competition were independent of each other for all but one species. Our results suggest that at any given level of soil nutrients, relaxation of above-ground competition allows for higher seedling survival in grasslands. At the same time, even at low levels of above-ground competition, nutrient enrichment negatively affects survival as compared to nutrient-poor conditions. Therefore, although maintaining low above-ground competition appears essential for species’ recruitment, for instance through mowing or herbivory, these management efforts are likely to be insufficient and we conclude that environmental policies aimed to reduce both excess nitrogen and particularly phosphorus inputs are also necessary.

人类活动已严重改变全球生态系统的氮、磷通量。在草地生态系统中，由此引发的负面效应通常被归因于地上生物量增加后发生的植物物种竞争排除现象。然而，部分研究表明这一机制无法完全解释养分富集的影响，这引发了疑问：通过刈割或植食作用降低草地生产力以缓解种间竞争，是否能够减轻养分污染的环境危害？此外，迄今鲜有研究对氮污染与磷污染的效应进行区分。我们在温室中宇宙实验系统（mesocosms）中开展了一项全因子实验，设置氮、磷添加处理，并搭配两种刈割方案以弱化地上种间竞争。随后，我们针对8种欧洲常见草地植物的幼苗存活与生长状况展开研究，结果发现8个物种中有5个在地上竞争强度最低的刈割方案下存活率更高。磷添加对7种植物产生了负面效应，氮添加则对4种植物产生负面效应。值得注意的是，除1个物种外，其余所有物种的养分添加负面效应与地上竞争加剧的负面效应均相互独立。我们的研究结果表明，在任一土壤养分水平下，弱化地上种间竞争均可提升草地生态系统中的幼苗存活率。与此同时，即便在较低的地上竞争强度下，与养分匮乏条件相比，养分富集仍会对幼苗存活产生负面影响。因此，尽管维持较低的地上竞争强度（例如通过刈割或植食作用）似乎对物种补充至关重要，但此类管理措施可能仍显不足；我们认为，旨在减少过量氮输入、尤其是磷输入的环境政策同样不可或缺。