Data from: Interactions among nutrients govern the global grassland biomass, precipitation relationship

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially co-limit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass-MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass-MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were co-limited by both in additive, or synergistic forms. Unexpectedly, we found little evidence for mediation of biomass–MAP relationships by plant community diversity because relationships of species richness, evenness and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass-MAP relationships. These findings reveal multiple nutrient co-limitation as a defining feature of the global grassland biomass-MAP relationship.

全球各类生态系统正经历年平均降水量（mean annual precipitation, MAP）格局的改变，同时面临多种养分的富集过程；这些养分或可共同限制植物生物量生产。在草原生态系统中，地上植物总生物量的均值与年平均降水量密切相关，但在多种养分富集后，这一关联会发生何种变化仍不明朗。我们提出假说：随着添加的养分数量增加，全球生物量-年平均降水量关联的斜率会变得更陡，且斜率的增幅与添加养分间的相互作用形式有关，同时会因植物群落多样性变化的介导作用增强而改变。我们在六大洲的71处草原样地开展了调查，涵盖了全球草原在年平均降水量、物种多样性、管理方式及土壤类型上的全部变异梯度，测定了各样地的地上植物生物量生产与物种多样性。我们对所有样地施加氮、磷、钾及微量元素的全组合施肥处理，以明确各站点的生物量受哪些养分限制。正如所假说的那样，施加1种、2种或3种养分，会逐步提升全球生物量-年平均降水量关联的斜率。斜率增幅的大小，与样地是否受氮或磷限制、仅受其中一种养分限制，或是受二者共同限制（以加和或协同形式）存在对应关系。出乎意料的是，我们几乎未发现植物群落多样性介导生物量-年平均降水量关联的证据——物种丰富度、均匀度及β多样性与年平均降水量和生物量的关联均较弱，甚至存在反向关系。样地的基础属性，包括初始生物量生产、土壤特征及管理方式，对生物量-年平均降水量关联的变异解释度极低。这些研究结果表明，多种养分的共同限制是全球草原生物量-年平均降水量关联的核心特征。