Above-ground and below-ground responses to long-term nutrient addition across a retrogressive chronosequence

1. There is much interest in understanding ecosystem responses to local-scale soil fertility variation, which has often been studied using retrogressive chronosequences that span thousands of years and show declining fertility and plant productivity over time. There have been few attempts to experimentally test how plant nutrient limitation changes during retrogression. 2. We studied a well-characterized system of 30 forested lake islands in northern Sweden that collectively represent a 5350-year post-fire retrogressive chronosequence, with fertility and productivity decreasing as time since fire increases. For each island we set up four plots on understorey vegetation, each subjected to a different fertilizer treatment over six years: no additions, nitrogen (N) only, phosphorus (P) only, and N + P. 3. We found that both N and P additions reduced feather moss and thus total plant biomass. Meanwhile the three dominant vascular plant species showed contrasting biomass responses, but similar responses of foliar nutrient concentrations to nutrient additions. Fertilization reduced most microbial groups and altered CO2 fluxes, most likely through feather moss reduction. Against expectations, the majority of interactive effects of N and P were antagonistic. 4. Changes in effects of nutrient additions during retrogression were usually modest. Empetrum hermaphroditum biomass was increasingly promoted by P and N + P addition while vascular plant N to P ratios were increasingly reduced by P addition, indicating increasing plant limitation by nutrients (notably P) during retrogression. Below-ground, positive effects of N addition on soil mineral N increased while negative effects of N addition on soil fungi decreased during retrogression; no other below-ground effects of fertilization changed along the gradient. 5. Synthesis. Our results show that forest understorey communities on islands of different fire history and thus stages of retrogression show relatively modest differences in how they respond to nutrient addition despite large changes in ecosystem productivity and soil fertility, probably because of high species turnover and adaptation of communities to infertile conditions. While increased nutrient availability (as expected through global change) may have important ecological consequences, these effects are likely, especially below-ground, to be rather similar across ecosystems that differ greatly in nutrient availability and productivity.

1. 学界对理解生态系统响应局域尺度土壤肥力变化的机制颇具兴趣，此类研究常借助跨度达数千年的退化演替序列（retrogressive chronosequence）开展，这类序列随时间推移会呈现土壤肥力与植物生产力持续下降的特征。但目前针对退化过程中植物养分限制如何变化的实验性研究仍较为匮乏。
2. 本研究以瑞典北部30个有林湖泊岛屿组成的典型研究系统为对象，该系统整体构成了一个跨度达5350年的火后退化演替序列，随火后时间增加，土壤肥力与植物生产力持续降低。我们在每个岛屿的林下植被区域设置4个样方，分别施加6年的不同肥料处理：不施肥、仅施氮（N）、仅施磷（P）以及氮磷配施。
3. 研究发现，施氮与施磷均会抑制羽藓（feather moss）的生长，进而降低总植物生物量。与此同时，3种优势维管植物的生物量响应模式存在显著差异，但它们的叶片养分浓度对养分添加的响应趋势较为一致。施肥会抑制多数微生物类群并改变二氧化碳（CO₂）通量，这一效应大概率由羽藓生物量下降介导。与预期相悖的是，氮磷交互作用大多表现为拮抗效应。
4. 退化过程中养分添加的效应变化通常较为温和。雌雄同株岩高兰（Empetrum hermaphroditum）的生物量随施磷以及氮磷配施的促进作用逐渐增强，而维管植物的氮磷比随施磷处理持续降低，这表明退化过程中植物受到的养分限制（尤其是磷限制）愈发严重。地下层面，施氮对土壤矿质氮的正效应随退化梯度逐渐增强，而施氮对土壤真菌的负效应随退化梯度逐渐减弱；其余施肥的地下效应未沿该梯度发生显著变化。
5. 综合分析：本研究结果表明，尽管不同火历史、进而处于不同退化阶段的岛屿上的森林林下群落，其生态系统生产力与土壤肥力变化幅度极大，但它们对养分添加的响应差异相对较小，这可能源于较高的物种周转速率以及群落对贫瘠生境的适应性。尽管养分有效性提升（正如全球变化预期的那样）可能带来重要的生态后果，但此类效应（尤其是地下生态过程相关的效应）在养分有效性与生产力差异悬殊的各类生态系统中，大概率较为相似。