Amazonian understory forests change phosphorus acquisition strategies under elevated CO2

The potential for the Amazon forest to continue functioning as a carbon (C) sink strongly depends on soil nutrient availability, particularly phosphorus (P), and on plants’ ability to adjust nutrient acquisition strategies. However, limited experimental evidence constrains the mechanistic representation of nutrient–carbon interactions in climate change models. We conducted an experiment in a P-depleted Amazonian understory forest, increasing atmospheric CO₂ in-situ by approximately 300 ppm using open-top chambers. Eight polypropylene open-top chambers (OTCs), each with a diameter of 2.4 m and a height of 3 m, were randomly installed in the forest understory. To prevent root proliferation and other effects from plants outside the OTCs, each chamber was surrounded by a circular soil trench measuring 30 cm in width and 50 cm in depth. The chambers were arranged in pairs, with four control chambers maintained at ambient CO2 (aCO2) and four elevated CO2 (eCO2) treatments, in which CO2 concentration was increased on average by ~ 300 ppm relative to the respective ambient OTC. Within the OTCs, we conducted various experiments to understand the effects of eCO2 on root parameters in the litter layer and in the soil, soil nutrient concentrations, microbial activity, and leaf litter decomposition. Our findings suggest that eCO₂ induced contrasting responses by roots along the litter–soil continuum that could facilitate nutrient uptake. In addition, intensifies the competition for P between plant roots and soil microorganisms, leading to changes in litter and soil P pools, exacerbating already strong nutrient constraints.亚马逊森林持续作为碳（C）汇发挥功能的潜力，极大程度依赖于土壤养分有效性，尤其是磷（P），以及植物调整养分获取策略的能力。然而，有限的实验证据制约了气候变化模型中养分-碳相互作用的机制表征。我们在磷素匮乏的亚马逊林下森林开展了一项原位实验，利用开顶箱（open-top chambers, OTCs）将大气CO₂浓度原位提升约300 ppm。共设置8个聚丙烯材质的开顶箱，每个直径2.4米、高3米，随机安装于该林下区域。为避免开顶箱外植物的根系扩散及其他非目标效应，每个开顶箱周围均开挖宽30厘米、深50厘米的环形土壤壕沟。实验采用配对分组设计，其中4个为对照箱，维持环境CO₂（ambient CO₂, aCO₂）浓度，另外4个为CO₂升高处理组（elevated CO₂, eCO₂），后者的CO₂浓度较对应环境对照开顶箱平均提升约300 ppm。在开顶箱内，我们开展了多项实验，以探究eCO₂对枯落物层及土壤中根系参数、土壤养分浓度、微生物活性以及叶片枯落物分解的影响。研究结果表明，eCO₂会沿枯落物-土壤连续体引发根系的差异化响应，这可能促进养分吸收。此外，eCO₂加剧了植物根系与土壤微生物之间的磷素竞争，进而改变枯落物与土壤中的磷素库，加重了本已强烈的养分限制状况。