The role of soil redox conditions in microbial phosphorus cycling in humid tropical forests

Humid tropical forests are among the most productive ecosystems globally, yet they often occur on soils with high phosphorus (P) sorption capacity, lowering P availability to biota. Short-term anoxic events are thought to release sorbed P and enhance its acquisition by soil microbes. However, the actual effects of anoxic conditions on microbial P acquisition in humid tropical forest soils are surprisingly poorly studied. We used laboratory incubations of bulk soils, NanoSIMS analysis of single microbial cells, and landscape scale measurements in the Luquillo Experimental Forest (LEF), Puerto Rico to test the hypothesis that anoxic conditions increase microbial P acquisition in humid tropical forests. In laboratory and field experiments we found that microbial P uptake generally decreased under anoxic conditions, leading to high microbial carbon (C) to P ratios in anoxic soils. The decreased P acquisition under anoxic conditions was correlated with lower microbial C use efficiency (CUE), an index of microbial energy transfer in ecosystems. Phosphorus amendments to anoxic soils led to increased microbial P uptake and higher CUE suggesting that microbes were less able to access and utilize P under natural low redox conditions. Under oxic conditions, microbial C:P ratios and CUE did not respond to changes in substrate stoichiometry. These results challenge the existing paradigm by showing that anoxic conditions can decrease microbial P uptake and ultimately constrain microbial CUE. Our findings indicate that soil redox conditions tightly couple soil P and C cycles and advance our understanding of controls on P cycling in humid tropical forest ecosystems.

湿润热带森林是全球生产力最高的生态系统之一，却往往分布在磷（phosphorus, P）吸附能力极强的土壤中，这会降低生物可获取的有效磷含量。学界普遍认为，短期缺氧事件可解吸吸附态磷，并提升土壤微生物对磷的获取能力。然而，针对缺氧条件如何影响湿润热带森林土壤中微生物磷获取的研究却极为匮乏。本研究采用原状土壤室内培养、纳米二次离子质谱（NanoSIMS）单细胞分析，以及在波多黎各卢奎洛实验林（Luquillo Experimental Forest, LEF）开展景观尺度测量的方法，以验证“缺氧条件可提升湿润热带森林中微生物磷获取能力”这一假说。室内与野外实验结果显示，缺氧条件下微生物的磷摄取量普遍下降，导致缺氧土壤中微生物碳（carbon, C）磷比显著升高。缺氧条件下微生物磷获取能力的下降，与更低的微生物碳利用效率（Carbon Use Efficiency, CUE）呈显著相关——碳利用效率是表征生态系统中微生物能量传递的核心指标。向缺氧土壤施加磷素后，微生物的磷摄取量与碳利用效率均显著提升，这表明在自然低氧化还原条件下，微生物对磷的获取与利用能力受到限制。而在有氧条件下，微生物碳磷比与碳利用效率均未随底物化学计量比的变化产生响应。本研究结果挑战了现有认知范式：缺氧条件可降低微生物磷摄取量，并最终制约微生物碳利用效率。本研究表明，土壤氧化还原条件可紧密耦合土壤磷与碳循环，从而加深了学界对湿润热带森林生态系统磷循环调控机制的认知。