Multiple constraints cause positive and negative feedbacks limiting grassland soil CO2 efflux under CO2 enrichment

Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO2 that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the feedbacks from other limiting resources and plant community change, which remain poorly understood for soil CO2 efflux, JCO2, a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO2 enrichment gradient (250 to 500 µL L-1) for eight years to grassland plant communities on soils from different landscape positions. We identified the trajectory of JCO2 responses and feedbacks from other resources, plant diversity (effective species richness, exp(H)), and community change (plant species turnover). We found linear increases in JCO2 on an alluvial sandy loam and a lowland clay soil, and an asymptotic increase on an upland silty clay soil. Structural equation modelling identified CO2 as the dominant limitation on JCO2 on the clay soil. In contrast with theory predicting limitation from a si...陆地生态系统（Terrestrial ecosystems）正日益被大气二氧化碳（atmospheric CO2）等限制生态系统过程的资源所富集。生态系统碳循环（ecosystem carbon cycling）的相关效应，取决于其他限制资源与植物群落变化（plant community change）所产生的反馈作用，但针对土壤二氧化碳排放通量（soil CO2 efflux, JCO2）——这一从生物圈（biosphere）向大气排放的主要碳通量——的相关反馈机制，目前仍未得到充分解析。我们针对采自不同景观位置（landscape positions）的草原植物群落（grassland plant communities），开展了为期8年的独特二氧化碳富集梯度（CO2 enrichment gradient）实验，梯度范围为250至500微升每升（µL L-1）。我们明确了JCO2的响应轨迹，以及其他限制资源、植物多样性（有效物种丰富度，exp(H)）与群落变化（plant species turnover）带来的反馈效应。研究发现，冲积砂壤土（alluvial sandy loam）与低地黏土（lowland clay soil）样地中的JCO2呈线性增长趋势，而高地粉质黏土（upland silty clay soil）样地中的JCO2则呈现渐近增长模式。结构方程模型（Structural equation modelling）分析表明，在黏土样地中，二氧化碳是JCO2的主导限制因子。这与预测某一限制因子的理论形成了鲜明对比（原文后续内容未完整呈现）