The use of stable isotope analysis to resolve ecosystem respiration into its plant and soil components rests on how well the end-member isotope signatures (δ¹³C) are characterised. In general, it is assumed that end-member values are constant over time. However, there are necessarily diurnal and other transient variations in end-members with environmental conditions. We analyse diurnal and seasonal patterns of ecosystem respiration and its δ¹³C in a C₄ grass growing in a C₃ soil using fixed and diurnally varying plant and soil δ¹³C end-members. We measure the end-members independently, and we assess the effects of expected variation in values. We show that variation in end-members within realistic ranges, particularly diurnal changes in the plant end-member, can cause partitioning errors of 40% during periods of high plant growth. The effect depends on how close the end-member is to the measured net respiration δ¹³C, that is, the proportion of the respiration due to that end-member. We show light-driven variation in plant end-members can cause substantial distortion of partitioned soil organic matter (SOM) flux patterns on a diurnal scale and cause underestimation of daily to annual SOM turnover of approximately 25%. We conclude that, while it is not practicable to independently measure the full temporal variation in end-member values over a growing season, this error may be adjusted for by using a diurnally varying δ¹³C_plant.

中文翻译：

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在从净生态系统呼吸分配土壤 C 通量时允许端元 δ13C 值的瞬时变化

使用稳定同位素分析将生态系统呼吸分解为其植物和土壤成分取决于端元同位素特征 ( δ ¹³ C) 的表征情况。通常，假定端元值随时间保持恒定。然而，终端成员在环境条件下必然存在昼夜变化和其他瞬态变化。我们使用固定和昼夜变化的植物和土壤δ ^{13 分析}生长在 C ₃土壤中的 C ₄草中生态系统呼吸的昼夜和季节模式及其δ ¹³ CC 终端成员。我们独立测量最终成员，并评估预期值变化的影响。我们表明，在实际范围内端部成员的变化，特别是植物端部成员的昼夜变化，在植物高生长期间会导致 40% 的分区错误。效果取决于末端成员与测量的净呼吸δ ^{13 的}接近程度C，即由该端元引起的呼吸作用的比例。我们表明，植物末端成员的光驱动变化会导致昼夜尺度上分配的土壤有机质 (SOM) 通量模式发生实质性扭曲，并导致对每日至年度 SOM 周转率的低估约 25%。我们得出的结论是，虽然独立测量生长季节中端元值的全部时间变化是不切实际的，但可以通过使用昼夜变化的δ ¹³ C_植物来调整该误差。