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Uncertainties in interpretation of isotope signals for estimation of fine root longevity: theoretical considerations

机译：同位素信号估算细根寿命的不确定性：理论上的考虑

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This paper examines uncertainties in the interpretation of isotope signals when estimating fine root longevity, particularly in forests. The isotope signals are depleted delta(13)C values from elevated CO2 experiments and enriched Delta(14)C values from bomb C-14 in atmospheric CO2. For the CO2 experiments, I explored the effects of six root mortality patterns (on-off, proportional, constant, normal, left skew, and right skew distributions), five levels of nonstructural carbohydrate (NSC) reserves, and increased root growth on root delta(13)C values after CO2 fumigation. My analysis indicates that fitting a linear equation to delta(13)C data provides unbiased estimates of longevity only if root mortality follows an on-off model, without dilution of isotope signals by pretreatment NSC reserves, and under a steady state between growth and death. If root mortality follows the other patterns, the linear extrapolation considerably overestimates root longevity. In contrast, fitting an exponential equation to delta(13)C data underestimates longevity with all the mortality patterns except the proportional one. With either linear or exponential extrapolation, dilution of isotope signals by pretreatment NSC reserves could result in overestimation of root longevity by several-fold. Root longevity is underestimated if elevated CO2 stimulates fine root growth. For the bomb C-14 approach, I examined the effects of four mortality patterns (on-off, proportional, constant, and normal distribution) on root Delta(14)C values. For a given Delta(14)C value, the proportional pattern usually provides a shorter estimate of root longevity than the other patterns. Overall, we have to improve our understanding of root growth and mortality patterns and to measure NSC reserves in order to reduce uncertainties in estimated fine root longevity from isotope data. [References: 36]
机译：本文在估算细根寿命（特别是在森林中）时研究了同位素信号解释中的不确定性。同位素信号是二氧化碳浓度升高实验中的delta（13）C值耗尽，大气CO2中炸弹C-14的Delta（14）C值丰富。对于二氧化碳实验，我探索了六种根系死亡率模式（开，关，比例，恒定，正态，左偏和右偏分布），五种非结构性碳水化合物（NSC）储备水平以及根上根生长增加的影响。二氧化碳熏蒸后的delta（13）C值。我的分析表明，仅当根系死亡率遵循开-关模型，且未经过预处理的NSC储备稀释同位素信号且在生长与死亡之间处于稳定状态时，将线性方程式拟合到delta（13）C数据才能提供长寿的无偏估计。 。如果根系死亡率遵循其他模式，则线性外推法会大大高估根系寿命。相比之下，将指数方程拟合到delta（13）C数据会低估寿命，除了比例成比例的死亡率以外，所有其他死亡率均如此。使用线性或指数外推法，通过预处理的NSC储备稀释同位素信号可能导致根寿命的高估数倍。如果升高的CO2刺激了良好的根系生长，则根系寿命被低估了。对于C-14炸弹方法，我检查了四种死亡率模式（开-关，成比例，恒定和正态分布）对根Delta（14）C值的影响。对于给定的Delta（14）C值，比例模式通常比其他模式提供更短的根寿命估计。总体而言，我们必须提高对根生长和死亡率模式的了解，并测量NSC储量，以减少同位素数据估算的细根寿命的不确定性。 [参考：36]

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