The question of why some types of organic matter are more persistent while others decompose quickly in soils has motivated a large amount of research in recent years. Persistence is commonly characterized as turnover or mean residence time of soil organic matter (SOM). However, turnover and residence times are ambiguous measures of persistence, because they could represent the concept of either age or transit time. To disambiguate these concepts and propose a metric to assess SOM persistence, we calculated age and transit time distributions for a wide range of soil organic carbon models. Furthermore, we show how age and transit time distributions can be obtained from a stochastic approach that takes a deterministic model of mass transfers among different pools and creates an equivalent stochastic model at the level of atoms. Using this approach we show the following: (1) Age distributions have relatively old mean values and long tails in relation to transit time distributions, suggesting that carbon stored in soils is on average much older than carbon in the release flux. (2) The difference between mean ages and mean transit times is large, with estimates of soil organic carbon persistence on the order of centuries or millennia when assessed using ages and on the order of decades when using transit or turnover times. (3) The age distribution is an appropriate metric to characterize persistence of SOM. An important implication of our analysis is that random chance is a factor that helps to explain why some organic matter persists for millennia in soil.

中文翻译：

---

土壤有机质持久性作为一个随机过程：土壤中碳的年龄和传播时间分布

近年来，为什么某些类型的有机物更持久，而另一些有机物在土壤中分解得很快，这一问题引发了大量研究。持久性通常被描述为土壤有机质 (SOM) 的周转或平均停留时间。然而，周转时间和停留时间是持久性的模糊度量，因为它们可以代表年龄或运输时间的概念。为了消除这些概念的歧义并提出评估 SOM 持久性的指标，我们计算了各种土壤有机碳模型的年龄和传播时间分布。此外，我们展示了如何通过随机方法获得年龄和渡越时间分布，该方法采用不同池之间质量传递的确定性模型，并在原子水平上创建等效的随机模型。使用这种方法，我们显示以下内容：（1）年龄分布具有相对较旧的平均值和与传输时间分布相关的长尾，这表明土壤中储存的碳平均比释放通量中的碳古老得多。(2) 平均年龄和平均运输时间之间的差异很大，使用年龄评估时，土壤有机碳持久性估计约为数百年或数千年，而使用运输或周转时间评估时，土壤有机碳持久性估计约为几十年。(3) 年龄分布是表征 SOM 持续性的适当指标。我们分析的一个重要含义是，随机机会是一个有助于解释为什么某些有机物在土壤中持续存在数千年的因素。