Department of Ecology, College of Urban and Environmental Sciences, Peking University, 100871, Beijing, China.
Oecologia. 2010 Jun;163(2):509-15. doi: 10.1007/s00442-009-1541-4. Epub 2010 Jan 8.
Among tree fine roots, the distal small-diameter lateral branches comprising first- and second-order roots lack secondary (wood) development. Therefore, these roots are expected to decompose more rapidly than higher order woody roots. But this prediction has not been tested and may not be correct. Current evidence suggests that lower order roots may decompose more slowly than higher order roots in tree species associated with ectomycorrhizal (EM) fungi because they are preferentially colonized by fungi and encased by a fungal sheath rich in chitin (a recalcitrant compound). In trees associated with arbuscular mycorrhizal (AM) fungi, lower order roots do not form fungal sheaths, but they may have poorer C quality, e.g. lower concentrations of soluble carbohydrates and higher concentrations of acid-insolubles than higher order roots, thus may decompose more slowly. In addition, litter with high concentrations of acid insolubles decomposes more slowly under higher N concentrations (such as lower order roots). Therefore, we propose that in both AM and EM trees, lower order roots decompose more slowly than higher order roots due to the combination of poor C quality and high N concentrations. To test this hypothesis, we examined decomposition of the first six root orders in Fraxinus mandshurica (an AM species) and Larix gmelinii (an EM species) using litterbag method in northeastern China. We found that lower order roots of both species decomposed more slowly than higher order roots, and this pattern appears to be associated mainly with initial C quality and N concentrations. Because these lower order roots have short life spans and thus dominate root mortality, their slow decomposition implies that a substantial fraction of the stable soil organic matter pool is derived from these lower order roots, at least in the two species we studied.
在细根中，包含一级和二级根的远端小直径侧枝缺乏次生（木质）发育。因此，这些根预计比更高阶的木质根分解得更快。但这一预测尚未得到验证，也可能不正确。目前的证据表明，在与外生菌根（EM）真菌相关的树种中，较低阶的根可能比高阶的木质根分解得更慢，因为它们更容易被真菌定殖，并被富含几丁质（一种难分解的化合物）的真菌鞘包裹。在与丛枝菌根（AM）真菌相关的树种中，较低阶的根不会形成真菌鞘，但它们的 C 质量可能较差，例如可溶性碳水化合物的浓度较低，酸不溶性物质的浓度较高，因此可能分解得更慢。此外，高浓度酸不溶物的凋落物在高氮浓度下（如较低阶的根）分解得更慢。因此，我们提出，在 AM 和 EM 树中，较低阶的根比高阶的根分解得更慢，这是由于 C 质量差和氮浓度高的共同作用。为了验证这一假设，我们在中国东北地区使用凋落物袋法研究了灰楸（一种 AM 树种）和落叶松（一种 EM 树种）的前六个根阶的分解情况。我们发现，两种树种的较低阶根比高阶根分解得更慢，这种模式似乎主要与初始 C 质量和 N 浓度有关。由于这些较低阶的根寿命较短，因此主导着根的死亡，它们的缓慢分解意味着稳定土壤有机物质库的很大一部分来自于这些较低阶的根，至少在我们研究的两个树种中是这样。
