Root functional parameters predict fine root decomposability at the community level

1. Root quality is one of the main drivers of fine root decomposition, an important process controlling soil carbon (C) and nutrient cycling in most terrestrial ecosystems. Root quality is defined by chemical and morphological traits, which differ across species and thus communities. This trait variation is assumed to follow a trade-off between resource acquisition and conservation (i.e. the root economics spectrum). To what extent root quality or the economics spectrum influence fine root decomposition rates at the community level remains poorly understood, particularly within the context of land use change. 2. Changes in land use induce shifts in plant community composition, which also affect root distribution within the soil profile, resulting in changes in root quality. We hypothesize that at the community level, i) root decomposability is driven by community root functional parameters (i.e. root traits measured at the community level), ii) changes in root functional parameters among land use types and with soil depth translate into changes in root decomposability. 3. We collected shallow and deep fine roots (< 2 mm) from 20 plant communities across contrasting land use types in 7 sites worldwide, ranging from agricultural crops to natural forests and determined their decomposition rate in standard conditions. Fine root quality was related to known values of functional parameters for these communities, including carbon (C), nitrogen (N) and lignin concentrations. 4. A combination of chemical functional parameters (lignin, C and N concentrations) best explained root decomposition rates at the community level, whereas root economics remained a poorer predictor of decomposability rates. Among land use gradients, roots from agricultural and agroforestry communities decomposed faster than roots from forest sites. Across and within plant communities, a consistently greater decomposability in shallow roots was observed. Both land use and depth effects were explained by changes in root chemical traits at the community level. 5. Synthesis. Our results suggest that the conversion of plant communities from forests to agricultural lands leads to changes in root functional parameters, that drastically increase root decomposition rates and may lead to major soil C losses, especially in shallow soil layers.

1. 细根质量是调控细根分解的主要驱动因子之一，而细根分解是控制多数陆地生态系统土壤碳（C）与养分循环的关键过程。细根质量由化学性状与形态性状决定，不同物种乃至不同群落间的性状存在差异。这种性状变异被认为遵循资源获取与资源保存之间的权衡关系（即根经济谱（root economics spectrum））。目前，在群落水平上，细根质量或根经济谱对细根分解速率的影响程度仍不甚明确，在土地利用变化的背景下这一问题尤为突出。
2. 土地利用变化会引发植物群落组成的改变，同时也会影响根系在土壤剖面中的分布，进而导致细根质量发生变化。我们提出如下群落水平假说：i）细根分解能力由群落水平的根系功能参数（即于群落尺度测定的根系性状）驱动；ii）不同土地利用类型间以及随土壤深度变化的根系功能参数差异，会转化为细根分解能力的改变。
3. 我们从全球7个研究位点的20个植物群落中采集了浅部与深部细根（直径<2 mm），这些群落涵盖了从农作物到天然林的多种土地利用类型，并在标准条件下测定了它们的分解速率。我们将细根质量与这些群落已知的功能参数（包括碳（C）、氮（N）与木质素浓度）进行了关联分析。
4. 化学功能参数（木质素、碳与氮浓度）的组合能够最佳解释群落水平的细根分解速率，而根经济谱对分解能力的预测效果则较差。在土地利用梯度上，农业与农林业群落的细根分解速率快于森林群落。在植物群落间及群落内部，浅部细根的分解能力始终更高。土地利用与土壤深度的影响均可通过群落水平的根系化学性状变化得到解释。
5. 综合分析与结论。本研究结果表明，将植物群落从森林转换为农业用地会改变根系功能参数，显著提高细根分解速率，并可能导致大量土壤碳损失，尤其是在浅层土壤中。