How do fine root traits of fast-growing trees promote soil organic carbon stabilization?

Soil represents a larger reservoir of soil organic carbon (SOC) than terrestrial vegetation, offering a great potential for reducing the widespread adverse consequences of climate change. In forests and tree plantations, fine roots significantly impact SOC stabilization through their functional traits. However, it is not obvious which fine root traits between those related to chemistry (easily decomposable or recalcitrant), to architecture or morphology are the most conducive to SOC stabilization in phylogenetically related fast-growing trees. We assessed the effects of root functional traits on SOC storage and stabilization by studying five hybrid poplar clones (Populus spp.) with different root traits in plantations located in New Liskeard, ON, Canada. We collected soil cores at depths of 0-20, 20-40 and 40-60 cm, and determined bulk soil organic carbon, particulate organic carbon (> 53 μm, POC) and mineral-associated organic carbon (< 53 μm, MAOC) fractions and fine root (< 2 mm diameter) traits. We found that root length density (RLD) was the best predictor of increased SOC stocks and MAOC among all root traits. Soil organic C stocks and MAOC were also positively correlated with root traits indicative of low chemical recalcitrance (i.e. high N and soluble compounds concentrations and low lignin/N). Such easily decomposed root matter could be readily consumed by soil microorganisms and promote adsorption of microbial by-products onto mineral surfaces. Thus, root traits that increase the soil volume explored by fine roots and are associated with easily decomposed organic compounds play a key role in SOC accumulation and persistence.土壤是陆地植被的更大土壤有机碳（soil organic carbon, SOC）储库，在缓解气候变化引发的广泛负面影响方面具备巨大潜力。在森林与人工林生态系统中，细根通过自身功能性状对土壤有机碳固存产生显著调控作用。然而，在系统发育关联的速生树种中，究竟哪些与化学特性（易分解或难降解、结构或形态相关的细根性状最有助于土壤有机碳固存，目前尚无定论。本研究以加拿大安大略省纽利斯卡尔地区人工林中的5个具有不同根性状的杨属（Populus spp.）杂交杨树无性系为研究对象，探究了根功能性状对土壤有机碳储量与固存的调控效应。我们分别在0-20 cm、20-40 cm及40-60 cm三层采集土壤钻芯，测定了土壤总有机碳、颗粒态有机碳（>53 μm, POC）与矿质结合态有机碳（<53 μm, MAOC）组分，以及直径小于2毫米的细根性状。研究发现，在所有根性状中，根长密度（root length density, RLD）是提升土壤有机碳储量与矿质结合态有机碳含量的最佳预测因子。土壤有机碳储量与矿质结合态有机碳含量同样与表征低化学难降解性的根性状呈正相关（即高氮与可溶性化合物浓度、低木质素/氮比值）。这类易分解的根系物质可被土壤微生物快速降解利用，并促进微生物代谢产物在矿物表面的吸附固持。因此，能够扩大细根探查土壤体积、且与易分解有机化合物相关的根性状，在土壤有机碳积累与持久性维持中发挥关键作用。