Root ‘collaboration’ dimension of the root economics space drives variations in soil organic carbon among forest plantations by regulating particulate organic carbon

Growing evidence shows that plant roots play a more important role than aboveground organs in soil organic carbon (SOC) storage and stabilization. The root economics space defines root resource acquisition strategies along two axes: the ‘conservation’ and ‘collaboration’ dimensions, which are widely used to investigate the relationships between root traits and ecosystem functions. However, it remains unclear whether and how the two-dimensional root resource acquisition strategies of trees are linked to the concentrations of SOC and its fractions (i.e., particulate organic carbon, POC, and mineral-associated organic carbon, MAOC). In this study, we measured four core traits of the root economics space, absorptive root biomass, SOC, POC and MAOC concentrations, soil microbial necromass carbon (MNC) concentrations and two soil C-degrading enzyme activities in a 19-year common garden of six temperate tree species in northeastern China. We found that the root ‘conservation’ dimension had limited impacts on POC, MAOC or SOC concentrations. In comparison, the root ‘collaboration’ dimension was positively correlated with POC and SOC concentrations but not associated with MAOC concentrations. That is, angiosperm tree species with the ‘do-it-yourself’ strategy along the root ‘collaboration’ dimension had higher POC concentrations than gymnosperm tree species with the ‘outsourcing’ strategy, thus increasing SOC concentrations. Moreover, the root resource acquisition strategies did not affect SOC by regulating MNC, except that the root ‘collaboration’ dimension was positively correlated with bacterial necromass carbon (BNC) concentrations. Overall, our results demonstrate that root ‘collaboration’ rather than ‘conservation’ dimension of the root economics space drives forest SOC sequestration by regulating POC, which highlights that symbiotic fungi play an important role in regulating the relationships between plant ecological strategies and soil functions. Our findings provide a basis for tree species selection in afforestation to enhance SOC storage via root traits and for incorporating root traits into global forest SOC cycling models.

越来越多的研究证据表明，植物根系在土壤有机碳（soil organic carbon, SOC）的储存与稳定过程中，发挥着比地上器官更为关键的作用。根系经济空间（root economics space）将根系资源获取策略划分为两个维度：「保守型」与「协作型」，该分析框架被广泛用于探究根系性状与生态系统功能之间的关联。然而，目前仍不清楚树木的二维根系资源获取策略是否以及如何与土壤有机碳及其组分[即颗粒态有机碳（particulate organic carbon, POC）和矿质结合态有机碳（mineral-associated organic carbon, MAOC）]的浓度存在关联。本研究在中国东北一处种植有6种温带树种的19年生共同栽培试验圃中，测定了根系经济空间的4项核心性状、吸收根生物量、土壤有机碳、颗粒态有机碳与矿质结合态有机碳浓度、土壤微生物残体碳（microbial necromass carbon, MNC）浓度，以及2种土壤碳降解酶的活性。研究发现，根系「保守型」维度对颗粒态有机碳、矿质结合态有机碳或土壤有机碳浓度的影响较为有限。相较而言，根系「协作型」维度与颗粒态有机碳及土壤有机碳浓度呈显著正相关，但与矿质结合态有机碳浓度无明显关联。具体而言，沿根系「协作型」维度采取「自力更生」策略的被子植物树种，其颗粒态有机碳浓度高于采取「外包」策略的裸子植物树种，进而提升了土壤有机碳浓度。此外，根系资源获取策略并未通过调控微生物残体碳来影响土壤有机碳水平，仅根系「协作型」维度与细菌残体碳（bacterial necromass carbon, BNC）浓度呈正相关。总体而言，本研究结果证实，根系经济空间的「协作型」维度而非「保守型」维度，通过调控颗粒态有机碳水平驱动森林土壤有机碳固存，这凸显了共生真菌在调控植物生态策略与土壤功能之间关联中的重要作用。本研究结果为通过根系性状筛选造林树种以提升土壤有机碳储量，以及将根系性状纳入全球森林土壤有机碳循环模型提供了理论依据。