Intensive plantations decouple fine root C:N:P in subtropical forests

Fine roots play key roles in carbon and nutrient cycling in terrestrial ecosystems. Despite the global conversion of natural forests to tree plantations, a quantitative evaluation of the dynamics of fine root nutrients and C:N:P stoichiometry in plantations and natural forests is lacking. This study examined the fine root elemental concentrations and C:N:P stoichiometry of Chinese fir plantations and natural secondary forests (5 to 41 years old), employing primary forests as a reference. Live fine roots had higher P, but lower Ca concentrations in plantations than natural forests across all age classes. Dead roots contained higher N, P, Ca, and Mg in young plantations than in natural forests. Live and dead fine root elemental concentrations and C:N:P stoichiometry did not change with stand age in both forest types, except for a decrease in dead root P and Mg concentrations in 19-41-year-old plantations. Live root C:N, C:P, and N:P ratios did not differ between forest types; however, dead root C:N, C:P, and N:P ratios were higher in natural forests than plantations. Our findings suggest that intensively managed forest monoculture plantations have a negative impact on the resorption proficiency of fine root nutrients and decouple the C:N:P of fine roots in subtropical forests. Consequently, we recommend reducing fertilization and increasing plant diversity in plantations to optimize the C:N:P of plants.

细根在陆地生态系统的碳与养分循环中发挥关键作用。尽管全球范围内天然林正向人工林转型，但目前仍缺乏针对人工林与天然林细根养分动态及碳氮磷化学计量比（C:N:P stoichiometry）的定量评估。本研究以原始林为参照，对5至41年生的杉木人工林与天然次生林的细根元素浓度及碳氮磷化学计量比进行了测定分析。跨所有林龄层级，人工林活体细根的磷（P）浓度更高，但钙（Ca）浓度低于天然次生林。幼龄人工林的枯死细根氮（N）、磷（P）、钙（Ca）及镁（Mg）浓度均高于天然次生林。两种林型的活体与枯死细根元素浓度及碳氮磷化学计量比均未随林龄发生显著变化，仅19-41年生人工林的枯死细根磷与镁浓度出现下降。两种林型的活体细根碳氮比（C:N）、碳磷比（C:P）及氮磷比（N:P）均无显著差异；但天然次生林的枯死细根碳氮比、碳磷比及氮磷比均高于人工林。本研究结果表明，亚热带森林中集约化经营的单一人工林会对细根养分的回收能力产生负面影响，并打破细根的碳氮磷化学计量耦合关系。据此，我们建议在人工林经营中减少施肥、提升植物多样性，以优化植物的碳氮磷化学计量比。