退化红壤区人工林林下植物根系生长与凋落物分解的互作机制

Understory vegetation is an important component of forests, and the phenomenon of root foraging in litters is common in forest ecosystems. The interactive mechanism between root growth and litter decomposition is unclear as an important scientific issue. Understory vegetation is an important component of forest, and the root foraging in litters was observed as a kind of relatively widespread phenomenon in research sites. The research on interaction between the roots and forest litters would be helpful to understand the impacts of litter input on understory root traits and rhizosphere effects, and to assess the role of understory roots on litter carbon, nitrogen and phosphorus return into soils. We selected >20 year-old Pinus massoniana and Liquidambar formosana plantations to study the responses of understory root turnover, root traits, rhizosphere carbon, nitrogen and phosphorus transformations and microbial processes to two types of litter quality (evergreen coniferous vs. deciduous broadleaved), three levels of litter input amount (natural, removal and double) using the root window method, excavation method, isotope tracer method, 454 pyrosequencing, to reveal the influences of root growth on litter decomposition, and carbon, nitrogen and phosphorus dynamics using the associated methods of understory removal, induction with root cutting and in situ buried bag, and to comprehensively understand the coupling effects among understory root traits, litter decomposition and plant physiological characteristics in mid-subtropical forest plantations. This project plans to test the hypothesis that understory root growth variously interacts with litter decomposition. The results will help promote the development of forest ecology, and provide scientific basis for managing understory vegetation and litters in restored plantations.

林下植物是森林的重要组成部分，其大量根系生长进入到凋落物层是一种较为普遍的现象，而植物根系生长和凋落物分解的互作机制是尚未明确的科学问题。林下植物根系与凋落物互作研究既可分析凋落物输入对林下植物根系性状和根际效应的影响，又可评价林下植物根系在凋落物碳及养分归还中的作用。以退化红壤恢复20余年的针叶林和阔叶林为对象，采用根窗法、挖掘法、同位素示踪法、454高通量测序等剖析主要林下植物细根周转、根系性状、根际碳氮磷转化和微生物功能群对凋落物质量（不同树种）和输入量（自然、移除、加倍3种处理）的响应机制；联合林下植物剔除、根系切断诱导和原位埋袋分解法揭示根系生长对凋落物分解及碳氮磷动态的作用机理；综合分析人工林林下植物根系生态、凋落物分解与植物生理过程的耦合效应。重点检验林下植物根系生长与凋落物分解的互作模式假说。研究结果有助于推动森林生态学的发展，并为人工林的管理提供支撑。

林下植物管理是人工林经营中重要的生产实践问题，目前尚缺乏统一的认识。本项目从林下植物根系生长与凋落物分解养分释放的内在关联及互作机制为切入点，布设了不同凋落物质量及其输入量两因素多水平的完全随机化试验，结合林下植物剔除、根系切断诱导、降水格局改变等因素，开展凋落物分解速率及养分释放、根系性状及增殖速率、微生物功能多样性等方面的研究。结果表明，1）檵木根系的参与对枫香凋落叶的分解有显著的促进作用，但主要表现在分解前期，而对中、后期的影响较小，作用机制可能是在分解前期根系为其共生微生物提供了所需的氮源，微生物活跃程度增高，加快了凋落物重量衰减，碳、氮和磷的释放速率；而檵木根系的参与对湿地松凋落物的分解无显著作用，且表现出一定的抑制能力，可能与湿地松凋落叶氮含量较低，C/N比较高，对提高根系共生微生物的活跃不利有关。2）无论是枫香还是湿地松凋落叶，根系参与分解虽然会使其有机碳、全氮和全磷含量的变化加快或者减慢，但并不会对分解过程中凋落物C:N:P化学计量比的动态变化造成影响，较好地支持了凋落物分解趋于同质化的理论。3）凋落物的输入促进了植物吸收根的分化生长，抑制贮藏根的生长，而水分添加则促进了运输根和贮藏根的分化生长。而在凋落物和水分同时供应条件下，植物会减少对运输根和贮藏根的投入，而分化产生更多的吸收根。总体来看，在丘陵红壤人工林中，有许多因素共同控制着林下植物根系生长和凋落叶分解的相互作用；林下植物根系生长对凋落物分解的作用不如凋落物自身化学计量比的控制能力，但根系的参与会通过加快凋落物有机碳、氮和磷的释放等改变凋落物分解的时间动态格局。