microbial succession on decomposing Scots pine root litter under two irrigation regimes. Litterbag Root Decomposition

Decomposition is a major component of the carbon cycle in forest soils and understanding the underlying processes is key for budgeting carbon turnover. Decomposition rates in forests are constrained by the presence of biological mediators and the underlying environmental conditions (e.g. water availability). Identifying soil microbial key-players during decomposition combined with measuring the chemical derivatives of this process can deepen our understanding of this complex process. For this purpose, high-throughput sequencing techniques offer unique opportunities to comprehensively survey these key players under different environmental conditions and in space and time. In this study, we used a metabarcoding approach of ribosomal markers to study the succession of bacterial and fungal decomposers on decomposing root litter over two years. Litterbags containing pine roots were buried in a mature pine forest for two years and have been gradually sampled. Decomposition and the associated communities were surveyed under two different precipitation regimes, i.e. under ambient semi-arid conditions as well as under long-term irrigation. Early stages of decomposition (<1 year) were characterized by the presence of largely copiotrophic microorganisms (e.g. Proteobacteria, yeast and molds). The largely copiotrophic populations decreased at later stages of decomposition (1-2 years) and were replaced by more oligotrophic bacteria as well as specialized litter-associated (e.g. white-rot fungi, Planctomycetes) or parasitic (e.g. Pleosporales) fungi. This succession can likely be explained by the decline of easy degradable sources and a relative increase of more recalcitrant compounds such as lignin at later stages of decomposition. The effect of irrigation had minor impact on the microbial decomposition. Despite contrasting water contents, neither fine nor coarse roots experienced slower degradation under dry soil conditions. We hypothesize that – despite the pronounced differences in community composition – the lack of difference in root decomposition rates between the ambient and irrigated forest plots has been caused by a sufficient degree of functional redundancy among the resident microbial taxa. These findings have important implications under future climate scenarios, since dry condition in drought prone Alpine forest does not reduce microbial decomposition, hence limits the primary litter input of the trees.

分解是森林土壤碳循环的核心组成部分，明确其内在过程是精准核算碳周转（carbon turnover）的关键。森林土壤的分解速率受生物介导物与潜在环境条件（如水分可利用性）的制约。明确分解过程中的土壤微生物关键类群，并结合该过程的化学衍生物检测，可深化我们对这一复杂过程的认知。为此，高通量测序（high-throughput sequencing）技术为我们在不同环境条件下，以及随空间和时间尺度全面调查这些关键类群提供了独特契机。本研究采用核糖体标记物的宏条形码（metabarcoding）测序方法，对两年内分解过程中细菌与真菌分解者的群落演替进行了研究。将装载松根的凋落袋（litterbag）埋藏于一片成熟松林，为期两年，并进行梯度采样。我们在两种降水格局下开展调查，即自然半干旱条件与长期灌溉处理下，分别监测了分解过程及其伴随的微生物群落。分解早期（<1年）以富营养微生物（copiotrophic microorganisms）为主，如变形菌门（Proteobacteria）、酵母与霉菌。在分解后期（1~2年），这类富营养类群丰度下降，转而被寡营养细菌，以及专门与凋落物相关的（如白腐真菌（white-rot fungi）、浮霉菌门（Planctomycetes））或寄生真菌（如格孢菌目（Pleosporales））所替代。这一演替过程或可归因于易降解底物的消耗，以及后期难降解化合物（如木质素（lignin））相对占比的提升。灌溉处理对微生物分解过程的影响微弱。尽管土壤含水量存在显著差异，但无论是细根还是粗根，在干旱土壤条件下均未表现出更慢的降解速率。我们提出如下假说：尽管群落组成存在显著差异，但自然与灌溉样地间根分解速率无显著差异，这一现象源于原位微生物类群间存在足够的功能冗余性（functional redundancy）。本研究结果对未来气候情景具有重要启示：在易发生干旱的阿尔卑斯森林中，干旱条件并不会抑制微生物分解过程，因此会限制树木的初级凋落物输入。