Melanization of mycorrhizal fungal necromass structures microbial decomposer communities

Mycorrhizal fungal necromass is increasingly recognized as an important contributor to soil organic carbon pools, particularly in forest ecosystems. While its decomposition rate is primarily determined by biochemical composition, how traits such as melanin content affect the structure of necromass decomposer communities remains poorly understood.
To assess the role of biochemical traits on microbial decomposer community composition and functioning, we incubated melanized and non-melanized necromass of the mycorrhizal fungus Meliniomyces bicolor in Pinus- and Quercus-dominated forests in Minnesota, USA and then assessed the associated fungal and bacterial decomposer communities after 1, 2 and 3 months using high-throughput sequencing.
Melanized necromass decomposed significantly slower than non-melanized necromass in both forests. The structure of the microbial decomposer communities depended significantly on necromass melanin content, although the effect was stronger for fungi than bacteria. On non-melanized necromass, fungal communities were dominated by r-selected ascomycete and mucoromycete microfungi early and then replaced by basidiomycete ectomycorrhizal fungi, while on melanized necromass these groups were co-dominant throughout the incubation. Bacterial communities were dominated by both specialist mycophageous and generalist taxa.
Synthesis. Our results indicate that necromass biochemistry not only strongly affects rates of decomposition but also the structure of the associated decomposer communities. Furthermore, the observed colonization patterns suggest that fungi, and particularly ectomycorrhizal fungi, may play a more important role in necromass decomposition than previously recognized.

菌根真菌残体（mycorrhizal fungal necromass）日益被视作土壤有机碳库（soil organic carbon pools）的重要贡献组分，在森林生态系统中尤为关键。尽管其分解速率主要由生化组成决定，但诸如黑色素含量这类功能性状如何调控残体分解者群落结构，目前仍缺乏深入认知。
为探明生化性状对微生物分解者群落组成与功能的作用，我们将菌根真菌双色梅氏霉（Meliniomyces bicolor）的黑色素化与非黑色素化残体，分别置于美国明尼苏达州的松属（Pinus）主导林与栎属（Quercus）主导林中进行培养，并于培养1、2、3个月后，采用高通量测序（high-throughput sequencing）技术对相关联的真菌与细菌分解者群落进行分析。
在两种林分中，黑色素化残体的分解速率均显著慢于非黑色素化残体。微生物分解者群落结构显著受残体黑色素含量的影响，不过该效应在真菌群落中表现得更为突出。在非黑色素化残体上，真菌群落在培养早期以r-选择型子囊菌（ascomycete）与毛霉门（mucoromycete）微型真菌为主，随后被担子菌（basidiomycete）外生菌根真菌（ectomycorrhizal fungi）取代；而在黑色素化残体上，这两类类群在整个培养周期内均为共优势类群。细菌群落则由专食性食真菌类群（mycophageous taxa）与广适性类群共同主导。
综合分析。本研究结果表明，残体生物化学特性不仅显著影响分解速率，同时也会重塑相关分解者群落的结构。此外，观测到的定殖模式提示，真菌（尤其是外生菌根真菌）在残体分解过程中所发挥的作用可能比此前认知的更为重要。