Litter Mixing Alters Microbial Decomposer Community to Accelerate Tomato Root Litter Decomposition

Abstract

Mixing plant litters of multiple species can alter litter decomposition, a key driver of carbon and nutrient cycling in terrestrial ecosystems. Changes in microbial decomposer communities is proposed as one of the mechanisms explaining this litter-mixture effect, but the underlying mechanism is unclear. In a microcosm litterbag experiment, we found that, at the early stage of decomposition, litter mixing promoted tomato root litter decomposition, thus generating a synergistic nonadditive litter-mixture effect. The transplanting decomposer community experiment showed that changes in microbial decomposer communities contributed to the nonadditive litter-mixture effect on tomato root litter decomposition. Moreover, litter mixing altered the abundance and diversity of bacterial and fungal communities on tomato root litter. Litter mixing also stimulated several putative keystone operational taxonomic units (OTUs) in the microbial correlation network, such as Fusarium sp. fOTU761 and Microbacterium sp. bOTU6632. Then, we isolated and cultured representative isolates of these two taxa, named Fusarium sp. F13 and Microbacterium sp. B26. Subsequent in vitro tests found that F13, but not B26, had strong decomposing ability; moreover, these two isolates developed synergistic interaction, thus promoted litter decomposition in coculture. Addition of F13 or B26 both promoted the decomposing activity of the resident decomposer community on tomato root litter, confirming their importance for litter decomposition. Overall, litter mixing promoted tomato root litter decomposition through altering microbial decomposers, especially through stimulating certain putative keystone taxa. IMPORTANCE Microbial decomposer community plays a key role in litter decomposition, which is an important regulator of soil carbon and nutrient cycling. Though changes in decomposer communities has been proposed as one of the potential underlying mechanisms driving the litter-mixture effects, direct evidence is still lacking. Here, we demonstrated that litter mixing stimulated litter decomposition through altering microbial decomposers at the early stage of decomposition. Moreover, certain putative keystone taxa stimulated by litter mixing contributed to the nonadditive litter-mixture effect. In vitro culturing validated the role of these taxa in litter decomposition. This study also highlights the possibility of regulating litter decomposition through manipulating certain microbial taxa.

Keywords: litter decomposition; litter mixing; litterbags; microbial community; nonadditive effects.

FIG 1

The experimental design. Litter of six plant species were used in this study. For the litterbag experiment, monospecific litter of each six species and all the possible two, four and six-way combinations of tomato with the other five species were included. All treatments were used for determining mass loss, while treatments containing tomato were used to analyze microbial community abundance and diversity, isolate bacteria and fungi. In the transplanting decomposer community experiment, decomposed tomato root litter from the litterbag experiment were used as inoculum to test the function of decomposer communities.

FIG 2

Litter mass loss in the litterbag experiment. (A) Mass loss of all litter for each treatment. For monospecific treatments, different letters indicate significant differences (Tukey's HSD test, P < 0.05). Black dots indicate the expected mass loss (the mean mass loss of the component litter species in isolation). * indicate significant difference between the observed and the expected mass loss (Student's t test, P < 0.05). (B) Effects of litter species richness on mass loss of all litter. (C) Tomato root litter mass loss for each treatment. * indicate significant different with the monospecific treatment (Student's t test, P < 0.05). (D) Effects of litter species richness on tomato root litter mass loss. Different letters indicate significant differences (Tukey's HSD test, P < 0.05). For (A) and (C), values are represented as mean ± SE (n = 3). For (B) and (D), dashed red lines show the linear or log-linear regression fittings and shaded areas represent 95% confidence intervals. T, tomato; C, cucumber; E, eggplant; M, maize; W, wheat; D, wild rocket.

FIG 3

Tomato root litter mass loss in the transplanting decomposer community experiment. (A) Tomato root litter mass loss for each treatment. * indicate significant different with the monospecific treatment (Student's t test, P < 0.05). Values are represented as mean ± SE (n = 3). (B) Effects of litter species richness on the mass loss of tomato root litter. Different letters indicate significant differences (Tukey's HSD test, P < 0.05). (C) Relationship between tomato root litter mass loss in the double-compartment litterbag experiment and that in the transplanting decomposer community experiment. Dashed red lines show the linear or log-linear regression fittings and shaded areas represent 95% confidence intervals. T, tomato; C, cucumber; E, eggplant; M, maize; W, wheat; D, wild rocket.

FIG 4

The abundance and diversity of microbial communities on tomato root litter. (A) Effects of litter species richness on abundances and Shannon index of microbial communities. Different letters indicate significant differences (Tukey's HSD test, P < 0.05). Dashed red lines show the linear or log-linear regression fittings and shaded areas represent 95% confidence intervals. (B) The β-diversities of microbial communities. *** indicates P < 0.001. (C) Microbial OTUs that were both stimulated by litter mixing and belonged to top-ranking mass loss-predicative OTUs. Venn plots show the numbers of shared and unique OTUs that were stimulated by litter mixing and belonged to top-ranking mass loss-predicative OTUs. The heatmap shows the relative abundances of OTUs that were both stimulated by litter mixing and belonged to top-ranking mass loss-predicative OTUs. The bubbles on the left panel show the Spearman’s correlations between the relative abundance of each OTU and tomato root litter mass loss. (D) The co-occurrence network showing significant correlations (ρ > 0.6, BH-corrected P < 0.01) between OTUs. The size of each node is proportional to the relative abundance of the OTU. (E) Degree-betweenness centrality plot of OTUs in the network. Keystone OTUs have gray background. Side panels show the distributions of node degrees and betweenness centrality for OTUs stimulated by litter mixing compared to the density of all OTUs in the network. For (D) and (E), OTUs stimulated by litter mixing are in red color.

FIG 5

Isolated Microbacterium and Fusarium spp. and their decomposing abilities. (A) The neighbor-joining trees showing the phylogenetic relationships of isolated Microbacterium and Fusarium spp. Isolates from this study are in bold letters. Reference strains from the NCBI database with their accession numbers are in regular letters. Numbers in parentheses are the sequence similarities of each Microbacterium and Fusarium spp. strain with bOTU6632 or fOTU761, respectively. Bootstrap values are based on 1,000 resampling and shown at the branching points. The photographs show the colony morphologies of B26 and F13 grown on Luria-Bertani agar and potato dextrose agar, respectively. (B) The abilities of B26 and F13 in isolate, and their mixture to decompose autoclaved tomato root litter. (C) Effects of addition of B26 and F13 on the decomposing ability of resident microbial community on tomato root litter and Fusarium sp. abundance Decomposing tomato root litter as an inoculant of resident decomposing community (In). Different letters indicate significant differences (Tukey's HSD test, P < 0.05).