Enhancement of nitrification rates in vitro by interacting species of saprotrophic fungi

Linking enhanced soil nitrogen mineralization to increased fungal decomposition capacity with Moso bamboo invasion of broadleaf forests

Plant invasion can markedly alter soil fungal communities and nitrogen (N) availability; however, the linkage between the fungal decomposition capacity and N mineralization during plant invasion remains largely unknown. Here, we examined the relationship between net mineralization rates and relevant functional genes, as well as fungal species composition and function following Moso bamboo (Phyllostachys edulis) invasion of evergreen broadleaf forests, by studying broadleaf forests (non-invaded), mixed bamboo-broadleaf forests (moderately invaded) and bamboo forests (heavily invaded). Fungal species composition and functional genes involved in organic matter decomposition (laccase and cellobiohydrolase), N mineralization (alkaline peptidases) and nitrification (ammonia monooxygenase) were determined via high-throughput sequencing and real-time PCR. Both net ammonification and nitrification rates were generally increased with bamboo invasion into the broadleaf forest, where the net ammonification rate, on average, was 10.8 times higher than the nitrification rate across the three forest types. The fungal species composition and ecological guilds were altered with bamboo invasion, as demonstrated by the increased proportion of saprotrophs but decreased proportion of symbiotrophs in the bamboo forest. The increased net ammonification rate in bamboo forest was positively correlated with both fungal species composition and functional groups, and the fungal lcc gene (for lignin breakdown) abundance explained 67% of the variation of the net ammonification rate. In addition, the gene abundance of ammonia-oxidizing bacteria (AOB) explained 62% of the variation of net nitrification rate across the three forest types. The increased soil ammonification and nitrification rates following bamboo invasion of broadleaf forests suggest that the bamboo-invasion associated increase in soil N supply provided a positive feedback that facilitated bamboo invasion into broadleaf forests.

Decomposition rate of organic substrates in relation to the species diversity of soil saprophytic fungi

Despite the great interest concerning the relationship between species diversity and ecosystem functioning, there is virtually no knowledge as to how the diversity of decomposer microbes influences the decomposition rate of soil organic matter. We established a microcosm study in which the number of soil fungi was investigated in relation to the system's ability to (i) degrade raw coniferous forest humus, and (ii) use resources that were either added to the systems or released into the soils after a disturbance (drought). With the exception of the most diverse treatment, in each of the six replicates of each of the six diversity treatments (1, 3, 6, 12, 24 or 43 taxa), fungal taxa were randomly chosen from a pool of 43 commonly isolated fungal species of raw humus. Two months after initiation of the study CO2 production increased as fungal diversity increased, but in the species-poor end of the diversity gradient only. Addition of various energy resources to the microcosms generally increased the level of soil respiration but did not affect the shape of the diversity-CO2-production curve. Rewetting the soil after severe drought resulted in a rapid flush of CO2, particularly in the most diverse communities. The biomass of the fungi in the non-disturbed soils, and soil NH4-N concentration and soil pH in both disturbed and non-disturbed systems were slightly but significantly higher in the diverse than in the simple systems. Fungal species richness had no influence on the organic matter content of the humus at the end of the experiment. The results suggest that the functional efficiency of fungal communities can increase with the number of fungal taxa. This diversity effect was, however, significant at the species-poor end of the diversity gradient only, which implies considerable functional equivalency (redundancy) among the decomposer fungi.