Different Responses of Soil Bacterial Communities to Nitrogen Addition in Moss Crust

Bacterial communities in soil serve an important role in controlling terrestrial biogeochemical cycles and ecosystem processes. Increased nitrogen (N) deposition in Northwest China is generating quantifiable changes in many elements of the desert environment, but the impacts of N deposition, as well as seasonal variations, on soil bacterial community composition and structure are poorly understood. We used high-throughput sequencing of bacterial 16S rRNA genes from Gurbantünggüt Desert moss crust soils to study the impacts of N addition on soil bacterial communities in March, May, and November. In November, we discovered that the OTU richness and diversity of soil bacterial community dropped linearly with increasing N input. In November and March, the diversity of the soil bacterial community decreased significantly in the medium-N and high-N treatments. In May, N addition caused a substantial change in the makeup of the soil bacterial composition, while the impacts were far less apparent in November and March. Furthermore, the relative abundance of major bacterial phyla reacted non-linearly to N addition, with high-N additions decreasing the relative richness of Proteobacteria, Bacteroidetes, and Acidobacteria while increasing the relative abundance of Actinobacteria and Chloroflexi. We also discovered that seasonality, as characterized by changes in soil moisture, pH, SOC, and AK content, had a significant impact on soil bacterial communities. Significant variations in the makeup of the community were discovered at the phylum and genus levels throughout the various months. In May, the variety of soil bacterial community was at its peak. Further investigation showed that the decrease in soil bacterial diversity was mostly attributed to a drop in soil pH. These results indicated that the impact of N deposition on the soil bacterial community was seasonally dependent, suggesting that future research should evaluate more than one sample season at the same time.

Microbial community structure and function prediction of two typical biocrusts in the Mu Us Sandland, Northwest China

<p>Biocrusts (Biological soil crusts) are a living ground cover widely distributed in arid and semi-arid regions worldwide and provide important ecological functions in ecosystems. As an important part of biocrusts, the microorganisms in the formation and succession of biocrusts should not be underestimated. However, the microbial processes among different types of biocrusts are poorly understood. We used high-throughput sequencing to identify soil bacteria and fungal community in two types of biocrusts, lichen crust and moss crust, in the Mu Us Sandland. The aims were to explore the composition, diversity, and ecological function of the microbial community in two types of biocrusts. Our study found that (1) The diversity of bacterial and fungal communities was significantly different between the two types of biocrusts. The Shannon index (6.18) of fungi in moss crust was higher than that (5.75) in lichen crust, and the operational taxonomic units of bacteria and fungi in moss crust were also higher than those in lichen crust by 3.22% and 30.61%, respectively. The bacteria and fungi community structure in two types of biocrusts were significantly different, while the differences were not significant. (2) In the microbiomes of lichen and moss biocrusts, Actinomycetes, Cyanobacteria, and Proteobacteria, the sum of which accounted for 68.01% in lichen crust and 59.88% in moss crust at operational taxonomic units level, were dominant phylum of bacteria, while the dominant phylum of fungi was mainly Ascomycota. Microcoleus (11.10%) and Exophiala (7.37%) were dominant genera in lichen crust, while the dominant genus in moss crust was RB41 (5.16%). (3) The pH, soil dissolved organic carbon, and soil organic carbon were the top three factors that correlated with both bacterial and fungal community structures. (4) The metabolic function of the bacterial community in two types of biocrusts was quite different. The relative abundances of metabolic pathways in moss crust, such as chemoheterotrophy, ureolysis, aromatic compound degradation, and nitrate reduction, were significantly higher than those in lichen crust, however, the relative abundances of cyanobacteria, oxygenic photoautotrophy, photoautotrophy, and phototrophy were significantly lower (ANOVA, P<0.05). Altogether, our study suggests that the biocrust types have significant effects on the pH, taxonomic, and metabolic diversity, providing a theoretical basis for improving the physicochemical properties of the surface soil in the desertification land ecosystem.</p>

Effect of sand burial on the subcritical water repellency of a dominant moss crust in a revegetated area of the Tengger Desert, Northern China

Sand burial is a ubiquitous disturbance that influences the ecological and hydrological properties of moss crusts in many sandy desert areas. There is little available information regarding the effect of sand burial on the water repellency (WR) of moss crusts in desert areas. Therefore, this study evaluated the effects of sand burial (sand depths of 0 (control), 0.5, 1, 2, 4 and 10 mm) followed by three simulated precipitation regimes (through applying 4 and 6 mm, 2 and 3 mm, and 1 and 1.5 mm of distilled water at 8-day intervals in spring and autumn, respectively) on the WR of a widespread moss crust dominated by Bryum argenteum Hedw. in a revegetated area of the Tengger Desert, China. The results showed moss crust WR remained subcritical during the whole experiment, and that it considerably decreased immediately after sand burial, even though the values of WR were significantly higher in autumn than those in spring under the same treatment (p < 0.05). Furthermore, the depth threshold (TD) values for sand burials that reduced WR to zero were 1 and 2 mm in spring and autumn, respectively. After a recovery period of nearly one-season (72 days), the WR of the moss crust significantly increased (p < 0.05). In addition, sand burial had two separate effects on moss crust WR. Specifically, shallower sand burial (burial depth less than 0.5 mm) increased moss crust WR, whereas deeper sand burial (burial depth exceeds 0.5 mm) decreased it. The TD values also significantly increased to 2 and 4 mm in spring and autumn, respectively. These findings about the effects of sand burial on moss crust WR provide additional information that can be used to better understand the influence of sand burial on moss crust colonization and maintenance in arid sand-burial-stressed ecosystems, and to help explain why there are some contrasting viewpoints on biocrust WR.

Moss-dominated biological soil crusts improve stability of soil organic carbon on the Loess Plateau, China

The succession of biological soil crust (biocrust) may alter soil organic carbon (SOC) stability by affecting SOC fractions in arid and semi-arid regions. In the study, the SOC fractions were measured including soil easily oxidizable carbon (SEOC), soil microbial biomass carbon (SMBC), soil water soluble carbon (SWSC), and soil mineralizable carbon (SMC) at the Loess Plateau of China by using four biocrusts. The results show that SOC fractions in the biocrust layer were consistently higher than that in the subsoil layers. The average SOC content of moss crust was approximately 1.3–2.0 fold that of three other biocrusts. Moss crusts contain the lowest ratio of SEOC to SOC compared with other biocrusts. The ratio of SMC to SOC was the highest in light cyanobacteria biocrust and the lowest in moss crust, but no difference was observed in SMBC to SOC and SWSC to SOC in biocrust layers among four studied biocrusts. The results show that the moss crusts increase the accumulation of organic carbon into soil and reduce the ratio of SEOC to SOC and SMC to SOC. Together, these findings indicate that moss crusts increase the SOC stability and have important implications that SOC fractions and mineralization amount are good indicators for assessing the SOC stability.