[Responses of arbuscular mycorrhizal fungal morphological traits and the diversity of spore-associated bacteria to simulated nitrogen deposition and drought in a Cunninghamia lanceolata plantation soil]

Arbuscular mycorrhizal (AM) fungi play an important role in plant nutrient absorption and stress resistance. AM fungal spore-associated bacteria are essential for mycorrhizal colonization, but their responses to environmental changes remain largely unknown. We collected surface soil samples from a Chinese fir plantation in both summer and winter to investigate the responses of AM fungal morphological traits and spore-associated bacterial communities to simulated nitrogen deposition (40 kg N·hm-2·a-1 addition) and drought (-50% precipitation exclusion). Our results showed that nitrogen addition and precipitation exclusion significantly affected AM fungal spore density and extraradical hyphal length, respectively. AM fungal intraradical colonization rate, extraradical hyphal length and spore density were significantly differed between the two seasons. Compared to control (no nitrogen addition and no precipitation exclusion treatment), both nitrogen addition and precipitation exclusion significantly reduced spore density in winter, while precipitation exclusion alone and the combined nitrogen addition and precipitation exclusion significantly increased extraradical hyphal length in summer. The dominant spore-associated bacterial phyla were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes. Nitrogen addition and precipitation exclusion did not affect the diversity of spore-associated bacteria. However, the combined nitrogen addition and precipitation exclusion treatment altered the composition of the bacterial community, with significant variations between the two seasons. The spore-associated bacterial diversity was significantly higher and community variability (or turnover) was lower during summer than winter. Soil total nitrogen, nitrate nitrogen and dissolved organic carbon were important factors influencing the bacterial community composition. In all, the effects of nitrogen addition and precipitation exclusion on the morphological traits of AM fungi are seasonally dependent. The combination of nitrogen addition and precipitation exclusion has a significant impact on AM fungal spore-associated bacterial community structure.

[Responses of soil microbial biomass and carbon source utilization to simulated nitrogen deposition and drought in a Cunninghamia lanceolata plantation]

Chinese fir (Cunninghamia lanceolata) plantation is a dominant forest type and carbon sink in the subtropical region in China. An experiment with simulated nitrogen deposition (addition of 40 kg N·hm^-2·a^-1) and drought (50% of precipitation exclusion, PE) was established in Chinese fir plantation in 2018. Soil samples (0-15 cm) were collected in summer (July 2020) and winter (January 2021). Soil microbial biomass, colony forming units (CFUs) and carbon source utilization were determined through phospholipid fatty acids (PLFAs), plate count, and Biolog methods, respectively. The results showed significant seasonal variations of PLFAs-related microbial biomass and composition. Soil bacterial and fungal CFUs tended to be decreased by nitrogen addition or precipitation exclusion treatment, and bacterial CFUs were more sensitive to the two treatments than fungal CFUs. Soil microbial function (i.e. carbon source utilization) was not affected by nitrogen addition, but significantly decreased by precipitation exclusion. There was a significant positive correlation between bacterial CFUs and microbial function, indicating the crucial roles of culturable bacteria in microbial carbon transformation. Our results highlight the critical effects of nitrogen deposition and 50% reduced precipitation on microbes in topsoil of fir plantation, with implications for unraveling soil microbial ecological function of subtropical forest ecosystem under global changes in future.

Responses of soil dissolved organic matter to nitrogen addition and its correlation with bacterial communities in Pinus taiwanensis forest

We explored the responses of the quantity and quality of soil dissolved organic matter (DOM) to short-term nitrogen (N) addition across an addition gradient (0, 40, and 80 kg N·hm^-2·a^-1) and their correlation with bacterial communities in Pinus taiwanensis forest in the Daiyunshan Nature Reserve, Fujian, China. Soil DOM and bacterial community composition were measured using three-dimensional fluorescence and parallel factor analyses combined with high-throughput sequencing. The results showed that compared with the control, N-addition reduced the content of dissolved organic carbon (DOC) and the humification index (HIX) of DOM in the 0-10 and 10-20 cm soil layers, with significant reduction under the high N addition (80 kg N·hm^-2·a^-1). Results of parallel factor analysis further showed that the relative content of humus-like components (C1, C2) in DOM decreased following N addition. Short-term N addition reduced the relative abundance of copiotrophic bacteria (Proteobacteria and Acidimicrobiia), but increased the relative abundance of oligotrophic bacteria (Spartobacteria). The relative abundance of copiotrophic bacteria were positively correlated with the HIX and refractory humus-like components (C1, C2) and negatively correlated with the relatively easily decomposed fulvic acid-like component (C3), whereas the pattern was opposite for oligotrophic bacteria. It is indicated that bacterial communities with different life strategies had obvious preference for difficult- and easy-to-decompose components in DOM under N addition treatments. We speculated that the transformation of soil microbial life strategies under the background of increasing N deposition might alter DOM components.

[Seasonal Effect of Simulated Nitrogen Deposition on Soil Respiration and Soil Enzyme Activity in Masson Pine Forest in Mt. Jinyun, Chongqing, China]

Soil enzymes involved in the conversion of soil carbon and nitrogen, meanwhile the availability of soil carbon and nitrogen is the base of soil enzymes, yet atmospheric N deposition influences the release of soil CO₂ by reduce the activities of soil enzyme. The objective of this study was to investigate the effect of different nitrogen deposition on soil respiration and soil enzymes, and explore the relationship among soil respiration, soil temperature, soil moisture and soil enzymes in the Masson pine forest. The results might provide a reference for further study on the effects of nitrogen deposition on pine forest ecosystem. From May 2014 to July 2015, three nitrogen application treatments and a control treatment were set up: low nitrogen [N₅, 20 g·(m²·a)^-1], moderate nitrogen [N₁₀, 40 g·(m²·a)^-1], high nitrogen [N₁₅, 60 g·(m²·a)^-1] and control treatment [N₀, 0 g·(m²·a)^-1) in the Masson pine forest. We measured soil respiration, soil temperature, and soil moisture simultaneously by using the Automated Soil CO₂ Exchange Station (ACE, UK). The results showed that: 1 Soil enzymes and soil respiration had obvious seasonal variation, soil respiration of N₀, N₅, N₁₀ and N₁₅ was the highest in Summer, followed by the Spring and Autumn, and the lowest in Winter, and no consistent change rule was found in soil enzymes. 2 Generally, nitrogen deposition suppressed soil respiration and soil enzymes, and these inhibitory effects were strengthened with increasing levels of nitrogen deposition. The only exception in which nitrogen deposition enhanced soil respiration was in the Masson pine forest in Winter, In Spring, Summer and Autumn, nitrogen deposition suppressed soil enzymes, while there was difference among Ure, Ive, CAT and ACP in Winter. 3 stepwise regression showed that in control treatment and low nitrogen treatment, T, Ure and Ive made great contributions to the R_s, and R_s rapidly increased with the increase of T, Ure and Ive. In middle nitrogen treatment, T, Ure and CAT made great contributions to the R_s, and R_s increased with the increase of T, Ure and CAT. In high nitrogen treatment, R_s decreased with the increase of Ure, yet R_s increased with the increase of CAT and W.