[Effects of Simulated Acid Rain and Nitrogen Deposition on Soil Bacterial Community Structure and Diversity in the Masson Pine Forest]

By analyzing the effects of acid rain and nitrogen deposition on the structure and diversity of soil bacterial communities, the response mechanism of Masson pine forests to environmental stress was investigated, providing a theoretical reference basis for resource management and conservation in Tianmu Mountain National Nature Reserve. Four treatments of the simulated acid rain and nitrogen deposition were set up in 2017 to 2021 in Tianmu Mountain National Nature Reserve (pH value of 5.5 and 0 kg·(hm²·a)^-1, CK; pH value of 4.5 and 30 kg·(hm²·a)^-1, T1; pH value of 3.5 and 60 kg·(hm²·a)^-1, T2; pH value of 2.5 and 120 kg·(hm²·a)^-1, T3). The differences in soil bacterial community composition and structure among treatments and their influencing factors were analyzed by collecting soils from four treatments, using the Illumina MiSeq PE300 second-generation high-throughput sequencing platform. The results showed that acid rain and nitrogen deposition significantly reduced soil bacterial α-diversity (P<0.05) in a Masson pine forest. The Masson pine forest soils consisted of 36 phylum groups of mycota, with Acidobacteria, Proteobacteria, Actinobacteria, and Chloroflexi as the main bacterial phyla (relative abundance>1%) in the Masson pine forest soils. Flavobacterium, Nitrospira, Haliangium, Candidatus_Koribacter, Bryobacter, Occallatibacter, Acidipla, Singulisphaera, Pajaroellobacter, and Acidothermus, which showed significant changes in relative abundance under the four treatments, could be used as indicator species for changes in soil bacterial communities under acid rain and nitrogen deposition stress. Soil pH and total nitrogen were influential factors in the diversity of soil bacterial communities. As a result, acid rain and nitrogen deposition increased the potential ecological risk, and the loss of microbial diversity will change the ecosystem function as well as reduce the stability of the ecosystem.

Soil Nitrogen Transformation Process Influenced by Litterfall Manipulation in Two Subtropical Forest Types

Nitrogen (N) is often recognized as the primary limiting nutrient element for the growth and production of forests worldwide. Litterfall represents a significant pathway for returning nutrients from aboveground parts of trees to the soils and plays an essential role in N availability in different forest ecosystems. This study explores the N transformation processes under litterfall manipulation treatments in a Masson pine pure forest (MPPF), and Masson pine and Camphor tree mixed forest (MCMF) stands in subtropical southern China. The litterfall manipulation included litterfall addition (LA), litterfall removal (LR), and litterfall control (LC) treatments. The project aimed to examine how litterfall inputs affect the soil N process in different forest types in the study region. Results showed that soil ammonium N (NH₄ ⁺-N) and nitrate N (NO₃ ^--N) content increased under LA treatment and decreased under LR treatment compared to LC treatment. LA treatment significantly increased soil total inorganic N (TIN) content by 41.86 and 22.19% in MPPF and MCMF, respectively. In contrast, LR treatment decreased the TIN content by 10 and 24% in MPPF and MCMF compared to LC treatment. Overall, the soil net ammonification, nitrification, and N mineralization rates were higher in MCMF than in MPPF; however, values in both forests were not significantly different. LA treatment significantly increased annual net ammonification, nitrification, and mineralization in both forest types (p < 0.05) compared to LC treatment. LR treatment significantly decreased the values (p < 0.05), except for ammonification, where LR treatment did not differ substantially compared to LC treatment. Our results suggested that changes in litterfall inputs would significantly alter soil N dynamics in studied forests of sub-tropical region. Moreover, mixed forest stands have higher nutrient returns due to mixed litter and higher decomposition compared to pure forest stands.

[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.