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Mixing native broadleaved tree species is a widely used method for renovating Pinus massoniana plantations. Soil microbial necromass carbon and organic carbon fractions are important parameters for evaluating the impacts of tree species mixing and soil organic carbon (SOC) stability. However, their responses to the mixing and renovation of P. massoniana plantation has not been understood yet. Here, we selected a pure P. massoniana plantation (PP) and a mixed P. massoniana and Castanopsis hystrix plantation, with ages of 16 (MP16) and 38 years (MP38), respectively, as the research objects. We quantified soil physical and chemical properties, microbial necromass carbon content, and organic carbon components at different soil layers to reveal whether and how the introduction of C. hystrix into P. massoniana plantation affected soil microbial necromass carbon and organic carbon components. The results showed that the mixed P. massoniana and C. hystrix plantation significantly reduced fungal necromass carbon content and the ratio of fungal/bacterial necromass carbon in the 0-20 cm and 20-40 cm soil layers. There were no significant differences in microbial necromass carbon contents, bacterial necromass carbon contents, and their contributions to SOC among the different plantations. The contribution of fungal necromass carbon to SOC was higher than that of bacterial necromass carbon in all plantation types. The contribution of soil mineral-associated organic carbon (MAOC) to SOC was higher than that of occluded particulate organic carbon (oPOC) and light-free particulate organic carbon (fPOC) for all plantation types. Mixing the precious broadleaved tree species (i.e., C. hystrix) with coniferous species (P. massoniana) significantly increased MAOC content and the contribution of MAOC, oPOC, and fPOC to SOC in the 0-20 cm and 20-40 cm soil layers. The MAOC of MP38 was significantly higher than that of PP in all soil layers and the MAOC of MP38 stands were significantly higher than MP16 stands in the 20-40 cm, 40-60 cm, and 60-100 cm soil layers, indicating that hybridization enhanced SOC stability and that the SOC of MP38 stands were more stable than MP16 stands. SOC and total nitrogen contents were the main environmental factors driving the changes in soil microbial necromass carbon, while soil total nitrogen and organically complexed Fe-Al oxides were the primary factors affecting organic carbon fraction. Therefore, SOC stability can be enhanced by introducing native broadleaved species, such as C. hystrix, during the management of the P. massoniana plantation. [ABSTRACT FROM AUTHOR]

Elevated nitrogen (N) deposition has induced substantial impacts on the emissions of nitrous oxide (N2O) from forest ecosystems, but how soil microbes regulate the production/consumption of N2O under elevated N deposition remains poorly understood, particularly in high N deposition subtropical forests that are characterized by distinct wet‐dry seasonality. We established a field N addition experiment in a subtropical forest in southern China to explore the influences of low, medium and high (35, 70, and 105 kg N ha‐1 yr‐1, respectively) N addition on N2O efflux and its associated microbial functional genes [amoA for nitrifiers (ammonia‐oxidizing bacteria (AOB) and ammonia‐oxidizing archaea (AOA)) and nirK and nosZ for denitrifiers]. The results showed the following: (1) The N2O emissions were stimulated by N addition in the dry season but were depressed in the wet season. (2) The nirK and nosZ abundances were generally stimulated by N addition, whereas the AOB‐amoA and AOA‐amoA abundances showed divergent responses to N addition. (3) Based on the results of principal component and Pearson correlation analyses, N2O effluxes were associated with microbial biomass in the wet season but with nirK and nosZ abundances in the dry season. Structural equation modeling analyses further indicated that both nitrifiers and denitrifiers under N addition contributed to the generation of N2O in the dry season, whereas the decreased production of N2O in the wet season was primarily caused by denitrifiers. Therefore, seasonally specific strategies should be developed to mitigate the emissions of N2O from subtropical forests with distinct seasonal precipitation patterns. Plain Language Summary: Tropical and subtropical forests are considerable sources of nitrous oxide (N2O), one of the greenhouse gases mainly produced via microbial nitrification and denitrification in soil. In recent decades, the emissions of N2O have been influenced by increasing nitrogen (N) deposition due to enhanced atmospheric reactive N derived primarily from human activities. Moreover, among the types of N deposition, wet N deposition depends greatly on precipitation. This dependence makes predicting future change trends in N2O emissions challenging, especially in tropical and subtropical forests in southern China due to the large amount of natural N deposition and the distinct wet‐dry seasonality caused by unevenly distributed annual precipitation in this region. We studied the responses of N2O emissions to increasing N deposition by establishing a field N addition experiment in a subtropical forest in southern China. We explored the responsive differences in N2O emissions between wet and dry seasons from microbial regulation aspects. The N addition depressed N2O emissions in the wet season but stimulated N2O emissions in the dry season. Moreover, microbial biomass in the wet season and denitrifier abundance in the dry season were the indicators of N2O emissions in response to elevated N deposition. Key Points: N2O emissions from the subtropical forest soils were stimulated in the dry season but depressed in the wet season by N additionThe abundances of nitrifying/denitrifying functional genes were more responsive to N addition than to wet‐dry seasonal changesN2O effluxes were associated with microbial biomass in the wet season but with denitrifying gene abundances in the dry season [ABSTRACT FROM AUTHOR]

Using a culture-based nitrous oxide (N2O) emission assay, three active N2O emitters were isolated from Sphagnum fuscum leaves and all identified as members of Burkholderia. These isolates showed N2O emission in the medium supplemented with but not with , and Burkholderia sp. SF-E2 showed the most efficient N2O emission (0.20 μg·vial−1·day−1) at 1.0 mM KNO3. In Burkholderia sp. SF-E2, the optimum pH for N2O production was 5.0, close to that of the phyllosphere of Sphagnum mosses, while the optimum temperature was uniquely over 30 °C. The stimulating effect of additional 1.5 mM sucrose on N2O emission was ignorable, but Burkholderia sp. SF-E2 upon exposure to 100 mg·L−1E-caffeic acid showed uniquely 67-fold higher N2O emission. All of the three N2O emitters were negative in both acetylene inhibition assay and PCR assay for nosZ-detection, suggesting that N2O reductase or the gene itself is missing in the N2O-emitting Burkholderia. [ABSTRACT FROM AUTHOR]

Using a culture-based nitrous oxide (N2O) emission assay, three active N2O emitters were isolated from Sphagnum fuscum leaves and all identified as members of Burkholderia. These isolates showed N2O emission in the medium supplemented with but not with , and Burkholderia sp. SF-E2 showed the most efficient N2O emission (0.20 μg·vial−1·day−1) at 1.0 mM KNO3. In Burkholderia sp. SF-E2, the optimum pH for N2O production was 5.0, close to that of the phyllosphere of Sphagnum mosses, while the optimum temperature was uniquely over 30 °C. The stimulating effect of additional 1.5 mM sucrose on N2O emission was ignorable, but Burkholderia sp. SF-E2 upon exposure to 100 mg·L−1E-caffeic acid showed uniquely 67-fold higher N2O emission. All of the three N2O emitters were negative in both acetylene inhibition assay and PCR assay for nosZ-detection, suggesting that N2O reductase or the gene itself is missing in the N2O-emitting Burkholderia. [ABSTRACT FROM AUTHOR]

Using a culture-based nitrous oxide (N2O) emission assay, three active N2O emitters were isolated from Sphagnum fuscum leaves and all identified as members of Burkholderia. These isolates showed N2O emission in the medium supplemented with but not with , and Burkholderia sp. SF-E2 showed the most efficient N2O emission (0.20 μg·vial−1·day−1) at 1.0 mM KNO3. In Burkholderia sp. SF-E2, the optimum pH for N2O production was 5.0, close to that of the phyllosphere of Sphagnum mosses, while the optimum temperature was uniquely over 30 °C. The stimulating effect of additional 1.5 mM sucrose on N2O emission was ignorable, but Burkholderia sp. SF-E2 upon exposure to 100 mg·L−1E-caffeic acid showed uniquely 67-fold higher N2O emission. All of the three N2O emitters were negative in both acetylene inhibition assay and PCR assay for nosZ-detection, suggesting that N2O reductase or the gene itself is missing in the N2O-emitting Burkholderia. [ABSTRACT FROM AUTHOR]

Intensified anthropogenic activities will increase rates of nitrogen (N) deposition over the next decades, especially in the tropics. There are urgent needs to know how soil microbial community in N-rich tropical forests responds to long-term N deposition. This study examined effects of long-term N additions on soil microbial biomass (determined by chloroform fumigation), microbial community composition (based on phospholipid fatty acids, PLFAs), and microbial enzyme activities, using an ongoing experimental N additions field in an N-rich tropical forest of South China. There were four N additions levels: no additions (Control); 50 kg N ha −1 yr −1 (Low-N); 100 kg N ha −1 yr −1 (Medium-N), and 150 kg N ha −1 yr −1 (High-N). Results showed that long-term N additions significantly decreased microbial biomass carbon (MBC) and nitrogen (MBN), but had little effects on total PLFAs. However, elevated N inputs significantly reduced the relative abundance of bacterial PLFAs, especially gram-negative bacterial PLFAs with higher gram-positive bacteria: gram-negative bacteria ratio in N treatment plots. Although N additions did not change fungi: bacteria ratio, the proportion of arbuscular mycorrhizal fungi increased significantly with N additions. Long-term N additions greatly increased bacterial stress indexes and enhanced specific enzyme activity (activity per unit of microbial biomass) involved in carbon, nitrogen and phosphorus mineralization. Meanwhile, shifts in microbial community composition and specific enzyme activity were correlated well with soil pH and available N. These results suggest that N-mediated environmental stresses can play an important role in shaping microbial community, and that soil microbes will invest more resources on enzyme production in N-rich forest under elevated N deposition. [ABSTRACT FROM AUTHOR]