Subtropical Natural Forest Research Articles

Soil microbes deal with the nitrogen deposition enhanced phosphorus limitation by shifting community structure in an old-growth subtropical forest

Elevated atmospheric nitrogen (N) deposition potentially enhances the degree of phosphorus (P) limitation in tropical and subtropical forests. However, it remains elusive that how soil microorganisms deal with the N deposition-enhanced P limitation. We collected soils experienced 9 years of manipulative N input at various rates (0, 40, and 80 kg N ha−1 y−1) in an old-growth subtropical natural forest. We measured soil total and available carbon (C), N and P, microbial biomass C, N and P, enzyme activities involved in C, N and P acquisition, microbial community structure, as well as net N and P mineralization. Additionally, we calculated element use efficiency and evaluated microbial homeostasis index. Our findings revealed that N input increased microbial biomass C:P (MBC:P) and N:P (MBN:P) ratios. The homeostasis indexes of MBC:P and MBN:P were 0.68 and 0.75, respectively, indicating stoichiometric flexibility. Interestingly, MBC:P and MBN:P correlated significantly with the fungi:bacteria ratio (F:B), not with N and P use efficiencies, net N and P mineralization, and enzyme C:P (EEAC:P) and N:P (EEAN:P) ratios. Furthermore, EEAC:P and EEAN:P correlated positively with F:B but did not negatively correlate with the C:P and N:P ratios of available resources and microbial biomass. The effects of N deposition on MBC:P, MBN:P and EEAN:P became insignificant when including F:B as a covariate. These findings suggest that microbes flexibly adapted to the N deposition enhanced P limitation by changing microbial community structure, which not only alter microbial biomass C:N:P stoichiometry, but also the enzyme production strategy. In summary, our research advances our understanding of how soil microorganisms deal with the N deposition-enhanced soil P limitation in subtropical forests.

Biochar produced at high temperature mitigates N2O emission and promotes nitrogen retention in subtropical forest soils

AbstractBiochar is produced by burning biomass under oxygen‐limited conditions, and it has been widely used as a soil amendment to improve soil functions such as nutrient retention. However, whether the impact of biochar application on soil nitrogen (N) transformation and N2O emission varies with the pyrolysis temperature remains unclear, especially in different forest types in subtropical regions. In this study, a 60‐day laboratory incubation experiment was conducted to evaluate the impact of biochar with different pyrolysis temperatures (300°C [BC300], 500°C [BC500], and 800°C [BC800]) on net N transformation rates and N2O emission in soils collected from Castanopsis kawakamii dominated natural forest (NF) and Chinese fir (Cunninghamia lanceolate, CF) plantation in subtropical China. The results showed that the application of biochar significantly increased soil ammonium (NH4+) content (p < 0.001) but reduced nitrate (NO3−) content (p < 0.001) compared with the control. The soil NH4+ content of the BC800 treatment was significantly higher than that of other treatments (p < 0.001). Biochar application significantly reduced soil net N mineralization (NRmin) and nitrification (NRnit) rate (p < 0.001), but increased net ammonification (NRamm) rate (p < 0.001). The application of biochar led to a remarkable decrease in cumulative N2O emission compared to the control (p < 0.001). In particular, soils treated with high‐temperature biochar emitted significantly lower N2O compared to other treatments (p < 0.001). The partial least squares path model demonstrated that biochar influenced N2O emission through a direct effect in NF soil and an indirect effect in CF soil. This study highlights the distinct role of biochar, particularly that produced under high pyrolysis temperatures as a soil amendment to mitigate N2O emission and promote N retention in both subtropical natural and planted forests.

Tree Species Classification in Subtropical Natural Forests Using High-Resolution UAV RGB and SuperView-1 Multispectral Imageries Based on Deep Learning Network Approaches: A Case Study within the Baima Snow Mountain National Nature Reserve, China

Accurate information on dominant tree species and their spatial distribution in subtropical natural forests are key ecological monitoring factors for accurately characterizing forest biodiversity, depicting the tree competition mechanism and quantitatively evaluating forest ecosystem stability. In this study, the subtropical natural forest in northwest Yunnan province of China was selected as the study area. Firstly, an object-oriented multi-resolution segmentation (MRS) algorithm was used to segment individual tree crowns from the UAV RGB imagery and satellite multispectral imagery in the forests with different densities (low (547 n/ha), middle (753 n/ha) and high (1040 n/ha)), and parameters of the MRS algorithm were tested and optimized for accurately extracting the tree crown and position information of the individual tree. Secondly, the texture metrics of the UAV RGB imagery and the spectral metrics of the satellite multispectral imagery within the individual tree crown were extracted, and the random forest algorithm and three deep learning networks constructed in this study were utilized to classify the five dominant tree species. Finally, we compared and evaluated the performance of the random forest algorithm and three deep learning networks for dominant tree species classification using the field measurement data, and the influence of the number of training samples on the accuracy of dominant tree species classification using deep learning networks was investigated. The results showed that: (1) Stand density had little influence on individual tree segmentation using the object-oriented MRS algorithm. In the forests with different stand densities, the F1 score of individual tree segmentation based on satellite multispectral imagery was 71.3–74.7%, and that based on UAV high-resolution RGB imagery was 75.4–79.2%. (2) The overall accuracy of dominant tree species classification using the light-weight network MobileNetV2 (OA = 71.11–82.22%), residual network ResNet34 (OA = 78.89–91.11%) and dense network DenseNet121 (OA = 81.11–94.44%) was higher than that of the random forest algorithm (OA = 60.00–64.44%), among which DenseNet121 had the highest overall accuracy. Texture metrics improved the overall accuracy of dominant tree species classification. (3) For the three deep learning networks, the changes in overall accuracy of dominant tree species classification influenced by the number of training samples were 2.69–4.28%.

Effects of Tree Diversity, Functional Composition, and Large Trees on the Aboveground Biomass of an Old-Growth Subtropical Forest in Southern China

Forest aboveground biomass (AGB) plays an important role in regulating the global carbon cycle and is thus an essential component of ecosystem functioning. In the relationships between biodiversity and ecosystem functioning (BEF), studies have shown that many biotic factors (e.g., species, functional traits, and large trees) and abiotic factors have significant impacts on AGB. However, the relative strength of these affecting factors remains unclear. In this study, we analyzed woody plants (diameter at breast height [DBH] ≥ 1 cm) within a 1.6 ha plot in an old-growth subtropical natural forest in southern China. We used structural equation models to test the effects of tree diversity (species, phylogenetic, functional, and size inequality), functional composition, large trees, and environmental factors (topography, soil nutrients, and understory light) on AGB. Our results indicated that size inequality, the community-weighted mean of maximum DBH (CWM_MDBH), and large trees had significant, positive effects on AGB (p < 0.001), while lower soil phosphorus content was found to promote an increase in AGB. Furthermore, large trees, which were mostly composed of dominant tree species, were the main driver of AGB, and the effect of functional composition (e.g., CWM_MDBH) on AGB was substantially reduced by large trees. We argue that the selection effect plays a key role in regulating BEF relationships in subtropical natural forests and conclude that retaining large-diameter trees and dominant species, along with sustaining a complex stand structure, are key measures for improving productivity.

Relationship between species diversity and tree size in natural forests around the Tropic of Cancer

Although numerous hypotheses have been proposed to explain the patterns of species diversity of forests at local and landscape levels, it is still difficult to predict the alpha diversity of species, especially in tropical and subtropical natural forests where trees of different sizes and shapes are highly mixed. Size might characterize species diversity, and the possible correlation between these variables may contribute to the development of easy-to-use growth indicators to predict diversity and to understand the status of trees within a stand. In this study, diameter classes were divided using the equal diameter class (EDC) and minimum measured diameter (MMD) methods, and five species diversity indices were calculated for each diameter class of 14 permanent plots in five national nature reserves surrounding the Tropic of Cancer (23.5°), southwest China. The results show that species richness, abundance, and spatial diversity indices decreased in a typical inverted J-shape pattern with increasing diameter class and MMD, and could be easily modeled by a negative exponential function. The Shannon–Wiener index showed a linear decrease while Pielou’s evenness index displayed a linear increase, with a small degree of instability. The results suggest that species diversity in subtropical forests is closely related to tree size, and the relationship is possibly independent of habitat. Measuring DBH of some trees in a stand could be informative regarding species diversity and contribute to the investigation and assessment of biodiversity.

Long-Term Nitrogen Addition Exerts Minor Effects on Microbial Community but Alters Sensitive Microbial Species in a Subtropical Natural Forest

Increasing nitrogen (N) deposition profoundly affects nutrient cycling in soil, thereby influencing forest ecosystem productivity and function. Soil microorganisms are integral in driving nutrient turnover; the changes in microbial communities in response to N deposition and the associated soil nutrient availability, especially of limited nutrients, are far from clear. To explore the changes in soil bacterial and fungal communities and their key environmental drivers under N deposition, we conducted a multilevel field N addition experiment in a Castanopsis carlesii natural forest. Soil properties and bacterial and fungal communities were investigated. There were no significant changes in alpha diversities (presented as Chao1 and Shannon’s indexes) and beta diversities of bacteria and fungi among the three treatments. Consistently, the relative abundances of dominant bacterial phyla (i.e., Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes) and fungal phyla (i.e., Basidiomycota, Ascomycota, and Rozellomycota) did not change following N addition. These results suggest that N deposition did not alter microbial community diversity and structure. In addition, the results of the Mantel test showed that soil pH, NO3−-N, dissolved organic N (DON), and total phosphorus (TP) predominantly influenced the community diversity and structure in bacteria, but not in fungi. Meanwhile, the relative abundance of some sensitive microbial genera, such as Bryobacter, Bradyrhizobium, Sorangium, and Archaeorhizomyces, were significantly decreased. These results indicate a decreased microbial ability for N fixation and P mobilization induced by N deposition. Moreover, there were significant relationships between Bryobacter, Bradyrhizobium, and Archaeorhizomyces and NO3−-N and available P (AP), suggesting that the responses of sensitive microbial groups to N deposition likely depend on the changes in available nutrients in soil, especially limited N or P. Collectively, 6 years of N addition had no significant influence on microbial communities, but some sensitive microbial groups were associated with N or P turnover. This finding emphasizes the critical roles of sensitive microbial species in meditating limited nutrient cycling in soil under climate change.

Canopy Gaps Control Litter Decomposition and Nutrient Release in Subtropical Forests

The formation of a canopy gap results in changes to the microenvironment which, in turn, affect litter decomposition and nutrient release. However, the mechanisms underlying these effects in differently sized gaps and non-gaps remain poorly understood. To address this gap in knowledge, we selected three large gaps (above 150 m2), three medium gaps (50–100 m2), three small gaps (30–50 m2), and three non-gaps with basically the same site conditions. We then used the litter bag method to investigate leaf and branch litter decomposition over a year in a Castanopsis kawakamii natural forest with the aim of characterizing the litter mass remaining and the nutrient release in canopy gaps and non-gaps. Our results revealed that the remaining litter mass of leaf and branch litter was lower in medium gaps compared to other gaps, and leaf litter decomposed faster than branch litter. Environmental factors were identified as the primary drivers of total carbon and nitrogen release during litter decomposition. Gap size (canopy openness), taxonomic Margalef index, the Brillouin index of soil microbes, soil total nitrogen content, soil pH value, and average air temperature were identified as the main factors driving carbon and nitrogen release from branch litter. In the late decomposition stage, the taxonomic Pielou index, soil total potassium content, soil water content, and average relative air humidity were the main drivers of nutrient release from branch litter. The soil water content and average relative air humidity were also found to be the main factors affecting the nutrient release from leaf litter throughout the different stages of decomposition. Overall, our study highlights the impact of canopy gaps on microenvironmental variation, taxonomic community diversity, and soil microbial functional diversity and how these factors ultimately influence litter decomposition and nutrient release. Our findings provide an important foundation for further research into soil nutrient cycling in subtropical natural forests.

Soil protist functional composition shifts with atmospheric nitrogen deposition in subtropical forests

Abstract Soil protists play a key role in driving ecological functions through predation and parasitism. However, little is known about how nitrogen (N) deposition and seasonal variation influence soil protist functions in forest soils. Here, we assessed first the impacts of N deposition (control, 50, 100 and 150 kg N ha−1 year−1) on the functional composition of the soil protist community in summer and winter, using amplicon sequencing of environmental DNA from a subtropical natural forest. We found that soil protists were dominated by consumers (42.6%–51.6%), followed by parasites (32.9%–40.9%) and phototrophs (3.2%–13.1%), implying a predominant role of consumers and potential top‐down effects on the other trophic groups in subtropical forest soils. The functional composition of soil protists was greatly influenced by N deposition, but these responses were dependent on seasonal variation. The diversity of phototrophs was lower in summer than in winter. Instead, an opposite pattern was observed for consumers, resulting in a significantly higher protist diversity in summer than in winter. Furthermore, low and high N deposition simplified the structural complexity of soil protist communities, suggesting a nonlinear response of protist structural stability to N deposition. Synthesis and applications. This study provides unprecedented evidence that seasonal variation plays an important role in regulating responses of soil protist functional composition to N deposition, and highlights the nonlinear effects of rising N deposition on the soil food web.

Depth-dependent response of particulate and mineral-associated organic carbon to long-term throughfall reduction in a subtropical natural forest

Climate change has altered the precipitation patterns and prolonged the drought season in subtropical areas, which affects forest productivity and soil carbon (C) cycling. However, the response of different soil organic C (SOC) fractions to the reduction in precipitation and the potential mechanisms remain poorly understood in subtropical forests. In this study, we examined the effects of long-term (9 years) throughfall reduction on SOC fractions, including particulate organic C (POC) and mineral-associated organic C (MAOC), dissolved organic C (DOC), microbial biomass C (MBC), mycorrhizal fungal biomass, and fine-root biomass of two soil layers (0–10 cm and 40–60 cm) in a subtropical Castanopsis carlesii forest. The results showed that throughfall reduction had a significant effect on soil C cycling in the topsoil but not in the subsoil. Throughfall reduction significantly decreased soil POC but increased soil MAOC in the topsoil. Meanwhile, soil MBC, DOC, fine-root biomass, and mycorrhizal fungal biomass were reduced with throughfall reduction. Regression and partial least squares path modeling analyses revealed that soil POC was positively correlated with fine-root biomass, necromass, and mycorrhizal fungal biomass, whereas negative relationships were observed for topsoil MAOC. The results from this study implied that fine roots play an important role in the contrasting responses of POC and MAOC to throughfall reduction in the topsoil. Such information is essential for understanding the roles of plant roots and mycorrhizal fungi in the soil C cycle in subtropical forests under drought conditions.

Dynamic of labile, recalcitrant carbon and nitrogen during the litter decomposition in a subtropical natural broadleaf forest

Dynamic of labile, recalcitrant carbon and nitrogen during the litter decomposition in a subtropical natural broadleaf forest

Higher Soil Aggregate Stability in Subtropical Coniferous Plantations Than Natural Forests Due to Microbial and Aggregate Factors

Forest restoration and soil structure stabilization are the focus of forestry and ecology. However, the combined mechanisms of soil microorganisms and organic and inorganic aggregate binding agents on soil aggregation is unclear. In order to explore the effects of subtropical forest restoration types on soil aggregates and the underlying mechanisms, we collected soil samples from subtropical natural forests and coniferous and broad-leaved plantations that are commonly used for forest restoration. The mean weight diameter (MWD) of the soil aggregate was used to indicate the aggregates’ stability. The soil microbial diversity and structure, the organic and inorganic aggregate binding agents including the mycorrhizal density, the glomalin-related soil protein and the Fe and Al oxides were investigated. Results showed that the Shannon and Simpson indices of soil microbial communities in the coniferous plantations were both significantly higher than those in the natural forests. At the annual level, compared with the natural forests, the plantations decreased the proportion of 0.25–1 mm aggregates while the MWD significantly increased. The forest type also significantly affected the mycorrhizal density, the easily extractable glomalin-related soil proteins (EEG) and the Fe oxide. A variance decomposition analysis showed that soil microbial communities, organic and inorganic binding agents, and their interactions together contributed to the aggregates’ composition and stability by 75.07%. The MWD was positively correlated with the microbial diversity, mycorrhizal density and Fe oxide. We therefore suggest that the combined effects of the soil microbial communities and the organic (mycorrhizal density) and inorganic binding agents (Fe oxide) can be the main mechanisms of soil aggregation in the study area, resulting in a higher soil aggregate stability in the subtropical coniferous plantation than in the natural forest.

Soil warming delays leaf litter decomposition but exerts no effect on litter nutrient release in a subtropical natural forest over 450 days

Litter decomposition is a fundamental ecosystem process, influencing soil carbon storage, nutrient availability, and forest productivity. Climate change may affect litter decomposition and thus nutrient dynamics via altering plant phenology, litter quality, and the composition of soil microbial communities. However, the effects of climate change on litter decomposition are not well understood, especially in tropical and subtropical forest ecosystems, which are less temperature limited. We conducted a manipulative study to assess how soil warming affects litter decomposition rates and its relation to litter chemistry, extracellular enzyme activities, and microbial biomass in an evergreen broad-leaved forest in subtropical China. The temperature at 0–10 cm soil depth was experimentally increased by 4 °C, starting from June 2016 to October 2017. Soil warming did not affect litter mass loss during the initial stage (0–270 day), but reduced litter mass loss by 12.9 % at the later stages (days 350 to 450). Structural equation modeling showed that litter moisture content was reduced by warming, but this was not the main effector leading to the reduction in late-stage litter decomposition in the warming treatment. The model suggested that warming reduced litter decomposition rates likely indirectly, through its negative effects on extractable organic carbon and microbial biomass (e.g., microbial carbon and nitrogen), and on litter enzyme activities (a composite variable of β-glucosidase, cellobiohydrolase, acid phosphatase, and phenoloxidase). These results show that warming may slow down litter carbon cycling, but this subtropical forest ecosystem did not affect litter N and P cycling and soil nutrient availability.

Tree roots exert greater influence on soil microbial necromass carbon than above-ground litter in subtropical natural and plantation forests

The stabilization of soil organic carbon (SOC) relies heavily on both the production of microbial necromass carbon (MNC) and its protection by clay minerals. Above- and below-ground carbon (C) sources differ not only in the C quality and quantity that drive MNC production but also in MNC-mineral interactions. Here we test the hypothesis that root litter and exudates play a more important role in the formation and stabilization of soil MNC than above-ground litterfall in a natural subtropical moist forest and a tree (Castanopsis carlesii) plantation of southeastern China. We employed a two-factorial design of field experiment consisting of four treatments: control (CT), litterfall exclusion (NL), root exclusion (NR), and no C input (NI). We found that NR treatment substantially reduced total soil MNC and both bacterial (BNC) and fungal (FNC) necromass C as well as fractions of MNC and FNC to total SOC in both natural and plantation forests, but did not alter ratios of FNC to BNC or BNC to total SOC. In comparison, NL treatment had no effect on the ratios of MNC, FNC or BNC to total SOC in both forests. Furthermore, NR treatment reduced both bacterial and fungal biomass, but NL treatment had no significant effect on bacterial or fungal biomass in the natural forests. Whereas the NR and NI treatments significantly reduced MNC and FNC in both forests, the NL treatment reduced MNC and FNC only in the natural forest and reduced BNC only in the plantation forest. Analysis of structural equation model revealed that the C treatments could alter the accumulation of MNC directly or indirectly through changing soil available substrates (i.e., DOC, NH4+) and microbial community structure. Our data suggests that plant roots exert a stronger influence on the production and stabilization of MNC than above-ground C source.

Neighborhood diversity structure and neighborhood species richness effects differ across life stages in a subtropical natural secondary forest

Neighborhood diversity structure and neighborhood species richness effects differ across life stages in a subtropical natural secondary forest

Forest gaps regulate seed germination rate and radicle growth of an endangered plant species in a subtropical natural forest

The survival rate of Castanopsis kawakamii from seed to seedling is relatively low, leading to difficulties in the regeneration of its natural forests. Forest gaps play a vital role in plant regeneration and biodiversity maintenance in forest ecosystems. Unfortunately, our understanding of the effects of gap size and within-gap position on the seed germination and radicle growth of C. kawakamii is still limited. In particular, our knowledge on the relationship between gap size and environmental factors and their influence on seed germination and radicle growth is incomplete. In the present study, we studied the influences of forest gaps and within-gap position on seed regeneration on the germination and radicle growth of an endangered species C. kawakamii in a subtropical natural forest in China. We selected three large gaps (LG, gap size above 200 m2), three medium gaps (MG, gap size 50–100 m2), three small gaps (SG, gap size 30–50 m2), and non-gap (NG), and planted the seeds of C. kawakamii in five positions within each gap. The results showed that (1) the influence of forest gaps on seed germination rate was, from highest to lowest, medium gaps (51%), non-gap (47%), small gaps (40%) and large gaps (17%), and the seed germination rate was the highest in all positions in medium gaps, with the exception of the east position. (2) Radicle length in forest gaps was, from highest to lowest, medium gaps, small gaps, large gaps and non-gap, and it was the highest in the east, south, west and north positions of medium gaps. (3) Canopy openness (gap size) and air temperature were the main factors influencing seed germination and radicle growth of C. kawakamii. We concluded that medium-sized gaps were the most suitable for seed germination and radicle growth of C. kawakamii, and they promote the regeneration of this endangered species in the investigated natural forest.

Changes in soil carbon and nitrogen stocks after conversion of subtropical natural forest to loblolly pine plantations

Changes in soil carbon and nitrogen stocks after conversion of subtropical natural forest to loblolly pine plantations

Two Decades Progress on the Application of Remote Sensing for Monitoring Tropical and Sub-Tropical Natural Forests: A Review

Forest covers about a third of terrestrial land surface, with tropical and subtropical zones being a major part. Remote sensing applications constitute a significant approach to monitoring forests. Thus, this paper reviews the progress made by remote sensing data applications to tropical and sub-tropical natural forest monitoring over the last two decades (2000–2020). The review focuses on the thematic areas of aboveground biomass and carbon estimations, tree species identification, tree species diversity, and forest cover and change mapping. A systematic search of articles was performed on Web of Science, Science Direct, and Google Scholar by applying a Boolean operator and using keywords related to the thematic areas. We identified 50 peer-reviewed articles that studied tropical and subtropical natural forests using remote sensing data. Asian and South American natural forests are the most highly researched natural forests, while African natural forests are the least studied. Medium spatial resolution imagery was extensively utilized for forest cover and change mapping as well as aboveground biomass and carbon estimation. In the latest studies, high spatial resolution imagery and machine learning algorithms, such as Random Forest and Support Vector Machine, were jointly utilized for tree species identification. In this review, we noted the promising potential of the emerging high spatial resolution satellite imagery for the monitoring of natural forests. We recommend more research to identify approaches to overcome the challenges of remote sensing applications to these thematic areas so that further and sustainable progress can be made to effectively monitor and manage sustainable forest benefits.

Changes in the stoichiometry of Castanopsis fargesii along an elevation gradient in a Chinese subtropical forest.

Elevation is important for determining the nutrient biogeochemical cycle in forest ecosystems. Changes in the ecological stoichiometry of nutrients along an elevation gradient can be used to predict how an element cycle responds in the midst of global climate change. We investigated changes in concentrations of and relationships between nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in the leaves and roots of the dominant tree species, Castanopsis fargesii, along an elevation gradient (from 500 to 1,000 m above mean sea level) in a subtropical natural forest in China. We analyzed correlations between C. fargesii’s above-ground biomass and stoichiometry with environmental factors. We also analyzed the soil and plant stoichiometry of this C. fargesii population. Our results showed that leaf N decreased while leaf K and Ca increased at higher elevations. Meanwhile, leaf P showed no relationship with elevation. The leaf N:P indicated that C. fargesii was limited by N. Elevation gradients contributed 46.40% of the total variance of ecological stoichiometry when assessing environmental factors. Our research may provide a theoretical basis for the biogeochemical cycle along with better forest management and fertilization for this C. fargesii population.

Latitudinal patterns of soil extracellular enzyme activities and their controlling factors in Pinus massoniana plantations in subtropical China

Soil extracellular enzyme activities (EEAs) and enzymatic acquisition ratios are significantly affected by both abiotic and biotic factors across various scales and ecosystems. However, what determines the soil EEAs and their acquisition ratios in planted forests remains poorly understood. In this study, we measured the activities of β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG) and acid phosphatase (AP) and explored their key controlling factors in Pinus massoniana plantations along a 1100 km latitudinal gradient in subtropical China. The three enzyme activities increased with an increase in latitude and were significantly and negatively affected by mean annual temperature (MAT) and mean annual precipitation (MAP), but were positively correlated with soil available P content. From south to north, the enzymatic C:N (ln(BG):ln(NAG)) acquisition ratio decreased significantly, while the enzymatic C:P (ln(BG):ln(AP)) and N:P (ln(NAG):ln(AP)) acquisition ratios increased significantly. The latitudinal patterns of the enzymatic C:P and N:P acquisition ratios were significantly and negatively affected by MAT and MAP, but were positively affected by soil available P content. In total, climatic factors (e.g., MAT and MAP) accounted for more (14.2% and 25.5%, respectively) of the latitudinal variations in soil EEAs and their acquisition ratios than soil (e.g., available P) and stand (e.g., diameter at breast height). Therefore, these results illustrate that planted forests are susceptible to variations in soil EEAs and their acquisition ratios along a latitudinal gradient and that such changes are mainly driven by climate. Moreover, the similar average enzymatic C:P and N:P acquisition ratios in this pine plantation with the literature values reported for tropical and subtropical natural forests indicate that the pine plantations in subtropical China may be strongly P limited.

How do tree species characteristics affect the bacterial community structure of subtropical natural mixed forests?

The effects of tree species on bacterial community structure have attracted much attention, but few studies have been done in natural mixed forests. In this study, we selected 12 sampling sites in the subtropical natural mixed forest (mainly distributed by Chinese sweet gum, chestnut, Oriental oak, Masson pine, Chinese fir, etc.). The fermentation layer (OF) and humified layer (OH) were mixed as forest floor samples, and the topsoil samples (0–10 cm) were taken. Bacterial composition was studied by 16S rRNA gene sequencing. Coniferous canopy area ratio (Pc), broadleaved and shrubby canopy area ratio (Phwd), elevation, soil properties were tested. The objective is to reveal which soil properties are significantly affected by tree species characteristics, which soil properties significantly affect bacterial community structure, and whether the bacterial community structure is the same in forest floor and topsoil samples at the same sampling site. The results showed that: (1) Pc and Phwd could be used to represent tree species characteristics of natural mixed forests, and they significantly (P=0.05) affected the soil C/N ratio; (2) the soil C/N ratio was the main factor affecting the soil bacterial community composition, especially for the dominant heterotrophic bacteria (Acidothermus, Variibacter, Candidatus Solibacter, Acidibacter, and Bryobacter). The relative abundance (1.11–26.27%) of the dominant heterotrophic bacteria increases with an increase in the C/N ratio (6.33–10.76) within a certain range; (3) the dominant bacteria in topsoil samples were Nitrospira, Acidothermus, Arthrobacter, Bradyrhizobium, and Variibacter, while that in forest floor samples were Jatrophihabitans, Acidothermus, Burkholderia-Paraburkholderia, and Bradyrhizobium. Although the forest floor bacteria came from the topsoil at the same sampling site, the bacterial community structure had changed significantly. This study indicated that tree species drive the change of soil bacterial community by changing the soil C/N ratio, which may provide a new perspective for maintaining the stability of regional ecosystem structure.