Seasonal variations in soil microbial community co-occurrence network complexity respond differently to field-simulated warming experiments in a northern subtropical forest.

Global warming may reshape seasonal changes in microbial community diversity and co-occurrence network patterns, with significant implications for terrestrial ecosystem function. We conducted a 2-year in situ field simulation of the effects of warming on the seasonal dynamics of soil microbial communities in a northern subtropical Quercus acutissima forest. Our study revealed that warming had no significant effect on the richness or diversity of soil bacteria or fungi in the growing season, whereas different warming gradients had different effects on their diversity in the nongrowing season. Warming also changed the microbial community structure, increasing the abundance of some thermophilic microbial species and decreasing the abundance of some symbiotrophic microorganisms. The co-occurrence network analysis of the microbial community showed that warming decreased the complexity of the intradomain network in the soil bacterial community in the growing and nongrowing seasons but increased it in the fungal community. Moreover, increasing warming temperatures increased the complexity of the interdomain network between bacteria and fungi in the growing season but decreased it in the nongrowing season, and the keystone species in the interdomain network changed with warming. Warming also reduced the proportion of positive microbial community interactions, indicating that warming reduced the mutualism, commensalism, and neutralism of microorganisms as they adapted to soil environmental stress. The factors affecting the fungal community varied considerably across warming gradients, with the bacterial community being significantly affected by soil temperature, MBC, NO3--N and NH4+-N, moreover, SOC and TN significantly affected fungal communities in the 4 °C warming treatment.. These results suggest that warming increases seasonal differences in the diversity and complexity of soil microbial communities in the northern subtropical region, significantly influencing soil dynamic processes regulating forest ecosystems under global warming.

The spatial distribution of tree-tree interaction effects on soil microbial biomass and respiration.

The capacity of forests to sequester carbon in both above- and belowground compartments is a crucial tool to mitigate rising atmospheric carbon concentrations. Belowground carbon storage in forests is strongly linked to soil microbial communities that are the key drivers of soil heterotrophic respiration, organic matter decomposition and thus nutrient cycling. However, the relationships between tree diversity and soil microbial properties such as biomass and respiration remain unclear with inconsistent findings among studies. It is unknown so far how the spatial configuration and soil depth affect the relationship between tree richness and microbial properties. Here, we studied the spatial distribution of soil microbial properties in the context of a tree diversity experiment by measuring soil microbial biomass and respiration in subtropical forests (BEF-China experiment). We sampled soil cores at two depths at five locations along a spatial transect between the trees in mono- and hetero-specific tree pairs of the native deciduous species Liquidambar formosana and Sapindus saponaria. Our analyses showed decreasing soil microbial biomass and respiration with increasing soil depth and distance from the tree in mono-specific tree pairs. We calculated belowground overyielding of soil microbial biomass and respiration - which is higher microbial biomass or respiration than expected from the monocultures - and analysed the distribution patterns along the transect. We found no general overyielding across all sampling positions and depths. Yet, we encountered a spatial pattern of microbial overyielding with a significant microbial overyielding close to L. formosana trees and microbial underyielding close to S. saponaria trees. We found similar spatial patterns across microbial properties and depths that only differed in the strength of their effects. Our results highlight the importance of small-scale variations of tree-tree interaction effects on soil microbial communities and functions and are calling for better integration of within-plot variability to understand biodiversity-ecosystem functioning relationships.

Spatiotemporal patterns and deposition of organophosphate esters (OPEs) in air, foliage and litter in a subtropical forest of South China.

Recent studies revealed the un-negligible impact of airborne organophosphate esters (OPEs) on phosphorus (P)-limited ecosystems. Subtropical forests, the global prevalence P-limited ecosystems, contain canopy structures that can effectively sequester OPEs from the atmosphere. However, little is known about the behavior and fate of OPEs in subtropical forest ecosystem, and the impact on the P cycling in this ecosystem. OPE concentrations in the understory air (at two heights), foliage, and litterfall were investigated in a subtropical forest in southern China. The median ∑OPE concentrations were 3149 and 2489 pg/m3 in the upper and bottom air, respectively. Foliage exhibited higher ∑OPE concentrations (median = 386 ng/g dry weight (dw)) compared to litter (median = 267 ng/g dw). The air OPE concentrations were ordered by broadleaved forest > mixed forest > coniferous forest, which corresponds to the results of canopy coverage or leaf area index. The spatial variation of OPEs in foliage and litter was likely caused by the leaf surface functional traits. Higher OPE concentrations were found in the wet season for understory air while in the dry season for foliage and litter, which were attributed to the changes in emission sources and meteorological conditions, respectively. The inverse temporal variation suggests the un-equilibrium partitioning of OPEs between leaf and air. The OPE concentrations during the litter-incubation presented similar temporal trends with those in foliage and litter, indicating the strong interaction of OPEs between the litter layer and the near-soil air, and the efficient buffer of litter layer played in the OPEs partitioning between soil and air. The median OPEs-associated P deposition fluxes through litterfall were 270, 186, and 249 µg P/m2·yr in the broadleaved, mixed, and coniferous forests, respectively. Although the fluxes accounted for approximately 0.2% of the total atmospheric P deposition, their significance to this P-limited ecosystem may not be negligible.

Soil fungal community has higher network stability than bacterial community in response to warming and nitrogen addition in a subtropical primary forest.

Global change factors are known to strongly affect soil microbial community function and composition. However, as of yet, the effects of warming and increased anthropogenic nitrogen deposition on soil microbial network complexity and stability are still unclear. Here, we examined the effects of experimental warming (3°C above ambient soil temperature) and nitrogen addition (5 g N m-2 year-1) on the complexity and stability of the soil microbial network in a subtropical primary forest. Compared to the control, warming increased |negative cohesion|:positive cohesion by 7% and decreased network vulnerability by 5%; nitrogen addition decreased |negative cohesion|:positive cohesion by 10% and increased network vulnerability by 11%. Warming and decreased soil moisture acted as strong filtering factors that led to higher bacterial network stability. Nitrogen addition reduced bacterial network stability by inhibiting soil respiration and increasing resource availability. Neither warming nor nitrogen addition changed fungal network complexity and stability. These findings suggest that the fungal community is more tolerant than the bacterial community to climate warming and nitrogen addition. The link between bacterial network stability and microbial community functional potential was significantly impacted by nitrogen addition and warming, while the response of soil microbial network stability to climate warming and nitrogen deposition may be independent of its complexity. Our findings demonstrate that changes in microbial network structure are crucial to ecosystem management and to predict the ecological consequences of global change in the future. IMPORTANCE: Soil microbes play a very important role in maintaining the function and health of forest ecosystems. Unfortunately, global change factors are profoundly affecting soil microbial structure and function. In this study, we found that climate warming promoted bacterial network stability and nitrogen deposition decreased bacterial network stability. Changes in bacterial network stability had strong effects on bacterial community functional potentials linked to metabolism, nitrogen cycling, and carbon cycling, which would change the biogeochemical cycle in primary forests.

Drought shortens subtropical understory growing season by advancing leaf senescence.

Subtropical forests, recognized for their intricate vertical canopy stratification, exhibit high resistance to extreme drought. However, the response of leaf phenology to drought in the species-rich understory remains poorly understood. In this study, we constructed a digital camera system, amassing over 360,000 images through a 70% throughfall exclusion experiment, to explore the drought response of understory leaf phenology. The results revealed a significant advancement in understory leaf senescence phenology under drought, with 11.75 and 15.76 days for the start and end of the leaf-falling event, respectively. Pre-season temperature primarily regulated leaf development phenology, whereas soil water dominated the variability in leaf senescence phenology. Under drought conditions, temperature sensitivities for the end of leaf emergence decreased from -13.72 to -11.06 days °C-1, with insignificance observed for the start of leaf emergence. Consequently, drought treatment shortened both the length of the growing season (15.69 days) and the peak growth season (9.80 days) for understory plants. Moreover, this study identified diverse responses among intraspecies and interspecies to drought, particularly during the leaf development phase. These findings underscore the pivotal role of water availability in shaping understory phenology patterns, especially in subtropical forests.

Effects of seasonal precipitation regimes on microbial biomass and extracellular enzyme activity during shrub foliar litter decomposition in a subtropical forest.

Elucidating the mechanisms underlying microbial biomass and extracellular enzyme activity responses to the seasonal precipitation regime during foliar litter decomposition is highly important for understanding the material cycle of forest ecosystems in the context of global climate change; however, the specific underlying mechanisms remain unclear. Hence, a precipitation manipulation experiment involving a control (CK) and treatments with decreased precipitation in the dry season and extremely increased precipitation in the wet season (IE) and decreased precipitation in the dry season and proportionally increased precipitation in the wet season (IP) was conducted in a subtropical evergreen broad-leaved forest in China from October 2020 to October 2021. The moisture, microbial biomass, and extracellular enzyme activities of foliar litter from two dominant shrub species, Phyllostachys violascens and Alangium chinense, were measured at six stages during the dry and wet seasons. The results showed that (1) both IE and IP significantly decreased the microbial biomass carbon and microbial biomass nitrogen content and the activities of ß-1,4-glucosidase, ß-1,4-N-acetylglucosaminidase, acid phosphatase and cellulase in the dry season, while the opposite effects were observed in the wet season. (2) Compared with those of IE, the effects of IP on foliar litter microbial biomass and extracellular enzyme activity were more significant. (3) The results from the partial least squares model indicated that extracellular enzyme activity during foliar litter decomposition was strongly controlled by the foliar litter water content, microbial biomass nitrogen, the ratio of total carbon to total phosphorus, foliar litter total carbon, and foliar litter total nitrogen. These results provide an important theoretical basis for elucidating the microbial mechanisms driving litter decomposition in a subtropical forest under global climate change scenarios.

Does local soil factor drive functional leaf trait variation? A test on Neilingding Island, South China.

Leaf traits were affected by soil factors and displayed varietal differences in forest. However, few examples have been reported on the Island ecosystems. We comprehensively investigated 9 leaf traits (leaf length, leaf width, leaf area, SLA, leaf fresh weight, leaf C content, leaf N content, leaf K content, leaf C:N ratio) of 54 main subtropical woody species and soil parameters (soil pH, total C content, total N content, total K content, available N content, available P content, available K content and soil moisture) in Neilingding Island, Shenzhen, southern China. Intra-and interspecific variation of leaf traits were measured and their correlations with soil parameters were explored. The interspecific variations of leaf C:N ratio, leaf N content and leaf fresh weight were higher than their intraspecific variations. The intraspecific variation of leaf K content was larger than that of interspecific one, accounting for 80.69% of the total variance. Positive correlations were found among intraspecific coefficients of variations in leaf morphological traits. The correlation analysis between the variation of intraspecific traits and the variation of soil parameters showed that changes in soil factors affected leaf morphology and stoichiometry. The interaction between soil moisture and soil available P content was the key factor on intraspecific variations of leaf traits including leaf area, leaf fresh weight, leaf C and leaf K content. We concluded that leaf traits of plants in the island were tightly related to soil parameters. Soil parameters, especially soil moisture and available P content, affected plant leaf morphology and stoichiometry at the local scale.

Unveiling the Sources and Transfer of Mercury in Forest Bird Food Chains Using Techniques of Vivo-Nest Video Recording and Stable Isotopes.

Knowledge gaps in mercury (Hg) biomagnification in forest birds, especially in the most species-rich tropical and subtropical forests, limit our understanding of the ecological risks of Hg deposition to forest birds. This study aimed to quantify Hg bioaccumulation and transfer in the food chains of forest birds in a subtropical montane forest using a bird diet recorded by video and stable Hg isotope signals of biological and environmental samples. Results show that inorganic mercury (IHg) does not biomagnify along food chains, whereas methylmercury (MeHg) has trophic magnification factors of 7.4-8.1 for the basal resource-invertebrate-bird food chain. The video observations and MeHg mass balance model suggest that Niltava (Niltava sundara) nestlings ingest 78% of their MeHg from forest floor invertebrates, while Flycatcher (Eumyias thalassinus) nestlings ingest 59% from emergent aquatic invertebrates (which fly onto the canopy) and 40% from canopy invertebrates. The diet of Niltava nestlings contains 40% more MeHg than that of Flycatcher nestlings, resulting in a 60% higher MeHg concentration in their feather. Hg isotopic model shows that atmospheric Hg0 is the main Hg source in the forest bird food chains and contributes >68% in most organisms. However, three categories of canopy invertebrates receive ∼50% Hg from atmospheric Hg2+. Overall, we highlight the ecological risk of MeHg exposure for understory insectivorous birds caused by atmospheric Hg0 deposition and methylation on the forest floor.

Effects of stand ages on soil enzyme activities in Chinese fir plantations and natural secondary forests.

Forest type and stand age are important biological factors affecting soil enzyme activities. However, the changes in soil enzyme activities across stand ages and underlying mechanisms under the two forest restoration strate-gies of plantations and natural secondary forests remain elusive. In this study, we investigated the variations of four soil enzyme activities including cello-biohydrolase (CBH), ß-1,4-glucosidase (ßG), acid phosphatase (AP) and ß-1,4-N-acetylglucosaminidase (NAG), which were closely associated with soil carbon, nitrogen, and phosphorus cycling, across Cunninghamia lanceolata plantations and natural secondary forests (5, 8, 21, 27 and 40 years old). The results showed that soil enzyme activities showed different patterns across different forest types. The acti-vities of AP, ßG and CBH in the C. lanceolata plantations were significantly higher than those in the natural secon-dary forests, and there was no significant difference in the NAG activity. In the plantations, AP activity showed a decreasing tendency with the increasing stand ages, with the AP activity in the 5-year-old plantations significantly higher than other stand ages by more than 62.3%. The activities of NAG and CBH decreased first and then increased, and ßG enzyme activity fluctuated with the increasing stand age. In the natural secondary forests, NAG enzyme activity fluctuated with the increasing stand age, with that in the 8-year-old and 27-year-old stand ages being significantly higher than the other stand ages by more than 14.9%. ßG and CBH enzyme activities increased first and then decreased, and no significant difference was observed in the AP activity. Results of the stepwise regression analyses showed that soil predictors explained more than 34% of the variation in the best-fitting models predicting soil enzyme activities in the C. lanceolata plantations and natural secondary forests. In conclusion, there would be a risk of soil fertility degradation C. lanceolata plantations with the increasing stand age, while natural secondary forests were more conducive to maintaining soil fertility.

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Soil N mineralization is a key process of nutrient cycling in ecosystems. The mechanism of the seasonal distribution of precipitation on soil N mineralization remains unclear. We conducted a precipitation manipulation experiment in a subtropical forest in the middle and lower reaches of the Yangtze River in China from 2020 to 2022, with three treatments, including control (CK), decreased precipitation in the dry season with extremely increased precipitation in the wet season (T1), and decreased precipitation in the dry season with proportionally increased precipitation in the wet season (T2). With in situ resin core method, we explored the effect of seasonal distribution of precipitation on soil N mineralization. The results showed that T1 and T2 significantly decreased dry season net nitrification rate by 57.9% and 72.5% and the net N mineralization rate by 82.5% and 89.6%, respectively, and significantly increased wet season net nitrification rate by 64.3% and 79.5% and net N mineralization rate by 64.2% and 81.1%, respectively. Proportionally increased precipitation in the wet season was more conducive to soil N mine-ralization process than extremely increased precipitation in the wet season. Results of the structural equation model showed that change in seasonal distribution of precipitation could significantly affect soil N mineralization processes in the subtropical forest by changing soil water content, ammonium nitrogen, microbial biomass nitrogen, and soil C:N. Our results had important reference for understanding soil nitrogen cycling and other ecological processes, and were conducive to more accurate assessment on the impacts of future changes in seasonal precipitation pattern on subtropical forest ecosystems.

Response of tree growth to nutrient addition is size dependent in a subtropical forest.

Understanding how nutrient addition affects the tree growth is critical for assessing forest ecosystem function and processes, especially in the context of increased nitrogen (N) and phosphorus (P) deposition. Subtropical forests are often considered N-rich and P-poor ecosystems, but few existing studies follow the traditional "P limitation" paradigm, possibly due to differences in nutrient requirements among trees of different size classes. We conducted a three-year fertilization experiment with four treatments (Control, N-treatment, P-treatment, and NP-treatment). We measured soil nutrient availability, leaf stoichiometry, and relative growth rate (RGR) of trees across three size classes (small, medium and large) in 64 plots. We found that N and NP-treatments increased the RGR of large trees. P-treatment increased the RGR of small trees. RGR was mainly affected by N addition, the total effect of P addition was only 10 % of that of N addition. The effect of nutrient addition on RGR was mainly regulated by leaf stoichiometry. This study reveals that nutrient limitation is size dependent, indicating that continuous unbalanced N and P deposition will inhibit the growth of small trees and increase the instability of subtropical forest stand structure, but may improve the carbon sink function of large trees.

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest.

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1 year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1 year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1 year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Long-term nitrogen and phosphorus addition have stronger negative effects on microbial residual carbon in subsoils than topsoils in subtropical forests.

Highly weathered lowland (sub)tropical forests are widely recognized as nitrogen (N)-rich and phosphorus (P)-poor, and the input of N and P affects soil carbon (C) cycling and storage in these ecosystems. Microbial residual C (MRC) plays a crucial role in regulating soil organic C (SOC) stability in forest soils. However, the effects of long-term N and P addition on soil MRC across different soil layers remain unclear. This study conducted a 12-year N and P addition experiment in two typical subtropical plantation forests dominated by Acacia auriculiformis and Eucalyptus urophylla trees, respectively. We measured plant C input (fine root biomass, fine root C, and litter C), microbial community structure, enzyme activity (C/N/P-cycling enzymes), mineral properties, and MRC. Our results showed that continuous P addition reduced MRC in the subsoil (20-40 cm) of both plantations (A. auriculiformis: 28.44% and E. urophylla: 28.29%), whereas no significant changes occurred in the topsoil (0-20 cm). N addition decreased MRC in the subsoil of E. urophylla (25.44%), but had no significant effects on A. auriculiformis. Combined N and P addition reduced MRC (34.63%) in the subsoil of A. auriculiformis but not in that of E. urophylla. The factors regulating MRC varied across soil layers. In the topsoil (0-10 cm), plant C input (the relative contributions to the total variance was 20%, hereafter) and mineral protection (47.2%) were dominant factors. In the soil layer of 10-20 cm, both microbial characteristics (41.3%) and mineral protection (32.3%) had substantial effects, whereas the deeper layer (20-40 cm) was predominantly regulated by microbial characteristics (37.9%) and mineral protection (18.8%). Understanding differential drivers of MRC across soil depth, particularly in deeper soil layers, is crucial for accurately predicting the stability and storage of SOC and its responses to chronic N enrichment and/or increased P limitation in (sub)tropical forests.

Community structure and species diversity dynamics of a subtropical evergreen broad-leaved forest in China: 2005 to 2020.

Here, we characterize the temporal and spatial dynamics of forest community structure and species diversity in a subtropical evergreen broad-leaved forest in China. We found that community structure in this forest changed over a 15-year period. Specifically, renewal and death of common species was large, with the renewal of individuals mainly concentrated within a few populations, especially those of Aidia canthioides and Cryptocarya concinna. The numbers of individual deaths for common species were concentrated in the small and mid-diameter level. The spatial distribution of community species diversity fluctuated in each monitoring period, showing a more dispersed diversity after the 15-year study period, and the coefficient of variation on quadrats increased. In 2010, the death and renewal of the community and the spatial variation of species diversity were different compared to other survey years. Extreme weather may have affected species regeneration and community stability in our subtropical monsoon evergreen broad-leaved forests. Our findings suggest that strengthening the monitoring and management of the forest community will help better understand the long- and short-term causes of dynamic fluctuations of community structure and species diversity, and reveal the factors that drive changes in community structure.

Long-term warming increased carbon sequestration capacity in a humid subtropical forest.

Tropical and subtropical forests play a crucial role in global carbon (C) pools, and their responses to warming can significantly impact C-climate feedback and predictions of future global warming. Despite earth system models projecting reductions in land C storage with warming, the magnitude of this response varies greatly between models, particularly in tropical and subtropical regions. Here, we conducted a field ecosystem-level warming experiment in a subtropical forest in southern China, by translocating mesocosms (ecosystem composed of soils and plants) across 600 m elevation gradients with temperature gradients of 2.1°C (moderate warming), to explore the response of ecosystem C dynamics of the subtropical forest to continuous 6-year warming. Compared with the control, the ecosystem C stock decreased by 3.8% under the first year of 2.1°C warming; but increased by 13.4% by the sixth year of 2.1°C warming. The increased ecosystem C stock by the sixth year of warming was mainly attributed to a combination of sustained increased plant C stock due to the maintenance of a high plant growth rate and unchanged soil C stock. The unchanged soil C stock was driven by compensating and offsetting thermal adaptation of soil microorganisms (unresponsive soil respiration and enzyme activity, and more stable microbial community), increased plant C input, and inhibitory C loss (decreased C leaching and inhibited temperature sensitivity of soil respiration) from soil drying. These results suggest that the humid subtropical forest C pool would not necessarily diminish consistently under future long-term warming. We highlight that differential and asynchronous responses of plant and soil C processes over relatively long-term periods should be considered when predicting the effects of climate warming on ecosystem C dynamics of subtropical forests.

Simulation and prediction of changes in tree species composition in subtropical forests of China using a nonlinear difference equation system model.

Changes in tree species composition are one of the key aspects of forest succession. In recent decades, significant changes have occurred in the tree species composition of subtropical forests in China, with a decrease in coniferous trees and an increase in broad-leaved trees. This study focuses on Zhejiang Province, located in the subtropical region of China, and utilizes seven inventories from the National Continuous Forest Inventory (NCFI) System spanning 30 years (1989-2019) for modeling and analysis. We categorized tree species into three groups: pine, fir, and broadleaf. We used the proportion of biomass in a sample plot as a measure of the relative abundance of each tree species group. A novel nonlinear difference equation system (NDES) model was proposed. A NDES model was established based on two consecutive survey datasets. A total of six models were established in this study. The results indicated that during the first two re-examination periods (1989-1994, 1994-1999), there was significant fluctuation in the trend of tree species abundance, with no consistent pattern of change. During the latter four re-examination periods (1999-2004, 2004-2009, 2009-2014, 2014-2019), a consistent trend was observed, whereby the abundance of the pine group and the fir group decreased while the abundance of the broad-leaved group increased. Moreover, over time, this pattern became increasingly stable. Although the abundances of the pine group and the fir group have been steadily declining, neither group is expected to become extinct. The NDES model not only facilitates short-term, medium-term, and even long-term predictions but also employs limit analysis to reveal currently obscure changing trends in tree species composition.

Climate change reshapes plant trait spectrum to explain biomass dynamics in an old-growth subtropical forest.

Climate change leads to novel species interactions and continues to reshuffle ecological communities, which significantly declines carbon accumulation rates in mature forests. Still, little is known about the potential influence of multiple global change factors on long-term biomass dynamics and functional trait combinations. We used temporal demographic records spanning 26 years and extensive databases of functional traits to assess how old-growth subtropical forest biomass dynamics respond to various climatic change scenarios (extreme drought, subsequent drought, warming, elevated CO2 concentrations, and windstorm). We found that the initial severe drought, subsequent drought and windstorm events increased biomass loss due to tree mortality, which exceeded the biomass gain produced by survivors and recruits, ultimately resulting in more negative net biomass balances. These drought and windstorm events caused massive biomass loss due to tree mortality that tended towards acquisition species with high hydraulic efficiency, whereas biomass growth from survivors and recruits tended to consist of acquisition species with high hydraulic safety. Compensatory growth in this natural forest provided good explanation for the increase in biomass growth after drought and windstorm events. Notably, these dominant-species transitions reduced carbon storage and residence time, forming a positive carbon-climate feedback loop. Our findings suggest that climate changes could alter functional strategies and cause shifts in new dominant species, which could greatly reduce ecological functions and carbon gains of old-growth subtropical forests.

Response of leaf functional traits to soil nutrients in the wet and dry seasons in a subtropical forest on an island.

Introduction: Island ecosystems often have a disproportionate number of endemic species and unique and fragile functional characteristics. However, few examples of this type of ecosystem have been reported. Methods: We conducted a comprehensive field study on Neilingding Island, southern China. The leaf samples of 79 subtropical forest tree species were obtained and their functional traits were studied in the dry and wet seasons to explain the relationships between plant functional traits and soil nutrients. Results: We found a greater availability of soil moisture content (SMC) and nutrients in the wet season than in the dry season. The values of wet season soil available phosphorus (5.97 mg·kg-1), SMC (17.67%), and soil available potassium (SAK, 266.96 mg·kg-1) were significantly higher than those of the dry season. The leaf dry matter content, specific leaf weight, leaf density, leaf total carbon, leaf total nitrogen, leaf total calcium, and the N/P and C/P ratios of leaves were all significantly higher in the dry season than in the wet season, being 18.06%, 12.90%, 12.00%, 0.17%, 3.41%, 9.02%, 26.80%, and 24.14% higher, respectively. In contrast, the leaf area (51.01 cm2), specific leaf area (152.76 cm2·g-1), leaf water content (0.59%), leaf total nitrogen (1.31%), leaf total phosphorus (0.14%), and leaf total magnesium (0.33%) were much lower in the dry season than in the wet one. There were significant pairwise correlations between leaf functional traits, but the number and strength of correlations were significantly different in the dry and wet seasons. The SAK, soil total phosphorus (STP), and pH impacted plant leaf functional traits in the dry season, whereas in the wet season, they were affected by SAK, STP, pH, and NO3- (nitrate). Discussion: Both soil nutrients and water availability varied seasonally and could cause variation in a number of leaf traits.

Labile carbon-induced soil organic matter turnover in a subtropical forest under different redox conditions.

Labile organic carbon (LOC) input strongly affects soil organic matter (SOM) dynamics, including gains and losses. However, it is unclear how redox fluctuations regulate these processes of SOM decomposition and formation induced by LOC input. The objective of this study was to explore the impacts of LOC input on SOM turnover under different redox conditions. Soil samples were collected in a subtropical forest. A single pulse of 13C-labeled glucose (i.e., LOC) was applied to the soil. Soil samples were incubated for 40 days under three redox treatments, including aerobic, anoxic, and 10-day aerobic followed by 10-day anoxic conditions. Results showed that LOC input affected soil priming and 13C-SOM accumulation differently under distinct redox conditions by altering the activities of various microorganisms. 13C-PLFAs (phospholipid fatty acids) were analyzed to determine the role of microbial groups in SOM turnover. Increased activities of fungi and gram-positive bacteria (i.e., the K-strategists) by LOC input could ingest metabolites or residues of the r-strategists (e.g., gram-negative bacteria) to result in positive priming. Fungi could use gram-negative bacteria to stimulate priming intensity via microbial turnover in aerobic conditions first. Reduced activities of K-strategists as a result of the aerobic to anoxic transition decreased priming intensity. The difference in LOC retention in SOM under different redox conditions was mainly attributable to 13C-particulate organic carbon (13C-POC) accumulation. Under aerobic conditions, fungi and gram-positive bacteria used derivatives from gram-negative bacteria to reduce newly formed POC. However, anoxic conditions were not conducive to the uptake of gram-negative bacteria by fungi and gram-positive bacteria, favoring SOM retention. This work indicated that redox-regulated microbial activities can control SOM decomposition and formation induced by LOC input. It is extremely valuable for understanding the contribution of soil affected by redox fluctuations to the carbon cycle.

Nutrient content and resorption efficiency of leaves of broad-leaved trees along altitudes in Wuyi Mountains, China.

To reveal the variation of leaf nutrient utilization strategies with altitude gradient in subtropical mountain broadleaved trees, 44 species of broadleaved trees at different altitudes (1400, 1600 and 1800 m) in Wuyi Mountains were selected to measure nutrient content, stoichiometric ratio, and nutrient resorption efficiency of green and senescent leaves, and analyzed their allometric growth relationships. The results showed that nitrogen (N) and phosphorus (P) contents in green leaves were significantly higher than those in senescent leaves, which increased with the increases of altitude. The average values of phosphorus resorption efficiency (PRE) and nitrogen resorption efficiency (NRE) were 48.3% and 34.9%, respectively. PRE was significantly higher than NRE. There was no significant difference in nutrient resorption efficiency with altitude. NRE had positive isokinetic growth with and mature leaf N content at low altitude (1400 m) and negative allometry growth with senescent leaf N content at high altitude (1800 m). PRE and N and P contents of senescent leaves had negative isokinetic growth at low altitude (1400 m) and negative allometry growth at high altitudes (1600 and 1800 m). PRE-NRE allometric growth index was 0.95 at each altitude. The nutrient contents of green and senescent leaves increased with the increases of altitude, but altitude did not affect nutrient resorption efficiency. Plants preferred to re-absorbed P from senescent leaves. Nutrient resorption efficiency of leaves at high altitude affected the nutrient status of senescent leaves.