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South China has been experiencing very high rate of acid deposition and severe soil acidification in recent decades, which has been proposed to exacerbate the regional ecosystem phosphorus (P) limitation. We conducted a 10‐year field experiment of simulated acid deposition to examine how acidification impacts seasonal changes of different soil P fractions in a tropical forest with highly acidic soils in south China. As expected, acid addition significantly increased occluded P pool but reduced the other more labile P pools in the dry season. In the wet season, however, acid addition did not change microbial P, soluble P and labile organic P pools. Acid addition significantly increased exchangeable Al3+ and Fe3+ and the activation of Fe oxides in both seasons. Different from the decline of microbial abundance in the dry season, acid addition increased ectomycorrhizal fungi and its ratio to arbuscular mycorrhiza fungi in the wet season, which significantly stimulated phosphomonoesterase activities and likely promoted the dissolution of occluded P. Our results suggest that, even in already highly acidic soils, the acidification‐induced P limitation could be alleviated by stimulating ectomycorrhizal fungi and phosphomonoesterase activities. The differential responses and microbial controls of seasonal soil P transformation revealed here should be implemented into ecosystem biogeochemical model for predicting plant productivity under future acid deposition scenarios. [ABSTRACT FROM AUTHOR]

Atmospheric acid deposition remains a widespread problem that may influence the protection of carbon (C) in soil by altering organo-mineral interactions. However, the impacts of additional acidity on organo-mineral interactions and soil C sequestration in naturally acidic tropical soils with a high content of reactive iron (Fe) phases have not been well studied. Here we conducted a long-term (9.5-year) field experiment with a gradient of acidity treatments (3, 9.6, 32, 96 mol H+ ha−1 year−1 as nitric + sulfuric acid, added to ambient deposition) to examine how acidification alters organo-mineral interactions and soil organic carbon (SOC) pools in a tropical forest of China where soils are already highly acidic (pH ≈ 4). As expected, soil acidification significantly enhanced the leaching of base cations (e.g., Ca2+), and it also altered the solubility and composition of Fe and Al phases. The acidity treatments decreased total Fe and converted more crystalline Fe (oxyhydr) oxides to short-range-ordered phases, evidenced by an increase in Fe extracted by citrate-ascorbate solution and 0.5 M hydrochloric acid, and a decrease in the difference in Fe extracted by citrate-dithionite and citrate-ascorbate. Together, these changes led to a large increase in Fe-bound C versus a relatively small decrease in Ca-bound C. Overall, the acidity treatments increased the mineral-associated C stock to 32.5–36.4 Mg C ha−1 versus 28.8 Mg C ha−1 in the control, accounting for 71–83% of the observed increase in total SOC stock. These findings highlight the importance of pH-sensitive geochemical changes and the key roles of Fe in regulating the response of SOC to further inputs of acid deposition even in highly weathered and naturally acidic soils. The magnitude of SOC changes observed here further reveals the necessity of including pH-sensitive geochemistry in Earth system models to better predict ecosystem C budgets under future acid deposition scenarios. [ABSTRACT FROM AUTHOR]

• Tree water uptake does not necessarily correspond to soil water content. • Deferred wet season onset by two months increases tree's groundwater uptake. • 25% increase in wet season precipitation reduces tree's shallow soil water uptake. • Trees adapt to precipitation changes through water uptake and stomatal regulation. Gaining insights into the impacts of altered precipitation patterns on tree species' water uptake sources and water usage strategies is crucial for water resource management, but a comprehensive understanding is lacking. We explored water uptake patterns and water-related traits of four dominant tree species (Evergreen: Carallia brachiate, Symplocos poilanei, Schefflera heptaphylla ; Deciduous: Tetradium glabrifolium) in two altered seasonal precipitation patterns (deferred wet season [DW] and wetter wet season [WW]) and a control (CK) in a secondary tropical forest. Using stable isotopes (δ2H and δ18O) and MixSIAR models, we determined water uptake sources of the target species from a 60 cm soil profile and groundwater. Our results showed that the DW treatment increased (2.3% to 11.5%) groundwater utilization by trees in June and November compared to the CK, and promoted greater water uptake by trees from 0 to 40 cm soil layer in March. However, the WW treatment had a mild impact on water utilization in plants, leading to a reduction in water uptake by 1% - 3.8% from the 0–40 cm soil layer for the four tree species in September. The four studied tree species primarily rely on soil water, with the deciduous T. glabrifolium exhibited more flexible water uptake strategies compared to the other three evergreen species. For plant traits, S. heptaphylla exhibited larger stomatal size under DW treatment, paralleling the elongated stomatal in C. brachiate under the DW treatment. Conversely, S. poilanei and T. glabrifolium exhibit greater flexibility in balancing midday water potential. Under altered precipitation patterns, these plants adopt different morphological and/or physiological strategies, along with shift their water uptake sources to adapt to fluctuating water conditions. Overall, this study improves our understanding of plant water use in secondary tropical forests and emphasizes the importance of considering species-specific responses in forest management under potential precipitation pattern changes. [ABSTRACT FROM AUTHOR]

Ecosystem functional responses such as soil CO2 emissions are constrained by microclimate, available carbon (C) substrates and their effects upon microbial activity. In tropical forests, phosphorus (P) is often considered as a limiting factor for plant growth, but it is still not clear whether P constrains microbial CO2 emissions from soils. In this study, we incubated seven tropical forest soils from Brazil and Puerto Rico with different nutrient addition treatments (no addition, Control; C, nitrogen (N) or P addition only; and combined C, N and P addition (CNP)). Cumulative soil CO2 emissions were fit with a Gompertz model to estimate potential maximum cumulative soil CO2 emission (Cm) and the rate of change of soil C decomposition (k). Quantitative polymerase chain reaction (qPCR) was conducted to quantify microbial biomass as bacteria and fungi. Results showed that P addition alone or in combination with C and N enhanced Cm, whereas N addition usually reduced Cm, and neither N nor P affected microbial biomass. Additions of CNP enhanced k, increased microbial abundances and altered fungal to bacterial ratios towards higher fungal abundance. Additions of CNP, however, tended to reduce Cm for most soils when compared to C additions alone, suggesting that microbial growth associated with nutrient additions may have occurred at the expense of C decomposition. Overall, this study demonstrates that soil CO2 emission is more limited by P than N in tropical forest soils and those effects were stronger in soils low in P. Highlights: A laboratory incubation study was conducted with nitrogen, phosphorus or carbon addition to tropical forest soils. Soil CO2 emission was fitted with a Gompertz model and soil microbial abundance was quantified using qPCR. Phosphorus addition increased model parameters Cm and soil CO2 emission, particularly in the Puerto Rico soils. Soil CO2 emission was more limited by phosphorus than nitrogen in tropical forest soils. [ABSTRACT FROM AUTHOR]

Elevated atmospheric N deposition has been well documented to enhance fine root production in N-limited temperate forests, but how fine roots respond to N deposition in N-rich tropical and subtropical forests remains poorly understood. The sequential coring and minirhizotron methods were applied to quantify fine root biomass, production, and turnover of a N-rich but P-limited subtropical forest in southern China and to assess the responses of these root variables to a gradient of N additions (control (0), low-N (35), medium-N (70), and high-N (105 kg N ha−1 year−1)) during the first 3 years of experimentation. The high- and medium-N additions significantly reduced fine root diameter by about 30% but increased the specific root length by 20–105%, i.e., fine roots became thinner and longer under the experimental N addition. Both low- and medium-N additions generally stimulated fine root production (10–88%) and turnover (3–40%), whereas high-N suppressed them by 32–70% and 8–54%, respectively, varying with sampling season and estimation method. The stimulatory effects were presumably ascribed to the increased fine root growth for P acquisition, the suppressive effect, to the deleterious damage to the root health and micronutrient availability. Overall, the N effects were more pronounced in the surface (0–10 cm) than in the deeper (10–40 cm) soil layers and for the first-order than the higher-order fine roots. Our results indicate that lower-order absorptive fine roots are responsive to elevated N deposition, and complex responses could emerge due to the interactive influences of the N deposition rate, seasonality, and soil depth. [ABSTRACT FROM AUTHOR]

Background and Aim: Subtropical soils are generally characterized as deficient in phosphorus (P), calcium (Ca) and magnesium (Mg), but rich in iron (Fe) and aluminum (Al). However, soils developed in phosphate rock are extremely P-rich in subtropical forests, southwestern China. Factors controlling plant stoichiometric traits across variable P sites are still not clear.Methods: We investigated leaf macroelements (C, N, P, K, Ca and Mg), microelements (Mn, Fe, Zn, and Cu), and non-essential elements (Na and Al) and their element:P ratios for 21 woody plant species at both P-rich and P-deficient sites.Results: Plants between the two P type sites were mainly discriminated by Mn, Al, N and their P ratios, and between functional groups by Cu, Fe, Zn and their P ratios. There were higher leaf N, P, K, Ca, Fe and Zn concentrations but lower Mn, Cu and Al at the P-rich sites. Evergreen conifers displayed strict homeostasis while evergreen and deciduous broadleaf were more plastic and had variable ratios across different nutrients.Conclusion: Microelements have strong influences on plant stoichiometry to differentiate geologic-derived P sites in subtropics, and three functional group plants have adopted different stoichiometric strategies under variable nutrient conditions. [ABSTRACT FROM AUTHOR]

Introduction: Soil heterotrophic respiration ( R , an indicator of soil organic carbon decomposition) is an important carbon efflux of terrestrial ecosystems. However, the dynamics of soil R and its empirical relations with climatic factors have not been well understood. Methods: We incubated soils of three subtropical forests at five temperatures (10, 17, 24, 31, and 38 °C) and five moistures (20, 40, 60, 80, and 100% water holding capacity (WHC)) over 90 days. R was measured throughout the course of the incubation. Three types of models (log-linear, exponential, and power model) were fitted to the measurements and evaluated based on the coefficient of determination ( r ) and Akaike Information Criterion (AIC) of the model. Further regression analysis was used to derive the empirical relations between model parameters and the two climatic factors. Results: Among the three models, the power function model ( R = R t ) performed the best in fitting the descending trend of soil R with incubation time ( r > 0.69 for 26 of 30 models). Both R and k generally increased linearly with soil temperature but varied quadratically with soil moisture in the three forest soils. Conclusions: This study demonstrated that the power function model was much more accurate than the exponential decay model in describing the decomposition dynamics of soil organic carbon (SOC) in mineral soils of subtropical forests. The empirical relations and parameter values derived from this incubation study may be incorporated into process-based ecosystem models to simulate R responses to climate changes. [ABSTRACT FROM AUTHOR]

Spatial and temporal patterns of carbon (C) storage in forest ecosystems significantly affect the terrestrial C budget, but such patterns are unclear in the forests in Hainan Province, the largest tropical island in China. Here, we estimated the spatial and temporal patterns of C storage from 1993–2008 in Hainan's forest ecosystems by combining our measured data with four consecutive national forest inventories data. Forest coverage increased from 20.7% in the 1950s to 56.4% in the 2010s. The average C density of 163.7 Mg C/ha in Hainan's forest ecosystems in this study was slightly higher than that of China's mainland forests, but was remarkably lower than that in the tropical forests worldwide. Total forest ecosystem C storage in Hainan increased from 109.51 Tg in 1993 to 279.17 Tg in 2008. Soil C accounted for more than 70% of total forest ecosystem C. The spatial distribution of forest C storage in Hainan was uneven, reflecting differences in land use change and forest management. The potential carbon sequestration of forest ecosystems was 77.3 Tg C if all forested lands were restored to natural tropical forests. To increase the C sequestration potential on Hainan Island, future forest management should focus on the conservation of natural forests, selection of tree species, planting of understory species, and implementation of sustainable practices. [ABSTRACT FROM AUTHOR]

• Soil C, N, and P were examined in tropical forests on Hainan Island, China. • Soil nutrient concentrations varied among depths. • Large spatial variations of C, N, P and their ratios were found on the Island. • Soil C, N and P were mostly influenced by habitat variables. • The findings are useful for ecosystem modeling and forest management. Soil carbon (C), nitrogen (N), and phosphorus (P) are three important elements. The study of stoichiometric relationships of soil C, N, and P in tropical forests on Hainan Island, China could improve our understanding of nutrient cycling and provide valuable information for forest management. Soil samples were collected at five different depths from 0 to 100 cm at 100 sites among four different forest types on Hainan Island, and total C, N, and P concentrations were measured. Soil C and N concentrations and soil C:P and N:P ratios declined from the surface soil layer to the deeper soil layers and soil P and C:N ratio had relatively small variations among different depths, due to that soil C and N were mostly controlled by biological processes such as photosynthesis and N 2 -fixation, while P was more influenced by bedrock. Large spatial variations were found for soil C, N, P concentrations and their ratios. Soil C and N concentrations were significantly influenced by longitude and vegetation cover, while soil P concentration and C:P and N:P ratios were significantly controlled by latitude. This study produced a comprehensive data set of soil C, N, and P stoichiometry, and their variation patterns and controls in the tropical forests. The information generated here could help improve ecosystem models for better understanding of forest element stoichiometry, ecosystem productivity, and plant-environment relationships. [ABSTRACT FROM AUTHOR]

The persistence of soil organic carbon (SOC) is primarily driven by microbial metabolic activities; however, how microbial effects on SOC sequestration are affected by soil nutrient status remains unclear. Here, we conducted a one-year-long in situ soil incubation experiment using mesh bags (with a mesh size of 38 μm, allowing bacterial colonization and fungal hyphal penetration while preventing root penetration). This experiment involved incubating fertile sugarcane soil and infertile sand across an elevational gradient, characterized by diverse climatic and biotic conditions within a tropical forest. Biomarkers, such as phospholipid fatty acids, carbon-, nitrogen-, and phosphorus-acquiring hydrolases, glomalin-related proteins, and amino sugars, were measured to characterize the production and accumulation of microbial biomass, exo-enzymes, extracellular glycoproteins, and microbial necromass. These measurements aimed to elucidate their respective contribution to the sequestration of SOC. We found that Gram-negative bacteria dominated the microbial community composition in fertile soil, and the higher nutrient availability was related to the production and accumulation of microbial necromass via promoting microbial biomass turnover, thus enhancing the accumulation of SOC in fertile soil. This process was negatively associated with phosphorus availability and carbon- and phosphorus-acquiring enzyme activities in fertile soil. In contrast, the SOC accumulation was positively correlated with nitrogen availability and stoichiometry (including C:N and C:P), as well as moisture content in infertile sand. However, more resources were preferentially allocated to stress-tolerant fungi and Gram-positive bacteria under nutrient deficiency in infertile sand used for microbial biomass maintenance, nutrient acquisition, and environmental adaption which further aggravated the consumption of SOC, resulting in SOC loss after one year of field incubation. Our results suggest that microbial effects on SOC persistence are highly context-dependent and nutrient availability-induced changes in microbial communities and microbial resource-allocation strategies are key processes for understanding and predicting the fate of carbon in tropical forest soils. [Display omitted] • Microbial communities and their role in soil carbon dynamics rely on soil fertility. • Mesh bags with fertile soil and infertile sand were incubated at varying elevations. • Fertile soil enhanced microbial growth and microbial metabolic activity. • The production and accumulation of microbial necromass promoted soil carbon content. • Microbial nutrient acquisition and environmental adaptation led to soil carbon loss. [ABSTRACT FROM AUTHOR]

Poor regeneration of natural vegetation is a major factor contributing to the degradation of tropical coral islands. Soil seed banks (SSB) are important for maintaining the resilience of plant communities. However, the community characteristics and spatial distribution of SSBs and the controlling factors along human disturbance on coral islands are unclear. To fill this gap, we measured the community structure and spatial distributions of forest SSBs on three coral islands in the South China Sea, with varying degrees of human disturbance. The results showed that strong human disturbance increased the diversity, richness, and density of SSBs, as well as increased the richness of invasive species. With increased human disturbance, the heterogeneity pattern of SSBs spatial distribution changed from difference between forest east and west to forest center and edge. The similarity between the SSBs and above-ground vegetation also increased, and the distribution of invasive species extended from the edge to the central area of the forests, demonstrating that human disturbance limited the outward dispersal of seeds of resident species but increased the inward dispersal of seeds of invasive species. Interaction between soil properties, plant characteristics, and human disturbance explained 23–45% of the spatial variation of forest SSBs on the coral islands. However, human disturbance reduced the correlations of plant communities and spatial distribution of SSBs with soil factors (i.e., available phosphorus and total nitrogen) and increased the correlations of the community characteristics of SSB with landscape heterogeneity index, road distance, and shrub and litter cover. Resident seed dispersal on tropical coral islands might be enhanced by reducing building height, constructing buildings in down-wind locations, and preserving corridors that support animal movement among forest fragments. [Display omitted] • Spatial variations of soil seed bank (SSB) are regulated by soil-plant-disturbance interactions. • Human disturbance affected the spatial distribution of SSB through two pathways. • Human disturbance weakened the effects of soil on SSB characteristics. • Human disturbance strengthened the effects of vegetation on SSB characteristics. [ABSTRACT FROM AUTHOR]

Canopies play an important role in nitrogen (N) redistribution in forest ecosystems, and ignoring the canopy's role might bias estimates of the ecological consequences of anthropogenic atmospheric N deposition. We investigated the effects of the approach of N addition (Canopy addition vs. Understory addition) and level of N addition (25 kg N ha−1yr−1 vs. 50 kg N ha−1yr−1) on microbial residual carbon (MRC) accumulation in topsoil and subsoil. We found that the response of MRC to both approach and level of N addition varied greatly with soil depth in a tropical forest over eight years of continuous N addition. Specifically, N addition enhanced the accumulation of fungal and total MRC and their contribution to soil organic C (SOC) pools in the topsoil, whereas it decreased the contribution of fungal and total MRC to SOC in the subsoil. The contrasting effects of N addition on MRC contribution at varying soil depths were associated with the distinct response of microbial residues production. Understory N addition showed overall greater effects on MRC accumulation than canopy N addition did. Our results suggest that the canopy plays an important role in buffering the impacts of anthropogenic atmospheric N deposition on soil C cycling in tropical forests. The depth-dependent response of microbial residues to N addition also highlights the urgent need for further studies of different response mechanisms at different soil depths. • Response of soil microbial residual C to N enrichment varied greatly with soil depth. • N enrichment enhanced microbial residual C accumulation in topsoil, but decreased in subsoil. • Microbial residue production and enzyme activities were important affecting factors. • Understory N addition showed greater effects on microbial residue than canopy N addition. [ABSTRACT FROM AUTHOR]

Tropical forests play an important role in global carbon (C) cycling due to high primary productivity and rapid litter and soil organic C decomposition. However, it is still unclear how changing rainfall will influence soil CO 2 losses (i.e. via soil respiration) in tropical forests. Here, using a rainfall and litter manipulation experiment in a tropical forest, we show that enhanced litter-leached dissolved organic carbon (DOC) production with increased rainfall frequency drives substantial CO 2 loss via soil respiration. A 50% increase in rainfall frequency (no change in total rainfall amount) enhanced inputs of DOC by 28%, total dissolved nitrogen (TDN) by 17%, and total dissolved phosphorus (TDP) by 34% through leaching from litter layer to soil surface likely due to faster litter decomposition rate, and stimulated soil respiration by ∼17% (about 1.16 t C ha −1 yr −1 ). Soil respiration responded to altered rainfall frequency with limited when litter layer was removed. Accordingly, soil microbial biomass C (MBC) and fine root biomass were increased by 23% and 20%, respectively only in the plots with litter layer. A 50% reduction in total rainfall (no change in rainfall frequency) did not change litter-leached DOC and nutrients fluxes, soil MBC, fine root biomass, or annual mean soil respiration rates. The new finding – that enhanced leached-DOC production with increased rainfall frequency drives profound increases in soil respiration in tropical forests – suggests that future climate changes may have significant impacts on soil C dynamics and global C budget, and argues for the importance of incorporating this underappreciated feedback into prognostic models used to predict future C-climate interactions. [ABSTRACT FROM AUTHOR]