Search

Aims: Soil warming significantly influences soil organic carbon (SOC) pools in terrestrial ecosystems through its impact on the processes of carbon (C) input and decomposition as well as the stabilization of SOC pools. Most studies demonstrated that soil warming reduces SOC pools, but the magnitude is highly variable, and the underlying mechanisms are poorly understood. Methods: The concentration, stability (dissolved, particulate, and mineral-associated SOC) and source (plant-derived vs. microbial-derived) of SOC, soil microbial community composition, and enzymatic activities were studied in a 10-year soil warming field experiment in an East Asian monsoon forest. Results: 10-year soil warming significantly enhanced SOC in the top 0–10 cm soil. The increased SOC induced by warming was mainly derived from plants, with lignin and phenol markers increasing by 60% on average, accompanied by a 27% decrease in microbial-derived SOC. However, the overall effect of warming on SOC stability was not statistically significant. Conclusions: The results suggest that the moist monsoon forest soil could sequester SOC upon long-term warming. The discrepancy between our findings and those from other regions highlights an urgent need for a better understanding of how the contrasting effects of plant- and microbial-derived C mediate the response of the SOC pool to warming across biomes. [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]

• 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]

• Reforestation on degraded land could recover soil carbon stock within a few decades. • Soil C turnover was still higher in the restored forests than in undisturbed forests. • Time rather than tree species constrains soil carbon dynamics following reforestation. • Afforestation has lower soil carbon turnover rate relative to natural restoration. • Changes in soil 13C natural abundance can be used to describe soil carbon turnover rate. As over half of the world's tropical forests are reforested or afforested, understanding the resilience of carbon (C) pool in these forests is critical for global C balance. While most previous studies on the reforested lands have focused on C stock recovery, soil C turnover has largely been overlooked. We evaluated soil C turnover rate by calculating the isotopic enrichment factors of α (defined as the slope of the regression between the δ13C difference and ln -transferred C concentrations in mineral soil samples relative to the surface litter) and β (defined as the slope of the regression between δ13C and log-transferred C concentrations) along 0–30 cm soil profiles in a 400-year-old monsoon evergreen broad-leaved forest (MEBF), a 51-year-old mixed-native plantation (NP1), a 31-year-old mixed-native plantation (NP2), a 31-year-old Acacia mangium plantation (AP), a 31-year-old mixed-conifer plantation (CP), and a 31-year-old secondary forest with natural restoration (SF). Results showed that soil C stocks did not differ among the six forests. The estimated α values ranged from 1.0023 to 1.0086 and increased in the order of MEBF = NP1 < NP2 = AP = CP < SF. The estimated β values ranged from −19.70 to −5.22 but showed an opposite trend to α values. Additionally, changes of the α and β values among these forests were mainly regulated by soil water content and bulk density. Our findings demonstrate that forest restoration could enhance soil C stock equivalent to the undisturbed old-growth forests within a few decades, but the rate of soil C turnover in these restored forests were still higher. [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]

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]

• Nine-year throughfall reduction significantly affected SOC fractions in the topsoil but not subsoil in a subtropical forest. • Soil POC content decreased but soil MAOC content increased in the topsoil. • Throughfall reduction reduced fine root biomass and mycorrhizal fungal biomass in the topsoil. • Fine root played an important role in the changes of SOC fractions in the topsoil. 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. [ABSTRACT FROM AUTHOR]

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, NH 4 +) 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. • Soil microbial necromass C is primarily controlled by below-ground C input. • Fungal necromass C responds sensitively to litter and root C exclusions. • Root C exclusion reduces the proportion of microbial and fungal necromass C to SOC. • Natural forests produce greater amount of microbial and fungal necromass C than plantation forests. [ABSTRACT FROM AUTHOR]

• Throughfall exclusion significantly decreased Resin-P and total extractable Po. • Throughfall exclusion significantly decreased fine root biomass P and microbial biomass P. • Resin-P was positively correlated with total extractable Po but uncorrelated to NaHCO 3 -P and total extractable Pi. • Microbial biomass P and fine root biomass P were positively correlated with Resin-P and total extractable Po. Long-term droughts can significantly reduce forest productivity due to water deficiency and associated changes in the availability of soil nutrients. Phosphorus (P) is a critical but limited element in subtropical forest ecosystems. However, how soil P availability responds to drought and the underlying mechanisms in subtropical forests remain poorly understood. In this study, we investigated the effects of drought treatment on soil P fractions, soil available N, microbial biomass N (MBN), microbial biomass P (MBP), soil microbial phospholipid fatty acids (PLFAs), enzyme activities, fine root biomass (FRB), and fine root biomass P (FRBP) in a subtropical Castanopsis carlesii forest after 7.5 years of throughfall exclusion. The results showed that throughfall exclusion significantly decreased resin extractable P (Resin-P) and total extractable organic P, suggesting that drought induced a reduction in soil available P and organic P. Meanwhile, Resin-P was positively correlated to total extractable organic P but not with NaHCO 3 extractable P (NaHCO 3 -P) and total extractable inorganic P, which indicated that the decline of available P in soil was mainly associated with the reduction of organic P. In addition, soil microbial biomass, MBN, MBP, FRB, and FRBP were reduced under the throughfall exclusion treatment. The MBP and FRBP were positively correlated with Resin-P and total extractable organic P. These results implied that the reductions of MBP and FRBP primarily contributed to the declines of soluble P and organic P. Such information is essential for understanding the roles of microbes and plant roots on soil P cycle in subtropical forests under drought. [ABSTRACT FROM AUTHOR]

Diazotrophs, nitrogen (N)-fixing microbes, play a pivotal role in N cycling in tropical/subtropical forests. However, little is known about the dynamics of diazotroph communities and the factors that drive their abundance during forest succession. Bulk soils were sampled across two chronosequences in two subtropical forests: a long-term natural succession (LS) and a short-term artificial-intervened succession (SS); both include an early-, mid- and late-successional stage. The results show that the diazotrophic diversity increased with forest succession for the SS, but did not change for the LS. The relative abundance of the dominant genus Bradyrhizobium was significantly greater in the early-successional stage than in the late-successional stage, and some occasional genera appeared in the late-successional stage, both for the LS and SS. Variation partitioning analyses showed that the diazotroph community composition was mainly correlated with soil phosphorus (P) concentration, especially the plant-available soil P concentration, which explained 33.3% of the diazotroph community variation. Our findings revealed the patterns of diazotroph community across forest succession and highlight the importance of soil P availability in mediating diazotroph community during succession in subtropical forests, which is valuable for guiding restoration practices in terms of nutrient management in subtropical forests. • Diazotroph diversity increased with short-term but not long-term forest succession. • The dominant diazotroph species were more abundant in the early than the late stage. • The diazotroph community was mainly affected by phosphorus availability. [ABSTRACT FROM AUTHOR]

• Plant in the urban forests have larger SLA and leaf N content, but lower leaf C:N ratio. • Specific leaf area is closely related to nutrient-related traits. • Both SLA and nutrient-related traits play an important role in species distribution along an urban-rural gradient. • Plant functional traits are closely related to soil pH, N, and copper content. Rapid urbanization in southern China has had significant impacts on the monsoon evergreen broad-leaved forests and caused a series of environmental issues. How plants, in particular plant functional traits, respond to urbanization is hence of importance for their acclimation to changing environments. In this study, we investigated the changes of plant functional traits associated with plant competition and nutrient utilization strategies and their relationships with environmental factors along an urban-rural gradient in Guangzhou, China. Results showed that plant species in the urban forests had larger specific leaf area (SLA), higher leaf nitrogen (LN) content, and lower leaf carbon/nitrogen ratio (C:N) than those in the suburban and rural forests. Principal component analysis revealed that species in the urban, suburban, and rural forests were significantly separated by the first principal component, which was strongly related to SLA and leaf nutrient traits. Plant functional traits were also found closely related to soil pH, nitrogen, and copper content. Species with higher SLA and higher LN content were better adapted to the urbanization environment. Our findings indicate that the process of urbanization may change the composition of lower subtropical monsoon evergreen broad-leaved forests by favoring acquisitive-strategy species, and further influence ecosystem services. [ABSTRACT FROM AUTHOR]