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Biological nitrogen (N) fixation, an important pathway of N, inputs from the atmosphere to Earth's ecosystems, is well demonstrated to decline under N input. However, it remains unclear why N fixers sustain N fixation in many forests under high atmospheric N deposition. To address this knowledge gap, we analyzed the response of the diazotroph community to low N loads (short-term and low N addition; 3-year N addition at the rates of 25-50 kg N ha -1 year -1 ) vs high loads (chronic and high N addition; 9-year N addition at the rate of 150 kg N ha -1 year -1 ) in forest soils using high-throughput sequencing. Rates of N fixation decreased under low and high N loads (by 13%-27% and 10%-12%, respectively). Richness and alpha diversity (ACE and Chao1) of the soil diazotroph community decreased under low but not high N loads. Approximately 67.1%-74.4% of the nifH gene sequences at the OTU level overlapped between the control and low N loads, but only 52.0%-53.6% of those overlapped between the control and high N loads, indicating a larger shift of diazotroph community composition under high N loads. Low N loads increased soil NH 4 + concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH 4 + concentrations under high N loads did not have negative impacts on the structure and function of the diazotroph community. These findings indicate that diazotrophs sustain N fixation under high N deposition via adjustment of their community composition in forest soils., Importance: This study examined the changes in soil diazotroph community under different loads of simulated N deposition and analyzed its relationship with N fixation rates in in five forests using high-throughput sequencing. The magnitudes of N fixation rates reduced by low N loads were higher than those by high N loads. Low N loads decreased richness and diversity of diazotroph community, whereas diazotroph community structure remained stable under high N loads. Compared with low N loads, high N loads resulted in a less similarity and overlap of nifH gene sequences among the treatments and a larger adjustment of diazotroph community. Low N loads increased soil NH4+ concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH4+ under high N loads did not have negative impacts on diazotroph community structure and N fixation. Based on these findings, it is urgently needed to incorporate the loads of N deposition and the composition of diazotroph community into terrestrial N-cycling models for accurate understanding of N inputs in forest ecosystems., Competing Interests: The authors declare no conflict of interest.

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., (© 2024 John Wiley & Sons Ltd.)

The traditional view holds that biological nitrogen (N) fixation is energetically expensive and thus, facultative N fixers reduce N fixation rates while obligate N fixers are excluded by non-N fixers as soil N becomes rich. This view, however, contradicts the phenomenon that N fixation does not decline in many terrestrial ecosystems under N enrichment. To address this paradoxical phenomenon, we conducted a meta-analysis of N fixation and diazotroph (N-fixing microorganism) community structure in response to N addition across terrestrial ecosystems. N addition inhibited N fixation, but the inhibitory effect weakened across increased soil organic carbon (SOC) concentrations. The response ratios of N fixation (including free-living, plant-associated, and symbiotic types) to N addition were lower in the ecosystems with low SOC concentrations (<10 mg/g) than in those with medium or high SOC concentrations (10-20 and > 20 mg/g, respectively). The negative N-addition effects on diazotroph abundance and diversity also weakened across increased SOC levels. Among the climatic and soil factors, SOC was the most important predictor regarding the responses of N fixation and diazotroph community structure to N addition. Overall, our study reveals the role of SOC in affecting the responses of N fixation to N addition, which helps understand the relationships of biological N fixation and N enrichment as well as the mechanisms of terrestrial C and N coupling., Competing Interests: Declaration of competing interest The authors declare no conflict of interests., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Soil microbial responses to anthropogenic nitrogen (N) enrichment at the overall community level has been extensively studied. However, the responses of community dynamics and assembly processes of the abundant versus rare bacterial taxa to N enrichment have rarely been assessed. Here, we present a study in which the effects of short- (2 years) and long-term (13 years) N additions to two nearby tropical forest sites on abundant and rare soil bacterial community composition and assembly were documented. The N addition, particularly in the long-term experiment, significantly decreased the bacterial α-diversity and shifted the community composition toward copiotrophic and N-sensitive species. The α-diversity and community composition of the rare taxa were more affected, and they were more closely clustered phylogenetically under N addition compared to the abundant taxa, suggesting the community assembly of the rare taxa was more governed by deterministic processes (e.g., environmental filtering). In contrast, the abundant taxa exhibited higher community abundance, broader environmental thresholds, and stronger phylogenetic signals under environmental changes than the rare taxa. Overall, these findings illustrate that the abundant and rare bacterial taxa respond distinctly to N addition in tropical forests, with higher sensitivity of the rare taxa, but potentially broader environmental acclimation of the abundant taxa. IMPORTANCE Atmospheric nitrogen (N) deposition is a worldwide environmental problem and threatens biodiversity and ecosystem functioning. Understanding the responses of community dynamics and assembly processes of abundant and rare soil bacterial taxa to anthropogenic N enrichment is vital for the management of N-polluted forest soils. Our sequence-based data revealed distinct responses in bacterial diversity, community composition, environmental acclimation, and assembly processes between abundant and rare taxa under N-addition soils in tropical forests. These findings provide new insight into the formation and maintenance of bacterial diversity and offer a way to better predict bacterial responses to the ongoing atmospheric N deposition in tropical forests.

A massive rise in atmospheric nitrogen deposition (ND) has threatened ecosystem health through accelerating soil nitrogen (N) cycling rates. While soil microbes serve a crucial function in soil N transformation, it remains poorly understood on how excess ND affects microbial functional populations regulating soil N transformation in tropical forests. To address this gap, we conducted 13-year N (as NH 4 NO 3 ) addition experiments in one N-rich tropical primary forest (PF) and two N-poor tropical reforested forests (rehabilitated and disturbed) in South China. Based on our data, 13-year N introduction markedly enhanced soil N 2 O generation in all forests, regardless of soil N status, but microbial functional groups showed divergent responses to excess N addition among the studied forests. In the PF, long-term N introduction markedly decreased presence of bacterial 16S rRNA gene, nitrifier (amoA) and denitrifier genes (nirK, nirS and nosZ) and bacteria/fungi ratio, which could be attributed to the decreases in soil pH, dissolved organic carbon to N ratio and understory plant richness. In the two reforested forests, however, long-term N introduction generally did neither alter soil properties nor the abundance of most microbial groups. We further found that the elevated N 2 O generation was related to the increased soil N availability and decreased nosZ abundance, and the PF has the highest N 2 O generation than the other two forests. Overall, our data indicates that the baseline soil N status may dominate response of microbial functional groups to ND in tropical forests, and N-rich forests are more responsive to excess N inputs, compared to those with low-N status. Forests with high soil N status can produce more N 2 O than those with low-N status. With the spread of elevated ND from temperate to tropical zones, tropical forests should merit more attention because ecosystem N saturation may be common and high N 2 O emission will occur., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In-situ soil cadmium (Cd) immobilization helps to reduce Cd accumulation in rice grain, while its effects on bioaccessibility of Cd in rice during digestion and the associated health risk from rice consumption remain unclear. Here, we combined in-situ soil Cd immobilization and bioaccessibility-corrected health risk assessment (HRA) to minimize both the risk and uncertainty of Cd exposure from rice consumption. Wollastonite with or without four different phosphates (P) were applied to immobilize soil Cd at paddy fields, and their influences on Cd, essential elements, and amino acids in rice grain were analyzed. Moreover, a bioaccessibility-corrected HRA was conducted to accurately reflect the Cd exposure risk from ingesting these rices. The results showed the co-application of wollastonite and four different P reduced Cd concentrations in rice grain equally, while their impacts on bioaccessibility of Cd in rice during simulated human digestion were inconsistent (53-71%). The HRA based on bioaccessibility of Cd in rice revealed that Cd exposure risk from rice consumption was lowest with the application of wollastonite, followed by the co-application of wollastonite and sodium hexametaphosphate. This work highlights the value of bioaccessibility-corrected HRA for screening the optimal Cd immobilization strategy to achieve safer rice consumption., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The responses of forests to nitrogen (N) deposition largely depend on the fates of deposited N within the ecosystem. Nitrogen-fixing legume trees widely occur in terrestrial forests, but the fates of deposited N in legume-dominated forests remain unclear, which limit a global evaluation of N deposition impacts and feedbacks on carbon sequestration. Here, we performed the first ecosystem-scale 15 N labeling experiment in a typical legume-dominated forest as well as in a nearby non-legume forest to determine the fates of N deposition between two different forest types and to explore their underlying mechanisms. The 15 N was sprayed bimonthly for 1 year to the forest floor in control and N addition (50 kg N ha -1  year -1 for 10 years) plots in both forests. We unexpectedly found a strong capacity of the legume forest to retain deposited N, with 75 ± 5% labeled N recovered in plants and soils, which was higher than that in the non-legume forest (56 ± 4%). The higher 15 N recovery in legume forest was mainly driven by uptake by the legume trees, in which 15 N recovery was approximately 15% more than that in the nearby non-legume trees. This indicates higher N-demand by the legume than non-legume trees. Mineral soil was the major sink for deposited N, with 39 ± 4% and 34 ± 3% labeled N retained in the legume and non-legume forests, respectively. Moreover, N addition did not significantly change the 15 N recovery patterns of both forests. Overall, these findings indicate that legume-dominated forests act as a strong sink for deposited N regardless of high soil N availability under long-term atmospheric N deposition, which suggest a necessity to incorporate legume-dominated forests into N-cycling models of Earth systems to improve the understanding and prediction of terrestrial N budgets and the global N deposition effects., (© 2021 John Wiley & Sons Ltd.)

Human activities have disturbed global phosphorus (P) cycling by introducing substantial amounts of P to natural ecosystems. Although natural P gradients and fertilization studies have found that plant community traits are closely related to P availability, it remains unclear how increased P supply affects plant growth and diversity in P-deficient tropical forests. We used a decadal P-addition experiment (2007-2017) to study the effects of increased P input on plant growth and diversity in understory layer in tropical forests. We monitored the dynamics of seedling growth, survival rate, and diversity of understory plants throughout the fertilization period under control and P addition at 15 g P m -2 yr -1 . To identify the drivers of responses, P concentration, photosynthesis rate and nonstructural carbon were analyzed. Results showed that long-term P addition significantly increased P concentrations both in soil pools and plant tissues. However, P addition did not increase the light-saturated photosynthesis rate or growth rate of the understory plants. Furthermore, P addition significantly decreased the survival rate of seedlings and reduced the species richness and density of understory plants. The negative effects of P addition may be attributed to an increased carbon cost due to the tissue maintenance of plants with higher P concentrations. These findings indicate that increased P supply alone is not necessary to benefit the growth of plants in ecosystems with low P availability, and P inputs can inhibit understory plants and may alter community composition. Therefore, we appeal to a need for caution when inputting P to tropical forests ecosystems., Competing Interests: Declaration of competing interest All authors have no conflict of interest to declare., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Soil fungi play critical roles in ecosystem processes and are sensitive to global changes. Elevated atmospheric nitrogen (N) deposition has been well documented to impact on fungal diversity and community composition, but how the fungal community assembly responds to the duration effects of experimental N addition remains poorly understood. Here, we aimed to investigate the soil fungal community variations and assembly processes under short- (2 years) versus long-term (13 years) exogenous N addition (∼100 kg N ha -1 yr -1 ) in a N-rich tropical forest of China. We observed that short-term N addition significantly increased fungal taxonomic and phylogenetic α-diversity and shifted fungal community composition with significant increases in the relative abundance of Ascomycota and decreases in that of Basidiomycota . Short-term N addition also significantly increased the relative abundance of saprotrophic fungi and decreased that of ectomycorrhizal fungi. However, unremarkable effects on these indices were found under long-term N addition. The variations of fungal α-diversity, community composition, and the relative abundance of major phyla, genera, and functional guilds were mainly correlated with soil pH and NO 3 - -N concentration, and these correlations were much stronger under short-term than long-term N addition. The results of null, neutral community models and the normalized stochasticity ratio ( NST ) index consistently revealed that stochastic processes played predominant roles in the assembly of soil fungal community in the tropical forest, and the relative contribution of stochastic processes was significantly increased by short-term N addition. These findings highlighted that the responses of fungal community to N addition were duration-dependent, i.e., fungal community structure and assembly would be sensitive to short-term N addition but become adaptive to long-term N enrichment., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 He, Jiao, Tan, Wei, Ma, Nie, Liu, Lu, Mo and Shen.)

Plant hydraulic traits have been shown to be sensitive to changes in nitrogen (N) availability in short-term studies largely using seedlings or saplings. The extent and the magnitude of N-sensitivity of the field grown mature trees in long-term experiments, however, are relatively unknown. Here, we investigated responses of leaf water relations and morphological and anatomical traits of two dominant tree species (Castanopsis chinensis and Schima superba) to a six-year canopy N addition in a subtropical forest. We found that N addition increased leaf hydraulic conductivity in both species along with higher transpiration rate and less negative water potential at 50% loss of leaf hydraulic conductivity and at leaf turgor loss point. Examination of leaf morphological and anatomical traits revealed that increased leaf hydraulic efficiency was at least in part due to increased vessel diameter which also compromised the hydraulic safety under increased water stress. Moreover, reduced vessel reinforcement and increased thickness shrinkage index further interpreted the increases in leaf hydraulic vulnerability under N addition. Our results demonstrated that N deposition may lead to increases of plant water loss to the atmosphere as well as tree vulnerability to drought., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO 2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Biological nitrogen (N) fixation plays an important role in terrestrial N cycling and represents a key driver of terrestrial net primary productivity (NPP). Despite the importance of N fixation in terrestrial ecosystems, our knowledge regarding the controls on terrestrial N fixation remains poor. Here, we conducted a meta-analysis (based on 852 observations from 158 studies) of N fixation across three types of ecosystems with different status of disturbance (no management, restoration [previously disturbed], and disturbance [currently disturbed]) and in response to multiple environmental change factors (warming, elevated carbon dioxide [CO 2 ], increased precipitation, increased drought, increased N deposition, and their combinations). We explored the mechanisms underlying the changes in N fixation by examining the variations in soil physicochemical properties (bulk density, texture, moisture, and pH), plant and microbial characteristics (dominant plant species numbers, plant coverage, and soil microbial biomass), and soil resources (total carbon, total N, total phosphorus (P), inorganic N, and inorganic P). Human disturbance inhibited non-symbiotic N fixation but not symbiotic N fixation. Terrestrial N fixation was stimulated by warming (+152.7%), elevated CO 2 (+19.6%), and increased precipitation (+73.1%) but inhibited by increased drought (-30.4%), N deposition (-31.0%), and combinations of available multiple environmental change factors (-14.5%), the extents of which varied among biomes and ecosystem compartments. Human disturbance reduced the N fixation responses to environmental change factors, which was associated with the changes in soil physicochemical properties (2%-56%, p < .001) and the declines in plant and microbial characteristics (3%-49%, p ≤ .003) and soil resources (6%-48%, p ≤ .03). Overall, our findings reveal for the first time the effects of multiple environmental change factors on terrestrial N fixation and indicate the role of human disturbance activities in inhibiting N fixation, which can improve our understanding, modeling, and prediction of terrestrial N budgets, NPP, and ecosystem feedbacks under global change scenarios., (© 2020 John Wiley & Sons Ltd.)

A mechanistic understanding of plant photosynthetic response is needed to reliably predict changes in terrestrial carbon (C) gain under conditions of chronically elevated atmospheric nitrogen (N) deposition. Here, using 2,683 observations from 240 journal articles, we conducted a global meta-analysis to reveal effects of N addition on 14 photosynthesis-related traits and affecting moderators. We found that across 320 terrestrial plant species, leaf N was enhanced comparably on mass basis (N mass , +18.4%) and area basis (N area , +14.3%), with no changes in specific leaf area or leaf mass per area. Total leaf area (TLA) was increased significantly, as indicated by the increases in total leaf biomass (+46.5%), leaf area per plant (+29.7%), and leaf area index (LAI, +24.4%). To a lesser extent than for TLA, N addition significantly enhanced leaf photosynthetic rate per area (A area , +12.6%), stomatal conductance (g s , +7.5%), and transpiration rate (E, +10.5%). The responses of A area were positively related with that of g s , with no changes in instantaneous water-use efficiency and only slight increases in long-term water-use efficiency (+2.5%) inferred from 13 C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and A area diminished while that of E increased significantly. The observed patterns of increases in both TLA and E indicate that N deposition will increase the amount of water used by plants. Taken together, N deposition will enhance gross photosynthetic C gain of the terrestrial plants while increasing their water loss to the atmosphere, but the effects on C gain might diminish over time and that on plant water use would be amplified if N deposition persists., (© 2020 John Wiley & Sons Ltd.)

Developing an understanding of the response of soil organic carbon (SOC) to N addition is critical to quantify and predict the terrestrial carbon uptake under increasing N deposition in the future. However, results from field studies on the response of SOC content and composition to N addition are highly variable across different ecosystems. The interpretation of SOC responses to N addition are often complicated by the differences in climate, soil substrate and other factors. To address this question, we measured SOC and its components in adjacent broadleaved and coniferous subtropical forests after 14 years of N addition. SOC in the top 50 cm increased by 2.1 kg m - 2 , 1.8 kg m - 2 and 1.2 kg m - 2 for low, medium and high rates of N addition in the broadleaved forest, but did not change significantly in the coniferous forest. Increased SOC in the broadleaved forest was contributed by the significant increases in particulate organic carbon (POC), humus organic carbon (HOC) in the 0-10 cm and 30-50 cm soil layers and resistant organic carbon (ROC) in the 0-10 cm soil layer. 13 C nuclear magnetic resonance (NMR) spectra of coarse SOC revealed a decrease in easily decomposed carbon (C) and a shift in recalcitrant C. The increased SOC accumulation in the broadleaved forest was largely driven by altered rates of organic matter decomposition, rather than C inputs to soil. Land-history and low nutrient availability may have contributed to the lack of significant impact of N addition on SOC in the coniferous forest. Our results suggested the different controls of SOC accumulation and less sensitivity of SOC chemical composition at the molecular level to N addition in the two subtropical forest soils., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N-limited temperate forests. In N-rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old-growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low-N), 100 (Medium-N), and 150 (High-N) kg N ha -1  year -1 . Soil organic carbon (SOC) content increased under High-N, corresponding to a 33% decrease in CO 2 efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N 2 O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High-N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N 2 O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems., (© 2019 John Wiley & Sons Ltd.)

Urbanization, the migration of populations from rural to urban areas, has been causing great stress on natural environments, leading to air pollution and nitrogen (N) deposition, negatively affecting forest health. Although there is evidence that urbanization has changed forest N cycling, little is known about whether urbanization also changes the availability of phosphorus (P), which is important for plant growth and forest productivity. To address this question, we carried out a survey in the Pearl River Delta region, the world's largest urban area in southern China, using two types of representative forests, the evergreen broadleaf forests (BFs) and pine plantations (PPs). The leaf N:P ratios in the two forest types were high (20-50) with a significant increasing pattern along the rural-to-urban gradient. The ratios of leaf P:K and P:Na declined along the rural-to-urban gradient, whereas leaf P content did not change in BF but decreased in PP along the rural-to-urban gradient, suggesting that leaf P became limiting along urbanization. The abundance of actinomycetes and gram-negative bacteria decreased along the rural-to-urban gradient, indicating the negative effects of urbanization on soil microorganisms. Principal component analysis indicated that divergent key factors respond to the urbanization and affect plant P limitation in the two forest types. In BF, broadleaf trees showed a greater response to N deposition from urbanization indicating direct leaf N uptake from N deposition is a key factor for plant P limitation. Alternatively, in PP, our findings suggest soil acidification is an important factor accelerating plant P limitation. Our study revealed that urbanization intensifies plant P limitation in subtropical forests, and the effects vary depending on forest types. Our findings provide empirical information to support the management of forest ecosystems and evaluation of urbanization effects on forest health., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Biological nitrogen (N) fixation (BNF), an important source of N in terrestrial ecosystems, plays a critical role in terrestrial nutrient cycling and net primary productivity. Currently, large uncertainty exists regarding how nutrient availability regulates terrestrial BNF and the drivers responsible for this process. We conducted a global meta-analysis of terrestrial BNF in response to N, phosphorus (P), and micronutrient (Micro) addition across different biomes (i.e, tropical/subtropical forest, savanna, temperate forest, grassland, boreal forest, and tundra) and explored whether the BNF responses were affected by fertilization regimes (nutrient-addition rates, duration, and total load) and environmental factors (mean annual temperature [MAT], mean annual precipitation [MAP], and N deposition). The results showed that N addition inhibited terrestrial BNF (by 19.0% (95% confidence interval [CI]: 17.7%-20.3%); hereafter), Micro addition stimulated terrestrial BNF (30.4% [25.7%-35.3%]), and P addition had an inconsistent effect on terrestrial BNF, i.e., inhibiting free-living N fixation (7.5% [4.4%-10.6%]) and stimulating symbiotic N fixation (85.5% [25.8%-158.7%]). Furthermore, the response ratios (i.e., effect sizes) of BNF to nutrient addition were smaller in low-latitude (<30°) biomes (8.5%-36.9%) than in mid-/high-latitude (≥30°) biomes (32.9%-61.3%), and the sensitivity (defined as the absolute value of response ratios) of BNF to nutrients in mid-/high-latitude biomes decreased with decreasing latitude (p ≤ 0.009; linear/logarithmic regression models). Fertilization regimes did not affect this phenomenon (p > 0.05), but environmental factors did affect it (p < 0.001) because MAT, MAP, and N deposition accounted for 5%-14%, 10%-32%, and 7%-18% of the variance in the BNF response ratios in cold (MAT < 15°C), low-rainfall (MAP < 2,500 mm), and low-N-deposition (<7 kg ha -1  year -1 ) biomes, respectively. Overall, our meta-analysis depicts a global pattern of nutrient impacts on terrestrial BNF and indicates that certain types of global change (i.e., warming, elevated precipitation and N deposition) may reduce the sensitivity of BNF in response to nutrient enrichment in mid-/high-latitude biomes., (© 2019 John Wiley & Sons Ltd.)

The data presented in this article are referred to the research article "A potential source of soil ecoenzymes: From the phylllosphere to soil via throughfall" (Mori et al., 2019). The data included the activities of β-1,4-glucosidase (BG, EC 3.2.1.21), β-d-cellobiosidase (CBH, EC 3.2.1.91), β-1,4-N-acetyl-glucosaminidase (NAG, EC 3.2.1.52), leucine amino peptidase (LAP, EC 3.4.11.1), polyphenol oxidase (PPO, EC 1.10.3.2), and phosphomonoesterase (PME, EC 3.1.3.2). The informatin of study sites and sampling method are shown in Fig. 1 and 2., (© 2019 The Author(s).)

Sulfur dioxide emissions have been regulated at a global scale; sulfur (S) deposition no longer contributes to soil acidification instead of an alleviation effect in temperate regions; however, it remains unclear whether S deposition still contributes to soil acidification in the tropics. The Pearl River Delta (PRD), South China, has been suffering serious soil acidification, but the contribution of S deposition was ignored because of the regulation of S emission since 2001. Here, we chose the evergreen broadleaf forests, which are the typical forest type at the regional scale in PRD to examine the contribution of S deposition and its characteristics in this acidification, based on an established urban-rural gradient in the range of 260 km. A substantial acidification was evidenced by the significant decline of soil pH from rural to urban sites, with mean pH values decreased by more than 0.60 U through the whole 40-cm depths. However, there was no significant difference in soil pH from 0-10 cm, 10-20 cm, and to 20-40 cm at each site (P > 0.05). Acid-neutralizing capacity (ANC) showed a similar trend to soil pH, with a significant decline along the urbanization gradient and no significant effect of soil depths. Soil sulfate (SO 4 2- ), as the most abundant species in ANC, contributed greatly to soil acidification for the whole 40-cm depth, as shown by the significant positive relationships between it with soil pH and base cations. Soils also exhibited the depletion of base cations with low base saturation (< 20%) and the release of Al and Fe. Our research demonstrated that the severe soil acidification in the PRD region has extended to the subsoil level (40-cm depth), and S deposition is still an important driver to this acidification. Therefore, both recovering the acidified soils and controlling the acidifying pollutants, especially S, are particularly difficult in southern China.

Lowland tropical forests with chronic nitrogen (N) deposition and/or abundant N-fixing organisms are commonly rich in N relative to other nutrients. The tropical N richness introduces a paradoxical relationship in which many tropical forests sustain high rates of asymbiotic N fixation despite the soil N richness and the higher energy cost of N fixation than of soil N uptake. However, the mechanism underlying this phenomenon remains unclear. Our study aims to test this phenomenon and examine potential mechanisms of nutrient concentrations vs. substrate stoichiometry in regulating N fixation using multiple linear regression models. We hypothesized that the rates of asymbiotic N fixation would be low in an N-rich forest under N deposition and substrate stoichiometry would explain the variation in N fixation better than nutrient concentrations. We conducted a chronic N-addition experiment in an N-saturated tropical forest in southern China and measured the N fixation rates, carbon (C), N, and phosphorus (P) concentrations, and stoichiometry in different substrates (soil, forest floor, mosses, and canopy leaves). Total N fixation rates were high (10.35-12.43 kg N·ha -1 ·yr -1 ) in this N-saturated forest because of the high substrate C:N and N:P stoichiometry (which explained 13-52% of the variation in N fixation, P < 0.037) rather than substrate nutrient concentrations (P > 0.05). Atmospheric N deposition (34-50 kg N·ha -1 ·yr -1 ) failed to down-regulate asymbiotic N fixation in this forest possibly because the N deposition rate was insufficient to inhibit N fixation or N deposition maintained high N fixation rates by increasing C sequestration in the substrates. Our N-addition experiment showed the insensitivity of N fixation in all the tested substrates to low N addition (50 kg N·ha -1 ·yr -1 ); however, medium and high N addition (100-150 kg N·ha -1 ·yr -1 ) stimulated the moss and foliar N fixation because of the increases in substrate C:N stoichiometry (which explained 30-34% of the variation in N fixation, P < 0.001). Overall, our results emphasize the importance of substrate (particularly mosses and foliage) stoichiometry as a driver of asymbiotic N fixation and sustained N richness in lowland tropical forests., (© 2018 by the Ecological Society of America.)

Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems., Competing Interests: The authors declare no conflict of interest.

Anthropogenic increase of nitrogen (N) deposition has threatened forest ecosystem health at both regional and global scales. In N-limited ecosystems, atmospheric N input is regarded as an important nutrient source for plant growth. However, it remains an open question on how elevated N deposition affects plant growth in N-rich forest ecosystems. To address this question, we used a simulated N deposition experiment in an N-rich mature tropical forest of southern China, with N addition levels as 0kgNha -1 yr -1 (Control), 50kgNha -1 yr -1 (Low-N), 100kgNha -1 yr -1 (Middle-N) and 150kgNha -1 yr -1 (High-N), respectively. We measured foliar nutrient element status (e.g., N, P, K, Ca and Mg), N metabolism and photosynthesis capacity of three dominant understory plant species (Cryptocarya concinna and Cryptocarya chinensis as medium-light species; and Randia canthioides as shade tolerant species) in this forest. Results showed that two years of N addition greatly increased foliar N content, but decreased the content of nutrient cations (e.g., K, Ca and Mg). Nitrogen addition also increased N accumulation as organic forms as soluble protein and/or free amino acid (FAA), but not as chlorophyll in all three species. We further found that the photosynthesis capacity (Pmax) of C. concinna and C. chinensis decreased significantly with elevated N addition, with no effects on R. canthioides. However, photosynthetic nitrogen use efficiency (PNUE) significantly declined with N addition for all three species, with significantly negative relationships between PNUE/Pmax and foliar N content. These findings suggest that excess N inputs can accelerate nutrient imbalance, and inhibit photosynthetic capacity of understory plant species, indicating continuous high N deposition can threat understory plant growth in N-rich tropical forests in the future. Meanwhile, PNUE can be used as a sensitive indicator to assess ecosystem N status under chronic N deposition., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Asymbiotic nitrogen (N) fixation is an important source of new N in ecosystems, and is sensitive to atmospheric N deposition. However, there is limited understanding of asymbiotic N fixation and its response to N deposition in the context of forest rehabilitation. In this study, we measured N fixation rates (acetylene reduction) in different ecosystem compartments (i.e. soil, forest floor, moss Syrrhopodon armatus, and canopy leaves) in a disturbed and a rehabilitated subtropical forest in southern China, under 12years of N treatments: control, low N addition (50kgNha -1 yr -1 ), and medium N addition (100kgNha -1 yr -1 ). The rehabilitated forest had higher nutrient (e.g. N) availability than the disturbed forest. In control plots, N fixation rates in forest floor were higher in the rehabilitated forest than in the disturbed forest, but N fixation rates in other compartments (soil, S. armatus, and canopy leaves) were comparable between the forests. Nitrogen addition significantly suppressed N fixation in soil, forest floor, S. armatus, and canopy leaves in the disturbed forest, but had no significant effect on those compartments in the rehabilitated forest. The main reasons for the negative effects of N addition on N fixation in the disturbed forest were NH 4 + inhibition (soil), the P and C limitation (forest floor), and the reduced N dependence on canopy N-fixers (S. armatus and canopy leaves). We conclude that asymbiotic N fixation does not decline with increasing N availability after rehabilitation in the study forests. The inhibitory effects of N addition on asymbiotic N fixation occurred in the disturbed forest but not in the rehabilitated forest, indicating that forest rehabilitation may change the response of ecosystem function (i.e. N fixation) to N deposition, which merits further study in other tropical and subtropical regions., (Copyright © 2017. Published by Elsevier B.V.)

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem 15 N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added 15 N tracer monthly as 15 NH 4 15 NO 3 for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50 kg N ha -1  yr -1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20 cm was 50 kg N ha -1  yr -1 in the control plots and was close to the N input (51 kg N ha -1  yr -1 ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha -1  yr -1 . However, of these only 7 and 23 kg N ha -1  yr -1 in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest., (© 2016 John Wiley & Sons Ltd.)

Nitrogen (N) deposition in China has increased greatly, but the general impact of elevated N deposition on carbon (C) dynamics in Chinese terrestrial ecosystems is not well documented. In this study we used a meta-analysis method to compile 88 studies on the effects of N deposition C cycling on Chinese terrestrial ecosystems. Our results showed that N addition did not change soil C pools but increased above-ground plant C pool. A large decrease in below-ground plant C pool was observed. Our result also showed that the impacts of N addition on ecosystem C dynamics depend on ecosystem type and rate of N addition. Overall, our findings suggest that 1) decreased below-ground plant C pool may limit long-term soil C sequestration; and 2) it is better to treat N-rich and N-limited ecosystems differently in modeling effects of N deposition on ecosystem C cycle., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Soil acidification instead of alkalization has become a new environmental issue caused by urbanization. However, it remains unclear the characters and main contributors of this acidification. We investigated the effects of an urbanization gradient on soil acidity of Pinus massoniana forests in Pearl River Delta, South China. The soil pH of pine forests at 20-cm depth had significantly positive linear correlations with the distance from the urban core of Guangzhou. Soil pH reduced by 0.44 unit at the 0-10 cm layer in urbanized areas compared to that in non-urbanized areas. Nitrogen deposition, mean annual temperature and mean annual precipitation were key factors influencing soil acidification based on a principal component analysis. Nitrogen deposition showed significant linear relationships with soil pH at the 0-10 cm (for ammonium N(NH4+(-N)), P < 0.05; for nitrate N(NO3-(-N)), P < 0.01) and 10-20 cm (for NO3-(-N), P < 0.05) layers. However, there was no significant loss of exchangeable non-acidic cations along the urbanization gradient, instead their levels were higher in urban than in urban/suburban area at the 0-10 cm layer. Our results suggested N deposition particularly under the climate of high temperature and rainfall, greatly contributed to a significant soil acidification occurred in the urbanized environment.

Elevated nitrogen (N) deposition may aggravate phosphorus (P) deficiency in forests in the warm humid regions of China. To our knowledge, the interactive effects of long-term N deposition and P availability on soil microorganisms in tropical replanted forests remain unclear. We conducted an N and P manipulation experiment with four treatments: control, N addition (15 g N m(-2)·yr(-1)), P addition (15 g P m(-2)·yr(-1)), and N and P addition (15 + 15 g N and P m(-2)·yr(-1), respectively) in disturbed (planted pine forest with recent harvests of understory vegetation and litter) and rehabilitated (planted with pine, but mixed with broadleaf returning by natural succession) forests in southern China. Nitrogen addition did not significantly affect soil microbial biomass, but significantly decreased the abundance of gram-negative bacteria PLFAs in both forest types. Microbial biomass increased significantly after P addition in the disturbed forest but not in the rehabilitated forest. No interactions between N and P additions on soil microorganisms were observed in either forest type. Our results suggest that microbial growth in replanted forests of southern China may be limited by P rather than by N, and this P limitation may be greater in disturbed forests.

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.

Nowadays, nitrogen (N) deposition has become a growing global concern due to urbanization activities increasing the large amount of reactive N in the atmosphere. However, it remains unclear whether urbanization affects the composition and main sources of N deposition in rapidly urbanizing areas such as in China. One-year measurement of wet inorganic N deposition was conducted using ion-exchange resin (IER) columns in the range of 260 km from urban to rural areas in the Pearl River Delta (PRD) region, south China. An increasing pattern of wet inorganic deposition along the urbanization gradient was observed and it increased in the order: rural (15.26 ± 0.20 kg N ha(-1) year(-1)) < suburban/rural (21.45 ± 3.73 kg N ha(-1) year(-1)) < urban (31.16 ± 0.44 kg N ha(-1) year(-1)) < urban/suburban sites (34.15 ± 5.73 kg N ha(-1) year(-1)). Nitrate N (NO3 (-)-N) accounted for 53.5-79.1 % of total wet inorganic N deposition, indicating a significant negative correlation with distance from the urban core. Based on moss δ(15)N-values the main source of NO3 (-)-N was considered to be emitted from vehicles. Our results demonstrate that urbanization has large impacts on the regional pattern of wet inorganic N deposition. Thus, controlling NOx emission, especially vehicle emission will become an effective strategy for N pollution abatement in China.

Elevated anthropogenic nitrogen (N) deposition has become an important driver of soil acidification at both regional and global scales. It remains unclear, however, how long-term N deposition affects soil buffering capacity in tropical forest ecosystems and in ecosystems of contrasting land-use history. Here, we expand on a long-term N deposition experiment in three tropical forests that vary in land-use history (primary, secondary, and planted forests) in Southern China, with N addition as NH4NO3 of 0, 50, 100, and 150 kg N ha(-1) yr(-1), respectively. Results showed that all three forests were acid-sensitive ecosystems with poor soil buffering capacity, while the primary forest had higher base saturation and cation exchange capacity than others. However, long-term N addition significantly accelerated soil acidification and decreased soil buffering capacity in the primary forest, but not in the degraded secondary and planted forests. We suggest that ecosystem N status, influenced by different land-use history, is primarily responsible for these divergent responses. N-rich primary forests may be more sensitive to external N inputs than others with low N status, and should be given more attention under global changes in the future, because lack of nutrient cations is irreversible.

Elevated nitrogen (N) deposition may constrain soil phosphorus (P) and base cation availability in tropical forests, for which limited evidence have yet been available. In this study, we reported responses of soil inorganic nutrients to full factorial N and P treatments in three tropical forests different in initial soil N status (N-saturated old-growth forest and two less-N-rich younger forests). Responses of microbial biomass, annual litterfall production and nutrient input were also monitored. Results showed that N treatments decreased soil inorganic nutrients (except N) in all three forests, but the underlying mechanisms varied depending on forests: through inhibition on litter decomposition in the old-growth forest and through Al(3+) replacement of Ca(2+) in the two younger forests. In contrast, besides great elevation in soil available P, P treatments induced 60%, 50%, 26% increases in sum of exchangeable (K(+)+Ca(2+)+Mg(2+)) in the old-growth and the two younger forests, respectively. These positive effects of P were closely related to P-stimulated microbial biomass and litter nutrient input, implying possible stimulation of nutrient return. Our results suggest that N deposition may result in decreases in soil inorganic nutrients (except N) and that P addition can enhance soil inorganic nutrients to support ecosystem processes in these tropical forests.

Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long-term N deposition on soil acidification processes in tropical forests. We have established a long-term N-deposition experiment in an N-rich lowland tropical forest of Southern China since 2002 with N addition as NH4 NO3 of 0, 50, 100 and 150 kg N ha(-1)  yr(-1) . We measured soil acidification status and element leaching in soil drainage solution after 6-year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid-sensitive showing high acidification (pH(H2O) <4.0), negative water-extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long-term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al(3+) , but increased cation exchange capacity (CEC). Nitrogen addition-induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al(3+) was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H(+) dominating changes of soil cation pool, an exhausted base cation pool, N-addition stimulating SOC production, and N saturation. Our results suggest that long-term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future., (© 2014 John Wiley & Sons Ltd.)

Global changes such as increasing CO2, rising temperature, and land-use change are likely to drive shifts in litter inputs to forest floors, but the effects of such changes on litter decomposition remain largely unknown. We initiated a litter manipulation experiment to test the response of litter decomposition to litter removal/addition in three successional forests in southern China, namely masson pine forest (MPF), mixed coniferous and broadleaved forest (MF) and monsoon evergreen broadleaved forest (MEBF). Results showed that litter removal decreased litter decomposition rates by 27%, 10% and 8% and litter addition increased litter decomposition rates by 55%, 36% and 14% in MEBF, MF and MPF, respectively. The magnitudes of changes in litter decomposition were more significant in MEBF forest and less significant in MF, but not significant in MPF. Our results suggest that change in litter quantity can affect litter decomposition, and this impact may become stronger with forest succession in tropical forest ecosystem.

We investigated soil CH4 fluxes from six forests along an urban-to-rural gradient in Guangzhou City metropolitan area, South China. The most significant CH4 consumption was found in the rural site, followed by suburban, and then urban forest sites. The rates of CH4 uptake were significantly higher (by 38% and 44%, respectively for mixed forest and broadleaf forest) in the rural than in the urban forest site. The results indicate that soil water filled pore space (WFPS) is the primary factor for controlling CH4 consumption in subtropical forests. The reductions of soil CH4 uptake in urban forests were also influenced by the higher rates of atmospheric nitrogen (N) deposition and increases in soil nitrate (NO3(-)) and aluminum (Al(3+)) contents as a result of urbanization. Results from this work suggest that environmental changes associated with urbanization could decrease soil CH4 consumption in subtropical forests and potentially contribute to increase of atmospheric CH4 concentration.

The responses of litter decomposition to nitrogen (N) and phosphorus (P) additions were examined in an old-growth tropical forest in southern China to test the following hypotheses: (1) N addition would decrease litter decomposition; (2) P addition would increase litter decomposition, and (3) P addition would mitigate the inhibitive effect of N addition. Two kinds of leaf litter, Schima superba Chardn. & Champ. (S.S.) and Castanopsis chinensis Hance (C.C.), were studied using the litterbag technique. Four treatments were conducted at the following levels: control, N-addition (150 kg N ha(-1) yr(-1)), P-addition (150 kg P ha(-1) yr(-1)) and NP-addition (150 kg N ha(-1) yr(-1) plus 150 kg P ha(-1) yr(-1)). While N addition significantly decreased the decomposition of both litters, P addition significantly inhibited decomposition of C.C., but did not affect the decomposition of S.S. The negative effect of N addition on litter decomposition might be related to the high N-saturation in this old-growth tropical forest; however, the negative effect of P addition might be due to the suppression of "microbial P mining". Significant interaction between N and P addition was found on litter decomposition, which was reflected by the less negative effect in NP-addition plots than those in N-addition plots. Our results suggest that P addition may also have negative effect on litter decomposition and that P addition would mitigate the negative effect of N deposition on litter decomposition in tropical forests.

Elevated nitrogen (N) deposition to tropical forests may accelerate ecosystem phosphorus (P) limitation. This study examined responses of fine root biomass, nutrient concentrations, and acid phosphatase activity (APA) of bulk soil to five years of N and P additions in one old-growth and two younger lowland tropical forests in southern China. The old-growth forest had higher N capital than the two younger forests from long-term N accumulation. From February 2007 to July 2012, four experimental treatments were established at the following levels: Control, N-addition (150 kg N ha(-1) yr(-1)), P-addition (150 kg P ha(-1) yr(-1)) and N+P-addition (150 kg N ha(-1) yr(-1) plus 150 kg P ha(-1) yr(-1)). We hypothesized that fine root growth in the N-rich old-growth forest would be limited by P availability, and in the two younger forests would primarily respond to N additions due to large plant N demand. Results showed that five years of N addition significantly decreased live fine root biomass only in the old-growth forest (by 31%), but significantly elevated dead fine root biomass in all the three forests (by 64% to 101%), causing decreased live fine root proportion in the old-growth and the pine forests. P addition significantly increased live fine root biomass in all three forests (by 20% to 76%). The combined N and P treatment significantly increased live fine root biomass in the two younger forests but not in the old-growth forest. These results suggest that fine root growth in all three study forests appeared to be P-limited. This was further confirmed by current status of fine root N:P ratios, APA in bulk soil, and their responses to N and P treatments. Moreover, N addition significantly increased APA only in the old-growth forest, consistent with the conclusion that the old-growth forest was more P-limited than the younger forests.

Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m(-2) yr(-1)), P addition (15 g P m(-2) yr(-1)), and N&P addition (15 g N m(-2) yr(-1) plus 15 g P m(-2) yr(-1)). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.

Nitrification has been believed to be performed only by autotrophic ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) until the recent discovery of ammonia-oxidizing archaea (AOA). Meanwhile, it has been questioned whether AOB are significantly responsible for NH(3) oxidation in acidic forest soils. Here, we investigated nitrifying communities and their activity in highly acidified soils of three subtropical forests in southern China that had received chronic high atmospheric N deposition. Nitrifying communities were analyzed using PCR- and culture (most probable number)-based approaches. Nitrification activity was analyzed by measuring gross soil nitrification rates using a (15) N isotope dilution technique. AOB were not detected in the three forest soils: neither via PCR of 16S rRNA and ammonia monooxygenase (amoA) genes nor via culture-based approaches. In contrast, an extraordinary abundance of the putative archaeal amoA was detected (3.2 × 10(8) -1.2 × 10(9)  g soil(-1) ). Moreover, this abundance was correlated with gross soil nitrification rates. This indicates that amoA-possessing archaea rather than bacteria were predominantly responsible for nitrification of the soils. Furthermore, sequences of the genus Nitrospira, a dominant group of soil NOB, were detected. Thus, nitrification of acidified subtropical forest soils in southern China could be performed by a combination of AOA and NOB., (© 2011 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.)

Nitrogen (N) deposition is an important component in the global N cycle that has induced large impacts on the health and services of terrestrial and aquatic ecosystems worldwide. Anthropogenic reactive N (N(r)) emissions to the atmosphere have increased dramatically in China due to rapid agricultural, industrial and urban development. Therefore increasing N deposition in China and its ecological impacts are of great concern since the 1980s. This paper synthesizes the data from various published papers to assess the status of the anthropogenic N(r) emissions and N deposition as well as their impacts on different ecosystems, including empirical critical loads for different ecosystems. Research challenges and policy implications on atmospheric N pollution and deposition are also discussed. China urgently needs to establish national networks for N deposition monitoring and cross-site N addition experiments in grasslands, forests and aquatic ecosystems. Critical loads and modeling tools will be further used in N(r) regulation., (Copyright © 2010. Published by Elsevier Ltd.)

Responses of understory plant diversity to nitrogen (N) additions were investigated in reforested forests of contrasting disturbance regimes in southern China from 2003 to 2008: disturbed forest (with harvesting of understory vegetation and litter) and rehabilitated forest (without harvesting). Experimental additions of N were administered as the following treatments: Control, 50 kg N ha(-1) yr(-1), and 100 kg N ha(-1) yr(-1). Nitrogen additions did not significantly affect understory plant richness, density, and cover in the disturbed forest. Similarly, no significant response was found for canopy closure in this forest. In the rehabilitated forest, species richness and density showed no significant response to N additions; however, understory cover decreased significantly in the N-treated plots, largely a function of a significant increase in canopy closure. Our results suggest that responses of plant diversity to N deposition may vary with different land-use history, and rehabilitated forests may be more sensitive to N deposition., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Old-growth forests have traditionally been considered negligible as carbon sinks because carbon uptake has been thought to be balanced by respiration. We show that the top 20-centimeter soil layer in preserved old-growth forests in southern China accumulated atmospheric carbon at an unexpectedly high average rate of 0.61 megagrams of carbon hectare-1 year-1 from 1979 to 2003. This study suggests that the carbon cycle processes in the belowground system of these forests are changing in response to the changing environment. The result directly challenges the prevailing belief in ecosystem ecology regarding carbon budget in old-growth forests and supports the establishment of a new, nonequilibrium conceptual framework to study soil carbon dynamics.

Plant allelopathy relates to many ecological factors. The deficit of soil nutrients can influence the production of plant allelochemicals, and thus, influence plant allelopathic potential, while plant allelochemicals can influence the form and level of soil nutrients by the ways of complexation, adsorption, acid dissolution, competition, inhibition, and others. In this paper, the interactive effects between plant allelochemicals, plant allelopathic potential and soil nutrients were summarized, and further research aspects in this field were prospected. It was suggested that following aspects should be strengthened: (1) the integration of plant allelopathy and soil-plant nutrition research to more precisely and deeply interpret the relationships between plant allelochemicals, plant allelopathic potential and soil nutrients, (2) the integration of plant allelopathy and ecosystem nutrient cycling research to simulate the plant nutrients disturbance in nature and make the allelopathy research results more true and more reliable, and (3) the allelopathy research with soils containing excessive nutrients or polluted to provide new ideas and scientific basis in revealing the mechanisms of plants interaction and biomass variation in agricultural and forestry production, and in ecological protection.

In this paper, simulated N deposition addition (0, 50, 100 and 150 kg x hm(-2) x yr(-1)) by spreading water or NH4NO3 was conducted to study the early responses of soil fauna in three typical native forests (monsoon evergreen broadleaf forest, pine forest, and broadleaf-pine mixed forest) of subtropical China. The results showed that in monsoon evergreen broadleaf forest, N deposition addition had an obviously negative effect on the three indexes for soil fauna, but in pine forest, the positive effect was significant (P < 0. 05), and the soil fauna community could reach the level in mixed forest, even that in monsoon evergreen broadleaf forest at sometime. The responses in mixed forest were not obvious. In monsoon evergreen broadleaf forest, the negative effects were significant (P < 0.05) under medium N deposition, but not under low N deposition. In pine forest, the positive effect was significant (P < 0.05) under high N deposition, especially for the number of soil fauna groups. The results obtained might imply the N saturation-response mechanisms of forest ecosystems in subtropical China, and the conclusions from this study were also consisted with some related researches.