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Response of plant biodiversity to increased availability of nitrogen (N) has been investigated in temperate and boreal forests, which are typically N-limited, but little is known in tropical forests. We examined the effects of artificial N additions on plant diversity (species richness, density and cover) of the understory layer in an N saturated old-growth tropical forest in southern China to test the following hypothesis: N additions decrease plant diversity in N saturated tropical forests primarily from N-mediated changes in soil properties. Experimental additions of N were administered at the following levels from July 2003 to July 2008: no addition (Control); 50 kg N ha−1 yr−1 (Low-N); 100 kg N ha−1 yr−1 (Medium-N), and 150 kg N ha−1 yr−1 (High-N). Results showed that no understory species exhibited positive growth response to any level of N addition during the study period. Although low-to-medium levels of N addition (≤100 kg N ha−1 yr−1) generally did not alter plant diversity through time, high levels of N addition significantly reduced species diversity. This decrease was most closely related to declines within tree seedling and fern functional groups, as well as to significant increases in soil acidity and Al mobility, and decreases in Ca availability and fine-root biomass. This mechanism for loss of biodiversity provides sharp contrast to competition-based mechanisms suggested in studies of understory communities in other forests. Our results suggest that high-N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition. [ABSTRACT FROM AUTHOR]

Indigenous broadleaf plantations are increasingly developing as a prospective silvicultural management approach for substituting in place of large pure conifer plantations in subtropical China. However, little information is known about the effects of tree species conversion on soil-atmosphere greenhouse gas (GHG) exchanges. Four adjacent monospecific plantations were selected in subtropical China to examine the effects of tree species on soil-atmosphere exchanges of NO, CH and CO. One coniferous plantation was composed of Pinus massoniana (PM), and the three broadleaf plantations were Castanopsis hystrix (CH), Michelia macclurei (MM) and Mytilaria laosensis (ML). We found that mean soil NO and CO emissions in the PM plantation were 4.34 μg N m h and 43.25 mg C m h, respectively, lower than those in the broadleaf plantations (>5.25 μg N m h and >56.38 mg C m h). The PM plantation soil had higher mean CH uptake (39.03 μg C m h) than the broadleaf plantation soils (<32.67 μg C m h). Variations in soil NO emissions among tree species could be primarily explained by the differences in litter C:N ratio and soil total N stock. Differences in soil CH uptake among tree species could be mostly attributed to the differences in mean soil CO flux and water filled pore space (WFPS). Litter C:N ratio could largely account for variations in soil CO emissions among tree species. This study confirms that there is no GHG benefit of converting PM plantation to broadleaf plantations in subtropical China. Therefore, the future strategy of tree species selection for substituting in place of large coniferous plantations in subtropical China needs to consider the potential effects of tree species on soil-atmosphere GHG exchanges. [ABSTRACT FROM AUTHOR]

Elucidating the processes of leaf litter and fine root decomposition has been a major research focus, while how the correlation between leaf litter and fine root decomposition is unclear. We studied the in situ decomposition and N dynamics of leaf litter and fine root of four subtropical tree species ( Pinus massoniana, Castanopsis hystrix, Michelia macclurei and Mytilaria laosensis) to determine whether leaf litter and fine root decomposition is correlated across species as well as which factors influence decomposition above versus below ground. Decomposition rate of leaf litter was related to that of fine root across species. The strong correlation between leaf litter and fine root decomposition rates arose largely for several reasons. First, soil moisture had the similar influences on both leaf litter and fine root decomposition rates. Second, traits (i.e., initial Ca concentration) important to both leaf litter and fine root decomposition rates showed significant similarity among species. Third, initial P, N and aromatic C concentrations, and C/N ratio were uniquely important for fine root decomposition rate, while no unique traits for leaf litter decomposition rate. This also could account for the strong correlation between leaf litter and fine root decomposition rates. Our study suggests that among these subtropical trees, species effects on in situ decomposition rates of leaf litter and fine root are very similar. Thus, species differences in decomposition rates may be as large as they would be if faster decomposition of leaf litter was correlated with faster decomposition of fine root. N immobilization rate of leaf litter was unrelated to that of fine root across species. Our results help explain some important mechanisms by which tree species influence litter in situ decomposition. [ABSTRACT FROM AUTHOR]

At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N2O), carbon dioxide (CO2) and methane (CH4) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N2O and CO2, but take up less atmospheric CH4 than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N2O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N2O ranged from 2.0 to 4.4 kg N ha−1 year−1, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N2O emissions in the laboratory incubated soils varied with slope positions. Soil CO2 release and CH4 uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N2O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling. [ABSTRACT FROM AUTHOR]

Emissions of nitrous oxide (N2O) from the soil following simulated nitrogen (N) deposition in a disturbed (pine), a rehabilitated (pine and broadleaf mixed) and a mature (monsoon evergreen broadleaf) tropical forest in southern China were studied. The following hypotheses were tested: (1) addition of N will increase soil N2O emission in tropical forests; and (2) any observed increase will be more pronounced in the mature forest than in the disturbed or rehabilitated forest due to the relatively high initial soil N concentration in the mature forest. The experiment was designed with four N treatment levels (three replicates; 0, 50, 100, 150 kg N ha−1 year−1 for C (Control), LN (Low-N), MN (Medium-N), and HN (High-N) treatment, respectively) in the mature forest, but only three levels in the disturbed and rehabilitated forests (C, LN and MN). Between October 2005 to September 2006, soil N2O flux was measured using static chamber and gas chromatography methodology. Nitrogen had been applied previously to the plots since July 2003 and continued during soil N2O flux measurement period. The annual mean rates of soil N2O emission in the C plots were 24.1 ± 1.5, 26.2 ± 1.4, and 29.3 ± 1.6 μg N2O–N m−2 h−1 in the disturbed, rehabilitated and mature forest, respectively. There was a significant increase in soil N2O emission following N additions in the mature forest (38%, 41%, and 58% when compared to the C plots for the LN, MN, and HN plots, respectively). In the disturbed forest a significant increase (35%) was observed in the MN plots, but not in the LN plots. The rehabilitated forest showed no significant response to N additions. Increases in soil N2O emission occurred primarily in the cool-dry season (November, December and January). Our results suggest that the response of soil N2O emission to N deposition in tropical forests in southern China may vary depending on the soil N status and land-use history of the forest. [ABSTRACT FROM AUTHOR]

Response of soil respiration (CO2 emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four-levels of N treatments (Control, no N addition; Low-N, 5 g N m−2 yr−1; Medium-N, 10 g N m−2 yr−1; and High-N, 15 g N m−2 yr−1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO2-C m−2 yr−1 in the Controls. Annual mean soil respiration rate in the Control, Low-N and Medium-N treatments (69±3, 72±3 and 63±1 mg CO2-C m−2 h−1, respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58±3 mg CO2-C m−2 h−1) compared with the Control treatment, also the temperature sensitivity of respiration, Q10 was reduced from 2.6 in the Control with 2.2 in the High-N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N-treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition. [ABSTRACT FROM AUTHOR]

Evaluation of litterfall production is important for understanding nutrient cycling, forest growth, successional pathways, and interactions with environmental variables in forest ecosystems. Litterfall was intensively studied during the period of 1982–2001 in two subtropical monsoon vegetation gradients in the Dinghushan Biosphere Reserve, Guangdong Province, China. The two gradients include: (1) a successional gradient composed of pine forest (PF), mixed pine and broadleaved forest (MF) and monsoon evergreen broadleaved forest (BF), and (2) an altitudinal gradient composed of Baiyunci ravine rain forest (BRF), Qingyunci ravine rain forest (QRF), BF and mountainous evergreen broadleaved forest (MMF). Mean annual litterfall production was 356, 861 and 849 g m−2 for PF, MF and BF of the successional gradient, and 1016, 1061, 849 and 489 g m−2 for BRF, QRF, BF and MMF of the altitudinal gradient, respectively. As expected, mean annual litterfall of the pioneer forest PF was the lowest, but rapidly increased over the observation period while those in other forests were relatively stable, confirming that forest litterfall production is closely related to successional stages and growth patterns. Leaf proportions of total litterfall in PF, MF, BF, BRF, QRF and MMF were 76.4%, 68.4%, 56.8%, 55.7%, 57.6% and 69.2%, respectively, which were consistent with the results from studies in other evergreen broadleaved forests. Our analysis on litterfall monthly distributions indicated that litterfall production was much higher during the period of April to September compared to other months for all studied forest types. Although there were significant impacts of some climate variables (maximum and effective temperatures) on litterfall production in some of the studied forests, the mechanisms of how climate factors (temperature and rainfall) interactively affect litterfall await further study. [ABSTRACT FROM AUTHOR]

Natural abundance of 15N (δ15N) in plants and soils can provide integrated information on ecosystem nitrogen (N) cycling, but it has not been well tested in warm and humid sub-tropical forests. In this study, we examined the measurement of δ15N for its ability to assess changes in N cycling due to increased N deposition in an old-growth broadleaved forest and a secondary pine forest in a high N deposition area in southern China. We measured δ15N of inorganic N in input and output fluxes under ambient N deposition, and N concentration (N%) and δ15N of major ecosystem compartments under ambient and after decadal N addition at 50 kg N ha-1 yr-1. Our results showed that the N deposition was δ15N-depleted (-12 ‰) mainly due to high input of depleted NH4+-N. Plant leafs in both forest were also δ15N-depleted (-4 to -6 ‰). The old-growth forest had higher plant and soil N %, and was more 15N-enriched in most ecosystem compartments relative to the pine forest. Nitrogen addition did not significantly affect N % in both forests, indicating that the ecosystem pools are already N-rich. Soil δ15N was not changed significantly by the N addition in both forests. However, the N addition significantly increased the δ15N of plants toward the 15N signature of the added N (~ 0 ‰), indicating incorporation of added N into plants. Thus, plant δ15N was sensitive to ecosystem N input manipulation although N % was unchanged in these N-rich sub-tropical forests. We interpret the depleted δ15N values of plants as an imprint from the high and δ15N-depleted N deposition. The signal from the input (deposition or N addition) may override the enrichment effects of fractionation during the steps of N cycling that are observed in most warm and humid forests. Thus, interpretation of ecosystem δ15N values from high N deposition regions need to include data on the deposition δ15N signal. [ABSTRACT FROM AUTHOR]

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 (Al3+) 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. [ABSTRACT FROM AUTHOR]

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δN can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha yr, while the throughfall covered a larger range of 11.7-65.1 kg N ha yr. Foliar N concentration, NO leaching to stream, and soil NO concentration were low and NO production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO leaching to stream, and negatively with pH values in soil and stream water. Foliar δN was between −6.6‰ and 0.7‰, and was negatively correlated with soil NO concentration and NO leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δN values. We proposed several potential mechanism that could contribute to the δN pattern, including (1) increased plant uptake of N-depleted soil NO, (2) foliage uptake of N-depleted NH, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability. [ABSTRACT FROM AUTHOR]

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha−1 y−1 as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha−1 y−1). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha−1 y−1. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO leaching in all three forests. We hypothesize that abiotic incorporation of excess NO (through chemically reactive NO) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. [ABSTRACT FROM AUTHOR]

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: −5 g N m−2 y−1, Medium-N: −10 g N m−2 y−1, and High-N: −15 g N m−2 y−1, were employed to evaluate decomposition. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem. [ABSTRACT FROM AUTHOR]

Nitrogen (N) availability limits plant growth and soil carbon (C) sequestration in N‐limited ecosystems, however, plant and soil C feedback on the free‐living nitrogen fixation (FLNF) process is poorly understood. Moreover, whether this feedback is influenced by initial N availability in leguminous and non‐leguminous forest soils has not been clarified. Here, we found that the addition of plant‐ and microbial‐derived C significantly enhanced soil nitrogenase activity (13∼28%) and that microbial‐derived C had a more positive impact. These positive effects were attributed to the direct C‐energy supply (0.49∼0.84) rather than variations in soil microbial activity (−0.01∼0.21) and substrate resources (−0.45∼0.27). Long‐term N addition did not inhibit FLNF. C addition promoted FLNF in soils of the two forests, but the response rate was higher in the leguminous forest soils. Our study reveals that increased soil C availability can drive FLNF in tropical forests, enhancing our understanding of the soil C‐N coupling mechanism. Plain Language Summary: Explicit consideration of controlling factors for free‐living nitrogen fixation (an important process of nitrogen biogeochemistry), is critical to understanding the response of ecosystems to environmental change. Our study reveals a previously overlooked but important process in which increased soil carbon availability exerts positive feedback on soil nitrogen inputs via free‐living nitrogen fixation. This insight enhances our understanding of soil carbon‐nitrogen coupling mechanisms and highlights an important nitrogen source in tropical forests based on increased vegetation and soil carbon sink conditions under global environmental change scenario. Key Points: Carbon (C) input affected free‐living nitrogen fixation (FLNF) more than nitrogen (N) application in tropical forest soilsBoth plant‐ and microbial‐derived C enhanced FLNF, but microbial‐derived C had a more positive impactC addition promoted FLNF in soils of the two forests, but the response rate was higher in the leguminous forest soils [ABSTRACT FROM AUTHOR]

A correction to the article "Cumulative effects of nitrogen additions on litter decomposition in three tropical forests in southern China" that was published in the 2007 issue is presented.

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. [ABSTRACT FROM AUTHOR]

Highly diverse exoenzymes mediate the energy flow from substrates to the multitrophic microbiota within the soil decomposer micro‐food web. Here, we used a "soil enzyme profile analysis" approach to establish a series of enzyme profile indices; those indices were hypothesized to reflect micro‐food web features. We systematically evaluated the shifts in enzyme profile indices in relation to the micro‐food web features in the restoration of an abandoned cropland to a natural area. We found that enzymatic C:N stoichiometry and decomposability index were significantly associated with substrate availability. Furthermore, the higher Shannon diversity index in the exoenzyme profile, especially for the C‐degrading hydrolase, corresponded to a greater microbiota community diversity. The increased complexity and stability of the exoenzyme network reflected similar changes with the micro‐food web networks. In addition, the gross activity of the enzyme profile as a parameter for soil multifunctionality, effectively predicted the substrate content, microbiota community size, diversity, and network complexity. Ultimately, the proposed enzymic channel index was closely associated with the traditional decomposition channel indices derived from microorganisms and nematodes. Our results showed that soil enzyme profile analysis reflected very well the decomposer food web features. Our study has important implications for projecting future climate change or anthropogenic disturbance impacts on soil decomposer micro‐food web features by using soil enzyme profile analysis. Highlights: Soil enzyme profile analysis for indicating decomposer micro‐food web features was first proposed.Diversity indication utilities for contrasting enzyme profiles were different.Protozoa channel index and enzymatic channel index were proposed. [ABSTRACT FROM AUTHOR]

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Nitrogen (N) deposition usually increases plant tissue N concentrations and thus phosphorus (P) demand in young and/or N‐limited forests, but the N deposition effect on plant P demand has rarely been assessed in N‐saturated forests.Impacts of 18‐year external N additions (Control: 0, Low N: 50, Moderate N:100 and High N: 150 kg N ha−1 year−1) on leaf P of four plant life‐forms (tree, shrub, herb and liana), P fractions of bulk and rhizosphere soils were examined in a N‐saturated mature tropical forest in southern China.Leaf N, P and N: P ratios of all plant life‐forms remained stable under three N additions. Among soil P fractions, moderate labile organic P increased by 25%–33% across three N additions; and soil total P was increased by 11.76% under Low N, and 8.87% under High N, compared with the control. The PLS‐PM results showed that path coefficient of microbial community to available P significantly increased and of inorganic P to available P significantly decreased under N additions than control. N additions improved soil P availability through microbe‐mediated P transformation: Low N significantly increased soil microbial taxonomic diversity, and a higher microbial diversity could enlarge the sources of nutrient acquisition and stimulate decomposition of recalcitrant organic matters; while High N significantly decreased soil microbial taxonomic diversity, the remaining microorganisms that were screened by N‐rich environments had the characteristics of resisting the N addition effects and maintained efficient P acquisition.Synthesis. Our findings provide a novel line of evidence that long‐term N deposition did not increase plant P demand in a N‐saturated mature tropical forest. The underlying mechanism is that plants did not increase N uptakes therefore nor increase P uptakes (a stable leaf N: P stoichiometry) in an already N‐saturated ecosystem. Different N addition rates regulated soil P transformation via microbial community transition. These findings help improve the understanding of plant P acquisition and modelling of biogeochemical N–P cycling and vegetation productivity in N‐rich forest ecosystems, particularly considering the fact that chronic N deposition may likely lead to soil N richness and even saturation of many forests in the future. [ABSTRACT FROM AUTHOR]

Soil microorganisms in tropical forests can adapt to phosphorus (P)-poor conditions by changing the activity ratios of different types of phosphatases. We tested whether microorganisms in P-poor tropical forest soils increased the phosphomonoesterase (PME) to phosphodiesterase (PDE) activity ratio, because a one-step enzymatic reaction of monoester P degradation might be more adaptive for microbial P acquisition than a two-step reaction of diester P degradation. A continuous 10-year P addition experiment was performed in three tropical forests. The activities of PME and PDE, and their ratio in soil, were determined under the hypothesis that the P-fertilized plots where P shortage is relieved would have lower PME:PDE ratios than the unfertilized controls. We demonstrated that long-term P addition in tropical forest soil did not alter the PME:PDE ratio in primary and secondary forests, whereas P fertilization elevated the PME:PDE ratio in planted forest. These results were in contrast to previous results. The long-term, large-scale P fertilization in our study may have reduced litter- and/or throughfall-derived PDE, which negated the lowered PME:PDE ratio via exogenous P inputs. [ABSTRACT FROM AUTHOR]

Soil enzyme activity can be affected by both production and degradation processes, as enzymes can be degraded by proteases. However, the impact of nutrient addition on enzyme activity is often solely attributed to changes in enzyme production without fully considering degradation. In this study, we demonstrate that the activities of β-1,4-glucosidase (BG), β-D-cellobiohydrolase (CBH), β-1,4-xylosidase (BX), and β-1,4-N-acetyl-glucosaminidase (NAG) in two tropical plantations exhibited comparable levels between nitrogen (N)- and phosphorus (P)-fertilized soils and the unfertilized control under field conditions. However, it was observed that the reduction in enzymatic activity was significantly higher in the fertilized soils during short-term laboratory incubation in the acacia plantation. Additionally, the eucalyptus plantation exhibited a similar tendency, although statistical significance was not achieved due to the high variance of the data. The results show that the interruption of the natural, continuous supply of organic matter or non-soil microbial-derived enzymes, which typically occurs under field conditions, leads to a more significant reduction in soil enzyme activities in fertilized soils compared to unfertilized control. This may be attributed to the higher abundance of protease in fertilized soils, resulting in faster enzyme degradation. Interestingly, P fertilization alone did not have a similar effect, indicating that N fertilization is likely the main cause of the larger decreases in enzyme activity during incubation in fertilized soils compared to unfertilized control soils, despite our study site being poor in P and rich in N. These findings highlight the importance of considering enzyme degradation when investigating material dynamics in forest ecosystems, including the impact of nutrient addition on enzyme activity, as enzyme production alone may not fully explain changes in soil enzyme activity. [ABSTRACT FROM AUTHOR]

Elevated nitrogen (N) deposition may further acidify soils in tropical forests. Yet, we have limited evidence on this prediction and it remained unclear how this would affect fine root growth therein. Here, we report responses of fine root biomass, vitality, and chemistry, as well as related soil parameters to eight years of N additions in three tropical forests different in initial soil N status, with one primary forest being N-saturated, and another two younger forests (one secondary forest and one planted forest) less N-rich. Results showed that in the primary forest, fine root biomass decreased and fine root necromass increased following N addition, resulting in lower live fine root proportion (fine root vitality). Declining fine root vitality was associated with fine root Fe accumulation and soil acidification indicated by regression analysis. These alterations of fine root growth and chemistry co-occurred with soil pH decline, soil exchangeable Fe3+ mobilization, exchangeable Ca2+, and Mg2+ depletion after N treatments in the primary forest. In contrast, N addition only elevated fine root K, Al, and Fe content in the secondary forest. In the planted forest, moderate but significant decreases in soil pH, soil exchangeable K+, and Mg2+ were found after N treatment, with fine root biomass negatively correlated with soil exchangeable Al3+ and Al3+/(Ca2+ + Mg2+) ratio. Our results suggested that long-term N fertilization may negatively affect fine root growth, via severed soil acidification, Fe mobilization, and base cation depletion in highly acidified, N-saturated primary tropical forests. Initial forest N status, influenced by different land-use history, mediates N deposition effects on fine root growth. [ABSTRACT FROM AUTHOR]

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 15N 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 15N 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 15N recovery in legume forest was mainly driven by uptake by the legume trees, in which 15N 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 15N 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. [ABSTRACT FROM AUTHOR]

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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 NO3––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. [ABSTRACT FROM AUTHOR]

The effects of deposited nitrogen (N) on forest ecosystems largely depend on the amount of N retained in the ecosystems and its partitioning among ecosystem pools. However, our understanding of the capacity of tropical plantations to retain deposited N is limited. To evaluate the retention of deposited N in a human-disturbed pine plantation in southern China and compare the result with previous findings in an adjacent old-growth forest, we added 15N-tracer monthly to the forest floor for one year and determined its recovery in ecosystem compartments four months after the last addition. We monitored 15N recoveries in soil solution monthly to quantify leaching losses. The pine forest retained about 58 ± 5% of the 15N-labeled deposited N, which is lower than that reported in the adjacent old-growth forest (72 ± 6%). Both forests experience chronic N deposition (recently measured at 51 kg N ha−1 yr−1) and we attribute the difference in retention to effects of previous disturbance mainly understory and litter harvesting in the pine plantation. Only 3 kg N ha−1 yr−1 (5% of the 15N-labeled deposited N) out of the measured total leaching (54 kg N ha−1 yr−1) originated from deposited (and labeled) N from the measurement year, suggesting that N leaching is dominated by unlabeled N sources. Furthermore, results from our study and other similar 15N labeling experiments together show similar patterns of total ecosystem retention of deposited N in tropical and temperate forests, but here we demonstrate a decreasing retention of N with increased N deposition in these forests. Our findings indicate that plantation forests that experience human-disturbance and chronic N deposition have lower N retention compared to old-growth forests, and thus elevated N inputs in such ecosystems can cause risk of hydrological N losses, soil acidification, and freshwater pollution. [ABSTRACT FROM AUTHOR]

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 [CO2], 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 CO2 (+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. [ABSTRACT FROM AUTHOR]

In tropical plantations, nutrients such as nitrogen (N) or phosphorus (P) are often applied as management practices. However, the effects of such nutrient additions on topsoil C- and N-acquiring enzymes activities are unclear. In this study, the impacts of fertilization on β-1,4-glucosidase (BG), β-D-cellobiosidase (CBH), β-1,4-xylosidase (BX), β-1,4-N-acetyl-glucosaminidase (NAG), and leucine amino peptidase (LAP) enzymes activities from topsoil and litter layer of two tropical plantations (Acacia auriculiformis and Eucalyptus urophylla) were measured. The results showed that N addition had neutral impact on topsoil enzymes, while significantly elevating the activities of BG, CBH, BX, and NAG in the litter layer. P fertilization had no impacts except for an elevation of NAG in litter sample. There was no interactions found between N and P additions on these enzyme activities. The clearer impacts of N over P fertilization were unexpected because that the study site receives a high rate of atmospheric N deposition, and has low soil P availability. The impact of P fertilization on hydrolytic enzyme activities may be less important compared with that of N. [ABSTRACT FROM AUTHOR]

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 (Nmass, +18.4%) and area basis (Narea, +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 (Aarea, +12.6%), stomatal conductance (gs, +7.5%), and transpiration rate (E, +10.5%). The responses of Aarea were positively related with that of gs, with no changes in instantaneous water‐use efficiency and only slight increases in long‐term water‐use efficiency (+2.5%) inferred from 13C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and Aarea 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. [ABSTRACT FROM AUTHOR]

The traditional view holds that biological nitrogen (N) fixation often peaks in early‐ or mid‐successional ecosystems and declines throughout succession based on the hypothesis that soil N richness and/or phosphorus (P) depletion become disadvantageous to N fixers. This view, however, fails to support the observation that N fixers can remain active in many old‐growth forests despite the presence of N‐rich and/or P‐limiting soils. Here, we found unexpected increases in N fixation rates in the soil, forest floor, and moss throughout three successional forests and along six age‐gradient forests in southern China. We further found that the variation in N fixation was controlled by substrate carbon(C) : N and C : (N : P) stoichiometry rather than by substrate N or P. Our findings highlight the utility of ecological stoichiometry in illuminating the mechanisms that couple forest succession and N cycling. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition alters the cycling of nutrients in terrestrial ecosystems. However, there is limited knowledge regarding how N addition affects foliar nutrients beyond N and phosphorus (P) and their stoichiometry. We conducted a meta‐analysis, including 2,004 observations from 134 fertilization studies, to synthesize the effects of N addition on multiple foliar nutrients and stoichiometry in terrestrial ecosystems and to examine their potential controls. Overall, we found that N addition significantly decreased foliar P, potassium (K), calcium (Ca), magnesium (Mg), and calcium:aluminium (Ca:Al) by 3.22%, 7.58%, 9.55%, 5.65%, and 15.17%, respectively, but significantly increased foliar N, Al, N:K, N:Ca, and N:Mg by 19.02%, 6.99%, 21.06%, 27.65%, and 11.33%, respectively. Among the forest ecosystems, foliar N and P exhibited greater changes in temperate or boreal forests, and foliar K, Mg, and Mn showed greater decreases or increases in tropical forests after N addition. Nitrogen addition significantly affected foliar K (−7.55%), Mg (−6.46%), and Al (+6.81%) in forests but not in grasslands. Similarly, foliar K, Mg, and Al showed significant changes in woody plants but not in herbaceous plants. In addition, we found that environmental factors (e.g., ambient N deposition, mean annual precipitation, and soil pH) affected foliar nutrient responses of N, K, and Mg to N addition. In summary, our findings indicate that N addition affects foliar nutrient contents, with the extent of these effects differing among ecosystem type. This is important for understanding and predicting plant growth and nutrient dynamics under N deposition scenarios. Plain Language Summary: Foliar nutrients, such as nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), aluminum (Al), manganese (Mn), and iron (Fe), play an important role in plant health and ecosystem function, and an imbalance in these elements affects plant growth. Although atmospheric N deposition has been known to affect foliar N and P contents, much uncertainty exists on how N deposition affects other foliar nutrients and stoichiometry. We conducted a meta‐analysis to synthesize the effects of N addition on eight foliar nutrient concentrations (e.g., N, P, K, Ca, Mg, Al, Mn, and Fe) and the stoichiometric ratios of N and these elements in terrestrial natural ecosystems. We found that N addition significantly decreased K, Ca, Mg, and P but increased N, Al, and N:K, N:Ca, and N:Mg across the tested biomes, and these N‐induced effects on foliar nutrients varied depending on the ecosystem type. In addition, we found that environmental factors (e.g., soil pH, ambient N deposition, and mean annual precipitation) regulated foliar nutrient responses of N, K, and Mg to N addition. Our findings provide direct evidence that high N deposition affects the nutrient status of plants in terrestrial natural ecosystems, particularly in tropical/subtropical forests. Key Points: A meta‐analysis was conducted with data from 134 papers to reveal the responses of multiple foliar elements and stoichiometry to N additionResponses of foliar N, P, K, Mg, and Mn to N addition varied depending on the ecosystem typeIncreases in background N deposition and mean annual precipitation decreased the responses of foliar N, K, and Mg to N addition [ABSTRACT FROM AUTHOR]

Purpose: Phosphorus (P) addition could largely alter soil microbial activity. However, effects of long-term P addition on soil extracellular enzyme activity are not well understood in tropical forests. Materials and methods: To address this question, we measured absolute activities (activity per unit of dry soil) and specific activities (activity per unit of microbial biomass carbon) of enzymes involved in carbon (C), nitrogen (N), and P cycling in a 10-year P addition experimental site in a tropical forest. Results and discussion: Phosphorus addition decreased acid phosphatase absolute and specific activity by 37% and 47%, respectively. As to N-acquisition enzymes, P addition increased leucine amino peptidase absolute activity but decreased β-1,4-N-acetylglucosaminidase absolute activity by 33%. Meanwhile, P addition had no effects on leucine amino peptidase specific activity but decreased β-1,4-N-acetylglucosaminidase specific activity by 43%. Among C-acquisition enzymes, cellobiohydrolase, α-glucosidase, and β-glycosidase absolute and specific activities showed no significant responses to P addition, while P addition decreased β-xylosidase absolute and specific activity by 15% and 27% respectively. Phosphorus addition also decreased phenol oxidase absolute activity by 30% and peroxidase absolute activity by 29%. Conclusions: These results suggest a strong P shortage for microorganisms and that P addition could decline microbial productions of enzymes in phosphorus-poor tropical forests. Results from this study emphasize again the important role of available P in tropical forests. [ABSTRACT FROM AUTHOR]

Because potassium (K) is a rock-derived essential element that can be depleted in highly-weathered tropical soils, K availability may limit some portion of soil microbial activity in tropical forest ecosystems. In this paper we tested if K limits microbial activity in the condition of sufficient labile C supply. An incubation experiment was performed using surface soil samples (0–10 cm depth) obtained from four permanent ecological research plots in a natural sub-tropical forest in southern China. Soil samples were taken in September 2016. Heterotrophic soil respiration rates and microbial biomass were measured after the addition of glucose (both D and L) with and without K (potassium chloride). We did not observe any effects of K addition on soil microbial respiration, suggesting that K does not limit the microbial activity in the condition of sufficient labile C supply. The lack of microbial response to added K can be attributed to the high mobility of K in forest ecosystems, which may have provided sufficient K to microbes in our soil samples (already provided at the beginning of the incubation). However, at the present stage, we cannot conclude that K is not a limiting factor of soil microbial activity in other tropical forest ecosystems because of the heterogeneity of tropical forest ecosystems and few observations. The hypothesis needs to be tested in larger numbers of tropical forests. [ABSTRACT FROM AUTHOR]

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 CO2 efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N2O 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 N2O 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. [ABSTRACT FROM AUTHOR]

The impact of nitrogen (N) deposition on dissolved organic carbon (DOC) fractions in throughfall is not well understood. We performed a laboratory experiment and compared DOC leaching from canopy leaves after dipping leaves in pure water (control) and NH4NO3 solution (N-treatment) for 18 h. Net changes of DOC, NH4+, NO3−, SO42−, K+, Mg2+, Ca2+ and H+ contents after dipping leaves were determined by comparing solutions with and without leaves. We recorded no differences of DOC leaching between control and N-treatment, implying that N deposition had minor impacts on canopy DOC production. This confirmed that previous experiments testing the effects of N addition on DOC dynamics without considering the effects of the canopy reaction successfully described the real situation. We also confirmed the previously-reported canopy exchange process in spite of a high background N deposition at our study site. N-treatment significantly increased base cation leaching, especially K+, and the increase was positively correlated with foliar NH4+ retention. Net leaching of H+ and SO42− was not affected by the N-treatment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Asymbiotic nitrogen (N) fixation (ANF) is an important source of N in pristine forests and is predicted to decrease with N deposition. Previous studies revealing N fixation in response to N deposition have mostly applied understory N addition approaches, neglecting the key processes (for example, N retention and uptake) occurring in forest canopy. This study evaluated the effects of N deposition on N fixation in the soil, forest floor, mosses, and canopy leaves in a temperate forest (in central China) and a tropical forest (in southern China) with different treatments: control, understory N addition, and canopy N addition. Results showed that total ANF rates were higher in the temperate forest (2.57 ± 0.19 mg N m−2 d−1) than in the tropical forest (1.34 ± 0.09 mg N m−2 d−1). N addition inhibited the soil, forest floor, moss, and foliar N fixation in the temperate forest, whereas it inhibited only the soil N fixation in the tropical forest. Compared to canopy N addition, understory N addition overestimated the inhibitory effects of N deposition on total ANF slightly in the tropical forest (by 35%) but severely in the temperate forest (by 375–472%) due to neglecting canopy retention of N. In summary, our findings indicate that ANF has different rates and sensitivities to N addition between tropical and temperate forests and that understory N addition overestimates the N deposition effects on ANF in forests, particularly in the temperate forest. These findings are important for our accurate understanding and estimate of terrestrial N fixation under N deposition scenarios. [ABSTRACT FROM AUTHOR]

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 (SO42−), 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: It has long been believed that soil microbial activity in tropical forest ecosystems is limited by phosphorus (P) rather than nitrogen (N) availability. In this study, we reviewed the methods used to determine the limiting nutrients and evaluated the validity of the widespread P‐limitation hypothesis in tropical forest soils. The most commonly used analysis method entails testing whether fertilization increased microbial biomass or soil respiration. Fertilization using microbial biomass as an indicator was not a satisfactory method because standing microbial biomass does not always signal microbial activity. An increase in soil respiration after fertilization was also an insufficient indicator because the negative response shown by organic matter decomposition to nutrient addition can also be driven by nutrient shortage (nutrient mining). Nutrient amendment can also shift microbial communities towards more copiotrophic organisms, which may exhibit lower microbial respiration rates. We suggest that P addition may accelerate soil organic matter decomposition compared with N, which is independent of the nutrient limitation of soil microbial activity. The negative response of organic matter decomposition to N addition through nutrient mining is driven by N shortage, which is less likely to occur with P. N addition inhibits microbial activity via chemical reactions, whereas P addition may stimulate activity through replacement by C bound to sorption sites in the soil, improving C availability. Thus, the P‐limitation hypothesis must be reconsidered because of the contrasting effects of exogenous N and P addition on soil microbial activity, which could lead to misdetection of P limitation on soil microbial activity in tropical forest ecosystems. We recommend caution in applying the statement that soil microbial activity in tropical forest ecosystems is limited by P until a novel method is established to accurately determine the nutrients limiting soil microbial activity at the ecosystem level. We proposed alternative ways to describe nutrient shortage for soil microbes. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Elevated anthropogenic nitrogen (N) deposition is suggested to affect ecosystem phosphorus (P) cycling through altered biotic P demand and soil acidification. To date, however, there has been little information on how long-term N deposition regulates P fluxes in tropical forests, where P is often depleted. To address this question, we conducted a long-term N addition experiment in a mature tropical forest in southern China, using the following N treatments: 0, 50, 100, and 150 kg N ha−1 year−1. We hypothesized that (i) tropical forest ecosystems have conservative P cycling with low P output, and (ii) long-term N addition decreases total dissolved phosphorus (TDP) leaching losses due to reduced litter decomposition rates and stimulated P sorption deriving from accelerated soil acidification. As hypothesized, we demonstrated a closed P cycling with low leaching outputs in our forest. Under experimental N addition, TDP flux in throughfall was significantly reduced, suggesting that N addition may result in a less internal P recycling. Contrary to our hypothesis, N addition did not decrease TDP leaching, despite reduced litter decomposition and accelerated soil acidification. We find that N addition might have negative impacts on biological P uptake without affecting TDP leaching, and that the amount of TDP leaching from soil could be lower than a minimum concentration for TDP retention. Overall, we conclude that long-term N deposition does not necessarily decrease P effluxes from tropical forest ecosystems with conservative P cycling. [ABSTRACT FROM AUTHOR]

It is well documented that phosphorus (P) input stimulates biological nitrogen (N) fixation (BNF) in tropical forests with non-legume trees. However, in tropical legume forests with soil N enrichment and P deficiency, the effects of P availability and its combination with N on BNF remain poorly understood. In this study, we measured BNF rate in different compartments, i.e., bulk soil, forest floor, rhizosphere, and nodules, in two tropical plantations with legume trees Acacia auriculiformis ( AA) versus non-legume trees Eucalyptus urophylla, ( EU) in southern China after 4 years of P addition and combined N and P additions. The objective was to investigate how P addition and its combination with N addition regulate BNF in a tropical legume plantation, and to compare the effects with those in a non-legume plantation. Our results showed that total BNF rates were significantly higher in the P-addition plots than in the control plots by 27.4 ± 4.3 and 23.3 ± 1.7 % in the EU and AA plantations, respectively. Total BNF rates were significantly higher in the NP-addition plots than in the control plots by 27.7 ± 5.0 and 8.5 ± 1.4 % in the EU and AA plantations, respectively, which contrasted to our previous result that total BNF rates were significantly lower in N-addition plots than in the control plots in the AA plantation. These findings suggest that P input can stimulate BNF in tropical forest biome dominated by legume trees, even in consideration of elevated atmospheric N deposition. Thus, our study revealed the important role of P in regulating biological N input, which should be taken into account in the modeling of biogeochemical cycles in the future. [ABSTRACT FROM AUTHOR]