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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.

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.

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 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 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.

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.

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 N2 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 N2 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.

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

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

Anthropogenic elevated nitrogen (N) deposition has an accelerated terrestrial N cycle, shaping soil carbon dynamics and storage through altering soil organic carbon mineralization processes. However, it remains unclear how long-term high N deposition affects soil carbon mineralization in tropical forests. To address this question, we established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China with N additions such as NH4NO3 of 0 (Control), 50 (Low-N), 100 (Medium-N) and 150 (High-N) kg N ha−1 yr−1, and laboratory incubation experiment, used to explore the response of soil carbon mineralization to the N additions therein. The results showed that 15 years of N additions significantly decreased soil carbon mineralization rates. During the incubation period from the 14th day to 56th day, the average decreases in soil CO2 emission rates were 18%, 33% and 47% in the low-N, medium-N and high-N treatments, respectively, compared with the Control. These negative effects were primarily aroused by the reduced soil microbial biomass and modified microbial functions (e.g., a decrease in bacteria relative abundance), which could be attributed to N-addition-induced soil acidification and potential phosphorus limitation in this forest. We further found that N additions greatly increased soil-dissolved organic carbon (DOC), and there were significantly negative relationships between microbial biomass and soil DOC, indicating that microbial consumption on soil-soluble carbon pool may decrease. These results suggests that long-term N deposition can increase soil carbon stability and benefit carbon sequestration through decreased carbon mineralization in N-rich tropical forests. This study can help us understand how microbes control soil carbon cycling and carbon sink in the tropics under both elevated N deposition and carbon dioxide in the future.

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.

Significance Forest soil carbon (C) storage plays a central role in sequestrating atmospheric CO2 on timescales from centuries to millennia. However, our current understanding of soil C sequestration in response to N deposition mainly focuses on mid-to-high latitudes in the Northern Hemisphere, where N supply typically constrains forest growth. We lack data about changes in soil C stocks in tropical forests, where most ecosystems are N-rich or N-saturated. Using more than a decade of continuous N addition experiment and a meta-analysis, we found that excess N deposition can significantly increase soil C in N-rich tropical forests. However, enhanced C sequestration in tropical soils is not a good reason to justify excess N emissions to the atmosphere., 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 CO2 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.

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.

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.

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.

In southern China, high levels of atmospheric nitrogen (N) are being deposited in forests. Soil acidification and high rates of soil nitrification and subsequent NO 3 − leaching have been observed in the N-saturated forests. We previously did not detect NH 3 -oxidizing bacteria in non-N-saturated or N-saturated forest soils. However, NH 3 -oxidizing archaea were present in both soils, more in the saturated ones. The purpose of this study was to investigate the roles of autotrophic NH 3 -oxidizing archaea and heterotrophic nitrifiers in soil N transformations in N-saturated and non-N-saturated forests in southern China. We investigated the contribution of heterotrophic nitrifiers in the soils by determining the gross nitrification rates with and without an inhibitor of autotrophic nitrification, acetylene (C 2 H 2 ). We also reevaluated nitrification by NH 3 -oxidizing archaea by correlating the C 2 H 2 -inhibited gross nitrification rates with the abundance of the amoA transcripts of NH 3 -oxidizing archaea. We further measured the gross NH 4 + production rates and analyzed the community composition of the NH 3 -oxidizing archaea. The results suggest that NH 3 -oxidizing archaea, rather than heterotrophic nitrifiers and NH 3 -oxidizing bacteria, are responsible for the nitrification in the N-saturated forest soils. NH 3 -oxidizing archaea in the soils could be acidophilic, having low amoA diversity, indicating their strong adaptation to the highly acidified soils. The gross NH 4 + production rate did not differ between N-saturated and non-N-saturated forests; however, the gross nitrification rate was higher in N-saturated forests. Consequently, the high abundance and NH 3 oxidation activity of NH 3 -oxidizing archaea caused the high rates of nitrification and subsequent leaching of NO 3 − in the N-saturated forest. This study suggests that acidophilic NH 3 -oxidizing archaea have a great impact on soil N cycling in N-saturated forests.

Intensified anthropogenic activities will increase rates of nitrogen (N) deposition over the next decades, especially in the tropics. There are urgent needs to know how soil microbial community in N-rich tropical forests responds to long-term N deposition. This study examined effects of long-term N additions on soil microbial biomass (determined by chloroform fumigation), microbial community composition (based on phospholipid fatty acids, PLFAs), and microbial enzyme activities, using an ongoing experimental N additions field in an N-rich tropical forest of South China. There were four N additions levels: no additions (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 long-term N additions significantly decreased microbial biomass carbon (MBC) and nitrogen (MBN), but had little effects on total PLFAs. However, elevated N inputs significantly reduced the relative abundance of bacterial PLFAs, especially gram-negative bacterial PLFAs with higher gram-positive bacteria: gram-negative bacteria ratio in N treatment plots. Although N additions did not change fungi: bacteria ratio, the proportion of arbuscular mycorrhizal fungi increased significantly with N additions. Long-term N additions greatly increased bacterial stress indexes and enhanced specific enzyme activity (activity per unit of microbial biomass) involved in carbon, nitrogen and phosphorus mineralization. Meanwhile, shifts in microbial community composition and specific enzyme activity were correlated well with soil pH and available N. These results suggest that N-mediated environmental stresses can play an important role in shaping microbial community, and that soil microbes will invest more resources on enzyme production in N-rich forest under elevated N deposition.

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.

Nitrogen (N) deposition induces soil acidification in natural forests; however, whether it increases soil acidity in tropical plantations with simple tree structures compared with natural forests remains unclear. This study aimed to investigate the effects of N deposition on the soil acidity of tropical broadleaf plantations dominated by Acacia auriculiformis and Eucalyptus urophylla in South China, which has been enduring N deposition for over 30 years, and investigate the reasons for the changes in soil acidity. Long-term N addition did not affect soil acidity in the two plantations, with no significant changes in soil pH values, and exchangeable non-acidic and acidic cation concentrations. Long-term N deposition did not significantly affect the plant and total soil N concentrations, but significantly increased the soil nitrous oxide emission rates and total dissolved N concentrations in the soil solutions. Our findings indicate that most of the added N was lost via leaching and emissions, such that long-term N addition did not exacerbate soil acidification in broadleaf plantations, thereby providing novel insight into the effects of atmospheric N deposition on forest ecosystems. Overall, our study indicates that long-term N deposition does not always lead to soil acidification in tropical forests, as previously expected.

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 12 years of N treatments: control, low N addition (50 kg N ha− 1 yr− 1), and medium N addition (100 kg N ha− 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 NH4+ 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.

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. 13C 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.

The prevailing paradigm for soil microbial activity in tropical forests is that microbial activity is limited by phosphorus (P) availability. Thus, exogenous P addition should increase rates of organic matter decomposition. Studies have also confirmed that soil respiration is accelerated when P is added experimentally. However, we hypothesize that the increased rates of soil microbial respiration could be due to the release of organic material from the surface of soil minerals when P is added, because P is more successful at binding to soil particles than organic compounds. In this study, we demonstrate that P addition to soil is associated with significantly higher dissolved organic carbon (DOC) content in a tropical evergreen forest in southern China. Our results indicate that P fertilization stimulated soil respiration but suppressed litter decomposition. Results from a second sorption experiment revealed that the recovery ratio of added DOC in the soil of a plot fertilized with P for 9 years was larger than the ratio in the soil of a non-fertilized plot, although the difference was small. We also conducted a literature review on the effects of P fertilization on the decomposition rates of litter and soil organic matter at our study site. Previous studies have consistently reported that P addition led to higher response ratios of soil microbial respiration than litter decomposition. Therefore, experiments based on P addition cannot be used to test whether microbial activity is P-limited in tropical forest soils, because organic carbon desorption occurs when P is added. Our findings suggest that the prevailing paradigm on the relationship between P and microbial activity in tropical forest soils should be re-evaluated.

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.

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 NH415 NO3 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.

Dissolved organic matter (DOM) drives many fundamental biogeochemical processes (e.g., carbon storage, nutrient cycling, and soil development) in forest soil. However, the molecular-level characteristics of DOM derived from different types of tropical forest soils are poorly understood. Here, water samples at different soil depths (0, 20, and 40 cm) from tropical legume (Acacia auriculiformis, AA) and non-legume (Eucalyptus urophylla, EU) tree plantations were analyzed using absorption and fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy. The FT-ICR MS results indicated that DOM persisted in the soil, but its molecular composition notably shifted from low-mass (150–300 Da) and more-aromatic molecules to middle- (300–450 Da) and high-mass (>450 Da) and less-aromatic molecules with increasing soil depth. This was primarily mediated by consumption and mineral sorption of low-mass plant-derived DOM (e.g., low-mass carbohydrates and polyphenols) and further formation of larger microbial products (e.g., protein-like and lipid-like compounds). In addition, a higher abundance of microbial-derived molecules (e.g., protein-like and carboxyl-rich alicyclic molecules) was found at the legume plantation relative to the non-legume plantation, which suggests a faster microbial turnover of DOM. Also, the legume plantation had greater enrichment of middle- and high-mass and condensed aromatic-like DOM components in soils. These findings improve our understanding of the drivers that mediate the response of DOM to soil depth and tree species in tropical plantations.

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.

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 (

Summary Nitrogen (N) saturation hypothesis suggests that when an ecosystem reaches N-saturation, continued N input will cause increased N leaching, nitrous oxide (N2O) emission, and N mineralization and nitrification rates. It also suggests that a different element will become the main limiting factor when N saturation has been reached. Although this hypothesis has been tested in temperate forests, whether they can be directly applied to N-saturated tropical forests remain poorly addressed. To test this hypothesis, soil inorganic N, soil N mineralization and nitrification rate, soil N2O emission rate and nitrate (NO3−) leaching rate were measured in an N-saturated old-growth tropical forest in southern China, after 6 years of N and P addition. We hypothesized that N addition would stimulate further N saturation, but P addition might alleviate N saturation. As expected, our results showed that six continuous years of experimental N addition did cause further N saturation, which was indicated by significant increases in soil inorganic N concentration, N2O emission and nitrate (NO3−) leaching. However, in contrast to our expectations, N addition significantly decreased in situ rates of net N mineralization and nitrification, which could be related to associated changes in enzyme activity and microbial community composition. On the other hand, P addition mitigated N saturation, as expected. Soil inorganic N concentration, N2O emission and NO3− leaching decreased significantly after P addition, but the net rates of N mineralization and nitrification were significantly increased. Our results provide a new understanding of the N saturation hypothesis, suggesting that the effects of long-term N deposition on net N mineralization and nitrification rates in N-saturated tropical forests can be negative and that P addition can alleviate N saturation in such tropical systems.

Atmospheric nitrogen (N) deposition has accelerated in the last several decades due to anthropogenic activities, such as nitrogen fertilization, N-fixing plants cultivation and fossil fuel and biomass combustion. Increasing N deposition has become one of the important factors regulating N cycle in forest ecosystems. Forest ecosystems can retain part of deposited N in soil by biotic and abiotic mechanisms, but when N inputs exceed the capacity of soil retention, N losses will aggravate in terms of N oxide emission and/or nitrate leaching. The excess N input has threatened ecosystem health via acidification and eutrophication, causing declines in terrestrial biodiversity and forest productivity in forest ecosystems of Europe and North America. Recently, China has become one of the three areas that undergo severe N deposition in the world. Impacts of N deposition on soil N cycle in Chinese forest ecosystems have received increasing concern. In this paper, we reviewed the processes of soil N cycle and their responses to atmospheric N deposition based on available literature. The objective is to enhance our understanding on how N deposition affects soil N cycle in forest ecosystems and provide scientific information for sustainable forest management. The review mainly includes the following four aspects: (1) processes of soil N cycle and their controlling factors. These processes include biological N fixation (BNF), decomposition, mineralization, nitrification, denitrification, N oxide emission and NO3−N leaching. The controlling factors of these processes are complicated and interactional. Only one of these factors altered may affect soil N cycle. For example, C/N is the factor that controls BNF, decomposition, mineralization and NO3−N leaching. (2) Research methods and current results about studies are related to the impact of N deposition on soil N cycle in forest ecosystems. In general, the research methods are long-term simulated N deposition experiment, N deposition gradient method, roof clean rain method and 15N tracing method. Effects of N deposition on soil N cycle vary depending on different initial N statuses and lengths of experiment. In “N-limited” forests, N deposition tended to have positive effect on soil N cycling processes, such as accelerating litter decomposition rate and N mineralization rate. However, such result generally showed in short-term fertilization experiments. In some long-term fertilization experiments, it showed that the negative effects would rise when the forests reached N saturation. Compared to “N-limited” forests in temperate region, N deposition tended to have negative or neutral effects in “N-rich” tropical forest. For example, N deposition promoted nitrification process in tropical forests. (3) Possible mechanisms for the effect of N deposition on soil N cycle: N deposition can affect soil N cycle through altering the chemical characteristic of forest substrates, the biomass and community composition of plant and microorganism. (4) Current problems and future research needs for the study about the effect of N deposition on soil N cycle: What role does regional diversity, changes in forest type, and interaction of carbon (C), N and phosphorus (P) play on the effect of N deposition on soil N cycle in forest ecosystems deserve our further study in the future.

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.

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.

Soil acidification is defined as the process in which exchangeable cations are leaching and soil H+ concentration is raising thereby increases soil acidity. Changes in soil pH value and acid neutralizing capacity are mainly indicators of soil acidification. Soil acidification is considered to be a serious ecological and environmental issue, which not only reduces soil quality, but also decreases biodiversity of forest ecosystem and induces forest decline. With nitrogen (N) deposition rapidly increasing, its contribution to soil acidification becomes a major concern in the world. However, the impact of increased N deposition on soil acidification is not well addressed highlighting the need for further attention to the issue. In this paper, the studies on forest soil acidification induced by N deposition were reviewed. The factors related to soil acidification driven by N deposition were classified and discussed, which included soil acidic buffering capacity, N components in atmospheric N deposition, climate, plant species in forests, and N status in ecosystem. Iron (Fe) buffering phase and the consequent Fe toxicity occurring to the acidified soil caused by high N deposition were concerned. The scarcity of phosphorus (P) element induced by soil acidification was particularly emphasized. The research methods used to study soil acidification driven by N deposition were also evaluated. In the end we stressed the importance of the study on soil acidification especially in tropical and subtropical regions driven by N deposition and its mechanisms. This paper can serve for maintaining sustainable forest and agricultural ecosystems.

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)

It is well established that tropical forest ecosystems are often limited by phosphorus (P) availability, and elevated atmospheric nitrogen (N) deposition may further enhance such P limitation. However, it is uncertain whether P availability would affect soil fluxes of greenhouse gases, such as methane (CH4) uptake, and how P interacts with N deposition. We examine the effects of N and P additions on soil CH4 uptake in an N saturated old-growth tropical forest in southern China to test the following hypotheses: (1) P addition would increase CH4 uptake; (2) N addition would decrease CH4 uptake; and (3) P addition would mitigate the inhibitive effect of N addition on soil CH4 uptake. Four treatments were conducted at the following levels from February 2007 to October 2009: 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). Static chamber and gas chromatography techniques were used to quantify soil CH4 uptake every month throughout the study period. Average CH4 uptake rate was 31.2 ± 1.1 μg CH4-C m−2 h−1 in the control plots. The mean CH4 uptake rate in the N-addition plots was 23.6 ± 0.9 μg CH4-C m−2 h−1, significantly lower than that in the controls. P-addition however, significantly increased CH4 uptake by 24% (38.8 ± 1.3 μg CH4-C m−2 h−1), whereas NP-addition (33.6 ± 1.0 μg CH4-C m−2 h−1) was not statistically different from the control. Our results suggest that increased P availability may enhance soil mathanotrophic activity and root growth, resulting in potentially mitigating the inhibitive effect of N deposition on CH4 uptake in tropical forests.

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 d 15 N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha � 1 yr � 1 , while the throughfall covered a larger range of 11.7–65.1 kg N ha � 1 yr � 1 . Foliar N concentration, NO3 leaching to stream, and soil NO3 concentration were low and NO3 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 NO3 leaching to stream, and negatively with pH values in soil and stream water. Foliar d 15 N was between � 6.6% and 0.7%, and was negatively correlated with soil NO3 concentration and NO3 leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest d 15 N values. We proposed several potential mechanism that could contribute to the d 15 N pattern, including (1) increased plant uptake of 15 N-depleted soil NO3 , (2) foliage uptake of 15 N-depleted NH4 1 , (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.

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); 50kgNha � 1 yr � 1 (Low-N); 100kgNha � 1 yr � 1 (Medium-N), and 150kgNha � 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 (� 100kgNha � 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 competitionbased 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.

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of 15 N( d 15 N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N2O emission and inorganic N leaching as well as N concentration and d 15 N of leaves, litters and soils. We found that foliar N concentration and d 15 N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and 13C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched 13C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.

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 longterm 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 (NO3 ) 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 NO3 leaching in all three forests. We hypothesize that abiotic incorporation of excess NO3 (through chemically reactive NO2 ) 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.

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 � 1m g CO2Cm � 2 h � 1 , respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58 � 3m g 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.

Nitrite dynamics could be highly associated with forest N cycles. However, they have often been overlooked mainly because of the experimental difficulties that occur owing to chemical reactive nature of NO2−. We investigated NO2− dynamics in an N-saturated forest soil with a recently developed method using 15N. Soils were aerobically incubated for 145 h after 15NO2− addition, and changes in 14N and 15N concentrations of NO2−, NO3−, NH4+, and dissolved organic N (DON) were monitored. Simultaneous production and consumption of NO2− were observed. The turnover rate of NO2− was even faster than that of NH4+ and NO3− calculated in other studies. Of the added 15NO2−, 28.5% was oxidized to NO3− and 17.8% was incorporated into the DON pool within 4 h. The remainder might be emitted as gas or fixed by insoluble soil organic matter. Our results suggested that rapid NO2− turnover could be a major driving force for N transformations in forest soil.

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.

The 15N-enrichment of plants and soils is believed to indicate characteristics of the open nitrogen (N) cycle in terrestrial ecosystems because N lost from an ecosystem is presumably 15N-depleted through isotopic fractionation. However, because of a lack of an appropriate analytical methodology to confirm that supposition, the δ15N value for total dissolved nitrogen (TDN, the sum of ammonium, nitrate, and dissolved organic N) in stream water from forests has been measured only rarely. This report describes the δ15N values for TDN, ammonium, and nitrate in precipitation and stream water, together with those for soil-emitted nitrous oxide (N2O; measured once) in an N-saturated subtropical forest in southern China. Concentration-weighted δ15N values of TDN were −0.7‰ in precipitation and +1.2‰ in stream water. The difference in δ15N between soil (+3.9‰) and TDN in the stream water was 2.7‰. In contrast, soil-emitted N2O was strongly 15N-depleted (−14.3‰): 18‰ lower than that of the soil. Our results demonstrate that the discharged N loss is 15N-depleted only slightly compared with soil N, and gaseous N losses can be a strong driver for raising the terrestrial ecosystem δ15N. Our findings suggest that the relation between ecosystem δ15N and the open N cycle can be interpreted better by considering the net discrimination against 15N determined by the balance between gaseous and discharge N losses. Steady state 15N budget calculations proposed by Houlton and Bai (2009) can provide important information about the gaseous N fluxes, which are difficult to measure directly. The steady state calculation for the relationships among gaseous N loss, apparent isotopic fractionation during gaseous N loss, and isotopic signature of N inputs suggests that precise measurements of unmeasured components (e.g., dry deposition, NO and N2 emission) are quite important for better estimation of gaseous N losses from the ecosystem.