Xiankai Lu

Nitrogen deposition and its ecological impact in China: an overview.

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

Divergent Responses of Soil Buffering Capacity to Long-Term N Deposition in Three Typical Tropical Forests with Different Land-Use History

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.

The 15N natural abundance of the N lost from an N‐saturated subtropical forest in southern China

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.

Effects of experimental nitrogen additions on plant diversity in tropical forests of contrasting disturbance regimes in southern China

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

Nutrient Limitation in Three Lowland Tropical Forests in Southern China Receiving High Nitrogen Deposition: Insights from Fine Root Responses to Nutrient Additions

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

δ15N of soil N and plants in a N‐saturated, subtropical forest of southern China

We investigated the delta(15)N profile of N (extractable NH(4)(+), NO(3)(-), and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of delta(15)N in the soil was EON > NH(4)(+) > NO(3)(-). Although the delta(15)N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5 per thousand. The difference in delta(15)N between bulk soil N and EON (Delta(15)N(bulk-EON)) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high delta(15)N of EON, as in the case of soil microbial biomass delta(15)N. A thorough delta(15)N survey of available N (NH(4)(+), NO(3)(-), and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the delta(15)N of the available N forms did not fully overlap with the delta(15)N of plants. This mismatch in delta(15)N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest.

Response of plant nutrient stoichiometry to fertilization varied with plant tissues in a tropical forest

Plant N:P ratios are widely used as indices of nutrient limitation in terrestrial ecosystems, but the response of these metrics in different plant tissues to altered N and P availability and their interactions remains largely unclear. We evaluated changes in N and P concentrations, N:P ratios of new leaves (<1 yr), older leaves (>1 yr), stems and mixed fine roots of seven species after 3-years of an N and P addition experiment in a tropical forest. Nitrogen addition only increased fine root N concentrations. P addition increased P concentrations among all tissues. The N × P interaction reduced leaf and stem P concentrations, suggesting a negative effect of N addition on P concentrations under P addition. The reliability of using nutrient ratios as indices of soil nutrient availability varied with tissues: the stoichiometric metrics of stems and older leaves were more responsive indicators of changed soil nutrient availability than those of new leaves and fine roots. However, leaf N:P ratios can be a useful indicator of inter-specific variation in plant response to nutrients availability. This study suggests that older leaf is a better choice than other tissues in the assessment of soil nutrient status and predicting plant response to altered nutrients using nutrients ratios.

Decomposition responses of pine (Pinus massoniana) needles with two different nutrient-status to N deposition in a tropical pine plantation in southern China

Abstract• The effect of nitrogen (N) deposition on the decomposition of pine (Pinus massoniana) needles in a tropical pine plantation was studied. The pine needles with two different nutrient status (nutrient-rich and nutrient-poor) were used, followed by 3-levels of N treatments (Control: no N addition, Low- N: 5 g N m−2 y−1, and Medium-N: 10 g N m−2 y−1 experimental inputs), which had been applied for 26 months continuously before this experiment and continued throughout the decomposition measurement.• The main objective was to test the hypothesis that decomposition of nutrient-rich needles would be more sensitive to cumulative N deposition than the decomposition of nutrient-poor needles.• Nitrogen addition had negative effect on mass loss, and the release of N and P from decomposing nutrient-rich needles but little or no effect on the decomposition of nutrient-poor needles. In addition, a negative effect in the initial decomposition phase and a positive effect in later decay stages were found on C release. The negative effect was stronger on nutrient-rich needles than on nutrient-poor needles, but the reverse was true for the positive effect.• Our results suggest that response of litter decomposition to N deposition may vary depending on the nutrient status of the litter.Résumé• Les effets des dépôts azotés (N) sur la décomposition des aiguilles de pin ont été étudiés dans une plantation de pins tropicaux (Pinus massoniana) du Sud de la Chine. Les aiguilles de pin différentes par leur statut nutritionnel (station riche en nutriments et station pauvre en nutriments) ont été utilisées dans une expérimentation comportant 3 niveaux de traitement azoté (témoin sans apport d’azote, apport faible d’azote : 5 g N m−2 an−1, apport moyen d’azote :10 g Nm−2 an−1) qui ont été appliqués pendant une période de 26 mois avant et pendant cette expérimentation.• L’objectif principal était de tester l’hypothèse que la décomposition des aiguilles riches en nutriments devrait être plus sensible à une déposition cumulée d’azote que la décomposition des aiguilles pauvres en nutriments.• L’apport répété d’azote a eu un effet négatif sur la perte de masse et la libération d’azote et de phosphore des aiguilles de la station riche et un effet faible ou nul sur la décomposition des aiguilles de la station la plus pauvre. En outre, un effet négatif dans la phase initiale de la décomposition et un effet positif dans les phases plus tardives de la décomposition ont été mis en évidence pour la libération de carbone. L’effet négatif a été plus fort pour les aiguilles riches en nutriments que pour les aiguilles pauvres en nutriments, mais l’inverse était vrai pour ce qui concerne l’effet positif.• Nos résultats, complétés par ceux obtenus au cours d’expérimentation antérieures sur le même site, suggèrent que l’accumulation de dépôts azotés peut changer les impacts sur la décomposition des litières et que cet effet peut varier en relation avec la fertilité de la station considérée.

Phosphate addition enhanced soil inorganic nutrients to a large extent in three tropical forests

Elevated nitrogen (N) deposition may constrain soil phosphorus (P) and base cation availability in tropical forests, for which limited evidence have yet been available. In this study, we reported responses of soil inorganic nutrients to full factorial N and P treatments in three tropical forests different in initial soil N status (N-saturated old-growth forest and two less-N-rich younger forests). Responses of microbial biomass, annual litterfall production and nutrient input were also monitored. Results showed that N treatments decreased soil inorganic nutrients (except N) in all three forests, but the underlying mechanisms varied depending on forests: through inhibition on litter decomposition in the old-growth forest and through Al3+ replacement of Ca2+ in the two younger forests. In contrast, besides great elevation in soil available P, P treatments induced 60%, 50%, 26% increases in sum of exchangeable (K++Ca2++Mg2+) in the old-growth and the two younger forests, respectively. These positive effects of P were closely related to P-stimulated microbial biomass and litter nutrient input, implying possible stimulation of nutrient return. Our results suggest that N deposition may result in decreases in soil inorganic nutrients (except N) and that P addition can enhance soil inorganic nutrients to support ecosystem processes in these tropical forests.

The effects of atmospheric nitrogen deposition on terrestrial and freshwater biodiversity

This chapter reports the findings of a Working Group on how atmospheric nitrogen (N) deposition affects both terrestrial and freshwater biodiversity. Regional and global scale impacts on biodiversity are addressed, together with potential indicators. Key conclusions are that: the rates of loss in biodiversity are greatest at the lowest and initial stages of N deposition increase; changes in species compositions are related to the relative amounts of N, carbon (C) and phosphorus (P) in the plant soil system; enhanced N inputs have implications for C cycling; N deposition is known to be having adverse effects on European and North American vegetation composition; very little is known about tropical ecosystem responses, while tropical ecosystems are major biodiversity hotspots and are increasingly recipients of very high N deposition rates; N deposition alters forest fungi and mycorrhyzal relations with plants; the rapid response of forest fungi and arthropods makes them good indicators of change; predictive tools (models) that address ecosystem scale processes are necessary to address complex drivers and responses, including the integration of N deposition, climate change and land use effects; criteria can be identified for projecting sensitivity of terrestrial and aquatic ecosystems to N deposition. Future research and policy-relevant recommendations are identified.