Nitrogen Deposition Rates Research Articles

Identifying factors that affect mountain lake sensitivity to atmospheric nitrogen deposition across multiple scales

Increased nitrogen (N) deposition rates over the past century have affected both North American and European mountain lake ecosystems. Ecological sensitivity of mountain lakes to N deposition varies, however, because chemical and biological responses are modulated by local watershed and lake properties. We evaluated predictors of mountain lake sensitivity to atmospheric N deposition across North American and European mountain ranges and included as response variables dissolved inorganic N (DIN = NNH4+ + NNO3–) concentrations and phytoplankton biomass. Predictors of these responses were evaluated at three different spatial scales (hemispheric, regional, subregional) using regression tree, random forest, and generalized additive model (GAM) analysis. Analyses agreed that Northern Hemisphere mountain lake DIN was related to N deposition rates and smaller scale spatial variability (e.g., regional variability between North American and European lakes, and subregional variability between mountain ranges). Analyses suggested that DIN, N deposition, and subregional variability were important for Northern Hemisphere mountain lake phytoplankton biomass. Together, these findings highlight the need for finer-scale, subregional analyses (by mountain range) of lake sensitivity to N deposition. Subregional analyses revealed differences in predictor variables of lake sensitivity. In addition to N deposition rates, lake and watershed features such as land cover, bedrock geology, maximum lake depth (Zmax), and elevation were common modulators of lake DIN. Subregional phytoplankton biomass was consistently positively related with total phosphorus (TP) in Europe, while North American locations showed variable relationships with N or P. This study reveals scale-dependent watershed and lake characteristics modulate mountain lake ecological responses to atmospheric N deposition and provides important context to inform empirically based management strategies.

Drivers of soil respiration in response to nitrogen addition in a Mediterranean mountain forest

Atmospheric nitrogen (N) deposition rates affect soil N dynamics, influencing soil respiration (RS) rates. However, for the Mediterranean region, the effect of changes in atmospheric N deposition on RS are not well constrained yet. We investigated the interplay between increased N deposition and tree species composition on RS at a Scots pine—Pyrenean oak ecotone in Central Spain, and whether the observed responses were mediated by changes on selected soil properties. Throughout 3 years, we simulated two N deposition rates—10 (medium) and 40 kg N ha−1 a−1 (high)—over the background deposition (control) in neighbouring stands in which tree species composition (pine or oak) shapes soil stoichiometry and microbial communities. We monitored RS on a monthly basis during 3 years; in addition, we performed targeted measurements 24 h after the N fertilization events to assess short-term soil responses. During winter and summer, RS did not respond to enhanced N deposition rates. In spring and autumn, higher RS rates were observed in the medium-fertilization, but the size and duration of this effect was tree species dependent. We suggest that climate seasonality modulates the response of RS to N availability, with tree species effects becoming relevant only when environmental conditions are adequate. RS in fertilized plots was larger from February to May and in September under pine, while under oak a response was observed only in April, probably due to differences in native soil stoichiometry under each tree species. Overall, RS showed high stability during 3 years of N enrichment in this Mediterranean ecotone area. However, we observed short-term soil responses after N fertilization events—loss of base cations, soil acidification and reduced microbial biomass—which emphasize the need to investigate consequences for the belowground C and N cycles if chronic N enrichment persists in the long run.

Assessment of grey water footprint in paddy rice cultivation: Effects of field water management policies

Excessive use of fertilizers and manure has detrimentally affected surface and ground water resources due to high rates of nitrogen (N) deposition. Reducing the nitrogen-related grey water footprint (GWF) is crucial, otherwise it may lead to severe food production-related water pollution and a reduction in freshwater absorption capacity. This study explored the feasibility of farmland water management policies (WMPs) for GWF regulation in paddy rice fields. Four WMPs—frequent shallow irrigation (FSI), wet and shallow irrigation (WSI), controlled irrigation (CI), and rain-catching and controlled irrigation (RCCI)—were analyzed based on daily field water balance and nitrogen transportation in Nanjing, China. The results showed that GWF varied among the four WMPs. During 2015–2017, the GWF values under FSI, WSI, CI, and RCCI policies were 303.6, 366.4, 369.5, and 218.3 mm, respectively. The GWF under RCCI was the smallest for all the years, that under CI was the highest in the wet year (2016), and that under FSI was the highest in the dry year (2017). GWF increased with an increase in total field water inflow and precipitation. Additionally, changes in the irrigation and drainage methods could regulate and decrease GWF. Pollutant leaching risk increased with rising precipitation, which could be mitigated by reducing drainage in the early stages of growth. In East China, RCCI was observed to be the best policy for managing field water in paddy production with regard to GWF reduction. The findings of this study can aid in agricultural water resource management.

The Ghd7 transcription factor represses ARE1 expression to enhance nitrogen utilization and grain yield in rice

The genetic improvement of nitrogen use efficiency (NUE) of crops is vital for grain productivity and sustainable agriculture. However, the regulatory mechanism of NUE remains largely elusive. Here, we report that the rice Grain number, plant height, and heading date7 (Ghd7) gene genetically acts upstream of ABC1 REPRESSOR1 (ARE1), a negative regulator of NUE, to positively regulate nitrogen utilization. As a transcriptional repressor, Ghd7 directly binds to two Evening Element-like motifs in the promoter and intron 1 of ARE1, likely in a cooperative manner, to repress its expression. Ghd7 and ARE1 display diurnal expression patterns in an inverse oscillation manner, mirroring a regulatory scheme based on these two loci. Analysis of a panel of 2656 rice varieties suggests that the elite alleles of Ghd7 and ARE1 have undergone diversifying selection during breeding. Moreover, the allelic distribution of Ghd7 and ARE1 is associated with the soil nitrogen deposition rate in East Asia and South Asia. Remarkably, the combination of the Ghd7 and ARE1 elite alleles substantially improves NUE and yield performance under nitrogen-limiting conditions. Collectively, these results define a Ghd7–ARE1-based regulatory mechanism of nitrogen utilization, providing useful targets for genetic improvement of rice NUE.

Spatio-Temporal Variation of Atmospheric Nutrient Deposition in Different Land Uses/Covers around Lake Kivu

Atmospheric deposition of nutrients, particularly nitrogen and phosphorus has the capacity to significantly affect the productivity and biogeochemistry of aquatic ecosystems. The objective of this study was to assess the impact of land use/cover types on the atmospheric deposition of nutrients around Lake Kivu. Dry and wet atmospheric deposition samples were collected from four different land use/cover types (forest, wetland, agricultural and urbanized area) at four stations (Goma, Lwiro, Bukavu and Iko) around the Lake Kivu basin. The highest annual loading of dry total phosphorus (TP) was recorded at a station located in an urban area at Goma (4.4 ± 3.9 μmol/m2/yr) and the highest dry deposition of total nitrogen (TN) was recorded at Iko (84.5 ± 41.2 μmol/m2/yr). High wet TP and TN were at Bukavu (0.7 ± 1.1 μmol/m2/yr) and Iko (21.7 ± 34.7 μmol/m2/yr) respectively. High dry TP loads were recorded in the forest area of Goma and the highest dry TN at Lwiro. High wet TP loads were record in agriculture at Goma and high values of wet TN in agriculture at Iko. Phosphorus and nitrogen deposition rates around Lake Kivu were similar to those reported for other African lakes. The highest rate of TP was recorded mainly in the forest area for dry deposition while high TN was recorded in all types of land use/cover in the basin. This study found out that forest traps high concentration of nutrient than other land uses/covers.

Study of titanium nitride film growth by plasma enhanced pulsed laser deposition at different experimental conditions

In this work, we have combined a conventional pulsed laser deposition (PLD) system with a dual radio frequency (RF) discharge to create a plasma-enhanced PLD system, known as (PE-PLD). This system was used to grow titanium nitride (TiN) thin films in order to study the feasibility of obtaining control over the nitrogen concentration and the deposition rate. The characterization of the RF plasma was performed using an RF compensated Langmuir probe, while atomic force microscopy (AFM), optical profilometry, Raman spectroscopy, and x-ray diffraction (XRD) were used to analyze the deposited thin films. We observed a significantly enhanced nitrogen concentration and deposition rate as a function of the RF plasma parameters, substrate temperature, and deposition time. Furthermore, we have observed that, using a dual RF configuration, it becomes possible to modify independently the flow and the bombardment energy of the nitrogen ions that impact the substrate. These means allow the control of the nitrogen concentration and deposition rate.

Low and High Nitrogen Deposition Rates in Northern Coniferous Forests Have Different Impacts on Aboveground Litter Production, Soil Respiration, and Soil Carbon Stocks

Nitrogen (N) deposition can change the carbon (C) sink of northern coniferous forests by changing the balance between net primary production and soil respiration. We used a field experiment in an N poor Pinus sylvestris forest where five levels of N (0, 3, 6, 12, and 50 kg N ha−1 yr−1, n = 6) had been added annually for 12–13 years to investigate how litter C inputs and soil respiration, divided into its autotrophic and heterotrophic sources, respond to different rates of N input, and its subsequent effect on soil C storage. The highest N addition rate (50 kg N ha−1 yr−1) stimulated soil C accumulation in the organic layer by 22.3 kg C kg−1 N added, increased litter inputs by 46%, and decreased soil respiration per mass unit of soil C by 31.2%, mainly by decreasing autotrophic respiration. Lower N addition rates (≤ 12 kg N ha−1 yr−1) had no effect on litter inputs or soil respiration. These results support previous studies reporting on increased litter inputs coupled to impeded soil C mineralization, contributing to enhancing the soil C sink when N is supplied at high rates, but add observations for lower N addition rates more realistic for N deposition. In doing so, we show that litter production in N poor northern coniferous forests can be relatively unresponsive to low N deposition levels, that stimulation of microbial activity at low N additions is unlikely to reduce the soil C sink, and that high levels of N deposition enhance the soil C sink by increasing litter inputs and decreasing soil respiration.

Characteristics, sources and environmental implications of atmospheric wet nitrogen and sulfur deposition in Yangtze River Delta

Nitrogen (N) and sulfur (S) deposition rates have changed rapidly in China during recent years. But few studies have simultaneously measured wet N and S deposition rates on different land use types in the same region. In present study, 238 precipitation samples were collected between May 2016 and April 2017 using wet-only auto-samplers, and the wet N and S deposition rates were measured at three sites with different land use types (urban, agriculture, and lake) in the Yangtze River Delta (YRD), Eastern China. The annual wet deposition rates of total N (TN), ammonium (NH4+), nitrate (NO3−), dissolved organic N (DON), water-insoluble particulate N (WIPN), and sulfate (SO42−) were 16.87–26.64, 6.38–10.81, 5.92–11.11, 1.43–1.96, 2.23–2.67, and 9.07–22.53 kg N (S) ha−1 yr−1, respectively. The wet N and S deposition rates were in the order: agriculture > urban > lake sites, and the seasonal variations in wet N and S deposition were similar at all sites. DON and WIPN contributed 6.4–11.8% and 10.7–16.2% of the total wet N deposition, respectively. Chemical N fertilization and fossil fuel were important sources of wet N and S deposition. Annual wet N and S deposition are important nutrient sources that stimulate the eutrophication and soil/water acidification in the YRD. To minimize the environmental cost of wet N and S deposition in the YRD, it is critical to investigate the sources of N and S.

Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States.

Atmospheric nitrogen and sulfur pollution increased over much of the United States during the twentieth century from fossil fuel combustion and industrial agriculture. Despite recent declines, nitrogen and sulfur deposition continue to affect many plant communities in the United States, although which species are at risk remains uncertain. We used species composition data from >14,000 survey sites across the contiguous United States to evaluate the association between nitrogen and sulfur deposition and the probability of occurrence for 348 herbaceous species. We found that the probability of occurrence for 70% of species was negatively associated with nitrogen or sulfur deposition somewhere in the contiguous United States (56% for N, 51% for S). Of the species, 15% and 51% potentially decreased at all nitrogen and sulfur deposition rates, respectively, suggesting thresholds below the minimum deposition they receive. Although more species potentially increased than decreased with nitrogen deposition, increasers tended to be introduced and decreasers tended to be higher-value native species. More vulnerable species tended to be shorter with lower tissue nitrogen and magnesium. These relationships constitute predictive equations to estimate critical loads. These results demonstrate that many herbaceous species may be at risk from atmospheric deposition and can inform improvements to air quality policies in the United States and globally.

Precipitation chemistry and deposition at a high-elevation site in the Pacific Northwest United States (1989–2015)

Emissions from fuel combustion and agricultural activities contribute significantly to the continuous atmospheric entrainment of pollutants into protected and vulnerable ecosystems where long-term monitoring is often a challenge. Here, results are presented from a 26-year study (1989–2015) of wet precipitation collected at Paradise Station (1654 m above sea level), Mount Rainier National Park, Washington, USA. Weekly samples were analyzed for pH, conductivity and major anions and cations. While precipitation concentrations for sulfate, nitrate, protons and conductivity peaked in the early 2000s, overall trends decreased by 54%, 46%, 41%, and 37%, respectively. Associated pH values increased from 5.2 to 5.6, and were largely controlled by non-sea-salt contributions of sulfate and neutralizing calcium, potassium and magnesium. Between 1999 and 2015, nitrogen (N) deposition rates from ammonium increased by a factor of 3.6, from 0.27 to 0.96 kg N ha−1 yr−1 (p = 0.02), while nitrate deposition did not change statistically (0.91–0.74 kg N ha−1 yr−1, p = 0.30). Combined, these N sources are reaching reported critical loads of 2.0 kg N ha−1 yr−1. Results indicate that emission regulations focused on stationary sources have effectively decreased apparent acid precipitation, however, increased nitrogen deposition from ammonium may lead to further fertilization and acidification of delicate soils and waters. Continued long-term monitoring is thus imperative to track continued anthropogenic inputs to vulnerable ecosystems.

Elemental and isotopic assessment for Colombian orchids from a montane cloud forest: a baseline for global environmental change

Orchidaceae is the largest family of plants, reaching its maximum diversity in Colombia where 4000 species have been registered. One particular ecosystem with high diversity of orchids is the tropical montane cloud forest characterized by high humidity and low air temperatures. However, due to anthropic pressure such as land use change its area has been reduced. This is not the only anthropic disturbance that can affect the continuity of this forest and orchids, climate change and nitrogen deposition also become threats. By means of elemental composition and isotopic measurements of carbon and nitrogen, we determined the photosynthetic pathway for orchids from a tropical cloud forest in Colombia, for which, we also evaluated the nutritional status of nitrogen and its relationship with atmospheric deposition. We found 46 species from 18 genera; of these, 19 species were epiphytic, 9 lithophytic and 18 terrestrials. The carbon content was not significantly different among the species evaluated, averaging 46.4 ± 0.41% (dry weight). In turn, only one terrestrial orchid had a nitrogen content above 2.2%, averaged 1.4 ± 0.07% for the remaining 45 species. The δ13C that averaged − 28.1 ± 0.4‰ for 45 species was typical of C3 plants, while only one species, whose δ13C of 18.8‰ can be considered CAM. The δ15N values for 44 orchids were negative, in total they ranged from − 8.1 to 4.5‰. It can be concluded that the temperatures in this area are not yet high enough to produce a noticeable change in photosynthesis as evidenced by the carbon content of orchids, however, the expected rise would bring harmful consequences for these populations of orchids, especially when considering that these orchids were collected at the highest altitudinal range of the region. Moreover, the rates of nitrogen deposition are not high enough yet to leave a significant signal in the vegetation.

Nitrogen-fixing trees could exacerbate climate change under elevated nitrogen deposition

Biological nitrogen fixation can fuel CO2 sequestration by forests but can also stimulate soil emissions of nitrous oxide (N2O), a potent greenhouse gas. Here we use a theoretical model to suggest that symbiotic nitrogen-fixing trees could either mitigate (CO2 sequestration outweighs soil N2O emissions) or exacerbate (vice versa) climate change relative to non-fixing trees, depending on their nitrogen fixation strategy (the degree to which they regulate nitrogen fixation to balance nitrogen supply and demand) and on nitrogen deposition. The model posits that nitrogen-fixing trees could exacerbate climate change globally relative to non-fixing trees by the radiative equivalent of 0.77 Pg C yr−1 under nitrogen deposition rates projected for 2030. This value is highly uncertain, but its magnitude suggests that this subject requires further study and that improving the representation of biological nitrogen fixation in climate models could substantially decrease estimates of the extent to which forests will mitigate climate change.

The Growth and N Retention of Two Annual Desert Plants Varied Under Different Nitrogen Deposition Rates.

Nitrogen (N) partitioning between plant and soil pools is closely related to biomass accumulation and allocation, and is of great importance for quantifying the biomass dynamics and N fluxes of ecosystems, especially in low N-availability desert ecosystems. However, partitioning can differ among species even when growing in the same habitat. To better understand the variation of plant biomass allocation and N retention within ephemeral and annual species we studied the responses of Malcolmia Africana (an ephemeral) and Salsola affinis (an annual) to N addition, including plant growth, N retention by the plant and soil, and N lost to the environment using 15N (double-labeled 15NH415NO3 (5.16% abundance) added at 0, 0.8, 1.6, 3.2, and 6.4 g pot-1, equivalent to 0, 15, 30, 60, and 120 kg N ha-1) in a pot experiment. Higher N addition (N120) inhibited plant growth and biomass accumulation of the ephemeral but not the annual. In addition, the aboveground:belowground partitioning of N (the R:S ratio) of the ephemeral decreased with increasing N addition, but that of the annual increased. The N input corresponding to maximum biomass and 15N retention of the ephemeral was significantly less than that of the annual. The aboveground and belowground retention of N in the ephemeral were significantly less than those of the annual, except at low N rates. The average plant–soil system recovery of added 15N by the ephemeral was 70%, significantly higher than that of the annual with an average of 50%. Although the whole plant–soil 15N recovery of this desert ecosystem decreased with increasing N deposition, our results suggested that it may vary with species composition and community change under future climate and elevated N deposition.

Quantitative analysis of impact factors on net primary productivity of Tahe forest based on InTEC model.

Net primary productivity (NPP) is an important character reflecting forest carbon source/sink capability, with the spatial-temporal change of which being affected by meteorological change (air temperature, precipitation, etc.), changes in atmospheric composition (CO2 concentration, nitrogen deposition) and various disturbances. However, the key factor driving the changes of forest NPP is still unclear. In order to explore this issue, the relationship between NPP and forest age under different site class index (SCI) was re-simulated and embedded the forest fire data in 1987-2015 to simulate the change characteristics of average NPP of Tahe forest from 1901 to 2015 by comprehensively considering the disturbance and non-disturbance factors of InTEC model, nine scenarios were considered to quantitatively analyze the contribution of different influence factors to the changes of NPP in Tahe forest from 1961 to 2015, and to explore the main influencing factors of the interannual and chronological changes of the Tahe forest NPP to provide a guiding strategy for forest management. The results showed that NPP in Tahe forest was relatively stable from 1901 to 1960, but had a significant trend with the disturbance factors after 1960. The embedding of forest fire and SCI data had different effects on the distribution characteristics of NPP at different times. After 1960, the main reason for the significant change of NPP in Tahe forest was stand age and fire disturbance, with an average annual contribution rate of -49%, followed by precipitation and CO2, which were -28% and 17% respectively. The average contribution rates of temperature and nitrogen deposition were only 5% and 1%, respectively.

Soil solution, foliar concentrations and tree growth response to 8 years of ammonium-nitrate additions in two boreal forests of Quebec, Canada

The boreal forest is recognised as one of the world’s most N-limited ecosystems. Increased N deposition is thought to stimulate boreal forest growth and carbon sequestration. However, evidence is lacking about the impact of increasing N supply over long time scales. Moreover, N deposition in forest ecosystems may cause N saturation, which leads to soil cation depletion and, ultimately, affects tree health. Ammonium nitrate (NH4NO3) was applied during an 8-year period in two stands in the boreal forest in Québec (Canada): one composed of balsam fir (Abies balsamea [Linné] Miller), and the other, of black spruce (Picea mariana [Mill.] B.S.P.). Application rates were 3 and 10 times the atmospheric nitrogen (N) deposition rate measured at each site, which was 6 kg ha−1 year−1 at the balsam fir site and 3 kg ha−1 year−1 at the black spruce site. Soil solution (at depths of 30 and 60 cm), mosses, soil and foliar chemistry as well as tree growth were analyzed. After 8 years of N additions, no signs of N saturation were observed in the soil solution of either site, and no treatment response was observed at the balsam fir site. By contrast, in the treated plots at the black spruce site, N concentration increased in mosses and tree needles. Tree basal areal increment also increased by 21% compared with the control. The different responses between the two forest types could be ascribed to the fact that the black spruce site is located further north and receives less N deposition than the balsam fir site. The tree growth increase at the black spruce site (concomitant with foliar N increase) suggests that C sequestration in tree biomass is more likely to increase in areas having soils with high C:N ratio and receiving low N deposition.

Global Estimates of Inorganic Nitrogen Deposition Across Four Decades

AbstractAtmospheric deposition of inorganic nitrogen is critical to the function of ecosystems and elemental cycles. During the industrial period, humans have doubled the amount of inorganic nitrogen in the biosphere and radically altered rates of atmospheric nitrogen deposition. Despite this rapid change, estimates of global nitrogen deposition patterns generally have low, centennial‐scale temporal resolution. Lack of information on annual‐ to decadal‐scale changes in global nitrogen deposition makes it difficult for scientists researching questions on these finer timescales to contextualize their work within the global nitrogen cycle. Here we use the GEOS‐Chem Chemical Transport Model to estimate wet and dry deposition of inorganic nitrogen globally at a spatial resolution of 2° × 2.5° for 12 individual years in the period from 1984 to 2016. During this time, we found an 8% increase in global inorganic nitrogen deposition from 86.6 to 93.6 TgN/year, a trend that comprised a balance of variable regional patterns. For example, inorganic nitrogen deposition increased in areas including East Asia and Southern Brazil, while inorganic nitrogen deposition declined in areas including Europe. Further, we found a global increase in the percentage of inorganic nitrogen deposited in chemically reduced forms from 30% to 35%, and this trend was largely driven by strong regional increases in the proportion of chemically reduced nitrogen deposited over the United States. This study provides spatially explicit estimates of inorganic nitrogen deposition over the last four decades and improves our understanding of short‐term human impacts on the global nitrogen cycle.

Divergent responses of nitrous oxide, methane and carbon dioxide exchange to pulses of nitrogen addition in a desert in Central Asia

Desert ecosystems are nitrogen-limited, and highly responsive to even small inputs of nitrogen. Recently, desert ecosystems have been affected by increasing levels of nitrogen deposition, which may lead to changes in greenhouse gas efflux. However, the effects of nitrogen deposition on these gases in the desert ecosystems remain poorly understood. To study the dynamics in greenhouse gases under increasing nitrogen, six rates of simulated nitrogen deposition, including 0(N0), 0.5(N0.5), 1.0(N1), 3.0(N3), 6.0(N6), 24.0 (N24) g nitrogen m−2 a−1, were applied in soils of the Gurbantunggut Desert in Central Asia. Efflux of nitrous oxide (N2O), methane (CH4), ecosystem respiration and net carbon ecosystem exchange were measured across two growing seasons for two years. The efflux of the four gases changed greatly in different seasons, which can explain more variation than nitrogen treatments except for N2O. Nitrogen input stimulated N2O emission, especially in the high nitrogen treatments, with N2O emission 1.6 (N6) and 2.6 (N24) times greater than control (N0). However, N2O exchange also depended on the seasons, while only minor changes were found in some seasons (late spring 2010 and late summer 2011). Overall, CH4 uptake (average 31.4 μg m−2 h−1) was not significantly affected by the nitrogen addition. At the high nitrogen treatment levels, ecosystem respiration decreased in the late spring and summer of the second year, with similar responses to nitrogen addition in mid-autumn between two years. Net ecosystem carbon exchange also showed gradual responses to nitrogen addition overall, but did not differ significantly across seasons and treatments. Overall, structural equation models showed that the dynamics in N2O were mostly attributed to variations in nitrogen addition, CH4 to soil moisture (or temperature), ecosystem respiration to plant species richness (or density) and net ecosystem carbon exchange to soil moisture (or temperature). Our study indicates that the changes in greenhouse gas emissions caused by nitrogen deposition in short time scale were small in these desert soils.

Responses of soil nutrients and biological characteristics to nitrogen deposition in Hulun Buir Grassland, China.

Studies on effects of nitrogen deposition were mainly focused on temperate grasslands in Inner Mongolia of China. In addition, there are substantial differences between the present simulation methods and the natural nitrogen deposition. A three-year experiment was carried out to compare the effects of simulation methods (common urea and slow-released urea) and nitrogen deposition rates (0, 25, 50, 75, 100, 150, 200 and 300 kg N·hm-2·a-1) on soil nutrients and biological characteristics in Hulun Buir Grassland. We found that simulated nitrogen deposition had significant influences on soil chemical properties, biological properties and enzyme activities. With the increases of nitrogen deposition, soil pH declined with the greatest extent of 0.2 units, while the highest concentrations of total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) increased by 5-7 times and 12%-36%, respectively. There was a decline trend for soil total phosphorus (TP) and organic phosphorus (TOP). Microbial biomass and metabolic activity increased firstly and then decreased. Moderate simulated nitrogen deposition rates significantly increased soil carbon, nitrogen and phosphorus related enzyme activities. Compared to common urea, using slow-released urea to simulate nitrogen deposition decelerate the decline of soil pH and the increase of dissolved nutrients, and smoothed the change of microbial biomass, metabolic activity, and nitrogen hydrolyzed enzyme activities. Overall, the results confirmed that continuous nitrogen input caused the decline of soil pH and the increase of bioavailable carbon and nitrogen, and then changed microbial biomass and activity.

Nutrient dynamics during photodegradation of plant litter in the Sonoran Desert

Arid and semi-arid ecosystems are experiencing increased rates of atmospheric nitrogen (N) deposition, but the ecological fates of excess nutrients in aridlands are unclear given the few studies conducted in deserts compared to mesic ecosystems. Altered decomposition resulting from increased N availability may not be important in arid ecosystems with significant photodegradation, which may be less influenced by nutrient content than biologically-mediated decomposition. Additionally, nutrient dynamics during decomposition have been studied almost exclusively in mesic ecosystems. To understand the potential impacts of N deposition on mass loss and chemical dynamics during aridland decomposition, we assessed N and phosphorus (P) dynamics of decomposing litter in a long-term N + P enrichment experiment in both urban (+N deposition) and outlying areas (-N deposition) of the Sonoran Desert. Litter was decomposed with and without UV radiation for 9 months, measuring mass loss, litter chemistry, and bacterial biomass. UV radiation significantly accelerated mass loss and altered N and P dynamics, and there was an impact of the urban environment and experimental fertilization on nutrient dynamics. Overall, these patterns suggest that the aridland urban environment, where rates of N deposition are elevated, alters nutrient dynamics during decomposition but not the fraction of litter lost to photodegradation.

Seabirds enhance coralreef productivity and functioning in the absence of invasive rats.

Biotic connectivity between ecosystems can provide major transport of organic matter and nutrients, influencing ecosystem structure and productivity1, yet the implications are poorly understood owing to human disruptions of natural flows2. When abundant, seabirds feeding in the open ocean transport large quantities of nutrients onto islands, enhancing the productivity of island fauna and flora3,4. Whether leaching of these nutrients back into the sea influences the productivity, structure and functioning of adjacent coralreef ecosystems is not known. Here we address this question using a rare natural experiment in the Chagos Archipelago, in which some islands are rat-infested and others are rat-free. We found that seabird densities and nitrogen deposition rates are 760 and 251 times higher, respectively, on islands where humans have not introduced rats. Consequently, rat-free islands had substantially higher nitrogen stable isotope (δ15N) values in soils and shrubs, reflecting pelagic nutrient sources. These higher values of δ15N were also apparent in macroalgae, filter-feeding sponges, turf algae and fish on adjacent coral reefs. Herbivorous damselfish on reefs adjacent to the rat-free islands grew faster, and fish communities had higher biomass across trophic feeding groups, with 48% greater overall biomass. Rates of two critical ecosystem functions, grazing and bioerosion, were 3.2 and 3.8 times higher, respectively, adjacent to rat-free islands. Collectively, these results reveal how rat introductions disrupt nutrient flows among pelagic, island and coralreef ecosystems. Thus, rat eradication on oceanic islands should be a high conservation priority as it is likely to benefit terrestrial ecosystems and enhance coralreef productivity and functioning by restoring seabird-derived nutrient subsidies from large areas of ocean.