Experimental Nitrogen Deposition Research Articles

Experimental warming and nitrogen deposition in clear‐cuts and forest edges: Assessing impacts on plant communities and tree seedling performance

AbstractQuestionsDo nitrogen deposition and climate warming affect tree seedlings and plant communities in different habitats? In these habitats, how do microenvironments, including soil properties and, when applicable, edge effects relate to plant performance?LocationThe University of British Columbia Malcolm Knapp Research Forest, Maple Ridge, British Columbia, Canada.MethodsWe assessed performance of Pseudotsuga menziesii (Mirb.) Franco, Thuja plicata Donn ex D. Don and Tsuga heterophylla (Raf.) Sarg. planted tree seedlings and associated vascular plant communities. Performance of tree seedlings (height) and under vascular plant communities (percent cover) were examined in an experiment with warming (open‐top chambers) and nitrogen deposition (ammonium nitrate applied at 10 kg N ha−1 year−1) treatments applied to subplots in six forest edges and six clear‐cuts (N = 298 total subplots).ResultsIn clear‐cuts, tree seedling height increased with experimental warming and differed among species, and vascular plant cover increased with warming. In clear‐cuts, species identity, not soil variables, was a strong predictor of height, and plant cover was negatively related to pH. In forest edges, edge position and some soil variables were related to height, but not plant cover. There were no interaction effects found between experimental nitrogen deposition and warming.ConclusionsOur results suggest that moderate warming can enhance tree seedling height and understorey vascular plant cover in clear‐cuts in Pacific Northwest coastal ecosystems, but low nitrogen deposition may not have an effect alone or synergistically with warming.

Resistance of soil bacterial communities from montane heathland ecosystems in the Cantabrian mountains (NW Spain) to a gradient of experimental nitrogen deposition

Elevated atmospheric nitrogen (N) deposition on terrestrial ecosystems has become one of the most important drivers of microbial diversity loss on a global scale, and has been reported to alter the soil function of nutrient-poor, montane Calluna vulgaris heathlands in the context of global change. In this work we analyze for the first time the shifts of bacterial communities in response to experimental addition of N in Calluna heathlands as a simulation of atmospheric deposition. Specifically, we evaluated the effects of five N addition treatments (0, 10, 20, and 50 kg N ha−1 yr−1 for 3-years; and 56 kg N ha−1 yr−1 for 10-years) on the resistance of soil bacterial communities as determined by changes in their composition and alpha and beta diversities. The study was conducted in montane Calluna heathlands at different development stages (young and mature phases) in the southern side of the Cantabrian Mountains (NW Spain). Our results evidenced a substantial increase of long-term (10-years) N inputs on soil extractable N-NH4+, particularly in young Calluna stands. The alpha diversity of soil bacterial communities in mature Calluna stands did not show a significant response to experimental N addition, whereas it was significantly higher under long-term chronic N addition (56 kg N ha−1 yr−1 for 10-years) in young Calluna stands. These bacterial community shifts are mainly attributable to a decrease in the dominance of Acidobacteria phylum, the most representative in montane Calluna ecosystems, in favor of copiotrophic taxa such as Actinobacteria or Proteobacteria phyla, favored under increased N availability. Future research should investigate what specific ecosystem functions performed by soil bacterial communities may be sensitive to increased nitrogen depositions, which may have substantial implications for the understanding of montane Calluna ecosystems' stability.

Effects of Experimental Nitrogen Deposition and Litter Manipulation on Soil Organic Components and Enzyme Activity of Latosol in Tropical Rubber Plantations

It is of great scientific significance in regulating plantation ecosystem restoration to investigate the effects of the nitrogen (N) deposition and litter manipulation on soil organic carbon components and enzyme activities. A micro-plot experiment was conducted with four nitrogen additions[CK (0 kg·hm-2·a-1, calculated by N), LN (50 kg·hm-2·a-1), MN (100 kg·hm-2·a-1), and HN (200 kg·hm-2·a-1)] and two litter treatments[LR (litter removal) and L (litter retained)] for tropical rubber plantations in western Hainan Island. The soil physico-chemical properties, soil organic carbon components, and enzyme activities in 0-10 cm and 10-20 cm depths were analyzed. The results showed that soil pH significantly decreased with elevated N addition and litter removal. The contents of NO3--N and NH4+-N significantly increased with elevated N addition. Moreover, there was a significant interaction between N addition and litter treatment on the contents of NO3--N and NH4+-N (P < 0.05). Compared to that with L, LR reduced SOC and its component contents; particularly, the largest decrease was in LFOC by 29.0%-81.4% in the 0-10 cm depth and 23.5%-58.4% in 10-20 cm, respectively. The contents of SOC and its components presented a trend of increasing first and then decreasing with elevated N addition irrespective of litter treatment, and those contents were significantly higher at LN than those at HN. There was a significant interaction between N addition and litter treatment on SOC, LFOC (0-10 cm), and HFOC contents. Compared with that under L, PPO activity was significantly reduced at LR under CK and LN but was significantly increased at LR under MN and HN, respectively. Variance analysis showed significant interactive effects between N addition and litter treatment on PPO and CBH (0-10 cm) activities, and the soil enzyme activity (BG, PPO, and CBH) responding to N addition was greater than that to the litter treatment. Pearson correlation analysis showed that SOC content was extremely positively correlated with MBC, POC, LFOC, and HFOC contents. To summarize, litter retained combined with low N deposition played an important synergistic role of improving SOC pool and soil enzyme activities for tropical rubber plantation systems.

Nitrogen deposition mediates more stochastic processes in structuring plant community than soil microbial community in the Eurasian steppe.

Anthropogenic environmental changes may affect community assembly through mediating both deterministic (e.g., competitive exclusion and environmental filtering) and stochastic processes (e.g., birth/death and dispersal/colonization). It is traditionally thought that environmental changes have a larger mediation effect on stochastic processes in structuring soil microbial community than aboveground plant community; however, this hypothesis remains largely untested. Here we report an unexpected pattern that nitrogen (N) deposition has a larger mediation effect on stochastic processes in structuring plant community than soil microbial community (those <2 mm in diameter, including archaea, bacteria, fungi, and protists) in the Eurasian steppe. We performed a ten-year nitrogen deposition experiment in a semiarid grassland ecosystem in Inner Mongolia, manipulating nine rates (0-50 g N m-2 per year) at two frequencies (nitrogen added twice or 12 times per year) under two grassland management strategies (fencing or mowing). We separated the compositional variation of plant and soil microbial communities caused by each treatment into the deterministic and stochastic components with a recently-developed method. As nitrogen addition rate increased, the relative importance of stochastic component of plant community first increased and then decreased, while that of soil microbial community first decreased and then increased. On the whole, the relative importance of stochastic component was significantly larger in plant community (0.552±0.035; mean±standard error) than in microbial community (0.427±0.035). Consistently, the proportion of compositional variation explained by the deterministic soil and community indices was smaller for plant community (0.172-0.186) than microbial community (0.240-0.767). Meanwhile, as nitrogen addition rate increased, the linkage between plant and microbial community composition first became weaker and then became stronger. The larger stochasticity in plant community relative to microbial community assembly suggested that more stochastic strategies (e.g., seeds addition) should be adopted to maintain above- than below-ground biodiversity under the pressure of nitrogen deposition.

Effects of Simulated Nitrogen Deposition on Soil Microbial Community Structure in Temperate Forest Based on PLFA Method

Soil microorganisms play a crucial role in the biogeochemical cycling of terrestrial ecosystems. However, previous studies on the effects of nitrogen deposition on microorganisms have primarily focused on nitrogen-sensitive tropical forest ecosystems. This study focused on soil in a temperate Korean pine plantation and conducted a field simulated nitrogen deposition experiment. The effects of different nitrogen application rates on the microbial community structure were analyzed using the phospholipid fatty acid (PLFA) method. The experiment included four nitrogen application rates: control (CK: 0 kg N ha−1 yr−1), low nitrogen treatment (LN: 20 kg N ha−1 yr−1), medium nitrogen treatment (MN: 40 kg N ha−1 yr−1), and high nitrogen treatment (HN: 80 kg N ha−1 yr−1). After seven years of continuous application of ammonium nitrate solution, the soil microbial community structure was determined using the PLFA method. The results showed that all nitrogen application rates significantly reduced the PLFA concentration of fungi and AM fungi (P &lt; 0.05), while bacterial biomass significantly decreased in the high nitrogen treatment group. The biomass of Gram-positive bacteria, Gram-negative bacteria (G-), and actinomycetes also significantly decreased in the HN treatment group. Furthermore, long-term application of high nitrogen concentration (80 kg N ha−1 yr−1) significantly reduced soil microbial biomass and changed the fungal to bacterial ratio, thus affecting the soil microbial community structure. Redundancy analysis (RDA) of soil microbial and soil chemical properties found that the long-term simulated nitrogen deposition experiment affected the soil microbial community structure by changing the content of soil N, P elements, and soil pH values. In summary, long-term simulated N deposition can negatively affect soil microbial biomass and its community structure, and the main reason for this analysis is related to long-term N application leading to soil acidification and changes in the conversion of soil N and P elements.

Differential responses of phosphorus accumulation and mobilization in Moso bamboo (Phyllostachys edulis (Carrière) J. Houz) seedlings to short-term experimental nitrogen deposition

Key messageShort-term nitrogen (N) deposition stimulates phosphorus (P) demand owing to the growth improvment of Phyllostachys edulis seedlings. Increased N loads led to the acquisition and utilization of sufficient P, while the limitation of P starvation could be alleviated by the higher activity of soil acid phosphatase and P use efficiency rather than P resorption from senescent organs.ContextPlants in most terrestrial ecosystems are usually subjected to natural phosphorus (P) deficiency or surplus by overfertilization associated with increasing global nitrogen (N) deposition. As the widely distributed gramineous plant in Southern China, moso bamboo (Phyllostachys edulis (Carrière) J. Houz) grows fast and it also shows a relatively good growth performance under the variable N and P conditions. However, few studies focus on the special mechanism of P mobilization and utilization of moso bamboo, especially with the N loads.AimsThe objective of this study was to figure out the mechanisms of P mobilization and utilization in P. edulis seedlings under varying levels of soil P and N deposition conditions in the subtropical region of China.MethodsWe grew P. edulis seedlings under 3 experimental N deposition rates (0 (N −), 30 (N +), and 60 (N + +) kg N ha−1·a−1) and 3 levels of soil P (2.99 mg·kg−1, soil available P content under natural conditions, denoted as P1; 20 mg·kg−1, P2; and 40 mg·kg−1, P3). We measured growth traits and analyzed the related P use indices.ResultsDry weight and P accumulation of new leaves and stems increased with increasing N loads under the 3 P treatments, with the positive effects of N deposition being stronger in the P2 and P3 treatments. Compared with N − , N + , and N + + significantly increased P use efficiency (PUE) (+ 15.54% and + 12.47%, respectively) regardless of soil P conditions. The P resorption efficiency showed a decreasing tendency under variable P conditions with increasing N, whereas PUE was further increased. Acid phosphatase (APase) activity and soil available P content were significantly improved by N loads in P1.ConclusionPhyllostachys edulis seedlings showed high growth plasticity and P mobilization mechanisms under different soil P availability with N loads. In general, N addition stimulated P demand of P. edulis seedlings owing to the growth promotion in the short period of experiment. A special P use mechanism in P deficiency conditions was that the limitation of P starvation was alleviated by the higher soil APase activity and PUE instead of P resorption from senescent organs. The long-term effect of N deposition on P mobilization and utilization in P. edulis forests requires further monitoring.

Effects of experimental nitrogen deposition on soil organic carbon storage in Southern California drylands.

Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.

Nitrogen deposition experiment mimicked with NH4NO3 overestimates the effect on soil microbial community composition and functional potential in the Eurasian steppe

BackgroundThe nitrogenous compound deposited from the atmosphere to the soil is complex, but most field experiments mimic nitrogen deposition with the acid NH4NO3 alone. Thus, whether the acid and non-acid nitrogenous compounds have similar effects on biodiversity and ecosystem functions remains understudied. We mimicked nitrogen deposition with acidic NH4NO3 and (NH4)2SO4, and non-acidic urea, slow-released urea and NH4HCO3 in a temperate steppe, and quantified soil microbial taxonomic and functional gene composition with amplicon sequencing and shotgun metagenomics, respectively.ResultsWhile NH4NO3 and (NH4)2SO4 significantly altered the soil microbial taxonomic and functional composition as well as their carbon decomposition potential, the other three compounds had smaller effects.ConclusionOur results suggested that previous nitrogen deposition experiments mimicked with NH4NO3 or (NH4)2SO4 alone may have overestimated the effect on biodiversity and ecosystem functions in the Eurasian steppe and similar ecosystems affected by mainly nonacidic nitrogen deposition.

Effects of simulated nitrogen deposition on the soil microbial community diversity of a Deyeuxia angustifolia wetland in the Sanjiang Plain, Northeastern China

PurposeThe soil microbial community is an important bioactive component of terrestrial ecosystems. Its structural and functional diversity directly affects carbon and nitrogen processes. This study aimed to investigate the variations in the diversity and composition of soil bacterial communities in a wetland with different nitrogen deposition conditions.MethodsA long-term simulated nitrogen deposition experiment was conducted in the Ecological Locating Research Station of the Institute of Nature and Ecology of Heilongjiang Academy of Sciences. Three different treatments were evaluated, including low nitrogen (LK; 40 kg N·hm-2·a-1), high nitrogen (HN; 80 kg N·hm-2·a-1), and control (CK; 0 kg N·hm-2·a-1). Bacterial 16S rDNA was then sequenced and analyzed using the next-generation sequencing technology.ResultHigher levels of N deposition resulted in an α-diversity increase followed by a decrease, with significant reductions in the HN treatment. Simulated nitrogen deposition resulted in changes in the structure and abundance of bacterial communities in wetland soils. The dominant phyla in all three plots were Proteobacteria and Acidobacteria. Compared with CK, the relative abundance of Chloroflexi increased significantly under the HN treatment (P < 0.05), whereas the relative abundance of Firmicutes and Bacteroidetes decreased significantly (P < 0.05). Nitrogen input changed the composition and relative abundance of the bacterial community, which was possibly due to N-induced soil acidification.ConclusionThis study thus provides a theoretical basis for predicting the effects of atmospheric nitrogen deposition on soil microorganisms, as well as changes in the wetland ecosystem in Sanjiang Plain.

Changes in soil enzymatic activity in a P-limited Mediterranean shrubland subject to experimental nitrogen deposition

N deposition is a key driver of global environmental change (GEC). The effects of N addition on soil enzymes which play a key role in biogeochemical cycles and functioning of terrestrial ecosystems remain unclear, especially in vulnerable ecosystems such as P- or/and water-limited Mediterranean ecosystems. Here, we studied seasonal effects of N deposition on the activities of five soil extracellular enzymes in a semi-arid, P-deficient Mediterranean shrubland, and the association with physical and nutritional soil variables. Over the study period N deposition caused direct effects on soil enzymatic activities due to N-accumulation that increased its availability and triggered ecosystem mobilization of less available nutrients, such as P, in accordance with optimum allocation theory. N deposition intensified P limitation, enhanced the activity of phosphatases and reduced the activity of N- and C-mobilizing enzymes; it is likely this could lead to a longer term increase in soil C storage, due to a reduction in soil organic matter decomposition. The prompt response of soil enzyme activity to experimental N deposition indicates the high vulnerability of soil biogeochemical processes in this Mediterranean shrubland if submitted to prolonged N deposition.

Plastic responses of fine root morphology and architecture traits to nitrogen addition in ectomycorrhizal and arbuscular mycorrhizal tree species in an evergreen broadleaved forest.

We measured the morphology traits (specific root length, specific root surface area, root tissue density, average root diameter) and architecture traits (root fork, root fork ratio, increase rate of root length, root tip density, root fork density) of fine roots in two mycorrhiza tree species, Castanopsis faberi (ectomycorrhizal) and Schima superba (arbuscular mycorrhizal), in an evergreen broadleaved forest in the middle subtropical zone. Root bags method was used in an in situ nitrogen deposition experiment. The aim of this study was to reveal the differences in the plastic responses of fine root morphology and architecture traits to nitrogen deposition between the different mycorrhizal trees. The plastic responses of specific root length, specific root surface area and root fork to nitrogen addition decreased from the first-order root to the fourth-order root, while root tissue density showed an opposite pattern. Such a result indicated a trade-off between nutrient acquisition and resource maintenance of different fine root orders. Different mycorrhizal tree species adopted diffe-rent adaptation strategies to the variations of soil nitrogen availability. C. faberi adopted an opportuni-stic strategy, which relied on fine root to improve nutrient absorption efficiency, enhanced the capacity of space expansion and in-situ nutrient absorption to focus on rapid nutrient absorption strategy. S. superba did not change fine root morphological traits through the trade-off between nutrient absorption efficiency and root construction cost, but relied more on the complementarity between mycorrhizal fungi and fine root architecture traits for nutrient acquisition. The differences in the cost of maintaining and constructing fine root C between different mycorrhizal trees led to fine root adopting the most suitable nutrient capture strategy.

Muted responses to chronic experimental nitrogen deposition on the Colorado Plateau.

Anthropogenic nitrogen (N) deposition is significantly altering both community structure and ecosystem processes in terrestrial ecosystems across the globe. However, our understanding of the consequences of N deposition in dryland systems remains relatively poor, despite evidence that drylands may be particularly vulnerable to increasing N inputs. In this study, we investigated the influence of 7 years of multiple levels of simulated N deposition (0, 2, 5, and 8kgNha-1year-1) on plant community structure and biological soil crust (biocrust) cover at three semi-arid grassland sites spanning a soil texture gradient. Biocrusts are a surface community of mosses, lichens, cyanobacteria, and/or algae, and have been shown to be sensitive to N inputs. We hypothesized that N additions would decrease plant diversity, increase abundance of the invasive annual grass Bromus tectorum, and decrease biocrust cover. Contrary to our expectations, we found that N additions did not affect plant diversity or B. tectorum abundance. In partial support of our hypotheses, N additions negatively affected biocrust cover in some years, perhaps driven in part by inter-annual differences in precipitation. Soil inorganic N concentrations showed rapid but ephemeral responses to N additions and plant foliar N concentrations showed no response, indicating that the magnitude of plant and biocrust responses to N fertilization may be buffered by endogenous N cycling. More work is needed to determine N critical load thresholds for plant community and biocrust dynamics in semi-arid systems and the factors that determine the fate of N inputs.

Plant invasion impacts on fungal community structure and function depend on soil warming and nitrogen enrichment

The impacts of invasive species on biodiversity may be mitigated or exacerbated by abiotic environmental changes. Invasive plants can restructure soil fungal communities with important implications for native biodiversity and nutrient cycling, yet fungal responses to invasion may depend on numerous anthropogenic stressors. In this study, we experimentally invaded a long-term soil warming and simulated nitrogen deposition experiment with the widespread invasive plant Alliaria petiolata (garlic mustard) and tested the responses of soil fungal communities to invasion, abiotic factors, and their interaction. We focused on the phytotoxic garlic mustard because it suppresses native mycorrhizae across forests of North America. We found that invasion in combination with warming, but not under ambient conditions or elevated nitrogen, significantly reduced soil fungal biomass and ectomycorrhizal relative abundances and increased relative abundances of general soil saprotrophs and fungal genes encoding for hydrolytic enzymes. These results suggest that warming potentially exacerbates fungal responses to plant invasion. Soils collected from uninvaded and invaded plots across eight forests spanning a 4 °C temperature gradient further demonstrated that the magnitude of fungal responses to invasion was positively correlated with mean annual temperature. Our study is one of the first empirical tests to show that the impacts of invasion on fungal communities depends on additional anthropogenic pressures and were greater in concert with warming than under elevated nitrogen or ambient conditions.

Potential soil extracellular enzyme activity is altered by long-term experimental nitrogen deposition in semiarid shrublands

Semi-arid shrublands in southern California are exposed to high levels of atmospheric dry nitrogen (N) deposition, which can alter soil microbial activity, and thus, ecosystem carbon (C) and N cycling and storage. We used archived soil samples to assess the relative effects of experimental N enrichment on the potential activity of key C, N, and phosphorus (P) cycling enzymes in chaparral and coastal sage scrub (CSS) shrublands over a 14 year period. Chaparral plots exposed to N exhibited a significant decline in β-glucosidase and phosphatase activity and an increase in peroxidase activity over time, while N exposure had no consistent effect on N-acetylglucosaminidase (NAGase) activity. In contrast, N exposure did not significantly affect the activity of any of the measured enzymes in CSS. However, ANCOVA results revealed that the potential response of some enzymes to cumulative N input was affected by annual rainfall, because significantly more N accumulated in the soil during times of low rainfall. These results indicate that chronic dry season N inputs to semi-arid shrublands can fundamentally alter potential enzyme activity, but the impact depends on climate and vegetation type.

Profile of foliar isoprenoid emissions from Mediterranean dominant shrub and tree species under experimental nitrogen deposition

Biogenic volatile organic compounds play important roles in atmospheric chemistry, and their emissions can be greatly influenced by the variations in environmental conditions and physiological activities caused by continuously increasing global nitrogen (N) deposition. However, this influence is still poorly understood, especially in a natural ecosystem. We conducted a one-year (2015–2016) experiment adding N deposition (60 kg N ha−1) with fertilization to a Mediterranean shrubland dominated by Erica multiflora and a Mediterranean forest dominated by Quercus ilex and compared the seasonal and daytime photosynthetic rates (A), stomatal conductances (gs) and rates of isoprenoid emission with control (2015–2016) and pre-treatment (2014–2015) plots. N fertilization increased A in warm weather as soil moisture increased, and assimilation became saturated when the environment was sufficiently favorable, and excess soil N significantly restrained A in cold weather. The plants were much more sensitive to soil water availability than N content and terpene emissions increased synergistically due to heat and drought stress in hot weather. N fertilization did not significantly affect isoprene emission but significantly increased total terpene emissions and decreased the diversity of terpenes. Our results suggest a successful acclimation of plants by emitting more isoprenoids under environmental stress and that N deposition will further stimulate emissions as the Mediterranean region becomes warmer and drier. The results highlight the necessity for predicting the most realistic future of ecosystems under global environmental change and for assessing the impacts of multiple factors acting in concert on plant physiological and ecosystem functioning including biogenic VOC emissions.

Nitrogen addition interacted with salinity-alkalinity to modify plant diversity, microbial PLFAs and soil coupled elements: A 5-year experiment

Atmospheric nitrogen (N) deposition has greatly increased in the past 50 years due to fossil fuel combustion and the use of synthetic fertilizers. Because N is the limiting nutrient for plant growth in terrestrial ecosystems, atmospheric deposition of N has the potential to increase biomass production, reduce species richness, inhibit microbial growth, and result in soil acidification. However, few studies have evaluated whether and how N inputs affect plant species diversity and the interactions among soil elements in saline-alkaline grassland ecosystems. The Songnen grassland of China is characterized by heterogeneous patches of saline-alkaline soils. A 5-year nitrogen deposition experiment was conducted in two contrasting saline-alkaline communities. Results showed that increasing N input increased the plant biomass production and reduced plant species richness in the low saline-alkaline stress community, but salinity-alkalinity was able to alleviate the negative effects of nitrogen input on community diversity. Plant communities were more sensitive to N addition than microbial communities. N addition induced a shift from N to P as the limiting factor in temperate grassland ecosystems. In particular, salinity-alkalinity in the high stress community increased the conversion rate from N to P limitation. These results highlight the importance of the integrative study of the plant-soil-microbe system in ecosystems under global N enrichment. Our study emphasizes the need for future research to understand the mechanisms that control biogeochemical cycling and maintain ecosystem stability under the combined conditions of N addition and other stress factors, such as saline-alkalinity.

Responses of greenhouse gas emission to simulated nitrogen deposition in Calamagrostis angustifolia wetlands of Sanjiang Plain, China.

A long-term simulated nitrogen deposition experiment was carried out in Ecological Locating Research Station of the Institute of Nature and Ecology of Heilongjiang Academy of Sciences, with three different treatments including low nitrogen treatment (40 kg N·hm-2·a-1), high nitrogen treatment (80 kg N·hm-2·a-1) and the control (0 kg N·hm-2·a-1). The greenhouse gas emission fluxes were measured using a static box-gas chromatography method, with environmental factors being simultaneously investigated to understand the responses of greenhouse gas emission to the nitrogen deposition in the Calamagrostis angustifolia wetland. The results showed that low and high nitrogen treatments significantly increased the greenhouse gas emission fluxes. The CO2 emission flux increased by 47.5% and 47.9%, the CH4 emission fluxes increased by 76.8% and 110.1%, and the N2O emission fluxes increased by 42.4% and 10.6% in low nitrogen treatment and high nitrogen treatment, respectively. Low nitrogen input changed the seasonal dynamics of N2O emission fluxes but had no significant effect on that of CO2 and CH4 emissions. High nitrogen treatment did not affect the seasonal dynamics of greenhouse gas emissions. Soil temperature significantly positively correlated with CO2 and CH4 emission fluxes. There was no correlation between soil temperature and N2O emission flux because the factors affecting N2O emission were complex.

Nitrous oxide emissions from a peatbog after 13 years of experimental nitrogen deposition

Abstract. Nitrogen deposition was experimentally increased on a Scottish peatbog over a period of 13 years (2002–2015). Nitrogen was applied in three forms, NH3 gas, NH4Cl solution, and NaNO3 solution, at rates ranging from 8 (ambient) to 64 kg N ha−1 yr−1, and higher near the NH3 fumigation source. An automated system was used to apply the nitrogen, such that the deposition was realistic in terms of rates and high frequency of deposition events. We measured the response of nitrous oxide (N2O) flux to the increased nitrogen input. Prior expectations, based on the IPCC default emission factor, were that 1 % of the added nitrogen would be emitted as N2O. In the plots treated with NH4+ and NO3− solution, no response was seen, and there was a tendency for N2O fluxes to be reduced by additional nitrogen, though this was not significant. Areas subjected to high NH3 emitted more N2O than expected, up to 8.5 % of the added nitrogen. Differences in the response are related to the impact of the nitrogen treatments on the vegetation. In the NH4+ and NO3− treatments, all the additional nitrogen is effectively immobilised in the vegetation and top 10 cm of peat. In the NH3 treatment, much of the vegetation was killed off by high doses of NH3, and the nitrogen was presumably more available to denitrifying bacteria. The design of the wet and dry experimental treatments meant that they differed in statistical power, and we are less likely to detect an effect of the NH4+ and NO3− treatments, though they avoid issues of pseudo-replication.

Chronic nitrogen deposition influences the chemical dynamics of leaf litter and fine roots during decomposition

Atmospheric nitrogen deposition induces a forest carbon sink across broad parts of the Northern Hemisphere; this carbon sink may partly result from slower litter decomposition. Although microbial responses to experimental nitrogen deposition have been well-studied, evidence linking these microbial responses to changes in the degradation of specific compounds in decaying litter is sparse. We used wet chemistry and Fourier transform infrared spectroscopy (FTIR) methods to study effects of chronic simulated nitrogen deposition on leaf litter and fine root chemistry during a three-year decomposition experiment at four northern hardwood forests in the north-central USA. Leaf litter and fine roots were highly different in initial chemistry, such as concentrations of acid-insoluble fraction (AIF, or Klason lignin) and condensed tannins (CTs). These initial differences persisted over the course of decomposition. Gravimetrically-defined AIF and lignin/carbohydrate reference IR peak ratios both provide evidence that lignin in fine roots was selectively preserved under simulated nitrogen deposition. Lignin/carbohydrate peak ratios were strongly correlated with AIF, suggesting that AIF is a good predictor of lignin. Because AIF is abundant in fine roots, slower AIF degradation was the major driver of the slower fine root decomposition under nitrogen enrichment, explaining 73.5% of the additional root mass retention. Nitrogen enrichment also slowed the loss of CTs and proteins in fine roots. Nitrogen additions initially slowed the loss of AIF, CTs, and proteins in leaf litter, which was comparatively low in AIF, but these effects disappeared at the later stage and did not affect leaf litter mass loss during the experiment. Our results suggest that decomposition of chemical classes subject to oxidative degradation, such as lignin and CTs, is generally inhibited by nitrogen enrichment; but whether this inhibition eventually slows litter mass loss and leads to organic matter accumulation depends on the initial quantities of these classes in litter.

High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought.

Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion.