Distinct effects of canopy vs understory and organic vs inorganic N deposition on root resource acquisition strategies of subtropical Moso bamboo plants.

Atmospheric nitrogen (N) deposition inevitably alters soil nutrient status, subsequently prompting plants to modify their root morphology (i.e., adopting a do-it-yourself strategy), mycorrhizal symbioses (i.e., outsourcing strategy), and root exudation (i.e., nutrient-mining strategy) linking with resource acquisition. However, how N deposition influences the integrated pattern of these resource-acquisition strategies remains unclear. Furthermore, most studies in forest ecosystems have focused on understory N and inorganic N deposition, neglecting canopy-associated processes (e.g., N interception and assimilation) and the impacts of organic N on root functional traits. In this study, we compared the effects of canopy vs understory, organic vs inorganic N deposition on eight root functional traits of Moso bamboo plants. Our results showed that N deposition significantly decreased arbuscular mycorrhizal fungi (AMF) colonization, altered root exudation rate and root foraging traits (branching intensity, specific root area, and length), but did not influence root tissue density and N concentration. Moreover, the impacts of N deposition on root functional traits varied significantly with deposition approach (canopy vs. understory), form (organic vs. inorganic), and their interaction, showing variations in both intensity and direction (positive/negative). Furthermore, specific root area and length were positively correlated with AMF colonization under canopy N deposition and root exudation rate in understory N deposition. Root trait variation under understory N deposition, but not under canopy N deposition, was classified into the collaboration gradient and the conservation gradient. These findings imply that coordination of nutrient-acquisition strategies dependent on N deposition approach. Overall, this study provides a holistic understanding of the impacts of N deposition on root resource-acquisition strategies. Our results indicate that the evaluation of N deposition on fine roots in forest ecosystems might be biased if N is added understory.

Climate warming impacts chewing Spodoptera litura negatively but sucking Corythucha marmorata positively on native Solidago canadensis.

Insect-plant interactions are among importantly ecological processes, and rapid environmental changes such as temperature and resource fluctuations can disrupt long-standing insect-plant interactions. While individual impacts of climate warming, atmospheric nitrogen (N) deposition, and plant provenance on insect-plant interactions are well studied, their joint effects on insect-plant interactions are less explored in ecologically realistic settings. To this end, we performed five experiments with native and invasive Solidago canadensis populations from home and introduced ranges and two insect herbivores (leaf-chewing Spodoptera litura and sap-sucking Corythucha marmorata) in the context of climate warming and N deposition. We determined leaf defensive traits, feeding preference, and insect growth and development, and quantified the possible associations among climate change, host-plant traits, and insect performance with structural equation modeling. First, native S. canadensis populations experienced higher damage by S. litura but lower damage by C. marmorata than invasive S. canadensis populations in the ambient environment. Second, warming decreased the leaf consumption, growth, and survival of S. litura on native S. canadensis populations, but did not affect these traits on invasive S. canadensis populations; warming increased the number of C. marmorata on native S. canadensis populations via direct facilitation, but decreased that on invasive S. canadensis populations via indirect suppression. Third, N addition enhanced the survival of S. litura on native S. canadensis populations, and its feeding preference and leaf consumption on invasive S. canadensis populations. Finally, warming plus N addition exhibited non-additive effects on insect-plant interactions. Based on these results, we tentatively conclude that climate warming could have contrasting effects on insect-plant interactions depending on host-plant provenance and that the effects of atmospheric N deposition on insects might be relatively weak compared to climate warming. Future studies should focus on the molecular mechanisms underlying these different patterns.

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest.

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1 year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1 year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1 year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Elevated CO2 interacts with nutrient inputs to restructure plant communities in phosphorus-limited grasslands.

Globally pervasive increases in atmospheric CO2 and nitrogen (N) deposition could have substantial effects on plant communities, either directly or mediated by their interactions with soil nutrient limitation. While the direct consequences of N enrichment on plant communities are well documented, potential interactions with rising CO2 and globally widespread phosphorus (P) limitation remain poorly understood. We investigated the consequences of simultaneous elevated CO2 (eCO2 ) and N and P additions on grassland biodiversity, community and functional composition in P-limited grasslands. We exposed soil-turf monoliths from limestone and acidic grasslands that have received >25 years of N additions (3.5 and 14 g m-2 year-1 ) and 11 (limestone) or 25 (acidic) years of P additions (3.5 g m-2 year-1 ) to eCO2 (600 ppm) for 3 years. Across both grasslands, eCO2 , N and P additions significantly changed community composition. Limestone communities were more responsive to eCO2 and saw significant functional shifts resulting from eCO2 -nutrient interactions. Here, legume cover tripled in response to combined eCO2 and P additions, and combined eCO2 and N treatments shifted functional dominance from grasses to sedges. We suggest that eCO2 may disproportionately benefit P acquisition by sedges by subsidising the carbon cost of locally intense root exudation at the expense of co-occurring grasses. In contrast, the functional composition of the acidic grassland was insensitive to eCO2 and its interactions with nutrient additions. Greater diversity of P-acquisition strategies in the limestone grassland, combined with a more functionally even and diverse community, may contribute to the stronger responses compared to the acidic grassland. Our work suggests we may see large changes in the composition and biodiversity of P-limited grasslands in response to eCO2 and its interactions with nutrient loading, particularly where these contain a high diversity of P-acquisition strategies or developmentally young soils with sufficient bioavailable mineral P.

Interactions between nitrogen and phosphorus in modulating soil respiration: A meta-analysis.

BACKGROUND: Economic and social development worldwide increases the input of nutrients, especially nitrogen (N) and phosphorus (P), to soils. These nutrients affect soil respiration (Rs) in terrestrial ecosystems. They may act independently or have interactive effects on Rs. The effect of N and P on Rs and its components (autotrophic respiration [Ra] and heterotrophic respiration [Rh]), however, either individually or together, is poorly understood. We performed a meta-analysis of 130 studies to examine the effects of different fertilization treatments on Rs and its components across terrestrial ecosystems. RESULTS: Our results showed that (1) The impact of fertilizer addition on Rs varies among different fertilizer types. N addition reduced Rs and Rh significantly but did not affect Ra; P addition had no significant effect on Rs, Rh, and Ra; NP addition increased Rs significantly but did not affect Rh and Ra. (2) Ecosystem type, duration of fertilization, fertilization rate, and fertilizer form influenced the response of Rs and its components to fertilizer application. (3) Based on our study, the annual average temperature may be a driving factor of Rs response to fertilizer addition, while soil total nitrogen may be an important predictor of Rs response to fertilizer addition. CONCLUSION: Overall, our study highlights the complex and multifaceted nature of the response of soil Rs and its components to fertilizer application, underscoring the importance of considering multiple factors when predicting and modeling future Rs and its feedback to global change.

Extraradical hyphae alleviate nitrogen deposition-induced phosphorus deficiency in ectomycorrhiza-dominated forests.

The continuous imbalance between nitrogen (N) and phosphorus (P) deposition is expected to shift many ecosystems from N- to P limitation. Extraradical hyphae of ectomycorrhizal (ECM) fungi play important roles in plant nutrient acquisition under nutrient deficiency. However, whether and how ECM hyphae enhance soil P availability to alleviate N-induced P deficiency remains unclear. We investigated the impacts of ECM hyphae on transformations among different soil P fractions and underlying mechanisms under N deposition in two ECM-dominated forests. Ectomycorrhizal hyphae enhanced soil P availability under N addition by stimulating mineralization of organic P (Po) and desorption and solubilization of secondary mineral P, as indicated by N-induced increase in positive hyphal effect on plant-available P pool and negative hyphal effects on Po and secondary mineral P pools. Moreover, ECM hyphae increased soil phosphatase activity and abundance of microbial genes associated with Po mineralization and inorganic P solubilization, while decreasing concentrations of Fe/Al oxides. Our results suggest that ECM hyphae can alleviate N-induced P deficiency in ECM-dominated forests by regulating interactions between microbial and abiotic factors involved in soil P transformations. This advances our understanding of plant acclimation strategies via mediating plant-mycorrhiza interactions to sustain forest production and functional stability under changing environments.

[Effects of Short-term Nitrogen Addition on Soil Organic Carbon Components in Robinia pseudoacacia L. Plantation].

Nitrogen (N) deposition in the context of human activities continuously affects the carbon cycle of ecosystems. The effect of N deposition on soil organic carbon is related to the differential responses of different carbon fractions. To investigate the changes in soil organic carbon fraction and its influencing factors in the context of short-term N deposition, four N addition gradients:0 (CK), 1.5 (N1), 3 (N2), and 6 (N3) g·(m2·a)-1 were set up in acacia plantations based on field N addition experiments, and the soil physicochemical properties, microbial biomass, and enzyme activities were measured in June and September. The results showed that:① exogenous N input reduced soil pH, promoted the increase in soluble organic carbon content, and increased soil nitrogen effectiveness. ② Short-term N addition significantly reduced soil organic carbon content, and the response of each component of organic carbon to N addition was different. Among them, the content of easily oxidized organic carbon was significantly reduced and reached the lowest value under the N2 treatment, with 54.4% and 48.2% reduction compared with that of the control, respectively, and the content of inert organic carbon increased, although the increase was not significant. Nitrogen addition reduced the soil carbon pool activity and improved the stability of the soil carbon pool. Soil carbon pool activity reached its lowest under the N3 and N2 treatments, with a decrease of 53.3% and 52.80%, respectively, compared to that of the control. ③Random forest modeling indicated that the soil microbial biomass stoichiometry ratio, microbial biomass carbon, and AP were the key factors driving the changes in soil organic carbon activity under short-term N addition, explaining 65.96% and 66.68% of the changes in oxidizable organic carbon and inert organic carbon, respectively. Structural equation modeling validated the results of the random forest modeling, and soil microbial biomass stoichiometric ratios significantly influenced carbon pool activity. Short-term nitrogen addition changed soil microbial biomass and its stoichiometric ratio in the acacia plantation forest mainly through two pathways, i.e., increasing soil nitrogen effectiveness and promoting soil acidification and inhibiting extracellular carbon hydrolase activity, thus changing the soil carbon fraction ratio and participating in the soil organic carbon cycling process.

Mechanisms underlying the responses of microbial carbon and nitrogen use efficiencies to nitrogen addition are mediated by topography in a subtropical forest.

Microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE) are key parameters determining the fate of C and N in soils. Atmospheric N deposition has been found to heavily impact multiple soil C and N transformations, but we lack understanding of the responses of CUE and NUE to N deposition, and it remains uncertain whether responses may be mediated by topography. Here, a N addition experiment with three treatment levels (0, 50 and 100 kg N ha-1 yr-1) was conducted in the valley and on the slope of a subtropical karst forest. Nitrogen addition increased microbial CUE and NUE at both topographic positions, but the underlying mechanisms differed. In the valley, the increase in CUE was associated with an increase in soil fungal richness:biomass and lower litter C:N, whereas on the slope, the response was linked with a reduced ratio of dissolved soil organic C (DOC) to available phosphorus (AVP) which reduced respiration, and increased root N:P stoichiometry. In the valley, the increase in NUE was explained by stimulated microbial N growth relative to gross N mineralization, which was associated with increased ratios of soil total dissolved N:AVP and fungal richness:biomass. In contrast, on the slope, the increase in NUE was attributed to reduced gross N mineralization, linked to increased DOC:AVP. Overall, our results highlight how topography-driven soil substrate availability and microbial properties can regulate microbial CUE and NUE.

Response of gene abundance of ammonia-oxidizing microorganisms and denitrifying microorganisms to nitrogen and phosphorus addition in subtropical forest.

We conducted a nitrogen (N) and phosphorus (P) addition experiment in Qianjiangyuan National Park in 2015, to investigate the response of ammonia-oxidizing microorganisms and denitrifying microorganisms. There were four treatments, including N addition (N), P addition (P), NP, and control (CK). Soil samples were collected in April (wet season) and November (dry season) of 2021. The abundance of amoA gene of ammonia-oxidizing microorganisms (i.e., ammonia-oxidizing archaea, AOA; ammonia-oxidizing bacteria, AOB; comammox) and denitrifying microbial genes (i.e., nirS, nirK, and nosZ) were determined using quantitative PCR approach. The results showed that soil pH was significantly decreased by long-term N addition, while soil ammonium and nitrate contents were significantly increased. Soil available P and total P contents were significantly increased with the long-term P addition. The addition of N (N and NP treatments) significantly increased the abundance of AOB-amoA gene in both seasons, and reached the highest in the N treatment around 8.30×107 copies·g-1 dry soil. The abundance of AOA-amoA gene was significantly higher in the NP treatment than that in CK, with the highest value around 1.17×109 copies·g-1 dry soil. There was no significant difference in N-related gene abundances between two seasons except for the abundance of comammox-amoA. Nitrogen addition exerted significant effect on the abundance of AOB-amoA, nirK and nosZ genes, especially in wet season. Phosphorus addition exerted significant effect on the abundance of AOA-amoA and AOB-amoA genes in both seasons, but did not affect denitrifying gene abundances. Soil pH, ammonium, nitrate, available P, and soil water contents were the main factors affecting the abundance of soil N-related functional genes. In summary, the response of soil ammonia-oxidizing microorganisms and denitrifying microorganisms was more sensitive to N addition than to P addition. These findings shed new light for evaluating soil nutrient availability as well as their response mechanism to global change in subtropical forests.

Study of the Pollen Grain Metabolome under Deposition of Nitrogen and Phosphorus in Taxus baccata L. and Juniperus communis L.

Nitrogen plays an important role in both quantitative and qualitative aspects of plant reproduction, including pollen grain compounds and seed production. Recent studies have pointed out that pollen grains produced by male plants of T. baccata and J. communis subjected to a long period of fertilizer supplementation have lower in vitro germination ability and higher nitrogen content. To gain molecular insights into these observations, we conducted GC-MS analysis of both species to characterize the metabolomes of dry, mature pollen grains, which allowed for the identification and quantification of more than 200 metabolites. The results demonstrated that fertilizer supplementation impacts the relative content of 14 metabolites in J. communis (9 downregulated and 5 upregulated) and 21 in T. baccata (6 downregulated and 15 upregulated). Although plants showed little similarity in patterns, in metabolite profiles, both up and down fold-changes were observed. This is the first report on the gymnosperm pollen grain metabolomic profile and changes induced by long-term nitrogen and phosphorus supplementation. Pollen grains produced by fertilizer-supplemented male individuals had significantly lower relative content of linolenic acid, 5,6-dihydrouracil, maltotriose, galactonic acid, D-xylulose, and glycerol-α-phosphate but higher content of sorbitol, glucosamine, and 1,5-anhydro-D-glucitol as well as n-acetyl-d-hexosamine, dimethyl phthalate, glycine, galactose-6-phosphate, D-fructose-6-phosphate, pyroglutamic acid, and 3-(3-hydroxyphenyl)-3-hydroxypropionic acid. Thus, in pollen grain samples earlier shown to have different germination abilities, the presence of different metabolites indicates a significant environmental impact on the quality of gymnosperm pollen grains.

Long-term alfalfa (Medicago sativa L.) establishment could alleviate phosphorus limitation induced by nitrogen deposition in the carbonate soil.

Phosphorus (P) limitation is a widespread problem of primary production in dryland submitted to persistent nitrogen (N) deposition. The legume alfalfa (Medicago sativa L.), which can fix N2, might potentially strengthen P limitation in dryland ecosystems and is widely distributed as forage. However, there is still unclear how alfalfa grassland mobilizes the soil P to meet its demand. In this experiment, alfalfa introduction was used for long-term revegetation to evaluate the P uptake of plants from deep soil and assess the P limitation induced by N deposition compared with fallow. Our results showed that alfalfa introduction increased the soil P storage significantly at 0-2.4 m soil depth (+0.74 Mg ha-1), whereas it decreased at 2.4-4.8 m soil depth (-0.21 Mg ha-1) after 15-year establishment. Alfalfa establishment increased soil organic P concentration (180.9 mg kg-1 vs. 67.2 mg kg-1) and its relative contribution to total P (19.64% vs. 8.08%) at 0-4.8 m. Alfalfa establishment also increased the concentration and proportion of labile and intermediate P fractions at 0-4.8 m (9.12 mg kg-1 vs. 6.87 mg kg-1, 1.12% vs. 0.98%; 16.06 mg kg-1 vs. 8.39 mg kg-1, 1.69% vs. 1.17%). Alfalfa introduction decreased the concentrated HCl-Pi (250.66 mg kg-1 vs. 229.32 mg kg-1, 36.81% vs. 28.91%) in 2.4-4.8 m soil depth. These results indicated that the deep root system of alfalfa grassland could promote the P mobilization from deep to shallow soil. The concentrated HCl-Pi may be the main potential P source of alfalfa from 2.4-4.8 m to 0-2.4 m of soil depth, and long-term establishment of alfalfa can alleviate P limitation caused by N deposition in carbonate soil. Our results suggested that species with deep roots (such as alfalfa) could be selected as an economical way to mitigate nitrogen deposition in drylands.

Differential aboveground-belowground adaptive strategies to alleviate N addition-induced P deficiency in two alpine coniferous forests.

Increasing atmospheric nitrogen (N) deposition has resulted in phosphorus (P) limitation in multiple terrestrial ecosystems, yet how plants coordinate aboveground and belowground strategies to adapt to such P deficiency remains unclear. In this study, we conducted a field N fertilization experiment in two alpine coniferous plantations (Picea asperata Mast. and Pinus armandii Franch.) with different soil N availability on the eastern Tibetan Plateau of China, to examine N addition effects on plant nutrient limiting status and plant adaptive strategies corresponding to aboveground P conservation and belowground P acquisition. The results showed that N addition aggravated P deficiency in both plantations, as indicated by decreased needle P concentrations and increased N:P ratios, and that plant strategies for addressing such P deficiency differed in the two plantations with different initial soil N availabilities. In the P. asperata plantation with relatively high N availability, significantly enhanced needle phosphatase activity and shifts in P fraction allocation (downregulation of the structural P fraction and increased allocation to the residual P fraction) co-occurred with increased rhizosphere effects on phosphatase activity under N addition, indicating a synergistic strategy of aboveground P conservation and belowground P mining to alleviate P deficiency. In the P. armandii plantation with relatively low N availability, however, N addition only enhanced phosphatase activity and increased allocation to residual P fraction in the aboveground but had little effect on belowground P acquisition-associated traits, suggesting a decoupling relationship between aboveground P conservation and belowground P acquisition. This study highlights the vital significance of initial soil nutrient availability in regulating the coordination of aboveground and belowground strategic alternatives, emphasizing the need to integrate soil nutrient conditions for a holistic understanding of forest adaptation to anthropogenic N enrichment.

[Effects of nitrogen addition on the contents and stoichiometric ratio of nitrogen and potassium in a meadow steppe of Hulunbuir, China]. / æ°®ç´ æ·»åŠ å¯¹å‘¼ä¼¦è´å°”è‰ç”¸è‰åŽŸæ¤ç‰©æ°®é’¾å ƒç´ å«é‡å’Œè®¡é‡æ¯”çš„å½±å“.

Potassium (K) is the second most abundant nutrient in plant leaves after nitrogen (N) and the most abundant cation in plant cells. It plays an important role in plant growth regulation, homeostasis maintenance, and stress response. Previous studies on the effects of N input on plant nutrient status mainly focus on N and phosphorus (P), but less on K and its stoichiometry. We examined the effects of N input and mowing on K content and N:K at both plant functional group and community levels. We analyzed the relative contribution of changes in functional groups and community composition to changes of community level nutrition status. The results showed that N input increased N content of each plant functional group and increased K content of rhizomatous grasses and legumes. Mowing reduced N content of rhizomatous grasses and bunchgrass, but did not affect K content and N:K of all functional groups. Nitrogen input significantly increased plant N and K contents at the community level, while mowing significantly increased plant N content. Both N input and mowing did not affect plant N:K at functional group and community levels. The contribution of nutritional changes in plant functional groups to the variation at the community level was greater than that of changes in community composition. For all the three examined nutritional traits, the contribution of nutrients at functional group level and that of community composition showed negative covariation. Our results indicated that plant N:K had high homeostasis in meadow steppe and that plants could regulate N and K balance, which was of great significance for maintaining N:K stoichiometry under the background of increasing N deposition.

The amounts and ratio of nitrogen and phosphorus addition drive the rate of litter decomposition in a subtropical forest.

Nitrogen (N) and phosphorus (P) control biogeochemical cycling in terrestrial ecosystems. However, N and P addition effects on litter decomposition, especially biological pathways in subtropical forests, remain unclear. Here, a two-year field litterbag experiment was employed in a subtropical forest in southwestern China to examine N and P addition effects on litter biological decomposition with nine treatments: low and high N- and P-only addition (LN, HN, LP, and HP), NP coaddition (LNLP, LNHP, HNLP, and HNHP), and a control (CK). The results showed that the decomposition coefficient (k) was higher in NP coaddition treatments (P < 0.05), and lower in N- and P-only addition treatments than in CK (P < 0.05). The highest k was observed with LNLP (P < 0.05). The N- and P-only addition treatments decreased the losses of litter mass, lignin, cellulose, and condensed tannins, litter microbial biomass carbon (MBC), litter cellulase, and soil pH (P < 0.05). The NP coaddition treatments increased the losses of litter mass, lignin, and cellulose, MBC concentration, litter invertase, urease, cellulase, and catalase activities, soil arthropod diversity (S) in litterbags, and soil pH (P < 0.05). Litter acid phosphatase activity and N:P ratio were lower in N-only addition treatments but higher in P-only addition and NP coaddition treatments than in CK (P < 0.05). Structural equation model showed that litter MBC, S, cellulase, acid phosphatase, and polyphenol oxidase contributed to the loss of litter mass (P < 0.05). The litter N:P ratio was negatively logarithmically correlated with mass loss (P < 0.01). In conclusion, the negative effect of N addition on litter decomposition was reversed when P was added by increasing decomposed litter soil arthropod diversity, MBC concentration, and invertase and cellulase activities. Finally, the results highlighted the important role of the N:P ratio in litter decomposition.

Nitrogen enrichment buffers phosphorus limitation by mobilizing mineral-bound soil phosphorus in grasslands.

Phosphorus (P) limitation is expected to increase due to nitrogen (N)-induced terrestrial eutrophication, although most soils contain large P pools immobilized in minerals (Pi ) and organic matter (Po ). Here we assessed whether transformations of these P pools could increase plant available pools alleviating P limitation under enhanced N availability. The mechanisms underlying these possible transformations were explored by combining results from a 10-year field N addition experiment and a 3700-km transect covering wide ranges in soil pH, soil N, aridity, leaching, and weathering that could affect soil P status in grasslands. Nitrogen addition promoted the dissolution of immobile Pi (mainly Ca-bound recalcitrant P) to more available forms of Pi (including Al- and Fe-bound P fractions and Olsen P) by decreasing soil pH from 7.6 to 4.7, but did not affect Po . Soil total P declined by 10% from 385 ± 6.8 to 346 ± 9.5 mg kg-1 , whereas available P increased by 546% from 3.5 ± 0.3 to 22.6 ± 2.4 mg kg-1 after the 10-year N addition, associated with an increase in Pi mobilization, plant uptake, and leaching. Similar to the N addition experiment, the drop in soil pH from 7.5 to 5.6 and increase in soil N concentration along the grassland transect were associated with an increased ratio between relatively mobile Pi and immobile Pi . Our results provide a new mechanistic understanding of the important role of soil Pi mobilization in maintaining plant P supply and accelerating biogeochemical P cycles under anthropogenic N enrichment. This mobilization process temporarily buffers ecosystem P limitation or even causes P eutrophication, but will extensively deplete soil P pools in the long run.

Atmospheric nitrogen enrichment changes nutrient stoichiometry and reduces fungal N supply to peatland ericoid mycorrhizal shrubs.

Peatlands store one third of global soil carbon (C) and up to 15% of global soil nitrogen (N) but often have low plant nutrient availability owing to slow organic matter decomposition under acidic and waterlogged conditions. In rainwater-fed ombrotrophic peatlands, elevated atmospheric N deposition has increased N availability with potential consequences to ecosystem nutrient cycling. Here, we studied how 14 years of continuous N addition with either nitrate or ammonium had affected ericoid mycorrhizal (ERM) shrubs at Whim Bog, Scotland. We examined whether enrichment has influenced foliar nutrient stoichiometry and assessed using N stable isotopes whether potential changes in plant nutrient constraints are linked with plant N uptake through ERM fungi versus direct plant uptake. High doses of ammonium alleviated N deficiency in Calluna vulgaris and Erica tetralix, whereas low doses of ammonium and nitrate improved plant phosphorus (P) nutrition, indicated by the lowered foliar N:P ratios. Root acid phosphatase activities correlated positively with foliar N:P ratios, suggesting enhanced P uptake as a result of improved N nutrition. Elevated foliar δ15N of fertilized shrubs suggested that ERM fungi were less important for N supply with N fertilization. Increases in N availability in peat porewater and in direct nonmycorrhizal N uptake likely have reduced plant nitrogen uptake via mycorrhizal pathways. As the mycorrhizal N uptake correlates with the reciprocal C supply from host plants to the soil, such reduction in ERM activity may affect peat microbial communities and even accelerate C loss via decreased ERM activity and enhanced saprotrophic activity. Our results thus introduce a previously unrecognized mechanism for how anthropogenic N pollution may affect nutrient and carbon cycling within peatland ecosystems.

Spatially divergent trends of nitrogen versus phosphorus limitation across European forests.

Nitrogen (N) and phosphorus (P) are essential nutrients that widely limit plant growth in global terrestrial ecosystems. Rising atmospheric CO2 concentration generally stimulates terrestrial net primary productivity and consequently may cause or aggravate N and P limitation due to a dilution effect, but the spatial variation of temporal trends in N versus P limitation and its key regulating factors is poorly understood. Using the leaf N:P ratio of 15 dominant tree species as an indicator, we analysed the spatial variation of plot-level shift towards N or P limitation across 163 European forest plots during 1995-2017. Phosphorus limitation increased from 25% to 33% of the studied plots between 1995-1997 and 2015-2017, while N limitation occurred in a negligible number of plots. A major proportion (56%) of the plots showed no significant trend in leaf N:P ratio, implying no shifts in N versus P limitation status. In the remaining plots, 38% of the plots showed a significant increase of leaf N:P ratio and only 6% of the plots showed a significant decrease of leaf N:P ratio. The spatial variation in the rate of decrease in leaf N:P ratio was associated with a significant decrease in leaf N concentration and mainly explained by the rate of decrease in N deposition. In contrast, the spatial variation in the rate of increase in leaf N:P ratio was associated with a significant decrease in leaf P concentration and mainly explained by forest category (broadleaf vs. conifer), mean annual temperature and soil C:N ratio. Our findings highlight a remarkable spatial divergence in temporal trends of nutrient limitation status across European forests over the past two decades, but overall, P is becoming more limiting versus N, especially in broadleaved forests.

Ozone impairs the response of isoprene emission to foliar nitrogen and phosphorus in poplar.

Tropospheric ozone (O3) impairs physiological processes of plants while nitrogen (N) deposition may cause imbalances in soil N and other nutrients such as phosphorus (P) suggesting an increase of P demand for plants. However, the combined effect of O3, soil N and P on isoprene emission from leaves has never been tested. We therefore examined isoprene emission in leaves of Oxford poplar clone exposed to O3 (ambient, AA [35.0 nmol mol-1 as daily mean]; 1.5 × AA; 2.0 × AA), soil N (0 and 80 kg N ha-1) and soil P (0, 40 and 80 kg P ha-1) in July and September in a Free-Air Controlled Exposure (FACE) facility. We also investigated the response of isoprene emission to foliar N, P and abscisic acid (ABA) contents in September because the 2-C-methylerythritol-5-phosphate (MEP) pathway of isoprenoid biosynthesis produces ABA. We found that O3 increased isoprene emission in July, which was associated to increased dark respiration, suggesting an activation of metabolism against O3 stress as an initial response. However, O3 decreased isoprene emission in September which was associated to reduced net photosynthesis. In September, isoprene emission was positively correlated with leaf N content and negatively correlated with leaf P content in AA. However, no response of isoprene emission to foliar N and P was found in elevated O3, suggesting that the isoprene responses to foliar N and P depended on the O3 exposure levels. Isoprene emission rate in 1.5 × AA and 2.0 × AA increased with increasing leaf ABA content, indicating accelerated senescence of injured leaves to favor new leaf growth when high O3 and nutritional availability in the soil were combined. Even though foliar N and P usually act as a proxy for isoprene emission rate, the impact of recent abiotic factors such as O3 should be always considered for modeling isoprene emission under climate change.

[Response of Soil Enzyme Activity and Chemical Properties to Nitrogen Addition in a Korean Pine Plantation].

Soil enzymes participate in numerous complex biochemical processes that take place in the soil and play an important role in the material circulation of terrestrial ecosystems. To explore the response of soil enzyme activities and chemical properties to nitrogen deposition in temperate forests, this study analyzed four soil enzyme activities based on the nitrogen addition experiment plot of Korean pine (Pinus koraiensis) plantation, which was located in the Liangshui National Natural Reserve, Heilongjiang Province. The results showed that the activities of N-acetyl-glucosidase (NAG) and alkaline phosphatase (AKP) increased significantly with increasing nitrogen application concentration. The activity of beta-glucosidase (BG) and acid phosphatase (ACP) was not significantly different among different nitrogen application treatments. The contents of total carbon, total nitrogen, total phosphorus, and available nitrogen and four enzyme activity in the upper soil (0-10 cm) under the same nitrogen application level were significantly higher than those in the lower soil (10-20 cm), but the pH values were not significantly different. Total carbon has an extremely significant positive correlation with NAG, BG, AKP, and ACP. Total nitrogen has an obvious or extremely significant positive correlation with BG, NAG, and AKP as well as ACP. The available nitrogen has an obvious and highly significant positive correlation with NAG and AKP. The total phosphorus has an obvious and extremely significant positive correlation with ACP and AKP, respectively. The nitrogen application level and the soil layer had different effects on soil enzyme activity and soil chemical properties. Long-term large input of nitrogen can directly or indirectly change soil chemical properties and affect soil enzyme activity.

Nutrients and warming interact to force mountain lakes into unprecedented ecological states.

While deposition of reactive nitrogen (N) in the twentieth century has been strongly linked to changes in diatom assemblages in high-elevation lakes, pronounced and contemporaneous changes in other algal groups suggest additional drivers. We explored the origin and magnitude of changes in two mountain lakes from the end of the Little Ice Age at ca 1850, to ca 2010, using lake sediments. We found dramatic changes in algal community abundance and composition. While diatoms remain the most abundant photosynthetic organisms, concentrations of diatom pigments decreased while pigments representing chlorophytes increased 200-300% since ca 1950 and total algal biomass more than doubled. Some algal changes began ca 1900 but shifts in most sedimentary proxies accelerated ca 1950 commensurate with many human-caused changes to the Earth System. In addition to N deposition, aeolian dust deposition may have contributed phosphorus. Strong increases in summer air and surface water temperatures since 1983 have direct and indirect consequences for high-elevation ecosystems. Such warming could have directly enhanced nutrient use and primary production. Indirect consequences of warming include enhanced leaching of nutrients from geologic and cryosphere sources, particularly as glaciers ablate. While we infer causal mechanisms, changes in primary producer communities appear to be without historical precedent and are commensurate with the post-1950 acceleration of global change.