Stoichiometry controls asymbiotic nitrogen fixation and its response to nitrogen inputs in a nitrogen-saturated forest

High biological N fixation potential dominated by heterotrophic diazotrophs in alpine permafrost rivers on the Qinghai‒Tibet Plateau

Biological nitrogen (N) fixation is a pivotal N source in N-deficient ecosystems. The Qinghai‒Tibet Plateau (QTP) region, which is assumed to be N limited and suboxic, is an ideal habitat for diazotrophs. However, the diazotrophic communities and associated N fixation rates in these high-altitude alpine permafrost QTP rivers remain largely unknown. Herein, we examined diazotrophic communities in the sediment and biofilm of QTP rivers via the nitrogenase (nifH) gene sequencing and assessed their N fixing activities via a 15N isotope incubation assay. Strikingly, anaerobic heterotrophic diazotrophs, such as sulfate- and iron-reducing bacteria, had emerged as dominant N fixers. Remarkably, the nifH gene abundance and N fixation rates increased with altitude, and the average nifH gene abundance (2.57 ± 2.60 × 108 copies g-1) and N fixation rate (2.29 ± 3.36 nmol N g-1d-1) surpassed that documented in most aquatic environments (nifH gene abundance: 1.31 × 105 ∼ 2.57 × 108 copies g-1, nitrogen fixation rates: 2.34 × 10-4 ∼ 4.11 nmol N g-1d-1). Such distinctive heterotrophic diazotrophic communities and high N fixation potential in QTP rivers were associated with low-nitrogen, abundant organic carbon and unique C:N:P stoichiometries. Additionally, the significant presence of psychrophilic bacteria within the diazotrophic communities, along with the enhanced stability and complexity of the diazotrophic networks at higher altitudes, clearly demonstrate the adaptability of diazotrophic communities to extreme cold and high-altitude conditions in QTP rivers. We further determined that altitude, coupled with organic carbon and phosphorus, was the predominant driver shaping diazotrophic communities and their N-fixing activities. Overall, our study reveals high N fixation potential in N-deficient QTP rivers, which provides novel insights into nitrogen dynamics in alpine permafrost rivers.

Soil diazotrophs sustain nitrogen fixation under high nitrogen enrichment via adjustment of community composition

Biological nitrogen (N) fixation, an important pathway of N, inputs from the atmosphere to Earth's ecosystems, is well demonstrated to decline under N input. However, it remains unclear why N fixers sustain N fixation in many forests under high atmospheric N deposition. To address this knowledge gap, we analyzed the response of the diazotroph community to low N loads (short-term and low N addition; 3-year N addition at the rates of 25-50 kg N ha-1 year-1) vs high loads (chronic and high N addition; 9-year N addition at the rate of 150 kg N ha-1 year-1) in forest soils using high-throughput sequencing. Rates of N fixation decreased under low and high N loads (by 13%-27% and 10%-12%, respectively). Richness and alpha diversity (ACE and Chao1) of the soil diazotroph community decreased under low but not high N loads. Approximately 67.1%-74.4% of the nifH gene sequences at the OTU level overlapped between the control and low N loads, but only 52.0%-53.6% of those overlapped between the control and high N loads, indicating a larger shift of diazotroph community composition under high N loads. Low N loads increased soil NH4+ concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH4+ concentrations under high N loads did not have negative impacts on the structure and function of the diazotroph community. These findings indicate that diazotrophs sustain N fixation under high N deposition via adjustment of their community composition in forest soils.

IMPORTANCE

This study examined the changes in soil diazotroph community under different loads of simulated N deposition and analyzed its relationship with N fixation rates in in five forests using high-throughput sequencing. The magnitudes of N fixation rates reduced by low N loads were higher than those by high N loads. Low N loads decreased richness and diversity of diazotroph community, whereas diazotroph community structure remained stable under high N loads. Compared with low N loads, high N loads resulted in a less similarity and overlap of nifH gene sequences among the treatments and a larger adjustment of diazotroph community. Low N loads increased soil NH4+ concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH4+ under high N loads did not have negative impacts on diazotroph community structure and N fixation. Based on these findings, it is urgently needed to incorporate the loads of N deposition and the composition of diazotroph community into terrestrial N-cycling models for accurate understanding of N inputs in forest ecosystems.

Land use modified impacts of global change factors on soil microbial structure and function: A global hierarchical meta-analysis

Nitrogen cycling in terrestrial ecosystems is critical for biodiversity, vegetation productivity and biogeochemical cycling. However, little is known about the response of functional nitrogen cycle genes to global change factors in soils under different land uses. Here, we conducted a multiple hierarchical mixed effects meta-analyses of global change factors (GCFs) including warming (W+), mean altered precipitation (MAP+/-), elevated carbon dioxide concentrations (eCO2), and nitrogen addition (N+), using 2706 observations extracted from 200 peer-reviewed publications. The results showed that GCFs had significant and different effects on soil microbial communities under different types of land use. Under different land use types, such as Wetland, Tundra, Grassland, Forest, Desert and Agriculture, the richness and diversity of soil microbial communities will change accordingly due to differences in vegetation cover, soil management practices and environmental conditions. Notably, soil bacterial diversity is positively correlated with richness, but soil fungal diversity is negatively correlated with richness, when differences are driven by GCFs. For functional genes involved in nitrification, eCO2 in agricultural soils and the interaction of N+ with other GCFs in grassland soils stimulate an increase in the abundance of the AOA-amoA gene. In agricultural soil, MAP+ increases the abundance of nifH. W+ in agricultural soils and N+ in grassland soils decreased the abundance of nifH. The abundance of the genes nirS and nirK, involved in denitrification, was mainly negatively affected by W+ and positively affected by eCO2 in agricultural soil, but negatively affected by N+ in grassland soil. This meta-analysis was important for subsequent research related to global climate change. Considering data limitations, it is recommended to conduct multiple long-term integrated observational experiments to establish a scientific basis for addressing global changes in this context.

Photosynthetic and biochemical responses of four subtropical tree seedlings to reduced dry season and increased wet season precipitation and variable N deposition

Interspecific variations in phenotypic plasticity of trees that are affected by climate change may alter the ecosystem function of forests. Seedlings of four common tree species (Castanopsis fissa, Michelia macclurei, Dalbergia odorifera and Ormosia pinnata) in subtropical plantations of southern China were grown in the field under rainout shelters and subjected to changing precipitation (48 L of water every 4 days in the dry season, 83 L of water every 1 day in the wet season; 4 g m-2 year-1 of nitrogen (N)), low N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 8 g m-2 year-1 N), high N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 10 g m-2 year-1 N) and their interactive effects. We found that the changes in seasonal precipitation reduced the light-saturated photosynthetic rate (Asat) for C. fissa due to declining area-based foliar N concentrations (Na). However, we also found that the interactive effects of changing precipitation and N deposition enhanced Asat for C. fissa by increasing foliar Na concentrations, suggesting that N deposition could alleviate N limitations associated with changing precipitation. Altered precipitation and high N deposition reduced Asat for D. odorifera by decreasing the maximum electron transport rate for RuBP regeneration (Jmax) and maximum rate of carboxylation of Rubisco (Vcmax). Ormosia pinnata under high N deposition exhibited increasing Asat due to higher stomatal conductance and Vcmax. The growth of D. odorifera might be inhibited by changes in seasonal precipitation and N deposition, while O. pinnata may benefit from increasing N deposition in future climates. Our study provides an important insight into the selection of tree species with high capacity to tolerate changing precipitation and N deposition in subtropical plantations.

Negative responses of terrestrial nitrogen fixation to nitrogen addition weaken across increased soil organic carbon levels

The traditional view holds that biological nitrogen (N) fixation is energetically expensive and thus, facultative N fixers reduce N fixation rates while obligate N fixers are excluded by non-N fixers as soil N becomes rich. This view, however, contradicts the phenomenon that N fixation does not decline in many terrestrial ecosystems under N enrichment. To address this paradoxical phenomenon, we conducted a meta-analysis of N fixation and diazotroph (N-fixing microorganism) community structure in response to N addition across terrestrial ecosystems. N addition inhibited N fixation, but the inhibitory effect weakened across increased soil organic carbon (SOC) concentrations. The response ratios of N fixation (including free-living, plant-associated, and symbiotic types) to N addition were lower in the ecosystems with low SOC concentrations (<10 mg/g) than in those with medium or high SOC concentrations (10-20 and > 20 mg/g, respectively). The negative N-addition effects on diazotroph abundance and diversity also weakened across increased SOC levels. Among the climatic and soil factors, SOC was the most important predictor regarding the responses of N fixation and diazotroph community structure to N addition. Overall, our study reveals the role of SOC in affecting the responses of N fixation to N addition, which helps understand the relationships of biological N fixation and N enrichment as well as the mechanisms of terrestrial C and N coupling.

Effects of Multiple Global Change Factors on Symbiotic and Asymbiotic N2 Fixation: Results Based on a Pot Experiment

Biological N2 fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N2 fixation to multiple global change factors remain poorly understood. Here, saplings of two N2-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N2 fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N2 fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N2 fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N2 fixation per plant for C. cajan and asymbiotic N2 fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N2 fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO2, and phosphorus deposition at the urban site. The responses of N2 fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N2-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N2 fixation under global change.

Biotic and abiotic controls of nitrogen fixation in cyanobacteria-moss associations

Most mosses are colonized by nitrogen (N)-fixing cyanobacteria. This discovery is relatively recent, which can explain the large knowledge gaps the field is now tackling. For instance, while we have a good understanding of the abiotic controls (e.g. nutrient availability, increased temperature), we still do not know much about the biotic controls of N₂ fixation in mosses. I propose here that we should endeavour to position moss-cyanobacteria associations along the mutualism-parasitism continuum under varying abiotic conditions (e.g. nutrient availability). This would finally unravel the nature of the relationship between the partners and will be a big leap in our understanding of the evolution of plant-bacteria interactions using moss-cyanobacteria associations as a model system.

Diversity and Variation of Asymbiotic Nitrogen-Fixing Microorganisms in Alpine Grasslands on the Tibetan Plateau

Asymbiotic nitrogen-fixing (ANF) bacteria contribute a substantial amount of nitrogen in ecosystems, especially in those with low symbiotic nitrogen fixation (SNF) capability. Degradation of alpine grassland is widespread on the Tibetan Plateau and sown grassland has become one of the main strategies for grassland restoration. However, the diversity and community structure of ANF bacteria in different grassland types remain unknown. The aim of this study was to fill this gap. Soil samples were obtained from 39 grassland plots selected from three counties in the eastern Tibetan Plateau. The plots were classified as natural grassland (NG), sown grassland (SG), lightly degraded grassland (LDG), and severely degraded grassland (SDG). ANF microbial communities of the four grassland types were compared at the level of community and species diversity by 16S rRNA high-throughput sequencing technology. The phylum Proteobacteria accounted for &gt;72% of the ANF bacteria. The community structures of soil ANF bacteria differed significantly (p &lt; 0.01) among grassland types. We concluded that: (1) planting gramineous forage could possibly mitigate the decrease in diversity of soil ANF bacteria caused by grassland degradation; and (2) the diversity of soil ANF bacteria in alpine grassland of the Tibetan Plateau is closely related to grassland degradation and restoration.

Restoring degraded microbiome function with self-assembled communities

The natural microbial functions of many soils are severely degraded. Current state-of-the-art technology to restore these functions is through the isolation, screening, formulation and application of microbial inoculants and synthetic consortia. These approaches have inconsistent success, in part due to the incompatibility between the biofertilizer, crop, climate, existing soil microbiome and physicochemical characteristics of the soils. Here, we review the current state of the art in biofertilization and identify two key deficiencies in current strategies: the difficulty in designing complex multispecies biofertilizers and the bottleneck in scaling the production of complex multispecies biofertilizers. To address the challenge of producing scalable, multispecies biofertilizers, we propose to merge ecological theory with bioprocess engineering to produce 'self-assembled communities' enriched for particular functional guilds and adapted to a target soil and host plant. Using the nitrogen problem as an anchor, we review relevant ecology (microbial, plant and environmental), as well as reactor design strategies and operational parameters for the production of functionally enriched self-assembled communities. The use of self-assembled communities for biofertilization addresses two major hurdles in microbiome engineering: the importance of enriching microbes indigenous to (and targeted for) a specific environment and the recognized potential benefits of microbial consortia over isolates (e.g. functional redundancy). The proposed community enrichment model could also be instrumental for other microbial functions such as phosphorus solubilization, plant growth promotion or disease suppression.

Effects of human disturbance activities and environmental change factors on terrestrial nitrogen fixation

Biological nitrogen (N) fixation plays an important role in terrestrial N cycling and represents a key driver of terrestrial net primary productivity (NPP). Despite the importance of N fixation in terrestrial ecosystems, our knowledge regarding the controls on terrestrial N fixation remains poor. Here, we conducted a meta-analysis (based on 852 observations from 158 studies) of N fixation across three types of ecosystems with different status of disturbance (no management, restoration [previously disturbed], and disturbance [currently disturbed]) and in response to multiple environmental change factors (warming, elevated carbon dioxide [CO₂ ], increased precipitation, increased drought, increased N deposition, and their combinations). We explored the mechanisms underlying the changes in N fixation by examining the variations in soil physicochemical properties (bulk density, texture, moisture, and pH), plant and microbial characteristics (dominant plant species numbers, plant coverage, and soil microbial biomass), and soil resources (total carbon, total N, total phosphorus (P), inorganic N, and inorganic P). Human disturbance inhibited non-symbiotic N fixation but not symbiotic N fixation. Terrestrial N fixation was stimulated by warming (+152.7%), elevated CO₂ (+19.6%), and increased precipitation (+73.1%) but inhibited by increased drought (-30.4%), N deposition (-31.0%), and combinations of available multiple environmental change factors (-14.5%), the extents of which varied among biomes and ecosystem compartments. Human disturbance reduced the N fixation responses to environmental change factors, which was associated with the changes in soil physicochemical properties (2%-56%, p < .001) and the declines in plant and microbial characteristics (3%-49%, p ≤ .003) and soil resources (6%-48%, p ≤ .03). Overall, our findings reveal for the first time the effects of multiple environmental change factors on terrestrial N fixation and indicate the role of human disturbance activities in inhibiting N fixation, which can improve our understanding, modeling, and prediction of terrestrial N budgets, NPP, and ecosystem feedbacks under global change scenarios.

Molybdenum, phosphorus, and pH do not constrain nitrogen fixation in a tropical forest in the southeastern Amazon

High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea-salt aerosols. Even in these regions, the low availability of Mo can constrain free-living BNF catalyzed by heterotrophic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and BNF with the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short-term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics.

Litter Management as a Key Factor Relieves Soil Respiration Decay in an Urban-Adjacent Camphor Forest under a Short-Term Nitrogen Increment

Increases in bioavailable nitrogen (N) level can impact the soil carbon (C) sequestration in many forest ecosystems through its influences on litter decomposition and soil respiration (Rs). This study aims to detect whether the litter management can affect the influence of N addition on Rs. We conducted a one-year field experiment in a camphor forest of central-south China to investigate the responses of available N status and soil Rs to N addition and litter manipulation. Four N addition plots (NH4NO3; 0, 5, 15, 30 g N m−2 year−1 as N0, N1, N2, N3, respectively) were established with three nested litter treatments: natural litter input (CK), double litter input (LA), and non-litter input (LR). We found a short-lived enhancement effect of N addition on soil (NO3-N) and net nitrification (RN), but not on (NH4-N), net ammonification (RA), or mineralization (RM). N addition also decreased Rs in CK spots, but not in LA or LR spots, in which the negative effects of N additions on Rs were alleviated by either litter addition or reduction. A priming effect was also observed in LA treatments. A structural equation modeling analysis showed that litter treatments had direct positive effects on soil available N contents and Rs, which suggested that litter decomposition may benefit from litter management when N is not a limiting factor in subtropical forests.

Substrate stoichiometry determines nitrogen fixation throughout succession in southern Chinese forests

The traditional view holds that biological nitrogen (N) fixation often peaks in early- or mid-successional ecosystems and declines throughout succession based on the hypothesis that soil N richness and/or phosphorus (P) depletion become disadvantageous to N fixers. This view, however, fails to support the observation that N fixers can remain active in many old-growth forests despite the presence of N-rich and/or P-limiting soils. Here, we found unexpected increases in N fixation rates in the soil, forest floor, and moss throughout three successional forests and along six age-gradient forests in southern China. We further found that the variation in N fixation was controlled by substrate carbon(C) : N and C : (N : P) stoichiometry rather than by substrate N or P. Our findings highlight the utility of ecological stoichiometry in illuminating the mechanisms that couple forest succession and N cycling.

Global pattern and controls of biological nitrogen fixation under nutrient enrichment: A meta-analysis

Biological nitrogen (N) fixation (BNF), an important source of N in terrestrial ecosystems, plays a critical role in terrestrial nutrient cycling and net primary productivity. Currently, large uncertainty exists regarding how nutrient availability regulates terrestrial BNF and the drivers responsible for this process. We conducted a global meta-analysis of terrestrial BNF in response to N, phosphorus (P), and micronutrient (Micro) addition across different biomes (i.e, tropical/subtropical forest, savanna, temperate forest, grassland, boreal forest, and tundra) and explored whether the BNF responses were affected by fertilization regimes (nutrient-addition rates, duration, and total load) and environmental factors (mean annual temperature [MAT], mean annual precipitation [MAP], and N deposition). The results showed that N addition inhibited terrestrial BNF (by 19.0% (95% confidence interval [CI]: 17.7%-20.3%); hereafter), Micro addition stimulated terrestrial BNF (30.4% [25.7%-35.3%]), and P addition had an inconsistent effect on terrestrial BNF, i.e., inhibiting free-living N fixation (7.5% [4.4%-10.6%]) and stimulating symbiotic N fixation (85.5% [25.8%-158.7%]). Furthermore, the response ratios (i.e., effect sizes) of BNF to nutrient addition were smaller in low-latitude (<30°) biomes (8.5%-36.9%) than in mid-/high-latitude (≥30°) biomes (32.9%-61.3%), and the sensitivity (defined as the absolute value of response ratios) of BNF to nutrients in mid-/high-latitude biomes decreased with decreasing latitude (p ≤ 0.009; linear/logarithmic regression models). Fertilization regimes did not affect this phenomenon (p > 0.05), but environmental factors did affect it (p < 0.001) because MAT, MAP, and N deposition accounted for 5%-14%, 10%-32%, and 7%-18% of the variance in the BNF response ratios in cold (MAT < 15°C), low-rainfall (MAP < 2,500 mm), and low-N-deposition (<7 kg ha^-1  year^-1 ) biomes, respectively. Overall, our meta-analysis depicts a global pattern of nutrient impacts on terrestrial BNF and indicates that certain types of global change (i.e., warming, elevated precipitation and N deposition) may reduce the sensitivity of BNF in response to nutrient enrichment in mid-/high-latitude biomes.

Leaf mechanical strength and photosynthetic capacity vary independently across 57 subtropical forest species with contrasting light requirements

Leaf mechanical strength and photosynthetic capacity are critical plant life-history traits associated with tolerance and growth under various biotic and abiotic stresses. In principle, higher mechanical resistance achieved via higher relative allocation to cell walls should slow photosynthetic rates. However, interspecific relationships among these two leaf functions have not been reported. We measured leaf traits of 57 dominant woody species in a subtropical evergreen forest in China, focusing especially on photosynthetic rates, mechanical properties, and leaf lifespan (LLS). These species were assigned to two ecological strategy groups: shade-tolerant species and light-demanding species. On average, shade-tolerant species had longer LLS, higher leaf mechanical strength but lower photosynthetic rates, and exhibited longer LLS for a given leaf mass per area (LMA) or mechanical strength than light-demanding species. Depending on the traits and the basis of expression (per area or per mass), leaf mechanical resistance and photosynthetic capacity were either deemed unrelated, or only weakly negatively correlated. We found only weak support for the proposed trade-off between leaf biomechanics and photosynthesis among co-occurring woody species. This suggests there is considerable flexibility in these properties, and the observed relationships may result more so from trait coordination than any physically or physiologically enforced trade-off.