N-cycling Processes Research Articles

Stratified Effects of Tillage and Crop Rotations on Soil Microbes in Carbon and Nitrogen Cycles at Different Soil Depths in Long-Term Corn, Soybean, and Wheat Cultivation.

Understanding the soil bacterial communities involved in carbon (C) and nitrogen (N) cycling can inform beneficial tillage and crop rotation practices for sustainability and crop production. This study evaluated soil bacterial diversity, compositional structure, and functions associated with C-N cycling at two soil depths (0-15 cm and 15-30 cm) under long-term tillage (conventional tillage [CT] and no-till [NT]) and crop rotation (monocultures of corn, soybean, and wheat and corn-soybean-wheat rotation) systems. The soil microbial communities were characterized by metabarcoding the 16S rRNA gene V4-V5 regions using Illumina MiSeq. The results showed that long-term NT reduced the soil bacterial diversity at 15-30 cm compared to CT, while no significant differences were found at 0-15 cm. The bacterial communities differed significantly at the two soil depths under NT but not under CT. Notably, over 70% of the tillage-responding KEGG orthologs (KOs) associated with C fixation (primarily in the reductive citric acid cycle) were more abundant under NT than under CT at both depths. The tillage practices significantly affected bacteria involved in biological nitrogen (N2) fixation at the 0-15 cm soil depth, as well as bacteria involved in denitrification at both soil depths. The crop type and rotation regimes had limited effects on bacterial diversity and structure but significantly affected specific C-N-cycling genes. For instance, three KOs associated with the Calvin-Benson cycle for C fixation and four KOs related to various N-cycling processes were more abundant in the soil of wheat than in that of corn or soybean. These findings indicate that the long-term tillage practices had a greater influence than crop rotation on the soil bacterial communities, particularly in the C- and N-cycling processes. Integrated management practices that consider the combined effects of tillage, crop rotation, and crop types on soil bacterial functional groups are essential for sustainable agriculture.

Groundwater flow regime shapes nitrogen functional traits by affecting microbial community assembly processes in the subsurface

The complex nitrogen (N) cycle in groundwater systems is affected by both biological and environmental factors. The interactions between hydrogeological conditions and the microbial community assembly processes that impact N-cycling processes remain poorly understood. We explored the assembly patterns of N-cycling microbial communities along the groundwater flow path. The environmental heterogeneity in different hydrological phases increased along the flow path (mean Ed: 0.16–0.49), accompanied by different microbial community assembly patterns. The assembly patterns that engaged in dissimilatory nitrate reduction to ammonium (DNRA) and denitrification changed across the water-sediment phases. Nitrifying microorganisms in the discharge area were mainly influenced by heterogeneous selection (41–69 %), and were closely correlated with dissolved oxygen (DO) concentrations. Homogeneity along flow-through increased stochastic assemblies, such as downstream drift of anammox bacterial (AnAOB) communities. Thus, the N removal pathway changed from “nitrification-denitrification” in the recharge area to “partial nitrification-anammox” in the discharge area. The increasing environmental heterogeneity brought more deterministic assembly patterns of N-cycling communities, linked to higher community turnover along the groundwater flow path. This study indicated that groundwater flow regime determined microbial community assembly patterns, providing valuable insight into the response of N transitions to environmental variations in groundwater systems.

The microbial-driven nitrogen cycle and its relevance for plant nutrition.

Nitrogen (N) is a vital nutrient and an essential component of biological macromolecules such as nucleic acids and proteins. Microorganisms are major drivers of N-cycling processes in all ecosystems, including the soil and plant environment. The availability of N is a major growth-limiting factor for plants and it is significantly affected by the plant microbiome. Plants and microorganisms form complex interaction networks resulting in molecular signaling, nutrient exchange, and other distinct metabolic responses. In these networks, microbial partners influence growth and N use efficiency of plants either positively or negatively. Harnessing the beneficial effects of specific players within crop microbiomes is a promising strategy to counteract the emerging threats to human and planetary health due to the overuse of industrial N fertilizers. However, in addition to N-providing activities (e.g. the well-known symbiosis of legumes and Rhizobium spp.), other plant-microorganism interactions must be considered to obtain a complete picture of how microbial-driven N transformations might affect plant nutrition. For this, we review recent insights into the tight interplay between plants and N-cycling microorganisms, focusing on microbial N-transformation processes representing N sources and sinks that ultimately shape plant N acquisition.

Belowground links between root properties of grassland species and N2O concentration across the topsoil profile

Plants can affect N2O emissions by enhancing nitrogen (N) uptake and other below-ground interactions. However, the specific effect of the root systems of different plant species on the production and accumulation of N2O within the soil profile remain largely unknown. The aim of this study was to investigate how plant species from different functional groups, their productivity and root traits affect N2O emissions and N2O concentrations within the soil profile in a fertilised grassland. We conducted a field experiment with two grasses (Phleum pratense, Lolium perenne), two legumes (Trifolium repens, Trifolium pratense), two forbs (Cichorium intybus, Plantago lanceolata), and the six-species mixture in a fertilised grassland. The effects of these plant communities on N-cycling processes were then assessed through the measurement of above- and below-ground plant traits, plant productivity, soil nutrient availability, N2O emissions and its distribution in the soil profile. We found that C. intybus and P. pratense had the lowest N2O emissions from the soil, which was mainly related to higher root biomass. The six-species mixture also showed lower N2O emissions compared to L. perenne monoculture which was explained by complementary effects between the different plant species. We did not find a relationship between N2O emission and its concentration in the soil profile. Higher specific root length and root length density coincided with higher N2O concentrations at 10–20 and 20–30 cm soil depths. Since these two traits have been previously linked to reductions in N2O emissions emitted from the soil, our results show that the relationships between root traits and N2O emissions may not be reflected down in the soil profile. Overall, this study underscores the often-neglected importance of root traits for N-cycling and emphasises the need to better understand how root traits modify N2O consumption within the soil profile to design more sustainable grasslands.

Aridity threshold for alpine soil nitrogen isotope signature and ecosystem nitrogen cycling.

Determination of tipping points in nitrogen (N) isotope (δ15N) natural abundance, especially soil δ15N, with increasing aridity, is critical for estimating N-cycling dynamics and N limitation in terrestrial ecosystems. However, whether there are linear or nonlinear responses of soil δ15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ15N and soil N-cycling characteristics in both topsoil and subsoil layers along a drought gradient across a 3000-km transect of drylands on the Qinghai-Tibetan Plateau. We found that the effect of increasing aridity on soil δ15N values shifted from negative to positive with thresholds at aridity index (AI) = 0.27 and 0.29 for the topsoil and subsoil, respectively, although soil N pools and N transformation rates linearly decreased with increasing aridity in both soil layers. Furthermore, we identified markedly different correlations between soil δ15N and soil N-cycling traits above and below the AI thresholds (0.27 and 0.29 for topsoil and subsoil, respectively). Specifically, in wetter regions, soil δ15N positively correlated with most soil N-cycling traits, suggesting that high soil δ15N may result from the "openness" of soil N cycling. Conversely, in drier regions, soil δ15N showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage δ15N, demonstrating that pathways other than typical soil N cycling may dominate soil δ15N under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil δ15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ15N under changing drought regimes. The aridity threshold of soil δ15N should be considered in terrestrial N-cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness.

Genetic variation in Zea mays influences microbial nitrification and denitrification in conventional agroecosystems

Abstract Background and Aims Nitrogenous fertilizers provide a short-lived benefit to crops in agroecosystems, but stimulate nitrification and denitrification, processes that result in nitrate pollution, N2O production, and reduced soil fertility. Recent advances in plant microbiome science suggest that genetic variation in plants can modulate the composition and activity of rhizosphere N-cycling microorganisms. Here we attempted to determine whether genetic variation exists in Zea mays for the ability to influence the rhizosphere nitrifier and denitrifier microbiome under “real-world” conventional agricultural conditions. Methods To capture an extensive amount of genetic diversity within maize we grew and sampled the rhizosphere microbiome of a diversity panel of germplasm that included ex-PVP inbreds (Z. mays ssp. mays), ex-PVP hybrids (Z. mays ssp. mays), and teosinte (Z. mays ssp. mexicana and Z. mays ssp. parviglumis). From these samples, we characterized the microbiome, a suite of microbial genes involved in nitrification and denitrification and carried out N-cycling potential assays. Results Here we are showing that populations/genotypes of a single species can vary in their ecological interaction with denitrifers and nitrifers. Some hybrid and teosinte genotypes supported microbial communities with lower potential nitrification and potential denitrification activity in the rhizosphere, while inbred genotypes stimulated/did not inhibit these N-cycling activities. These potential differences translated to functional differences in N2O fluxes, with teosinte plots producing less GHG than maize plots. Conclusion Taken together, these results suggest that Zea genetic variation can lead to changes in N-cycling processes that result in N leaching and N2O production, and thereby are selectable targets for crop improvement. Understanding the underlying genetic variation contributing to belowground microbiome N-cycling into our conventional agricultural system could be useful for sustainability.

Biodegradable film mulching increases soil microbial network complexity and decreases nitrogen-cycling gene abundance

Biodegradable plastic films have emerged as a substitute for conventional plastic films. Nevertheless, responses of plant-associated microbiomes to the application of biodegradable film mulching at field scale have received little attention. A field experiment was conducted to assess the influence of different film mulching treatments on various microbial attributes and nitrogen (N) cycling functional genes in bulk and rhizosphere soils. Biodegradable film mulching raised the bacterial Shannon index in bulk soils but not in rhizosphere soils. Biodegradable film mulching has led to an increase in the complexity and connectivity of microbial networks, as well as an enhancement in the positive association among microorganisms owing to raised soil nutrients and increased crop biomass. In biodegradable film-treated soils, both bacterial and fungal communities were primarily influenced by stochastic processes associated with dispersal limitation. Moreover, conventional plastic film mulching increased denitrification, anammox, N fixation, and dissimilatory nitrate-reduction (DNRA) gene abundance in bulk soils. In rhizosphere soils, biodegradable film mulching reduced nitrification, denitrification, anammox, N fixation, and DNRA gene abundance. Furthermore, keystone genera (e.g., Nitrosospira, Truepera, Adhaeribacter, Opitutus, and Fusarium) were affected by edaphic variables, contributing to decreased N-cycling gene abundance in biodegradable film-treated soils. Collectively, biodegradable film mulching transformed soil microbiome assembly and functional adaptation, and soil nutrient availability and plant biomass were the critical factors influencing the microbial community. These findings present a novel perspective on the diverse impacts of biodegradable and conventional film mulching on soil microbiome and N-cycling processes.

Climate change disrupts the seasonal coupling of plant and soil microbial nutrient cycling in an alpine ecosystem.

The seasonal coupling of plant and soil microbial nutrient demands is crucial for efficient ecosystem nutrient cycling and plant production, especially in strongly seasonal alpine ecosystems. Yet, how these seasonal nutrient cycling processes are modified by climate change and what the consequences are for nutrient loss and retention in alpine ecosystems remain unclear. Here, we explored how two pervasive climate change factors, reduced snow cover and shrub expansion, interactively modify the seasonal coupling of plant and soil microbial nitrogen (N) cycling in alpine grasslands, which are warming at double the rate of the global average. We found that the combination of reduced snow cover and shrub expansion disrupted the seasonal coupling of plant and soil N-cycling, with pronounced effects in spring (shortly after snow melt) and autumn (at the onset of plant senescence). In combination, both climate change factors decreased plant organic N-uptake by 70% and 82%, soil microbial biomass N by 19% and 38% and increased soil denitrifier abundances by 253% and 136% in spring and autumn, respectively. Shrub expansion also individually modified the seasonality of soil microbial community composition and stoichiometry towards more N-limited conditions and slower nutrient cycling in spring and autumn. In winter, snow removal markedly reduced the fungal:bacterial biomass ratio, soil N pools and shifted bacterial community composition. Taken together, our findings suggest that interactions between climate change factors can disrupt the temporal coupling of plant and soil microbial N-cycling processes in alpine grasslands. This could diminish the capacity of these globally widespread alpine ecosystems to retain N and support plant productivity under future climate change.

Seasonal changes in N-cycling functional genes in sediments and their influencing factors in a typical eutrophic shallow lake, China.

N-cycling processes mediated by microorganisms are directly linked to the eutrophication of lakes and ecosystem health. Exploring the variation and influencing factors of N-cycling-related genes is of great significance for controlling the eutrophication of lakes. However, seasonal dynamics of genomic information encoding nitrogen (N) cycling in sediments of eutrophic lakes have not yet been clearly addressed. We collected sediments in the Baiyangdian (BYD) Lake in four seasons to explore the dynamic variation of N-cycling functional genes based on a shotgun metagenome sequencing approach and to reveal their key influencing factors. Our results showed that dissimilatory nitrate reduction (DNRA), assimilatory nitrate reduction (ANRA), and denitrification were the dominant N-cycling processes, and the abundance of nirS and amoC were higher than other functional genes by at least one order of magnitude. Functional genes, such as nirS, nirK and amoC, generally showed a consistent decreasing trend from the warming season (i.e., spring, summer, fall) to the cold season (i.e., winter). Furthermore, a significantly higher abundance of nitrification functional genes (e.g., amoB, amoC and hao) in spring and denitrification functional genes (e.g., nirS, norC and nosZ) in fall were observed. N-cycling processes in four seasons were influenced by different dominant environmental factors. Generally, dissolved organic carbon (DOC) or sediment organic matter (SOM), water temperature (T) and antibiotics (e.g., Norfloxacin and ofloxacin) were significantly correlated with N-cycling processes. The findings imply that sediment organic carbon and antibiotics may be potentially key factors influencing N-cycling processes in lake ecosystems, which will provide a reference for nitrogen management in eutrophic lakes.

Comprehensive multi-gas study by means of fiber-enhanced Raman spectroscopy for the investigation of nitrogen cycle processes.

The extensive use of synthetic fertilizers has led to a considerable increase in reactive nitrogen input into agricultural and natural systems, resulting in negative effects in multiple ecosystems, the so-called nitrogen cascade. Since the global population relies on fertilization for food production, synthetic fertilizer use needs to be optimized by balancing crop yield and reactive nitrogen losses. Fiber-enhanced Raman spectroscopy (FERS) is introduced as a unique method for the simultaneous quantification of multiple gases to the study processes related to the nitrogen cycle. By monitoring changes in the headspace gas concentrations, processes such as denitrification, nitrification, respiration, and nitrogen fixation, as well as fertilizer addition were studied. The differences in concentration between the ambient and prepared process samples were evident in the Raman spectra, allowing for differentiation of process-specific spectra. Gas mixture concentrations were quantified within a range of low ppm to 100% for the gases N2, O2, CO2, N2O, and NH3. Compositional changes were attributed to processes of the nitrogen cycle. With help of multivariate curve resolution, it was possible to quantify N2O and CO2 simultaneously. The impact of fertilizers on N-cycle processes in soil was simulated and analyzed for identifying active processes. Thus, FERS was proven to be a suitable technique to optimize fertilizer composition and to quantify N2O and NH3 emissions, all with a single device and without further sample preparation.

Response of soil nitrogen mineralization to warming temperatures depends on soil management history

Rising global temperatures have the potential to increase soil nitrogen (N) mineralization from soil organic matter (SOM). By increasing SOM over time, management practices that increase SOM through the addition of soil amendments, such as compost, have been recognized as effective strategies for mitigating the effects of climate change and building resilience in agricultural ecosystems. However, the effects of these strategies on temperature-induced changes to soil N cycling are unclear, particularly when soils are managed to increase SOM. To determine how agricultural management history and compost amendments affect net N mineralization, net nitrification, and nitrous oxide (N2O) production, we performed a laboratory incubation of soils with two distinct agricultural management histories under three incubation temperatures. Three compost treatments (green-waste compost, food-waste compost, and no compost) were applied, each with and without the addition of synthetic urea fertilizer. We found that organically managed soil exhibited higher rates of net N mineralization and nitrification than conventionally managed soil, leading to greater nitrate production. The rate of N mineralization in organically managed soil was also more sensitive to temperature increases. Although compost addition stimulated microbial activity, it did not affect the N-cycling processes measured in this study at any temperature. Therefore, the implementation of climate change resilience and mitigation strategies aimed at augmenting stocks of soil carbon may render agricultural soils more susceptible to increased N mineralization and subsequent losses under warming, particularly if plant uptake of the mineralized N does not occur concurrently. Moreover, the effects of compost application to stimulate the immobilization of excess N is likely limited in soils with low background C.

Excessive manure application stimulates nitrogen cycling but only weakly promotes crop yields in an acidic Ultisol: Results from a 20-year field experiment

Population growth and growing demand for livestock products produce large amounts of manure, which can be harnessed to maintain soil sustainability and crop productivity. However, the impacts of excessive manure application on crop yields, nitrogen (N)-cycling processes and microorganisms remain unknown. Here, we explored the effects of 20-year of excessive rates (18 and 27 Mg ha–1 yr–1) of pig manure application on peanut crop yields, soil nutrient contents, N-cycling processes and the abundance of N-cycling microorganisms in an acidic Ultisol in summer and winter, compared with none and a regular rate (9 Mg ha–1 yr–1) of pig manure application. Long-term excessive pig manure application, especially at the high-rate, significantly increased soil nutrient contents, the abundance of N-cycling functional genes, potential nitrification and denitrification activity, while it had a weaker effect on peanut yield and plant biomass. Compared with manure application, seasonality had a much weaker effect on N-cycling gene abundance. Random forest analysis showed that available phosphorus (AP) content was the primary predictor for N-cycling gene abundance, with significant and positive associations with all tested N-cycling genes. Our study clearly illustrated that excessive manure application would increase N-cycling gene abundance and potential N loss with relatively weak promotion of crop yields, providing significant implications for sustainable agriculture in the acidic Ultisols.

Unraveling the impact of human perturbation on nitrogen cycling in terrestrial ecosystems of lower Himalaya, Pakistan.

Terrestrial ecosystems are under the enormous pressure of land use management regimes through human disturbances, resulting in the disruption of biogeochemical cycles and associated ecosystem services. Nitrogen (N) in soil ecosystems is of vital importance for primary productivity, hence estimating the extent of these human interventions on N-cycling processes becomes imperative from economic and environmental perspectives. This work investigated the impacts of variable anthropogenic activities on N cycling in three different terrestrial ecosystems (arable, grassland, and forest) in three regions of lower Himalaya, Pakistan. Potential nitrification (PNA) and denitrification (DEA) enzyme activities, relative distribution of inorganic N species (NH4, NO3), and the role of inherent edaphic factors were assessed. Results revealed high nitrification potentials and increased nitrous oxide (N2O) emissions in the incubated soil microcosms, in the order as arable > grassland > forest ecosystems. Notably, higher rates of both studied processes (~ 30-50%) and elevated soil mineral nitrogen pool were observed in arable ecosystems. Forest soils, assumed as pristine ecosystems relying mainly on natural N fixation, produced (de)nitrification rates relatively lower than grasslands, followed by arable soils which were moderately disturbed through long-term fertilization and intensive land-use regimes. Linear regression modeling revealed that the inorganic N species (particularly NO3), and inherent edaphic factors were the key determinants of high (de)nitrification rates, hence warn of accelerated N losses in these ecosystems. The study highlights that elevated PNA and DEA being proxies for the altered N cycling in the studied terrestrial ecosystems are of great ecological relevance in view of predicted N2O budget in the lower Himalaya.

Agritech to Tame the Nitrogen Cycle.

While the Haber-Bosch process for N-fixation has enabled a steady food supply for half of humanity, substantial use of synthetic fertilizers has caused a radical unevenness in the global N-cycle. The resulting increases in nitrate production and greenhouse gas (GHG) emissions have contributed to eutrophication of both ground and surface waters, the growth of oxygen minimum zones in coastal regions, ozone depletion, and rising global temperatures. As stated by the Food and Agriculture Organization of the United Nations, agriculture releases ∼9.3 Gt CO2 equivalents per year, of which methane (CH4) and nitrous oxide (N2O) account for 5.3 Gt CO2 equivalents. N-pollution and slowing the runaway N-cycle requires a combined effort to replace chemical fertilizers with biological alternatives, which after a 10-yr span of usage could eliminate a minimum of 30% of ag-related GHG emissions (∼1.59 Gt), protect waterways from nitrate pollution, and protect soils from further deterioration. Agritech solutions include bringing biological fertilizers and biological nitrification inhibitors to the marketplace to reduce the microbial conversion of fertilizer nitrogen into GHGs and other toxic intermediates. Worldwide adoption of these plant-derived molecules will substantially elevate nitrogen use efficiency by crops while blocking the dominant source of N2O to the atmosphere and simultaneously protecting the biological CH4 sink. Additional agritech solutions to curtail N-pollution, soil erosion, and deterioration of freshwater supplies include soil-free aquaponics systems that utilize improved microbial inocula to enhance nitrogen use efficiency without GHG production. With adequate and timely investment and scale-up, microbe-based agritech solutions emphasizing N-cycling processes can dramatically reduce GHG emissions on short time lines.

Arbuscular mycorrhizal fungi alleviate erosional soil nitrogen loss by regulating nitrogen cycling genes and enzymes in experimental agro-ecosystems

Nutrient losses from agricultural ecosystems are increasingly threatening global environmental and human health. Although arbuscular mycorrhizal (AM) fungi have the potential to regulate soil nitrogen (N) loss by enhancing plant uptake and soil particle immobilization, the microbial mechanism behind such mycorrhizal effect is unknown. Herein, by conducting a simulated erosion experiment, we compared the effects of exogenous AM fungal inoculation (Funneliformis mosseae) on the gene abundances and enzyme activities of N-cycling processes, and associated such effect to N uptake and loss. The experiment was composed of combinations of two AM fungal treatments (control vs. AM fungal inoculation), two crops (maize vs. soybean) and two slopes of the plots (6° vs. 20°). The experimental plots subjected to natural rainfalls to simulate the erosion events. We showed that the effects of AM fungi were greater in the maize soils than in the soybean soils. In the maize soils, AM fungi increased the abundances of N-fixing (+81.1 %) and nitrifying genes (+200.7 %) and N cycling enzyme activity (+22.3 %). In the soybean soils, AM fungi increased the N-fixing gene abundance (+36.9 %) but decreased the abundance of nitrifying genes (−18.9 %). The abundance of N-fixing gene was positively correlated with N uptake but negatively correlated with N loss. Additionally, AM fungi enhanced the effects of mycorrhizal colonization and moisture but decreased the effects of nutrients on soil microbial metrics related to N-cycling processes. Therefore, AM fungal inoculation enhanced N uptake and reduced N loss by increasing N-fixing gene abundance, and that AM fungi should be preferably used for the low N environments or for the ecosystems highly limited by or competing for N.

Medium-term effects of wildfire severity on soil physical, chemical and biological properties in Pinus halepensis Mill. woodland (Southern Italy): an opportunity for invasive Acacia saligna colonization?

Wildfire frequency and severity have greatly increased in Mediterranean areas in recent decades affecting ecosystems functioning due to alteration in the above- and below-ground process. This study aimed to investigate how wildfire severity, in the medium-term (2–5 years), impacts soil properties within a Pinus halepensis woodland located in the Special Area of Conservation (SAC) of the Natura 2000 network (IT9130006 - Pinewoods of the Ionian Arch). In 2021, four years after a large wildfire in 2017, the woodland still exhibited fire effects with evidence of low, medium or high burn severity in different sites (named LBS, MBS, HBS, respectively). In addition, we observed an area burnt at medium severity that was invaded by Acacia saligna (MBSA site), a fast-growing, highly invasive, drought-tolerant nitrogen-fixing plant, thus we also examined the combined effect of wildfire and A. saligna on the soil. We compared soil properties across burnt sites with a nearby unburnt site (control). Thickness, weight and organic carbon of litter (L) and fermentation (F) layers were measured, alongside physical, chemical and biological properties in the underlying mineral soil (0–10 cm). Our results show that wildfire destroyed the organic layers and these had not recovered four years after the wildfire (except for L-layer within LBS) with a consequent loss (∼2 t C ha−1) of this carbon pool. In mineral soil we identified fourfold increases in N mineralization and nitrification rates across all burnt sites, regardless of the burn severity and A. saligna presence, suggesting an alteration of N-cycle processes. On the contrary, total microbial biomass and soil respiration as well as most of the physical and chemical properties of the soil were comparable between the burnt and control soils. Principal Component Analysis (PCA) highlighted that burn severity affected soil variables with lower changes in LBS than in other burnt soils. Also, the HBS soil did not show greater negative impacts compared to MBS sites. This is probably due to the increased post-fire colonization by herbaceous plants in HBS, favoured by the complete destruction of trees. In this case, waiting for natural vegetation recovery can be a valid management option, but periodic monitoring of fire-soil-vegetation interaction mainly to avoid invasive species widespread is advocated.

Impact of seasonal change on dissimilatory nitrate reduction to ammonium (DNRA) triggering the retention of nitrogen in lake

Nitrogen (N) reduction processes including denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are critical for the eutrophication in the lake water. However, the understanding about the dominant pathways of N cycling keep limited due to the high complexity of N cycle processes in lacustrine environment. The N fractions in sediments collected from Shijiuhu Lake were measured using high-resolution (HR)-Peeper technique and chemical extraction method in varied seasons. The abundance and microbial community compositions of functional genes involved in various N-cycling processes were also obtained using high-throughput sequencing. The results showed that NH4+ concentrations in the pore water remarkably increased from the upper layer toward the deeper layer and from winter to spring. This trend suggested that higher temperature facilitated the accumulation of NH4+ in the water. Decreased NO3− concentrations were also detected at deeper sediment layers and higher temperature, indicating the intensification of N reduction on anaerobic conditions. The NH4+-N concentrations reduced in spring along with the slight change of NO3−-N in solid sediment, indicating the desorption and release of mobile NH4+ from solid phase to the solution. Remarkably decreased absolute abundances of functional genes were found in spring with DNRA bacteria nrfA gene as dominant genus and Anaeromyxobacter as the most dominant bacterium (21.67 ± 1.03%). Higher absolute abundance (146.2-788.1 × 105 Copies/g) of nrfA gene relative to other genes was mainly responsible for the increase of bio-available NH4+ in the sediments. Generally, microbial DNRA pathway predominated the N reduction and retention processes in the lake sediment at higher temperature and water depth even experiencing the suppression of DNRA bacteria abundance. These results suggested the existence of ecological risk via N retention by the action of the DNRA bacteria in the sediment on the condition of higher temperature, further provided valuable information for N management of eutrophic lakes.

Phosphorus addition promotes plant nitrogen uptake mainly via enhancing microbial activities: A global meta-analysis

Nitrogen (N) addition drives ecosystems toward phosphorus (P) limitation has gained increasing recognition, whereas how P addition affects ecosystem N-cycling processes remains elusive. We performed a meta-analysis of 1826 observational data from 222 independent studies to evaluate the responses of plant N pools, soil N pools, and N fluxes to P addition at the global scale. The results suggested that P addition had an overall positive effect on plant N uptake and ecosystem N retention, which were mainly reflected in increases of plant N pools (i.e., aboveground/belowground biomass and belowground biomass-N) and microbial N transformation (i.e., net N mineralization and nitrification), and decrease of soil N leaching (i.e., nitrate-N and total dissolved N). The contribution of microbial processes to plant N uptake was greater in the mid−/high-latitude ecosystems (≥30°) than in the low-latitude ecosystems (<30°), and the sensitivities of most N-cycling variables to P addition were stronger in the most/moderately acidic soils (pH < 6.5) or strongly/intermediately weathered soils than in the neutral/alkaline soils or slightly weathered soils. Our meta-analysis highlighted the key role of soil microbes in promoting plant N uptake, and identified several important soil properties (i.e., soil N status, initial pH, and weathering degree) regulating the N-cycling responses to P addition and should be incorporated into the nutrient-cycling models of terrestrial ecosystems.

Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland.

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N2 O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. -30%) influenced soil N2 O and carbon dioxide (CO2 ) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N2 O and CO2 emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N2 O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N2 O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N2 O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.

Effects of microplastics and nitrogen deposition on soil multifunctionality, particularly C and N cycling

Both nitrogen deposition (ND) and microplastics (MPs) pose global change challenges. The effects of MPs co-existing with ND on ecosystem functions are still largely unknown. Herein, we conducted a 10-month soil incubation experiment to explore the effects of polyethylene (PE) and polylactic acid (PLA) MPs on soil multifunctionality under different ND scenarios. We found that the interactions between ND and MPs affected soil multifucntionality. FAPROTAX function prediction indicated that both ND and MPs affected C and N cycling. ND increased some C-cycling processes, such as cellulolysis, ligninolysis, and plastic degradation. MPs also showed stimulating effects on these processes, particularly in the soil with ND. ND significantly decreased the abundance of functional genes NifH, amoA, and NirK, leading to inhibited N-fixation, nitrification, and denitrification. The addition of MPs also modified N-cycling processes: 0.1% PE enriched the bacterial groups for nitrate reduction, nitrate respiration, nitrite respiration, and nitrate ammonification, and 1% PLA MPs enriched N-fixation bacteria at all ND levels. We found that ND caused lower soil pH but higher soil N, decreased bacterial diversity and richness, and changed the composition and activity of functional bacteria, which explains why ND changed soil functions and regulated the impact of MPs.