Nitrogen Transformation Research Articles

Innovative insights into organic nitrogen degradation through protein family domains analysis in chicken and pig manure composting using metagenomic sequencing

The nitrogen loss in composting is primarily driven by the transformation of organic nitrogen, yet the mechanisms underlying the degradation process remain incompletely understood. This study employed protein family domains (Pfams) analysis based on metagenomic sequencing to investigate the functional characteristics, key microorganisms, and environmental parameters influencing organic nitrogen degradation in chicken manure and pig manure composting. 154 Pfams associated with ammonification function were identified. Predominant Pfams: proteolytic peptidase, followed by chitin/cell wall degraders, least involved in nucleic acid degradation. Ammonifying microbial diversity was basically consistent among compost types, particularly in the thermophilic stage with the peak of abundance of dominant ammonifying microorganisms. Viruses played an important role in ammonification process, especially Uroviricota. 26 key ammonifying genera were identified by the microbial network. pH dominated the metabolic activity of ammonifying microorganisms in various manure compost types, primarily consisting of protein-degrading bacteria with stable community structures.

Picocyanobacterial-bacterial interactions sustain cyanobacterial blooms in nutrient-limited aquatic environments

Cyanobacterial blooms (CBs) and concomitant water quality issues in oligotrophic/mesotrophic waters have been recently reported, challenging the conventional understanding that CBs are primarily caused by eutrophication. To elucidate the underlying mechanism of CBs in nutrition-deficient waters, the changes in Chlorophyll a (Chl-a), cyanobacterial-bacterial community composition, and certain microbial function in Qingcaosha Reservoir, the global largest tidal estuary storage reservoir, were analyzed systematically and comprehensively after its pilot run (2011–2019) in this study. Although the water quality was improved and stabilized, more frequent occurrences of bloom level of Chl-a (>20 μg L−1) in warm seasons were observed during recent years. The meteorological changes (CO2, sunshine duration, radiation, precipitation, evaporation, and relative humidity), water quality variations (pH, total organic carbon content, dissolved oxygen, and turbidity), accumulated sediments as an endogenous source, as well as unique estuarine conditions collectively facilitated picocyanobacterial-bacterial coexistence and community functional changes in this reservoir. A stable and tight co-occurrence pattern was established between dominant cyanobacteria (Synechococcus, Cyanobium, Planktothrix, Chroococcidiopsis, and Prochlorothrix) and certain heterotrophic bacteria (Proteobacteria, Actinobacteria, and Bacteroidetes), which contributed to the remineralization of organic matter for cyanobacteria utilization. The relative abundance of chemoorganoheterotrophs and bacteria related to nitrogen transformation (Paracoccus, Rhodoplanes, Nitrosomonas, and Zoogloea) increased, promoting the emergence of CBs in nutrient-limited conditions through enhanced nutrient recycling. In environments with limited nutrients, the interaction between photosynthetic autotrophic microorganisms and heterotrophic bacteria appears to be non-competitive. Instead, they adopt complementary roles within their ecological niche over long-term succession, mutually benefiting from this association. This long-term study confirmed that enhanced nutrient cycling, facilitated by cyanobacterial-bacterial symbiosis following long-term succession, could promote CBs in oligotrophic aquatic environments devoid of external nutrient inputs. This study advances understanding of the mechanisms that trigger and sustain CBs under nutritional constraints, contributing to developing more effective mitigation strategies, ensuring water safety, and maintaining ecological balance.

Microbial community dynamics and nitrogen transformation associated with turbid-turned culture stage of Pacific white shrimp, Litopenaeus vannamei in small greenhouse farms

Small greenhouse farms have experienced rapid expansion of Litopenaeus vannamei culture in China in recent years. To investigate microbial community dynamics and nitrogen transformation during the early critical stage, known as the turbid-turned stage in small greenhouse farms, microbial samples from the water column of small greenhouses were analyzed by 16S rRNA and 18S rRNA high-throughput sequencing, along with the detection of ammonia nitrogen in water. The results of water quality analysis revealed that accumulated ammonia nitrogen was converted to nitrite as the water color transitioned from clear green to turbid brown. High-throughput sequencing data demonstrated a shift in dominant microorganisms from zooplankton to algae and bacteria during the turbid-turned stage, and the main functional microbial taxa, such Marivita and Chlorella, were enriched. The results of Linear Discriminant Analysis Effect Size and canonical correlation analyses indicated that the compositions of key microbial communities were correlated with environmental factors. Furthermore, analysis of functional genes indicated that ammonia-oxidizing bacteria were present in the early growth stage, while the abundance of nitrite-oxidizing bacteria was low during the early culture stage. Overall, the findings suggest that microorganisms during the turbid-turned stage in small greenhouses may play a crucial role in regulating water quality and promoting shrimp health within a stable small greenhouse environment.

Microbial community dynamics in a two-stage treatment wetland: Insights from treating seasonal ski resort wastewater

Microbial community studies in treatment wetlands (TW) have not investigated cold-climate TW and have focused primarily on laboratory mesocosms. This study determined microbial community composition and dynamics using 16s rRNA sequencing in a partially-saturated two-stage, vertical-flow pilot TW treating ski resort wastewater from December–April. Indicator species analysis found organisms capable of the primary nitrogen transformations observed in each stage at 3 °C, with nitrifiers in the unsaturated second stage and heterotrophic denitrifiers in the first stage where recycled nitrate is removed. During seasonal TW operation, microbial communities developed significant differences between TW stages and α-diversity decreased. Microbial communities were compared to batch mesocosm TW communities, which were significantly different from the pilot. The study is the first to discern community dynamics and key microorganisms during seasonal loading of a cold-climate, field-scale TW. Results raise questions regarding comparability of mesocosms to pilot systems and indicate more field-scale TW investigations are needed.

Effects of agricultural mulch film on swine manure composting: Film degradation and nitrogen transformation

The utilization of biodegradable mulch films (bio-MFs) is essential for agricultural safety. This study explored the effects of no MF (CK), aging bio-MF (BM), non-aging bio-MF (NBM), and aging polyethylene (PE)-MF (PEM) on swine manure composting. The results demonstrated that outdoor aging (45 days) accelerated the macroscopic degradation of bio-MF in the BM. A reduction in NH4+-N and NH3 emissions in the initial composting was observed owing to an increase in the carbon source or the bulking effect provided by the MFs. N2O emissions from days 9 to 21 were higher in the PEM than other treatments because of the formation of anaerobic zone in the MF-based aggregates. An obvious increase of amoA in PEM indicated a promoted nitrification during the maturation phase, meanwhile the increase of NO2−-N and aggregate promoted denitrification. Altogether, MF influenced composting through the synergistic effects of increasing the carbon source, bulking effect, and aggregates.

Biocrusts enhance soil nitrogen mineralization and nitrification under experimental warming in a dryland ecosystem

Nitrogen (N) transformation is essential for soil nitrogen availability and plant growth in drylands, but climate warming may be altering the dynamics of this process. Biological soil crusts (biocrusts) are soil surface communities dominated by cyanobacteria, lichens, and mosses that perform many vital functions in dryland ecosystems. However, their role in regulating soil N transformations under climate warming is uncertain. To explore this, we conducted a one-year field warming experiment in the Tengger Desert of northwestern China to investigate whether and how biocrusts modulate the effects of warming on N transformations, microbial biomass and enzyme activity in the soil underneath them. We found increases in microbial biomass carbon and nitrogen, soil nitrate reductase and urease activities, and net nitrification and mineralization rates in the soil below the biocrusts in response to warming. Our results suggest that biocrusts, acting as a “living skin,” significantly enhance the soil's organic matter and total nitrogen content. Thus, the resultant soil “hot zones” formed by the nutrient increases, help to sustain microbial activity and to mitigate the negative effects of warming on the soil environment. This nutrient enhancement is closely related to microbial biomass and enzyme activity, which are key contributors to N transformation in drylands. Therefore, our study highlights the regulatory role of biocrusts in soil N transformation, emphasizing the necessity to consider their effects to more accurately predict the impact of warming on N cycling in dryland ecosystems.

Comprehensive agricultural ecological effects of aeration on regenerated liquid fertilizer of mini flush toilet

The high concentration of organic waste liquid obtained from the mini flush pipeline discharge technology based on source separation has the potential for fertilizer utilization. However, there are concerns about the risk of secondary pollution. This study proposes the idea of aeration treatment for regenerated liquid fertilizers to induce beneficial changes in their material composition and properties. Initially, this study compares the characteristic changes in nitrogen transformation of liquid fertilizer through aeration treatment. Subsequently, it examines the effects of different types of liquid fertilizers on soil properties, plant physiology, and soil microbial communities. Finally, we elucidate the flow and distribution of nitrogen in soil, plants, and nitrogen-containing gas emissions in agricultural ecosystems through material flow accounting. The study found that aeration treatment can reduce the ammonia nitrogen ratio while increasing the proportions of nitrite nitrogen and nitrate nitrogen. The regenerated liquid fertilizer through aeration treatment not only significantly increased the chlorophyll, protein, and polysaccharide content of vegetable leaves (P < 0.05) but also reduced nitrate accumulation. Moreover, it can reduce the risk of soil nitrate nitrogen leaching and increase the diversity of soil bacterial communities, enhancing the ecological functions of bacteria involved in carbon and nitrogen cycling. Material flow accounting indicated that aeration treatment for liquid fertilizer could reduce gaseous nitrogen loss by 50.0 %, improve the nitrogen utilization efficiency of vegetables by 95.5 %, and enhance soil nitrogen retention by 11.4 %. Overall, the results show that aeration treatment can improve the agricultural utilization of liquid fertilizer and reduce the risk of secondary pollution, providing preliminary decision-making support for optimizing resource treatment strategies for mini-flush toilet fecal waste to realize the agricultural cycle.

Using MODFLOW to Model Riparian Wetland Shallow Groundwater and Nutrient Dynamics in an Appalachian Watershed

Simulating shallow groundwater (SGW) flow dynamics and stream–SGW interactions using numerical modeling tools is necessary to develop a mechanistic understanding of water flow systems and improve confidence in water resource management practices. A three-dimensional (3D) SGW flow model was developed for a riparian wetland in a mixed forest and agricultural catchment in West Virginia (WV), Appalachia, USA, using a Modular 3D Groundwater Model (MODFLOW). The MODFLOW simulation was calibrated in steady (R2 = 0.98, ME = −0.21, and RMSE = 0.77), transient state (R2 = 0.97, ME = −0.41, and RMSE = 1.28) and validated (R2 = 0.97, ME = −0.28, and RMSE = 1.05) using observed SGW levels from thirteen nested piezometers under steady and transient states. An experimental MT3D transport scenario was developed to show the lateral transport of NO₃-N from the aquifer to stream cells. Relatively stable SGW head distribution was observed. In the downstream reach, SGW discharge varied from 948 m3/day to 907 m3/day in 2020, with creek seepage ranging from 802 m3/day to 790 m3/day. Similarly, SGW input to the stream ranged from 891 m3/day to 978 m3/day, while creek seepage ranged from 796 m3/day to 800 m3/day in 2021. In upstream reaches, losing stream conditions were observed in January, June, and September 2020 and January to April 2021, while gaining stream conditions prevailed during other months. Thus, an approximately monthly alternating gaining–losing stream condition was observed in the upstream area. An experimental MT3D transport scenario resulted in an advection–dispersion scenario, showing a cumulative loss of 947 g of NO3-N from SGW to the stream. Denitrification accounted for the cumulative loss of 1406 g of NO3-N from SGW, surpassing 639 g of nitrate from the SGW to the stream during the study period. Additionally, particle tracking using MODPATH indicated a long residence time for SGW nutrients, affirming the efficiency of nitrogen transformation through denitrification. This study is among the first to simulate hydrologic and nutrient interactions in riparian wetlands of a mixed land use catchment in the Appalachian region of the northeastern United States. The results better inform water resource management decisions and modeling efforts in the Appalachian region and similar physiographic regions globally.

Atomically Dispersed Metal–Nitrogen–Carbon Catalysts for Electrochemical Nitrogen Transformations to Ammonia and Beyond

Atomically Dispersed Metal–Nitrogen–Carbon Catalysts for Electrochemical Nitrogen Transformations to Ammonia and Beyond

Nitrous Oxide Emissions and Ammonia Volatilization from Pasture after Cattle Dung and Urine Applications in the Dry and Rainy Seasons of the Brazilian Cerrado

An important source of greenhouse gases in Brazil is the nitrous oxide (N2O) emission from pasture, and microorganisms play an important role in nitrogen transformations in the soil. This study aimed to evaluate N2O emission and NH3 volatilization from bovine excreta in pasture in an integrated crop–livestock system (ICL) in the Brazilian Cerrado. Three treatments (urine, dung and control) were performed in two pastures (Area 1—three-year pasture of Urochloa ruziziensis and Area 2—one-year pasture of Urochloa brizantha cv. Piatã), with two application times of the excreta (dry and rainy season), during two successive years of application. Compared to the control, the excreta deposition on ICL increased soil N2O and NH3 fluxes. In the dry season, N2O fluxes were associated with higher ammonium (NH4+) availability. In the rainy season, these fluxes were related to NO3− availability and water-filled pore space (WFPS). In both areas, NH3 volatilization was higher after urine than dung application, especially in the dry season. The highest N2O emission factors were obtained for urine (0.32%), the rainy season (0.36%), and older pasture (Area 1: 0.24%). All these values were below the mean IPCC default values (0.77%). These results indicate that N2O emissions in pasture should be evaluated in regional conditions.

A pilot study of Mn redox driven nitrogen removal in municipal tailwater: Nitrogen metabolic pathway and electron transport mechanism

Microbial-mediated manganese (Mn) cycle coupled with nitrogen removal in municipal tailwater has received increasing attention. Herein, this study successfully extended the application of this technique to an 800 L biofilter, achieving a daily capacity of 2.4 m3 for treating municipal tailwater with a low C/N ratio. The operation results over 180 days indicated that the MnOx biofilter performed more efficiently than gravel control biofilter, achieving ∼ 40.2 % for nitrate and ∼ 35.9 % for total nitrogen (TN) removal efficiencies, respectively, under the conditions of hydraulic retention time (HRT) of 4 ∼ 5 h and low temperature (6.3–10.5 °C). To better elucidate the potential mechanisms, various functional bacteria involved in nitrogen and Mn transformation were identified, including heterotrophic denitrifying bacteria, such as Denitratisoma, Thauera, and Anaerolineae (cumulative relative abundance: 16.94–23.14 % in MnOx system vs. 9.86–11.90 % in control system) and Mn(II) oxidizing denitrifying bacteria, such as Pseudomonas and Acinetobacter (12.90–18.13 % vs. 3.59–4.64 %). Furthermore, metagenomic analysis indicated that the Mn cycle enhanced the expression of genes responsible for electron generation in organics/Mn metabolic pathways (e.g. TCA cycle and Mn oxidation), promoted electron transfer by enriching the Mn(II)-oxidizing denitrifying bacteria, and improved the abundance of genes associated with electron utilization (e.g. nitrate and nitrite reduction). The enhancement of the electron generation-transport-utilization due to Mn circulation contributed to efficient nitrate removal. This study provides further guidance of large-scale application and expands the understanding of the nitrogen removal mechanism in the Mn cycle-mediated process in municipal tailwater.

Research Advances of Groundwater Nitrate Pollution and Source Apportionment in China

Groundwater nitrate (NO3-) contamination in China has become a serious environmental problem, especially in agricultural production areas, which greatly affects the safety of drinking groundwater and requires urgent attention. In this study, the main sources of groundwater nitrate in China were reviewed, including atmospheric deposition, soil nitrogen, agricultural fertilization, and fecal sewage, among which fecal sewage and agricultural fertilization were the main reasons for the excessive groundwater nitrate. The application of hydrochemical analysis, multivariate statistical analysis, stable isotope tracer method, and microbial source tracking in the source apportionment of groundwater nitrate was summarized. All types of source apportionment methods had certain limitations. It is suggested that a variety of methods should be used to identify the source of groundwater nitrate, and the contribution rate of different pollution sources should be calculated using multivariate statistical analysis and isotope quantitative analysis models. The source apportionment of nitrate pollution has undergone a process from qualitative to quantitative research. At present, the SIAR and MixSIAR models based on δ15N-NO3-and δ18O-NO3- have been used widely to analyze the source of nitrate. However, due to the overlap of isotope characteristic values of different input end-members, the difference in δ15N-NO3-and δ18O-NO3- values under different spatial and temporal changes, and the influence of isotope fractionation in the process of nitrogen migration and transformation, the results calculated by the model remain uncertain. It is necessary to further optimize the analytical method of the model to obtain the source of nitrate pollution and its contribution rate more accurately to further aid in the scientific management of groundwater resources.

Mechanisms linking triclocarban biotransformation to functional response and antimicrobial resistome evolution in wastewater treatment systems

Evaluating the role of antimicrobials biotransformation in the regulation of metabolic functions and antimicrobial resistance evolution in wastewater biotreatment systems is crucial to ensuring water security. However, the associated mechanisms remain poorly understood. Here, we investigate triclocarban (TCC, one of the typical antimicrobials) biotransformation mechanisms and the dynamic evolution of systemic function disturbance and antimicrobial resistance risk in a complex anaerobic hydrolytic acidification (HA)-anoxic (ANO)/oxic (O) process. We mined key functional genes involved in the TCC upstream (reductive dechlorination and amide bonds hydrolysis) and downstream (chloroanilines catabolism) biotransformation pathways by metagenomic sequencing. Acute and chronic stress of TCC inhibit the production of volatile fatty acids (VFAs), NH4+ assimilation, and nitrification. The biotransformation of TCC via a single pathway cannot effectively relieve the inhibition of metabolic functions (e.g., carbon and nitrogen transformation and cycling) and enrichment of antimicrobial resistance genes (ARGs). Importantly, the coexistence of TCC reductive dechlorination and hydrolysis pathways and subsequent ring-opening catabolism play a critical role for stabilization of systemic metabolic functions and partial control of antimicrobial resistance risk. This study provides new insights into the mechanisms linking TCC biotransformation to the dynamic evolution of systemic functions and risks, and highlights critical regulatory information for enhanced control of TCC risks in complex biotreatment systems.

Differential impacts of polyethylene microplastic and additives on soil nitrogen cycling: A deeper dive into microbial interactions and transformation mechanisms

The impact of microplastics and their additives on soil nutrient cycling, particularly through microbial mechanisms, remains underexplored. This study investigated the effects of polyethylene microplastics, polyethylene resin, and plastic additives on soil nitrogen content, physicochemical properties, nitrogen cycling functional genes, microbial composition, and nitrogen transformation rates. Results showed that all amendments increased total nitrogen but decreased dissolved total nitrogen. Polyethylene microplastics and additives increased dissolved organic nitrogen, while polyethylene resin reduced it and exhibited higher microbial biomass. Amendments reduced or did not change inorganic nitrogen levels, with additives showing the lowest values. Polyethylene resin favored microbial nitrogen immobilization, while additives were more inhibitory. Amendment type and content significantly interacted with nitrogen cycling genes and microbial composition. Distinct functional microbial biomarkers and network structures were identified for different amendments. Polyethylene microplastics had higher gross ammonification, nitrification, and immobilization rates, followed by polyethylene resin and additives. Nitrogen transformation was driven by multiple functional genes, with Proteobacteria playing a significant role. Soil physicochemical properties affected nitrogen content through transformation rates, with C/N ratio having an indirect effect and water holding capacity directly impacting it. In summary, plastic additives, compared to polyethylene microplastics and resin, are less conducive to nitrogen degradation and microbial immobilization, exert significant effects on microbial community structure, inhibit transformation rates, and ultimately impact nitrogen cycling.

Unveiling ammonium concentration ranges that determine competition for mineral nitrogen among soil nitrogen transformations under increased carbon availability

Globally, approximately 50% of nitrogen (N) fertilizer applied in agricultural practices escapes into the environment, resulting in water and air pollution and ozone depletion. Soil N transformation processes determine the chemical form of N and the amount of the various N forms, controlling where N fertilizer goes and how much of it is lost. Therefore, a comprehensive understanding of soil N transformation responses to N fertilizer is critical for developing effective management strategies to improve soil N retention capacity and minimize soil N losses. Using 15N tracing techniques with acetylene inhibition, three ranges for ammonium concentration in fertilizer were identified (I: 14–16, II: 46–59, and III: 90–115 mg N kg−1, depending on type of organic materials added) that determine competition for mineral N among N transformation processes under increased carbon availability in an agricultural soil. Increasing ammonium concentration caused a shift from a nitrate assimilation-dominated (<I) to an equally ammonium assimilation- and nitrification-dominated (I-II) to nitrification-dominated (II-III), and finally to nitrification-predominant (>III) period in organic amendment addition treatments. Structural equation modeling revealed that ammonium addition inhibited soil nitrate assimilation by enhancing nitrification and ammonium assimilation, resulting in the shift. Consequently, the ratios of nitrification to ammonium assimilation and to gross N assimilation (nitrate assimilation + ammonium assimilation) increased significantly in response to elevated ammonium concentration with organic amendment addition, indicating lower N retention capacity and higher potential risks of N loss. Overall, we present a comprehensive picture of how concurrent gross N transformation processes interact to compete for mineral N in response to ammonium-forming fertilizer application, and demonstrate that nitrification and ammonium assimilation weaken the stimulating effect of organic amendment on nitrate assimilation with increasing ammonium concentration.

Biochar immobilized hydrolase degrades PET microplastics and alleviates the disturbance of soil microbial function via modulating nitrogen and phosphorus cycles

Microplastics (MPs) pose an emerging threat to soil ecological function, yet effective solutions remain limited. This study introduces a novel approach using magnetic biochar immobilized PET hydrolase (MB-LCC-FDS) to degrade soil polyethylene terephthalate microplastics (PET-MPs). MB-LCC-FDS exhibited a 1.68-fold increase in relative activity in aquatic solutions and maintained 58.5 % residual activity after five consecutive cycles. Soil microcosm experiment amended with MB-LCC-FDS observed a 29.6 % weight loss of PET-MPs, converting PET into mono(2-hydroxyethyl) terephthalate (MHET). The generated MHET can subsequently be metabolized by soil microbiota to release terephthalic acid. The introduction of MB-LCC-FDS shifted the functional composition of soil microbiota, increasing the relative abundances of Microbacteriaceae and Skermanella while reducing Arthobacter and Vicinamibacteraceae. Metagenomic analysis revealed that MB-LCC-FDS enhanced nitrogen fixation, P-uptake and transport, and organic-P mineralization in PET-MPs contaminated soil, while weakening the denitrification and nitrification. Structural equation model indicated that changes in soil total carbon and Simpson index, induced by MB-LCC-FDS, were the driving factors for soil carbon and nitrogen transformation. Overall, this study highlights the synergistic role of magnetic biochar-immobilized PET hydrolase and soil microbiota in degrading soil PET-MPs, and enhances our understanding of the microbiome and functional gene responses to PET-MPs and MB-LCC-FDS in soil systems.

The Responses of Soil Nitrogen Transformations to Rainfall Reduction Mainly Occurred in the Early Growing‐Season in a Temperate Meadow

AbstractDrought is expected to increase in temperate regions in the coming decades due to global climate change and will strongly affect nitrogen (N) transformation and balance. However, the underlying mechanisms (soil microenvironment vs. biotic community) of rainfall reduction induced changes in soil N transformation rates, and whether the responses of soil N transformation rates to rainfall reduction vary over time remains poorly understood. In this study, we experimentally examined rainfall reduction effects on soil N transformation rates over two (the second and third experimental years) growing‐seasons in a temperate meadow. We applied four treatments: ambient rainfall, 30%, 50%, and 70% reduction in rainfall. Our findings demonstrated that, 70% rainfall reduction significantly decreased the average net N mineralization and nitrification rates by 68.4% and 72.0%, respectively. Soil nitrous oxide emission was profoundly varied between the experimental years, with it was sensitive no only to changes in annual precipitation amount, but also to seasonal distribution, the highest emissions appeared at the early growing‐season. Moreover, plant diversity, microbial biomass, and soil NO3−‐N content mainly regulated soil N transformation rates in response to rainfall reduction in the greater annual precipitation year; but soil N transformation rates were mainly controlled by soil water content in the less annual precipitation year. We suggest that to accurately project future ecosystem functions and improving the prediction of N cycling responses to changing precipitation patterns, stronger focus should be allocated not only to changes in precipitation amounts, but also to inter‐ and intra‐annual differences in precipitation distribution.

Effects of Canopy Nitrogen Addition and Understory Vegetation Removal on Nitrogen Transformations in a Subtropical Forest

The management of understory vegetation and anthropogenic nitrogen (N) deposition has significantly resulted in a nutrient imbalance in forest ecosystems. However, the effects of canopy nitrogen addition and understory vegetation removal on N transformation processes (mineralization, nitrification, ammonification, and leaching) along with seasonal variations (spring, summer, autumn, and winter) remain unclear in subtropical forests. To fill this research gap, a field manipulation experiment was conducted with four treatments, including: (i) CK, control; (ii) CN, canopy nitrogen addition (25 kg N ha−1 year−1); (iii) UR, understory vegetation removal; and (iv) CN+UR, canopy nitrogen addition plus understory vegetation removal. The results revealed that CN increased net mineralization and nitrification by 294 mg N m−2 month−1 in the spring and 126 mg N m−2 month−1 in the winter, respectively. UR increased N mineralization and nitrification rates by 618 mg N m−2 month−1 in the summer. In addition, CN effectively reduced N leaching in the spring, winter, and autumn, while UR increased it in the spring and winter. UR increased annual nitrification rates by 93.4%, 90.3%, and 38.9% in the winter, spring, and summer, respectively. Additionally, both net N ammonification and annual nitrification rates responded positively to phosphorus availability during the autumn. Overall, UR potentially boosted nitrification rates in the summer and ammonification in the spring and winter, while CN reduced N leaching in the spring, winter, and autumn. Future research should integrate canopy nitrogen addition, understory vegetation removal, and phosphorus availability to address the global N deposition challenges in forest ecosystems.

Effects of straw management and N levels on gross nitrogen transformations in fluvo-aquic soil of the North China Plain

Straw incorporation with nitrogen (N) fertilization is crucial for enhancing soil fertility and minimizing negative environmental impacts by altering the magnitude and direction of soil N transformation processes. However, the response of soil N transformations to long-term carbon (C) and N inputs, and their primary driving factors, remain poorly understood. Thus, a 15N tracing study was conducted to investigate the effects of straw incorporation (AS) and straw removal (NS) with N levels of 0, 150 and 250 kg N ha−1 per season (N0, N150 and N250) on gross N transformation rates in the North China Plain after 6-year trial. Results indicated that at N0, AS significantly increased soil microbial immobilization of nitrate (NO3−-N, INO3) and autotrophic nitrification rates (ONH4) compared to NS. With N fertilization, AS increased gross N immobilization (Itotal), ammonium-N immobilization (NH4+-N, INH4), net NH4+-N immobilization (InetNH4) and net NH4+-N absorption rates (AnetNH4). Specifically, at N150, AS significantly increased recalcitrant organic N mineralization rate (MNrec), while significantly reducing ONH4, labile organic N mineralization (MNlab), and gross N mineralization rates (Mtotal). At N250, AnetNH4, MNlab, MNrec and ONH4 under AS were significantly higher than under NS. Nitrogen application significantly increased ONH4, Itotal and INO3 under two straw management practices, and enhanced INH4 and InetNH4 under AS. Compared to N250, N150 significantly increased INH4 and InetNH4 under AS, while decreasing Mtotal. Opposite results were observed under NS. Meanwhile, NO3−-N and dissolved organic carbon (DOC) were master factors controlling immobilization, total nitrogen (TN), hydrolysable NH4+-N (HNN) and stable organic N significantly affected AnetNH4, while labile organic N were the key environmental factors affecting MNrec, all of which positively influenced the rates of assimilation, mineralization and clay mineral adsorption. Overall, this study provides new insights into reducing N fertilization under straw incorporation by quantifying soil N transformation processes.

Biochar Combined with Garbage Enzyme Enhances Nitrogen Conservation during Sewage Sludge Composting: Evidence from Microbial Community and Enzyme Activities Related to Ammoniation

Nitrogen loss is an unavoidable problem during composting processes, and the ammonia oxidation process significantly affects nitrogen transformation. The objective of this study was to evaluate nitrogen transformation when garbage enzyme (GE), biochar (BC), pelelith (PL) and combinations thereof were added during sewage sludge composting. Meanwhile, the succession of ammonia-oxidizing bacteria (AOB) and archaea (AOA) were also explored via quantitative polymerase chain reaction and high-throughput sequencing. The results showed that GE + BC and GE + PL treatments decreased ammonia (NH3) formation by 23.8% and 8.3%, and that of nitrous oxide (N2O) by 25.7% and 26.3% relative to the control, respectively. Simultaneously, the GE, GE + BC, and GE + PL treatments boosted the succession of AOA and AOB, and increased the activities of ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO) activities and the gene copies of AOA and AOB. The AMO activities, NH4-N, NO3-N, and C/N, significantly affect AOA and AOB community structures. The network analysis predicted that the AMO and HAO were secreted mainly by the unclassified_Archaea and norank_Crenarchaeota, whereas it also showed that the GE + BC improved microbial associations with AOA, enzymatic activity, and environmental factors. Thus, the addition of garbage enzyme and biochar appears to be a promising mitigation strategy to reduce nitrogen losses during the composting process.