Autotrophic Nitrification Rates Research Articles

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.

Elevated atmospheric CO2 promotes the contribution of autotrophic nitrification to N2O emissions in a typical summer maize field

Elevated atmospheric carbon dioxide (CO2) concentration generally stimulates nitrous oxide (N2O) emissions from upland soils, leading to a promoted feedback on climate change. However, the underlying mechanisms on how elevated CO2 (eCO2) stimulates N2O emissions from nitrogen (N)-fertilized upland soils remain poorly known. Here, we investigated the effects of eCO2 on soil N2O production pathways and associated gross N transformation rates, as well as on nitrifying and denitrifying microbes. Based on a 13 years of free-air CO2 enrichment (FACE) platform, we found that N2O emissions were significantly increased by 46.7% under eCO2 in a typical summer-maize field. The 15N tracing experiment showed that autotrophic nitrification rate was significantly increased by 11.6% under eCO2, coincided with the significantly higher abundance and shift composition of ammonia-oxidizing bacteria (AOB) under eCO2. Also, eCO2 enhanced gross ammonium (NH4+) immobilization rate but reduced gross nitrate (NO3−) immobilization rate. Autotrophic nitrification accounted for approximately 78% of N2O emissions, followed by heterotrophic nitrification (about 19%) and denitrification (about 3%). It was found that eCO2 increased carbon (C) deposition in soil, which could affect the availability of soil labile organic C and N and alter the composition and activity of nitrifiers. These changes promoted the N2O production derived from autotrophic nitrification, resulting in higher N2O emissions under eCO2. We could deduce that nitrifier denitrification contributed to the increased N2O emissions greatly under eCO2, derived by strongly stimulated ammonia oxidation and microbial respiration (i.e. higher soil CO2 emissions) caused suboxic conditions (i.e. lower soil oxygen (O2) concentration), along with higher abundance ratios of (nirK+nirS)/nosZ. Our results suggest that controlling autotrophic nitrification appropriately is crucial for mitigating N2O emissions from upland soils under rising atmospheric CO2 concentration.

Maize genotypes regulate the feedbacks between maize nitrogen uptake and soil nitrogen transformations

Maize (Zea mays L.) genotypes can directly affect gene expression and nitrogen (N) uptake capacity. However, the feedback between maize genotypes and soil gross N transformations, and their regulations on N uptake capacity are not well known. So, 15N tracing pot experiments were conducted in an acidic soil and an alkaline soil with six maize genotypes, i.e. Dika 007 (DK), Zhengda 999 (ZD1), Zhengdan 958 (ZD2), Jingke 968 (JK), Longdan 339 (LD), Liangyu 99 (LY). Maize N uptake and soil gross N transformation rates were quantified by the Ntraceplant tool to identify feedbacks between maize genotypes N uptake and soil N transformations and their mechanisms. Maize total N uptake rates (UTN) varied among genotypes and soils. UTN in alkaline soil was 1.1–2.1 times higher than that in acidic soil. NH4+ and NO3− uptake rates (UNH4 and UNO3, respectively) were regulated by specific N transport genes in acidic soil, supported by positive relationships between N uptake rates and relative expression of ZmNRT1.1, ZmNRT1.2 and ZmAMT1.1A genes. UTN, and UNO3 were positively correlated to soil gross N mineralization (M), and autotrophic nitrification (ONH4) rates, respectively. However, maize inhibited M compared to treatment without maize plantation (CK) due to effects on soil microbial community, and based on random forest analysis, the declined relative abundance of Acidicaldub, Gonytrichum and Nigrospora in acidic soil, Taeniolella in alkaline soil may cause the decrease of M by the presence of maize. Meanwhile, maize inhibited ONH4 to increase NH4+ residence time, which in turn was responsible for the positive relationship between UNH4 and ONH4. In contrast, the stimulation of heterotrophic nitrification (ONrec) by plants improved the NO3− availability. The maize genotypes, such as JK, could largely enhance N uptake by regulating the expression level of N transport genes in maize and stimulating soil N transformation to produce more inorganic N (Nmin) in the different soils. Our findings show that maize genotypes play a central role in regulating these feedbacks. These results are important for maize breeding and enhancing maize production.

Topography-driven differences in soil N transformation constrain N availability in karst ecosystems

Fragile karst ecosystems are characterized by complex topographic landscapes associated with high variations in vegetation restoration. Identifying the characteristics and driving factors of nitrogen (N) availability across the topographic gradient is essential to guide vegetation restoration in karst regions. In this study, we collected soil samples and plant leaves along the topographic gradient (ridge, upper slope, middle slope, and foot slope) of convex slopes in the karst fault basin of southwest China, and determined the indicators reflecting soil N availability, N transformation rates, and their controlling factors. Our results showed that foliar N content and δ15N value, soil inorganic N content and δ15N value, and foliar N:P ratio were substantially lower on the steep hillslopes than on the flat top ridge. Steep slope soils also had a lower enzyme C:N ratio but a higher enzyme N:P ratio than the flat ridge soils. Furthermore, the vector angles calculated by soil extracellular enzyme analysis were below 45o in all studied soils and decreased significantly with increasing slope, indicating that microbial growth was generally limited by N. These results jointly suggest the declines in soil N availability across the topographic gradient, which are further explained by the changes in soil inherent N transformation processes. As the slope became steeper, soil mineralization and autotrophic nitrification (ONH4) rates decreased significantly, while ratio of microbial NH4+ immobilization to ONH4 and NH4+ adsorption rate increased significantly, indicating the decrease in soil inorganic N supply capacity. We further found that deteriorated soil structure, decreased soil organic matter and calcium content, altered microbial abundance, and increased ratios of fungi to bacteria and gram-positive bacteria to gram-negative bacteria were the primary drivers of reduced N transformation rates and N availability across the topographic gradient. Overall, this study highlights the critical role of the topography in controlling soil N availability by regulating N transformation processes in karst regions. The topography should be considered an important factor affecting the functions and services of karst ecosystems.

Full substitution of chemical fertilizer by organic manure decreases soil N2 O emissions driven by ammonia oxidizers and gross nitrogen transformations.

Replacing synthetic fertilizer by organic manure has been shown to reduce emissions of nitrous oxide (N2 O), but the specific roles of ammonia oxidizing microorganisms and gross nitrogen (N) transformation in regulating N2 O remain unclear. Here, we examined the effect of completely replacing chemical fertilizer with organic manure on N2 O emissions, ammonia oxidizers, gross N transformation rates using a 13-year field manipulation experiment. Our results showed that organic manure reduced cumulative N2 O emissions by 16.3%-210.3% compared to chemical fertilizer. The abundance of ammonia oxidizing bacteria (AOB) was significantly lower in organic manure compared with chemical fertilizer during three growth stages of maize. Organic manure also significantly decreased AOB alpha diversity and changed their community structure. However, organic manure substitution increased the abundance of ammonia oxidizing archaea and the alpha diversity of comammox Nitrospira compared to chemical fertilizer. Interestingly, organic manure decreased organic N mineralization by 23.2%-32.9%, and autotrophic nitrification rate by 10.5%-45.4%, when compared with chemical fertilizer. This study also found a positive correlation between AOB abundance, organic N mineralization and gross autotrophic nitrification rate with N2 O emission, and their contribution to N2 O emission was supported by random forest analysis. Our study highlights the key roles of ammonia oxidizers and N transformation rates in predicting cropland N2 O.

Effects of 18 years repeated N fertilizer applications on gross N transformation rates in a subtropical rain-fed purple soil

In this study, a 15N tracing experiment was conducted to determine gross N transformation rates in soils with 18 years of repeated N fertilizer applications and to quantify the underlying mechanisms to explain the variability of crop yield in a subtropical rain-fed purple soil. Six N fertilizer treatments were evaluated: unfertilized control (CK), mineral N fertilizer (NPK), pig manure (OM), mineral N fertilizer plus pig manure (OM-NPK), crop residue (RSD) and mineral N fertilizer plus crop residue (RSD-NPK). The results showed that the soil organic carbon (SOC) concentration in OM (average 13.79 g kg−1) and RSD-NPK (average 13.15 g kg−1) were significantly higher than the other treatments (varied from 5.91 to 11.23 g kg−1) after 18 years. Higher maize yields were observed in NPK, OM, OM-NPK, and RSD-NPK, and especially in RSD-NPK and OM-NPK after 18 years, compared with the data measured in the 11th years. Soil gross N mineralization rates (M) were significantly higher in all N fertilizer treatments (varied from 13.00 to 31.17 mg N kg−1 soil d−1) compared to CK (average 5.53 mg N kg−1 soil d−1). After 18 years of repeated N fertilizer applications, M were significantly higher than those in the 11th years for the same N fertilizer treatment, indicating that long-term repeated N fertilizer application, whether mineral N or organic fertilizer, continuously enhances M. M was positively correlated with SOC (p < 0.05) and total N (TN) (p < 0.05), suggesting that soil organic substrates are key factors regulating M. Maize yield was positively correlated with M (p < 0.05), suggesting that available N produced through soil N mineralization played the important role in yield formation. All N fertilizer treatments continuously stimulated gross autotrophic nitrification rates (ONH4). ONH4 increased with increasing M (p < 0.01) and ammonia-oxidizing bacteria (AOB) abundance (p < 0.05). However, compared to the 11th year, rates of dissimilatory nitrate reduction to ammonium (DNRA) and immobilization of NH4+ (except RSD) and NO3− (except OM-NPK) decreased significantly in the N fertilizer treatments in the 18th year, indicating that N retention capacity is reduced over time and the risk of N losses increased. Considering the best balance among food security, soil C sequestration, and reduction of N leaching, mineral N in combination with straw return (RSD-NPK) can be recommended for regions with similar soils and climates.

Effects of Solidago canadensis L. on mineralization-immobilization turnover enhance its nitrogen competitiveness and invasiveness

The effects of exotic plants on soil nitrogen (N) transformations may influence species invasion success. However, the complex interplay between invasive plant N uptake and N transformation in soils remains unclear. In the present study, a series of 15N-labeled pot experiments were carried out with Solidago canadensis L. (S. canadensis), an invasive plant, and the Ntrace tool was used to clarify the preferred inorganic N form and its effects on soil N transformation. According to the results, nitrate-N (NO3−-N) uptake rates by S. canadensis were 2.38 and 2.28 mg N kg−1 d−1 in acidic and alkaline soil, respectively, which were significantly higher than the ammonium-N (NH4+-N) uptake rates (1.76 and 1.56 mg N kg−1 d−1, respectively), indicating that S. canadensis was a NO3−-N-preferring plant, irrespective of pH condition. Gross N mineralization rate was 0.41 mg N kg−1 d−1 in alkaline soil in the presence of S. canadensis L., which was significantly lower than that in the control (no plant, CK, 2.44 mg N kg−1 d−1). Gross autotrophic nitrification rate also decreased from 5.95 mg N kg−1 d−1 in the CK to 0.04 mg N kg−1 d−1 in the presence of S. canadensis in alkaline soil. However, microbial N immobilization rate increased significantly from 1.09 to 2.16 mg N kg−1 d−1, and from 0.02 to 2.73 mg N kg−1 d−1 after S. canadensis planting, in acidic and alkaline soil, respectively. Heterotrophic nitrification rate was stimulated in the presence of S. canadensis to provide NO3−-N to support the N requirements of plants and microbes. The results suggested that S. canadensis can influence the mineralization-immobilization turnover (MIT) to optimize its N requirements while limiting N supply for other plants in the system. The results of the present study enhance our understanding of the competitiveness and mechanisms of invasion of alien plants.

Invasive Spartina alterniflora accelerates soil gross nitrogen transformations to optimize its nitrogen acquisition in an estuarine and coastal wetland of China

Saltmarsh plants are important components of estuarine and coastal wetlands because they regulate ecosystem nitrogen (N) dynamics. However, complex interactions between the N uptake of saltmarsh plants and soil N transformation remain unclear. Here, we conducted a series of 15N tracing experiments with native Phragmites australis, invasive Spartina alterniflora, and bulk sediment without plants to explore the effect of plants on soil N cycling. The results showed that the NH4+ and NO3− uptake rates by the saltmarsh plants were 4.62–5.38 mg N kg⁻1 d⁻1 and 1.29–2.90 mg N kg⁻1 d⁻1, respectively, and the invasive S. alterniflora had a higher N uptake than the native P. australis. The presence of saltmarsh plants promoted N mineralization and dissimilatory NO3− reduction to NH4+, increasing the available NH4+ supply for the plants. Conversely, NH4+ immobilization and autotrophic nitrification rates were drastically reduced in the presence of the saltmarsh plants, indicating that the plants were able to outcompete soil microorganisms in NH4+ acquisition. Meanwhile, heterotrophic nitrification (organic N oxidation), which accounted for 66–82% of the total nitrification, was stimulated by the saltmarsh plants. Increased heterotrophic nitrification in the saltmarsh plants helped to provide NO3− substrates to meet the needs of the soil microorganisms and the plants. The regulatory effect of the invasive S. alterniflora on soil gross N transformation was more pronounced than that of the native P. australis due to the higher N requirements of the former. Microbial carbon sources and energy sources, relevant gene abundances and exoenzyme activities were the main factors by which the saltmarsh plants regulated gross N transformations. Overall, our results show that there are various interactions between soil microorganisms and saltmarsh plants and that S. alterniflora accelerates gross N transformations in the soil to meet its large demand for N. These findings provide valuable insights into the ecological management of invasive plants in estuarine and coastal ecosystems.

Gross N transformations and plant N use efficiency in intensive vegetable production soils

Long-term application of high doses of N fertilizer to achieve high yields has led to a significant accumulation of soil N (largely in the form of NO3−) in greenhouse vegetable production (GVP). At present, it is not clear how efficiently vegetables can use the NO3− accumulated in soil and from which source the N taken up by vegetables in GVP originates. In this study, three greenhouse soils with various degrees of soil NO3− accumulation (N1 <N2 <N3) and one abandoned land soil (AL) adjacent to greenhouses were analysed by 15N tracing pot experiments. The amount of N derived from soil (ANDFS), N use efficiency (NUE) of cucumbers, and gross N transformation rates were investigated to explore processes leading to the accumulation of NO3− in soil and how available this N is for N uptake by cucumbers. The results showed that the percentage of N derived from soil of cucumbers were 72–89% in greenhouse soils. ANDFS and NUE values of cucumbers decreased with NO3− accumulation. Plant NH4+ uptake (UNH4) was one of the main factors regulating NUE and ANDFS. Soil gross N mineralisation rates (M) were 2.5- and 3.0-fold higher in N1 than in N2 and N3, respectively, while autotrophic nitrification rates (ONH4) were significantly lower in N1 and N2 (by 4.31 and 4.25 mg N kg−1 day−1) than N3 (9.44 mg N kg−1 day−1). This resulted in significantly higher ONH4/M ratios in soils with relatively higher NO3− accumulation, which reduced the amount of NH4+ available to cucumbers and inhibited UNH4, in turn, reducing ANDFS and NUE. Higher ONH4 in soils with relatively high NO3− accumulation could result in fertilizer-derived NH4+ being rapidly oxidised to NO3−, leading not only to lower NUE but also to greater NO3− accumulated in soil due to a lower plant NO3− uptake rate. High soil electrical conductivity (EC) in N2 and N3 caused salt stress in cucumber plants and inhibited plant N uptake, resulting in low ANDFS and NUE. These results demonstrate that NO3− that is accumulated and newly mineralized in soil is available for cucumbers, and should be considered together with the prevailing soil N transformations in N fertilizer recommendations for GVP. Combined application of NH4+ fertilizer (e.g. in form of urea) with nitrification inhibitors is recommended to increase NH4+ retention and enhance NUE and utilisation of soil accumulated and newly mineralized NO3− by cucumbers.

Plastic-shed vegetable cultivation alters soil gross nitrogen transformation rates and nitrous oxide production pathways

Plastic-shed vegetable cultivation is characterized by high nitrogen (N) application rates, frequent irrigation and multiple cropping, probably leading to different soil N transformation rates compared to open-field cultivation. However, how continuous cultivation of vegetables under plastic sheds affects soil N transformation rates and associated nitrous oxide (N2O) emissions remains unclear. Here, a 15N-tracing experiment in combination with the qPCR technique was used to investigate the effects of conversion from conventional open-field cultivation to plastic-shed vegetable cultivation, as well as the age of this shift, on soil gross N transformation rates, N2O emission pathways and associated microbial abundances. Five typical fields, including a wheat–maize rotation system field and four adjacent plastic-shed vegetable fields under cultivation for 2, 5, 12 and 20 years, were selected. Soil gross N mineralization rates increased significantly after two years after conversion from a wheat–maize rotation system to a plastic-shed vegetable system, and then decreased with an increase in plantation age, which could be attributed to decreased soil total phosphorus availability. Long-term vegetable plantation under plastic-shed decreased soil gross autotrophic nitrification rates significantly by decreasing ammonia-oxidizing bacteria abundance probably due to decreasing soil pH. Such decreased rates of autotrophic nitrification further led to decreased N2O emissions under plastic-shed cultivation. Under long-term plastic-shed cultivation (>2 years), negligible gross nitrate-N (NO3−-N) immobilization rates were observed due to the preferential use of ammonium-N by soil microbes, which was probably responsible for rapid NO3−-N accumulation in soils. Overall, these results could be of help to elucidate the mechanisms of soil N2O emission and NO3−-N accumulation in plastic-shed vegetable fields.

Combined application of biochar and N fertilizer shifted nitrification rate and amoA gene abundance of ammonia-oxidizing microorganisms in salt-affected anthropogenic-alluvial soil

Biochar has been globally recognized as a novel and eco-friendly soil ameliorant to mitigate hazardous effects of salinization on soil health, and mediate carbon and nitrogen (N) nutrient cycling in agricultural ecosystems. However, knowledge pertaining to the regulatory mechanism of biochar application on nitrification in the salt-affected soil is limited. In this study, field plot experiment for three consecutive years and an aerobic incubation experiment with three biochar application rates (7.5, 15, and 30 ton ha−1) and N fertilizer (225 kg N ha−1 yr−1) were conducted. We investigated the effect of combined application of biochar and N fertilizer on soil chemical and microbial properties, nitrification kinetic parameters, and the abundance of ammonia monooxygenase (subunit A, amoA) gene of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Our results showed that biochar addition increased soil pH, cation exchange capacity, available potassium, and microbial biomass carbon, and decreased the numbers of amoA-AOB gene in the salt-affected soil applied with N fertilizer. The nitrification process and fitted kinetic parameters were negatively responsive to the combined application of biochar and N fertilizer, which reduced the nitrification potential and potential nitrification rate, and prolonged the duration of nitrification. The nitrification rate and abundance of amoA-AOB and amoA-AOA genes decreased under biochar and N fertilizer addition, and biochar played the predominant role in the inhibiting effect, which aggravated with the increase of biochar rate. The AOB dominated the autotrophic nitrification with the nitrification rate having higher positive correlation with abundance of amoA-AOB gene than with that of amoA-AOA gene. The abundance of both amoA-AOB and amoA-AOA genes showed significantly negative correlation with nitrate nitrogen (NO3−-N) concentration. In conclusion, biochar and N fertilizer addition inhibited the autotrophic nitrification rate mainly by greatly reducing the abundance of amoA-AOB gene, which was adaptive to the changing soil microhabitat traits induced by biochar application and N fertilization.

Field-aged biochar decreased N2O emissions by reducing autotrophic nitrification in a sandy loam soil

A 15N tracing incubation study was carried out using a sandy loam soil without (control) and with field-aged biochar (biochar) to investigate the mechanisms underlying the effects of field-aged biochar on nitrous oxide (N2O) emissions. During the incubation, carried out at 40%, 60%, and 80% water-filled pore space (WFPS), cumulative N2O emission decreased from 24.13–26.40 μg N kg−1 in the control soil to 18.27–23.94 μg N kg−1 in the biochar soil, with a reduction of 9.3–24.3%. The contribution of autotrophic nitrification to total N2O production was 81.8–87.6% in the control soil under 40–80% WFPS, which was significantly reduced by field-aged biochar to 67.1–78.6%. Under 60% WFPS, the gross rates of autotrophic nitrification and gross mineralization were reduced from 11.95 and 4.43 μg N g−1 d−1, respectively, in the control soil to 7.32 and 0.60 μg N g−1 d−1, respectively, in the biochar soil. The field-aged biochar increased the NH4+ immobilization rate by 440%, primarily by immobilizing NH4+ into the recalcitrant organic N pool. Both the turnover rate of NH4+ mineralization-immobilization and the ratio of nitrification to NH4+ immobilization were reduced under biochar amendment, consequently lowering the supply of NH4+ for nitrifiers. In addition, compared with the control soil, the gross rate of NH4+ adsorption was significantly higher in the biochar soil. Taken together, our results suggest that field-aged biochar contributes to mitigating N2O emissions, primarily by decreasing the autotrophic nitrification rate through a reduced NH4+ supply due to increased mineral N immobilization and adsorption and lowered organic N mineralization.

Soil gross nitrogen transformations in forestland and cropland of Regosols

Soil gross nitrogen (N) transformations could be influenced by land use change, however, the differences in inherent N transformations between different land use soils are still not well understood under subtropical conditions. In this study, an 15N tracing experiment was applied to determine the influence of land uses on gross N transformations in Regosols, widely distributed soils in Southwest China. Soil samples were taken from the dominant land use types of forestland and cropland. In the cropland soils, the gross autotrophic nitrification rates (mean 14.54 ± 1.66 mg N kg−1 day−1) were significantly higher, while the gross NH4+ immobilization rates (mean 0.34 ± 0.10 mg N kg−1 day−1) were significantly lower than those in the forestland soils (mean 1.99 ± 0.56 and 6.67 ± 0.74 mg N kg−1 day−1, respectively). The gross NO3− immobilization and dissimilatory NO3− reduction to NH4+ (DNRA) rates were not significantly different between the forestland and cropland soils. In comparison to the forestland soils (mean 0.51 ± 0.24), the cropland soils had significantly lower NO3− retention capacities (mean 0.01 ± 0.01), indicating that the potential N losses in the cropland soils were higher. The correlation analysis demonstrated that soil gross autotrophic nitrification rate was negatively and gross NH4+ immobilization rate was positively related to the SOC content and C/N ratio. Therefore, effective measures should be taken to increase soil SOC content and C/N ratio to enhance soil N immobilization ability and NO3− retention capacity and thus reduce NO3− losses from the Regosols.

Organic nitrogen addition causes decoupling of microbial nitrogen cycles by stimulating gross nitrogen transformation in a temperate forest soil

External inorganic and organic nitrogen (N) inputs can contrastingly affect the transformation and availability of N in forest soils. Studies have mainly focused on the effects of inorganic N enrichment, whereas little is known about the effects of organic N input on soil gross N transformation and the underlying microbial mechanisms. Here we conducted a laboratory 15N tracing study in a temperate needle-broadleaved mixed forest with a fertilization rate of 0, 20, 60, and 120 kg urea-N ha−1 yr−1 over three years. We investigated the key drivers of soil N transformation processes using a 15N tracing model in the context of selected soil chemical properties and microbial characteristics. Urea addition did not change soil gross N mineralization rates, while stimulating mineralization of labile organic N (MNlab). Urea addition at a rate of 120 kg N ha−1 yr−1 significantly increased autotrophic nitrification and gross nitrification rates by 88% and 96%, respectively. In contrast, all the three levels of urea addition significantly reduced gross microbial N immobilization by 28% to 52%, leading to an increase in the accumulation of soil NO3−-N in the top 10 cm soil layer by 38% to 88%. The changes in autotrophic nitrification were primarily driven by acid-tolerant ammonia-oxidizing archaea (AOA). Fungi were responsible for the change in heterotrophic nitrification under organic N enrichment. Gross N transformation rates were predominately regulated by AOA and fungal abundances as well as soil NO3−-N content under high level of organic N addition. The response of soil N transformation to exogenous organic N input depended on N addition level with the threshold rate being estimated to be 60–120 kg N ha−1 yr−1. All lines of evidence showed that the temperate needle-broadleaved mixed forest is moving towards an opener microbial N cycle under elevated organic N deposition. Our finding suggests that the effect of organic N input on soil gross N transformation is different from that of inorganic N input, which should be considered in ecosystem process models.

Tea plantation affects soil nitrogen transformations in subtropical China

This study proposed to investigate soil N transformation rates under different tea plantation ages. These plantations receive a great deal of nitrogen fertilizer each year. It was hypothesized that the inhibition of nitrification by increasing soil acidification would be counteracted by the N application. We aimed to explore whether this relationship would influence soil nitrogen transformations within tea plantations. Tea fields with different establishment periods (8 years, 36 years, and 105 years) and adjacent forest soils were collected from Hangzhou city, China. The 15N dilution technique and a 15N tracing model were used to investigate changes in soil gross N transformation rates, additionally combining the 15N dilution technique with acetylene inhibition to distinguish heterotrophic and autotrophic nitrification rates. Both 15N labeling methods demonstrated that land use conversion and fertilization significantly change the soil N transformation rates. The gross N mineralization rate was much higher in the tea soils compared with that in the adjacent forest soil and increased with planting age in the tea soils. Interestingly, appreciable NO3− production was detected in the forest soil caused by heterotrophic nitrification, whereas only a small amount of nitrate was formed by heterotrophs in the tea soils. Gross nitrification rates increased with the planting age from 8 to 36 years but decreased at 105 years. Nitrification was the main ammonium consuming process in the tea soils and a positive relationship was observed between ammonia oxidizing archaea (AOA) amoA abundance and nitrification rate, suggesting AOA as the dominant nitrification drivers in these tea soils. Overall, the conversion from forest to tea soil enhanced the gross rate of nitrification, N mineralization, and NH4+ immobilization, but N/I (nitrification/ammonium immobilization) also increased significantly in the tea plantations, indicating a high nitrate leaching and runoff risk.

Contrasting effects of different pH‐raising materials on N2O emissions in acidic upland soils

AbstractAcidic soils, occupying ca. 40% of the world's arable soils, often need to be managed (e.g., to raise their pH and to improve crop productivity); however, the environmental impact of raising soil pH is often difficult to assess. Increasing soil pH stimulates the reduction of N2O to N2, thus lowering N2O emissions associated with denitrification, but can also increase autotrophic nitrification rates and related N2O emission. Using a 15N tracing technique, we provide process‐based insights into the effects of two acid‐neutralizing materials (quicklime [CaO] vs. pig manure) on N2O emissions in an acidified upland soil that had experienced excessive N application. Without pH adjustments we found that N2O emissions, stimulated by supply of reactive N, were related to denitrification‐ and heterotrophic nitrification‐derived N2O emissions, whereas autotrophic nitrification‐derived N2O emissions declined with decreasing soil pH. These effects were reversed by increasing soil pH via liming. However, increasing the soil pH via application of pig manure significantly increased soil N2O emissions from both nitrification and denitrification. Our study highlights that pH‐amelioration practices may enhance N2O emissions depending on the type of material applied to the soil. Therefore, both pH remediation and greenhouse gas mitigation options need to be considered together to avoid adverse environmental effects. The effect of different acid‐neutralizing materials on soil N2O emissions should be incorporated into ecosystem models to better estimate global N2O emissions when pH amelioration is practised.Highlights Enhanced N2O emission by N input was from denitrification and heterotrophic nitrification. Chemical N input and liming have reversible effects on N2O emission. Soil N2O emission was decreased by liming but increased by animal manure input. Careful consideration of pH raising substrates is needed to avoid adverse effects.

Soil N transformation rates are not linked to fertilizer N losses in vegetable soils with high N input

Vegetable soils under plastic shed are characterized by multiple cropping and extremely high rates of nitrogen (N) application, but the N use efficiency (NUE) is generally low and the rate of N loss is often high. The underlying mechanisms behind these N losses following long-term N fertilizer applications remain elusive. Here, a 15N tracing incubation study in conjunction with in-situ monitoring was performed to investigate the effects of different N fertilization regimes on gross N transformations, microbial abundance and N losses via NO3− leaching and N2O emissions in a vegetable soil under plastic shed subjected to 14 years of repeated N fertilization. The field experiment comprised of six treatments: no N fertilization (control, CK), and applications of chicken manure (M), chemical N plus M (CM), CM plus straw (CMS), 42 % of chemical N plus chicken manure (RCM) and RCM plus straw (RCMS). The results showed that the total gross N mineralization rates were significantly higher in CMS, RCMS and M treatments than in CK, CM and RCM treatments. The total gross NH4+ immobilization rates followed the order of CK > CM > CMS > M > RCM ≈ RCMS, indicating a decreasing NH4+ immobilization rate after N application. Gross autotrophic nitrification rate was significantly enhanced by fertilizer application in the order: CK < CM < CMS ≈ RCM < M < RCMS, mainly due to both increased mineralization rates and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB). A significant negative relationship between gross rates of NH4+ immobilization and autotrophic nitrification suggests that higher autotrophic nitrification decreased NH4+ immobilization. Reduced chemical N input alleviated soil acidification, NO3− leaching and N2O emissions. There were significant positive relationships between rates of chemical N input and N loss but no relationships between gross N transformation rates and tomato yields, NO3− leaching losses and N2O emissions. Thus, soil N transformation rates do not reflect rate of fertilizer N losses in vegetable soil managed under plastic shed. We thus suggest that chemical N application rate could be reduced to decrease N losses while sustaining vegetable production.

Dissimilatory nitrate reduction to ammonium dominates soil nitrate retention capacity in subtropical forests

The abundance, community composition and activity of key nitrogen (N)-cycling functional guilds were monitored at a site where coniferous Cunninghamia lanceolate and broadleaved Mytilaria laosensis were planted in 1993. Leaf cellulose, litter C/N ratio, leaf dry matter content, soil inorganic N content were significantly higher under C. lanceolata, while specific leaf area, litter production, litter magnesium, soil C and soil C/NO3− ratio were higher under M. laosensis. The 15N tracing experiment together with quantitative PCR revealed that autotrophic nitrification rates, as well as the abundances of ammonia-oxidizing archaea and comammox Nitrospira were significantly higher under C. lanceolate than M. laosensis. However, M. laosensis exhibited substantially higher nitrate retention capacity via dissimilatory nitrate reduction to ammonium (DNRA), accompanied by a significantly higher abundance of nrfA gene than C. lanceolate. The Illumina sequencing indicated that tree species markedly affected soil bacterial community composition regardless of the soil layers. Redundancy analysis suggested that litter C/N ratio was the most influential factor explaining functional gene abundances and bacterial communities. Taken together, our findings showed that M. laosensis improved soil N retention capacity mainly through inhibiting autotrophic nitrification while enhancing DNRA activity. This study highlights the importance of tree species identity in influencing the microbially-mediated N cycling and bacterial community composition.

Continuous Cropping Affects Gross Nitrogen Transformations in Subtropical Acidic Soils Under Greenhouse Cultivation

Relationship between gross N transformation rate and continuous cropping years in the greenhouse soil is largely unknown. This study aimed to investigate the mechanism of the gross N transformation rate changes in greenhouse subtropical acidic soils under different continuous cropping years. The 15N isotope tracer method was used to study the effects of greenhouse soil under 4 years of continuous cropping (C4), 5 years of continuous cropping (C5), and 7 years of continuous cropping (C7) of lily (Lilium brownii), respectively, on soil N transformation with the N tracer model. The results showed that the main soil N transformation processes were N mineralization, autotrophic nitrification, and NO3−-N immobilization under the three continuous cropping years. After continuous cropping, the gross N mineralization rate, autotrophic nitrification rate, and NO3−-N immobilization rate increased with the increase of continuous cropping years. Specifically, the gross N mineralization rate was 0.62~0.85 mg N kg−1 day−1, the autotrophic nitrification rate was 1.08~3.24 mg N kg−1 day−1, and the NO3−-N immobilization rate was 0.30~0.62 mg N kg−1 day−1. Moreover, the positive difference between the autotrophic nitrification rate and immobilization rate of NO3−-N became greater with the increase of continuous cropping years, which might have been the main reason for the accumulation of NO3−-N in greenhouse subtropical soils. These findings contribute to the understanding of the mechanism of soil secondary salinization formation and provide ideas for prevention of secondary salinization under long-term continuous cropping.

Divergent gross nitrogen transformation paths in the topsoil and subsoil between abandoned and agricultural cultivation land in irrigated areas.

The nitrate concentration in groundwater has increased in many irrigated areas worldwide due to the excessive use of both water and fertilizers. Abandoned farmlands in such irrigated areas may alter the nitrogen (N) cycle because of drastically changed water and N inputs. However, the mechanisms of the N cycle in response to such changes remain unclear. We studied biogeochemical N cycling and microbiological responses from abandoned arable lands (AF), for the topsoil (20cm depth) and subsoil (100cm depth) layers, in comparison with irrigation-fertilization (control=CK) land, by using 15N tracing techniques, the 16S rRNA gene, and real-time PCR (qPCR) to reveal the mechanisms underpinning the N cycle. We found that the biogeochemical environment of abandoned soils shifted their N-cycling pathways. Except for reduced soil moisture, soil properties of total C and N, as well pH, showed improvement in the two layers of AF. But the microbial abundances of ammonia-oxidizing bacteria (AOB-amoA), archaea (AOA-amoA), bacteria and fungi were all significantly lower in the AF; and they presented a consistent trend in the subsoil of the two lands. Significant differences in gross N transformation rates were found for mineralization rates (MN) and autotrophic nitrification rate (ONH4) between lands or depths. Compared with AF, MN was increased by 1.45- and 11.75-times, and ONH4 by 1.69- and 2.89-times in the topsoil and subsoil of CK, respectively. Our results suggest that the SM×C/N interaction provides insight into the mechanisms underlying the soil microbe-driven changes to transformation rates in nitrogen dynamics after abandoning water-limited lands. The high moisture and N inputs reported here highlight the dynamics and prevalence of MN and ONH4, and an increasing the nitrate leaching rate in the unsaturated zone, which poses a major threat to groundwater quality.