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Soil gross nitrification (GN) is a critical process in the global nitrogen (N) cycle that results in the formation of nitrate through microbial oxidation of ammonium or organic N, and can both increase N availability to plants and nitrous oxide emissions. Soil GN is thought to be mainly controlled by soil characteristics and the climate, but a comprehensive analysis taking into account the climate, soil characteristics, including microbial characteristics, and their interactions to better understand the direct and indirect controlling factors of GN rates globally is lacking. Using a global meta-analysis based on 901 observations from 330 15 N-labeled studies, we show that GN differs significantly among ecosystem types, with the highest rates found in croplands, in association with higher pH which stimulates nitrifying bacteria activities. Autotrophic and heterotrophic nitrifications contribute 63% and 37%, respectively, to global GN. Soil GN increases significantly with soil total N, microbial biomass, and soil pH, but decreases significantly with soil carbon (C) to N ratio (C:N). Structural equation modeling suggested that GN is mainly controlled by C:N and soil total N. Microbial biomass and pH are also important factors controlling GN and their effects are similar. Precipitation and temperature affect GN by altering C:N and/or soil total N. Soil total N and temperature drive heterotrophic nitrification, whereas C:N and pH drive autotrophic nitrification. Moreover, GN is positively related to nitrous oxide and carbon dioxide emissions. This synthesis suggests that changes in soil C:N, soil total N, microbial population size, and/or soil pH due to anthropogenic activities may influence GN, which will affect nitrate accumulation and gaseous emissions of soils under global climate and land-use changes., (© 2021 John Wiley & Sons Ltd.)

The properties of agricultural soils in various regions of the world are variable and can have a significant but poorly understood impact on soil nitrogen (N) transformations and nitrous oxide (N 2 O) emissions. For this reason, we undertook a study of gross N transformations and related N 2 O emissions in contrasting agricultural soils from China and the UK. Seven Chinese and three UK agricultural soils were collected for study using a 15 N tracing approach. The soil pH ranged from 5.4 to 8.7, with three acidic soils collected from Jinjing, Lishu and Boghall; one neutral soil collected from Changshu, and the other six alkaline soils collected from Quzhou, Zhangye, Changwu, Jinzhong, Boxworth and Stetchworth. Our results showed that the main N transformation processes were oxidation of ammonium (NH 4 + ) to nitrate (NO 3 - . The gross autotrophic nitrification rates calculated in the three acidic soils were between 0.25 and 4.15 mg N kg NH4 ), and mineralization of organic N to NH 4 + . The gross autotrophic nitrification rates calculated in the three acidic soils were between 0.25 and 4.15 mg N kg -1 . Generally, soil pH was positively correlated (p < 0.001) with gross autotrophic nitrification rate and cumulative N -1 O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N -1 O emissions. There was also a significant positive correlation between the gross autotrophic nitrification rate and cumulative N -1 . Generally, soil pH was positively correlated (p < 0.001) with gross autotrophic nitrification rate and cumulative N 2 O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N 2 immobilization rates were very low in most soils except for the Jinjing soil with the lowest pH. In conclusion, the gross autotrophic nitrification rates and related N 2 O emissions, highlighting the importance of this process for producing N 2 O emissions in these agricultural soils under aerobic conditions. Gross NH 4 + immobilization rates were very low in most soils except for the Jinjing soil with the lowest pH. In conclusion, the gross autotrophic nitrification rates and related N 2 O emissions were controlled by soil pH irrespectively of the soil's origin in these agricultural soils., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Nitrogen (N) deposition has rapidly increased and is influencing forest ecosystem processes and functions on a global scale. Understanding process-specific N transformations, i.e., gross N transformations, in forest soils in response to N deposition is of great significance to gain mechanistic insights on the linkages between global N deposition and N availability or loss in forest soils. In this paper, we review factors controlling N mineralization, nitrification and N immobilization, particularly in relation to N deposition, discuss the limitations of net N transformation studies, and synthesize the literature on the effect of N deposition on gross N transformations in forest ecosystems. We found that more than 97% of published papers evaluating the effect of N deposition (including N addition experiments that simulate N deposition) on soil N cycle determined net rates of mineralization and nitrification, showing that N deposition significantly increased those rates by 24.9 and 153.9%, respectively. However, studies on net N transformation do not provide a mechanistic understanding of the effect of N deposition on N cycling. To date, a small number of studies (<20 published papers) have directly quantified the effect of N deposition on gross N transformation rates, limiting our understanding of the response of soil N cycling to N deposition. The responses to N deposition of specific N transformation processes such as autotrophic nitrification, heterotrophic nitrification, dissimilatory nitrate reduction to ammonium, N mineralization, and N immobilization are poorly studied. Future research needs to use more holistic approaches to study the impact of N deposition on gross N transformation rates, N loss and retention, and their microbial-driven mechanisms to provide a better understanding of the processes involved in N transformations, and to understand the differential responses between forest and other ecosystems., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

A better understanding of nitrogen transformation in soils could reveal the capacity for biological inorganic N supply and improve the efficiency of N fertilizers. In this study, a (15)N tracing study was carried out to investigate the effects of converting woodland to orchard, and orchard age on the gross rates of N transformation occurring simultaneously in subtropical soils in Eastern China. The results showed that inorganic N supply rate was remained constant with soil organic C and N contents increased after converting woodland into citrus orchard and with increasing orchard age. This phenomenon was most probably due to the increase in the turnover time of recalcitrant organic-N, which increased with decreasing soil pH along with increasing orchard age significantly. The amoA gene copy numbers of both archaeal and bacterial were stimulated by orchard planting and increased with increasing orchard age. The nitrification capacity (defined as the ratio of gross rate of nitrification to total gross rate of mineralization) increased following the Michaelis-Menten equation, sharply in the first 10 years after woodland conversion to orchard, and increased continuously but much more slowly till 30 years. Due to the increase in nitrification capacity and unchanged NO3(-) consumption, the dominance of ammonium in inorganic N in woodland soil was shifted to nitrate dominance in orchard soils. These results indicated that the risk of NO3(-) loss was expected to increase and the amount of N needed from fertilizers for fruit growth did not change although soil organic N accumulated with orchard age., (Copyright © 2015. Published by Elsevier B.V.)

Summary: Subtropical acidic forest soil is an important source of global nitrous oxide (N2O). However, the rates of autotrophic nitrification and denitrification in such soil are less than in temperate forest soil. We hypothesized that this difference is related to different N2O production pathways. We carried out paired 15N‐labelling experiments under aerobic conditions (60% water‐holding capacity) in the laboratory to compare N2O production pathways between subtropical and temperate forest soils to determine the differences in N2O production based on controlling factors. Our results showed that the contributions of NH4+ oxidation to NO3− to total N2O production (CNH4) were small in all soils studied (21–30%). Contributions of denitrification to total N2O production (CNO3) in subtropical forest soil (34%) were significantly less than those in temperate forest soil (54%). However, rates of N2O emission from denitrification (N2Od) were similar between subtropical and temperate forest soils, indicating that denitrification was probably not the predominant process causing the difference between the rates of N2O production in these soils. The average contribution of heterotrophic nitrification to total N2O production (CON) was significantly larger in subtropical forest soil (45%) than in temperate forest soil (15%). Soil pH and the C:N ratio were identified as key factors, with CON negatively correlated with soil pH (r = −0.60, P < 0.05) and positively correlated with soil C:N ratio (r = 0.78, P < 0.01), although the significance was probably weakened when forest types were taken into consideration. The rate of N2O production by heterotrophic nitrification (N2Oh) (average, 3.0 μg N kg−1 day−1) was also significantly larger in subtropical forest soil than in temperate forest soil (average, 0.7 μg N kg−1 day−1). Therefore, based on this study, the heterotrophic N2O pathway seems responsible for the larger N2O production in subtropical acid forest soil than in temperate forest soil. This study is among the first to elucidate in detail contributions to the processes of N2O production and to account for its large rates of production in subtropical acidic forest soil in China, which has implications for the prediction of N2O production in forest soil using ecosystem modelling. Highlights: Examined N2O production pathways in subtropical and temperate forest soils. Rate of N2O emission in subtropical forest soil was larger than temperate forest soil. Heterotrophic nitrification was responsible for large N2O production in subtropical soil. Soil pH and C:N ratio were identified as key factors affecting the N2O production pathway. [ABSTRACT FROM AUTHOR]

The nitrogen (N) cycle contains two different processes of dissimilatory nitrate (Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.) reduction, denitrification and dissimilatory Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. reduction to ammonium (DNRA). While there is general agreement that the denitrification process takes place in many soils, the occurrence and importance of DNRA is generally not considered. Two approaches have been used to investigate DNRA in soil, (1) microbiological techniques to identify soil microorganisms capable of DNRA and (2)15N tracing to elucidate the occurrence of DNRA and to quantify gross DNRA rates. There is evidence that many soil bacteria and fungi have the ability to perform DNRA. Redox status and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. ratio have been identified as the most important factors regulating DNRA in soil.15N tracing studies have shown that gross DNRA rates can be a significant or even a dominant Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. consumption process in some ecosystems. Moreover, a link between heterotrophic nitrification and DNRA provides an alternative pathway of ammonium (Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.) production to mineralisation. Numerical N tracing models are particularly useful when investigating DNRA in the context of other N cycling processes. The results of correlation and regression analyses show that highest gross DNRA rates can be expected in soils with high organic matter content in humid regions, while its relative importance is higher in temperate climates. With this review we summarise the importance and current knowledge of this often overlooked Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. consumption process within the terrestrial N cycle. We strongly encourage considering DNRA as a relevant process in future soil N cycling investigations. [ABSTRACT FROM AUTHOR]

The aim of the present study was to separate and quantify process-specific gross nitrite (NO2−) production in a permanent grassland soil and separate the observed NO2− concentration into process-specific sub-pools. We re-analysed data from a laboratory experiment where either the nitrate and/or ammonium ions were labelled with 15N. The simultaneously occurring N transformations were quantified with a 15N tracing tool in combination with a Monte Carlo sampling technique which has been extended by a NO2− sub-model. Three pathways of NO2− production were identified, that is autotrophic nitrification, denitrification and the oxidation of organic N. The full model contained nine pools and 14 N transformations. This is the first report where such a complex 15N tracing model has been used and where all parameters have been optimized simultaneously. Our analysis shows that on average 52.4% of the total soil NO2− concentration was directly derived from oxidation of organic N. Nitrification and denitrification contributed on average 18.1 and 29.5%, respectively, to the total NO2− pool. However, more than 99% of the gross NO2− production was related to the NH4+ oxidation, which was rapidly further oxidized to NO3−. Our re-analysis using a Monte Carlo sampling technique resulted in parameter profiles that differed from the original analysis. As the new technique is able to optimize all parameters in one optimization run we regard this technique as superior to the previous one. A process-based understanding of NO2− dynamics in soils has important implications for the understanding of the production of the greenhouse gas nitrous oxide (N2O), which is tightly linked to NO2− turnover. [ABSTRACT FROM AUTHOR]

The contribution of bacteria and fungi to NH4+ and organic N (Norg) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g−1 oven-dry soil. The size and enrichment of the mineral N pools from differentially (NH415NO3 and 15NH4NO3) and doubly labelled (15NH415NO3) NH4NO3 were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH4+-N and NO3−-N at 3.15 μmol N g−1 oven-dry soil. The N treatments were enriched to 60 atom % excess in 15N and acetate was added at 100 μmol C g−1 oven-dry soil, to provide a readily available carbon source. The oxidation rates of NH4+ and Norg were analysed separately for each inhibitor treatment with a 15N tracing model. In the absence of inhibitors, the rates of NH4+ oxidation and organic N oxidation were 0.0045 μmol N g−1 hour−1 and 0.0023 μmol N g−1 hour−1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH4+ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH4+ and organic N. [ABSTRACT FROM AUTHOR]

The objective of this study was to separate the observed nitrite (NO2–) concentration in a permanent grassland soil into process-specific subpools. A laboratory experiment was carried out where either the nitrate and/or ammonium pool was labelled with 15N at 60 atom % excess. The main N transformations that occurred in this experiment were analysed with a 15N tracing model extended with a NO2– submodel. Techniques that have been used to date have been only able to identify NO2– subpools related to nitrification (NO2–nit) and denitrification (NO2–den). With the analysis presented here, we were able to quantify the size of an additional NO2– pool in the soil related to organic N turnover (NO2–org). All transformations related to NO2– turnover of the three subpools occurred simultaneously. After non-linear parameter optimization the model predicted that on average NO2–den, NO2–nit and NO2–org pools contributed 57, 33 and 10% to the total soil NO2– concentration. The finding that heterotrophic processes can contribute to the NO2– dynamics in permanent grassland soils might also have important implications for the understanding of gaseous N production that are tightly linked to NO2– turnover. Further work is needed to find out how important the conversion of organic N to NO2– is in other soil-based ecosystems and to identify the microbial groups responsible for this process. [ABSTRACT FROM AUTHOR]

High nitrification rates which convert ammonium (NH4+) to the mobile ions NO2− and NO3− are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N. Selective nitrification inhibitors and 15N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a 15N tracing model was developed. This model consists of the three N-pools NH4+, NO3− and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH4+ to NO3− and not the conversion of organic N to NO3−. There was evidence that slurry-enhanced oxidation of NH4+ to NO3− was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH4+ by approximately a factor of three. [Copyright &y& Elsevier]

A laboratory incubation study was conducted with nine UK soils to determine the effect of soil physical and chemical properties, and temperature, on the efficacy the nitrification inhibitor dicyandiamide (DCD). Nitrogen was applied to soil as ammonium chloride at a rate of 100 μg N g −1 dry soil, and incubated at 60% water-filled-pore-space at either 5, 15 or 25 °C. The ammonium ( NH 4 + ) pool was enriched with 15 N to 60 atom% excess and DCD was applied at a rate of 10 μg g −1 dry soil. The concentrations and enrichments of the NH 4 + and nitrate ( NO 3 − ) pools, along with nitrous oxide (N 2 O) flux measurements, were determined regularly for 60 days after N application. Gross soil N transformation rates were quantified with a 15 N tracing model. The persistence of DCD was strongly related ( P < 0.001) to temperature with the measured half-life across all soils of 89, 37 and 18 days at 5, 15, and 25 °C, respectively. There was wide variation in the half-life of DCD among soils; which was predominantly associated with the soil oxalate extractable Fe concentration. Greater ( P < 0.001) inhibition in autotrophic nitrification by DCD occurred at 5 and 15 °C compared to 25 °C. Across all soils and temperatures DCD increased the rate of mineralisation of recalcitrant organic-N and the rate of adsorption of free NH 4 + , however, effects varied between soils. DCD did not have a significant effect on the rate of oxidation of recalcitrant organic-N to NO 3 − or on any of the immobilisation processes or mineralisation of labile N to NH 4 + . The efficacy of DCD in inhibiting net NO 3 − production best correlated with soil Cu ( r = −0.82), % clay ( r = −0.71), total N ( r = −0.66) and LOI ( r = −0.61). Stepwise multiple regression showed that Cu, oxalate extractable Fe and oxalate extractable Al explained 85.0% of the variation in the percentage inhibition of net NO 3 − production by DCD. The inhibitor also reduced cumulative N 2 O emissions, with reductions negatively correlated with a range of soil properties associated with organic matter. We provide evidence that the interaction between temperature, soil clay content and soil organic matter governs the efficacy of DCD. The grassland soils had higher native total N concentrations than the arable soils, hence the inhibition of net NO 3 − production by DCD was lower and this resulted in an overall inhibition in N 2 O emissions of 58% and 81% for grassland and arable soils respectively. [ABSTRACT FROM AUTHOR]

Abstract: A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH4 +), nitrate (NO3 −), labile (Nlab) and recalcitrant (Nrec) soil organic matter and adsorbed NH4 +), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of Nlab, while immobilization of NH4 + predominantly entered the Nrec pool. A fast exchange between the NH4 + and the adsorbed NH4 + pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH4 +. Moreover, high NH4 + immobilization rates into the Nrec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH4 + oxidation and organic N oxidation. More than 86% of the NO3 − produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO3 − consumption. DNRA rather than NO3 − immobilization ensures that NO3 − is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls. [Copyright &y& Elsevier]

A laboratory incubation study was conducted on a temperate grassland soil to quantify the main mineral nitrogen (N) transformation rates and pathways via a15N tracing approach. Soil samples were taken from a long-term phosphorus (P) trial to investigate the effects on gross N-transformations under high and low phosphorus amendment. The soils were incubated over a 2-week period and treated with ammonium-nitrate (NH 4 NO 3) which was applied to the soil both with and without a glucose amendment and labelled with 15N either on the ammonium (NH 4 +) or nitrate (NO 3 −) moiety at 50% atom enrichment. The results showed immobilisation to greatly outweigh mineralisation and that NO 3 − was predominantly produced via heterotrophic nitrification. Individual pathways for NO 3 − production were quantified including oxidation of NH 4 +, recalcitrant and labile organic N. Oxidation of labile organic N to NO 3 −, a newly considered pathway, accounted for between 63 and 83% of total NO 3 − production across the various treatments and P levels. This process was significantly higher in the low-P rather than the high-P soils (p < 0.05), highlighting the effect of soil P on the microbial community. • The relative abundance of C, N and P significantly influences N-transformations. • A novel nitrification pathway of oxidation of labile organic N to NO 3 − was found. • This pathway of accounted for between 63 and 83% of total NO 3 − production. • This novel pathway was significantly higher in the low-P soils when not C-limited. [ABSTRACT FROM AUTHOR]