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Nitrogen (N) deposition can profoundly alter soil N transformation processes and the long-term productivity of forest ecosystems. The response of soil gross N transformations to N deposition in forest ecosystems has been well studied through simulated N addition experiments. Simulated N addition experiments are conducted under a wide range of background N deposition rates. However, it remains unclear whether the response of soil gross N transformation rates to simulated N addition is dependent on background N deposition rates. Here, we collate results from the literature in forest ecosystems, and found, for the first time, that the responses of gross rates of N mineralization, nitrification, and NO3− immobilization to experimental N addition changed from positive to negative with increasing background N deposition rates with the thresholds for such changes were 3.23, 6.02, 1.90 kg N ha− 1 yr− 1, respectively. Our results suggest that background N deposition rates shall be incorporated into ecosystem models to better predict forest ecosystem N cycling under future N deposition scenarios. [ABSTRACT FROM AUTHOR]

Aims: Long-term application of pig manure can improve soil fertility and alleviate soil acidification, but also increase nitrogen (N) losses in subtropical upland red soils. However, mechanisms driving N losses via nitrate leaching or NO emissions remain unknown. Herein we investigated long-term pig manure applications in upland red soils by assessing soil N transformation dynamics. Methods: Pig manure was applied with or without lime over a 14-year period in four treatments: No manure (CK); Low-rate manure (LM, 150 kg N ha y); High-rate manure (HM, 600 kg N ha y); High-rate manure and lime (HML, 600 kg N ha y and 3000 kg Ca(OH) ha (3y)). N tracing was used to quantify gross N transformation and N dynamics. Results: Prolonged manure application increased soil gross N mineralization and NH immobilization, although the increase was only significant for HM. Both rates were further enhanced by lime addition. Gross autotrophic nitrification also increased with increasing manure application, and further increased with lime addition. In contrast, dissimilatory NO reduction to NH (DNRA) and NO immobilization were negligible irrespective of manure application. Thus, NO produced via autotrophic nitrification was not converted to NH and microbial biomass N, and accumulated in soil. Gross autotrophic nitrification was positively correlated with NO emissions and NO leaching, suggesting it largely determined N losses. Conclusions: Autotrophic nitrification governs N losses in upland red soils receiving repeated manure applications, and attempts to reduce N emissions or N leaching should therefore control this. [ABSTRACT FROM AUTHOR]

Purpose: Reductive soil disinfestations (RSDs), as an alternative to chemical fumigations, have been widely applied to improve degraded greenhouse vegetable soils. However, high nitrous oxide (NO) emission and nitrate (NO) leaching would occur during RSD treatment. The aim of this study was to investigate the effect of pH on the removal of NO and its transformed products during RSD treatment. Materials and methods: A N tracing incubation experiment was conducted in a degraded vegetable soil under 30 °C for 360 h. There were five treatments: control (flooding only, noted as CK), flooding + alfalfa (RSD), and RSD + lime at three application rates, which adjusted soil pH to 6.7, 7.7, and 8.4, noted as RSD, RSD, and RSD, respectively. Results and discussion: Increase in pH accelerated the removal of NO under flooding condition, with the decrease in Eh. It took 120 and 72 h for RSD and RSD + lime treatments, respectively, when soil NO content dropped from initial 147 to below 25 mg N kg, but NO content in the CK was still 32.1 mg N kg even after 360-h incubation. Liming inhibited dissimilatory NO reduction to ammonium (DNRA) process and immobilization of NO into organic nitrogen (N). After 360-h incubation, the ratios of N transformed into NH and organic N pools accounted for 0.46-1.20 % and 3.11-9.48 % of added KNO in RSD treatments, being lower than 2.17 and 16.8 % in CK, respectively. The ratio of cumulative NO emissions to NO disappeared from soil was greatly lower in RSD treatments (0.80-2.29 %) than in CK (5.20 %). Conclusions: RSD associated with liming accelerated the NO removal and reduced the NO emission but decreased the conversion rates of NO to NH and organic N. [ABSTRACT FROM AUTHOR]

Purpose: In the rice soils, the alternate wetting and drying conditions and the leakage of O from rice roots provides a favorable environment for many N transformation processes, such as DNRA, nitrification, mineralization, and denitrification. Since nitrification is an inevitable process providing available NO for DNRA and other NO loss pathways, it is logical to hypothesize that a relationship exists between nitrification and DNRA. Thus, we quantified the specific gross N transformation rates occurring simultaneously to investigate the correlation among the N transformation rates in the rice soil. Material and methods: In this study, three rice soils with different pH values were selected. The combination of N tracing experiment and numerical modeling method was performed to quantify the specific gross N transformation rates (e.g., DNRA), to analyze the correlation between DNRA and the other N transformations, and to estimate the functional role of DNRA comparing to other N transformation in the rice soil. Results and discussion: The results showed that there were two NH production pathways, organic N mineralization and DNRA, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, while the DNRA process was negligible in acidic soil (pH 4.7). The mineralization rate in the acidic soil (2.69 mg kgd) was significantly higher than that in neutral and alkaline soils. The DNRA rate was 0.48 and 1.09 mg kgd, which was almost equal to the mineralization rate, in the neutral (pH 6.2) and alkaline (pH 8.2) soils, offsetting efficiently the effect of low mineralization on N availability in ecosystems. The DNRA rate linearly increased with the increase in gross nitrification rate ( p < 0.05). This significant coupling relationship between DNRA and nitrification favors N conservation and availability for paddy growth in the rice ecosystem. Conclusions: DNRA plays an important role in soil N conservation and availability in neutral and alkaline but not acidic rice soil. [ABSTRACT FROM AUTHOR]

Denitrification and its products composition were evaluated in four typical Chinese paddy soils with pH (HO) ranging from 4.80 to 8.29 after application of 50 or 100 mg kg soil KNO and subsequent anaerobic incubation. Denitrification rates, which were indicated by nitric oxide (NO), nitrous oxide (NO), and dinitrogen gas (N) production, significantly varied among different paddy soils. The denitrification rates of the neutral and alkaline paddy soils were 2.6 to 16.6 times higher, respectively, than those of acidic paddies. Furthermore, denitrification in paddy soils could produce end products other than N, and the product composition depended on the paddy soil type. The percentage of total N gases (NO + NO + N) present as NO was negatively and linearly correlated with denitrification rate ( P < 0.05). Soil pH and C/N showed positive effects on denitrification rate ( r = 0.800 and r = 0.781, respectively, P < 0.05 for both), but negative effects on the percentage of total N gases present as NO ( r = −0.976, P < 0.01 and r = −0.781, P < 0.05, respectively). Denitrification rate and the percentage of total gases present as NO increased as the nitrate (NO) concentration increased. However, there was no effect of NO concentration on the percentage of total N gases present as NO. Our results indicate that the potential N loss through denitrification may be higher in alkaline paddies than that in neutral and acidic paddies. Moreover, the variation of the NO percentage in denitrification products of different paddy soils should be considered when estimating the denitrification-derived NO emission and when calculating the N budget in paddy soils. [ABSTRACT FROM AUTHOR]

Background and aims: Continuous vegetable cultivation in greenhouses can easily induce soil degradation, which considerably affects the development of sustainable vegetable production. Recently, the reductive soil disinfestation (RSD) is widely used as an alternative to chemical soil disinfestations to improve degraded greenhouse vegetable soils. Considering the importance of nitrogen (N) for plant growth and environment effect, the internal N transformation processes and rates should be well investigated in degraded vegetable soils treated by RSD, but few works have been undertaken. Methods: Three RSD-treated and three untreated degraded vegetable soils were chosen and a N tracing incubation experiment differentially labeled with NHNO or NHNO was conducted at 25 °C under 50 % water holding capacity (WHC) for 96 h. Soil gross N transformation rates were calculated using a N tracing model combined with Markov Chain Monte Carlo Metropolis algorithm (Müller et al. ), while the emissions of NO and NO were also measured. Results: RSD could significantly enhance the soil microbial NH immobilization rate, the heterotrophic and autotrophic nitrification rates, and the NO turnover time. The ratio of heterotrophic nitrification to total inorganic N supply rate (mineralization + heterotrophic nitrification) increased greatly from 5.4 % in untreated vegetable soil to 56.1 % in treated vegetable soil. In addition, low release potential of NO and NO was observed in RSD-treated vegetable soil, due to the decrease in the NO and NO product ratios from heterotrophic and autotrophic nitrifications. These significant differences in gross N transformation rates, the supply processes and capacity of inorganic N, and the NO and NO emissions between untreated and treated vegetable soils could be explained by the elimination of accumulated NO, increased pH, and decreased electrical conductivity (EC) caused by RSD. Noticeably, the NO consumption rates were still significantly lower than the NO production rates in RSD-treated vegetable soil. Conclusions: Except for improving soil chemical properties, RSD could significantly alter the supply processes of inorganic N and reduce the release potential of NO and NO in RSD-treated degraded vegetable soil. In order to retard the re-occurrence of NO accumulation, acidification and salinization and to promote the long-term productivity of greenhouse vegetable fields, the rational use of N fertilizer should be paid great attention to farmers in vegetable cultivation. [ABSTRACT FROM AUTHOR]

Temperature is an important factor that affects nitrogen (N) transformation in soils. In this study, variations in the gross and net N transformation rates between two Chinese paddy soils, namely, hydragric anthrosol (HA) and anthraquic cambisol (AC) in response to temperature (5, 15, 25, and 35 °C) were investigated by conducting an aerobic N dilution incubation experiment. The short-term (11 days) dynamics of gross N transformation rates was estimated using the numerical model FLUAZ. The results showed that increasing incubation temperature caused an increased gross and net transformation in the tested soils. However, the sensitivity to temperature fluctuation varied among the N transformation processes. Nitrogen mineralization was less sensitive than immobilization and nitrification at low temperatures (<15 °C). The optimum temperature for gross N nitrification in the studied paddy soils was approximately 25 °C. Higher gross N mineralization and gross N nitrification were observed in soil AC than in soil HA and were attributed to its higher pH (pH = 8.3), but further confirmation is needed. The ammonium (NH) in studied paddy soils was mostly consumed through the nitrification process compared with that in the immobilization process, especially in alkaline soil AC. High consumption of NH in alkaline soil AC might lead to a high risk of N loss through leaching and denitrification in case the generated nitrate (NO) was not absorbed by crops in time. [ABSTRACT FROM AUTHOR]

Nitrous oxide emission (NO) from applied fertilizer across the different agricultural landscapes especially those of rainfed area is extremely variable (both spatially and temporally), thus posing the greatest challenge to researchers, modelers, and policy makers to accurately predict NO emissions. Nitrous oxide emissions from a rainfed, maize-planted, black soil (Udic Mollisols) were monitored in the Harbin State Key Agroecological Experimental Station (Harbin, Heilongjiang Province, China). The four treatments were: a bare soil amended with no N (C0) or with 225 kg N ha (CN), and maize ( Zea mays L.)-planted soils fertilized with no N (P0) or with 225 kg N ha (PN). Nitrous oxide emissions significantly ( P < 0.05) increased from 141 ± 5 g NO-N ha (C0) to 570 ± 33 g NO-N ha (CN) in unplanted soil, and from 209 ± 29 g NO-N ha (P0) to 884 ± 45 g NO-N ha (PN) in planted soil. Approximately 75 % of NO emissions were from fertilizer N applied and the emission factor (EF) of applied fertilizer N as NO in unplanted and planted soils was 0.19 and 0.30 %, respectively. The presence of maize crop significantly ( P < 0.05) increased the NO emission by 55 % in the N-fertilized soil but not in the N-unfertilized soil. There was a significant ( P < 0.05) interaction effect of fertilization × maize on NO emissions. Nitrous oxide fluxes were significantly affected by soil moisture and soil temperature ( P < 0.05), with the temperature sensitivity of 1.73-2.24, which together explained 62-76 % of seasonal variation in NO fluxes. Our results demonstrated that NO emissions from rainfed arable black soils in Northeast China primarily depended on the application of fertilizer N; however, the EF of fertilizer N as NO was low, probably due to low precipitation and soil moisture. [ABSTRACT FROM AUTHOR]

study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a N tracing study. The results showed that N dynamics were dominated by NH turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH + NO) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH production in the broad-leaf forest soil to a balanced production of NH and NO in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH or also a high NO turnover is present. [ABSTRACT FROM AUTHOR]

Little information is available on the effects of urease inhibitor, N-( n-butyl)thiophosphoric triamide (NBPT), and nitrification inhibitor, dicyandiamide (DCD), on nitrous oxide (NO) emissions from fluvo-aquic soil in the North China Plain. A field experiment was conducted at the Fengqiu State Key Agro-Ecological Experimental Station, Henan Province, China, to study the influence of urea added with NBPT, DCD, and combination of both NBPT and DCD on NO emissions during the maize growing season in 2009. Two peaks of NO fluxes occurred during the maize growing season: the small one following irrigation and the big one after nitrogen (N) fertilizer application. There was a significant positive relationship between ln [NO flux] and soil moisture during the maize growing season excluding the 11-day datasets after N fertilizer application, indicating that NO flux was affected by soil moisture. Mean NO flux was the highest in the control with urea alone, while the application of urea together with NBPT, DCD, and NBPT + DCD significantly lowered the mean NO flux. Total NO emission in the NBPT + DCD, DCD, NBPT, and urea alone treatments during the experimental period was 0.41, 0.47, 0.48, and 0.77 kg NO-N ha, respectively. Application of urea with NBPT, DCD, and NBPT + DCD reduced NO emission by 37.7%, 39.0%, and 46.8%, respectively, over urea alone. Based on our findings, the combination of DCD and NBPT together with urea may reduce NO emission and improve the maize yield from fluvo-aquic soil in the North China Plain. [ABSTRACT FROM AUTHOR]

Abstract: Reactive forms of nitrogen (Nr) are accumulating at local, regional and global levels largely due to human activities, particularly N-fertilizer production and use as well as fossil fuel combustion. This has resulted in a change in the nitrogen (N) cycle and excess Nr in the environment, which has negative environmental effects. Therefore, characterizing denitrification and the edaphic variables controlling denitrification and its products is the first step in predicting the long-term effects of Nr accumulation. In the present study, six forest soil types in different climatic zones were collected from East China and evaluated for denitrification products following a K15NO3 amendment and subsequent incubation. The results showed that denitrification, indicated by production of nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2), was higher in the studied temperate forest soils than in the studied subtropical and tropical forest soils and was negatively correlated with soil redox potential at the beginning of incubation (r = −0.94, P < 0.01), but not with soil pH. The ratios of NO/total N gas and N2O/total N gas produced during denitrification varied among the soils, and were generally higher in the subtropical and tropical soils. Spearman''s correlation analysis showed that the NO ratio was positively correlated with soil oxidation capacity (OXC) (r = 0.94, P < 0.01) and redox potential at the beginning of incubation (r = 0.86, P < 0.05), but negatively correlated with soil pH (r = −0.83, P < 0.05). The N2O ratio was not significantly correlated with these edaphic variables, but showed a significant correlation to NO ratio (r = 0.83, P < 0.05). These results suggested that the OXC value might be the key factor affecting denitrification rates in soils. One possible explanation for these effects is that large OXC values would result in a higher level soil redox potential, thus suppressing denitrification and enhancing NO and N2O ratios during denitrification. [Copyright &y& Elsevier]

In the initial online publication the figure legends for Figures 1 and 2 were reversed. The original article has been corrected. [ABSTRACT FROM AUTHOR]

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 (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. • Levels of ammonium addition caused a shift of dominant N conversion process. • Ammonium addition inhibited soil nitrate assimilation rate by enhancing gross nitrification and ammonium assimilation rates. • Elevated ammonium content increased the ratios of nitrification to ammonium assimilation and to gross N assimilation. [ABSTRACT FROM AUTHOR]

Aims: Soil nitrogen (N) mineralization-immobilization turnover (MIT) regulates the inorganic N supply in terrestrial ecosystems. Much research has been done to understand the factors that control the MIT in soils, but how plants-soil-microbe interactions influence the MIT remains to be further explored.A series of 15N tracing experiments, including with and without maize (Zea mays L.) planting, maize shading, and maize root endophytes inoculation, were conducted to investigate the drivers of the change in MIT by the presence of plant.Soil gross N mineralization (M), especially the mineralization of recalcitrant organic-N to NH4+ (MNrec, 0.51 mg N kg−1 d−1) was significantly improved by maize compared with the control treatment (without maize) (0.07 mg N kg−1 d−1). MNrec significantly decreased after removing maize or covering maize with a black box (preventing photosynthesis). Soil dissolved organic carbon (DOC) concentration significantly decreased after preventing photosynthesis, and showed a significant positive relationship with MNrec, confirming that photosynthetic substrate supply was the dominating factor in stimulating MNrec. Simultaneously, the release of absorbed NH4+ on the cation exchange sites increased with decreasing MNrec when photosynthesis was prevented. Microbial N immobilization (I), especially NO3− immobilization rate (INO3), was significantly stimulated in all maize treatments compared to the control. The INO3 of unsterilized soil applied with unsterilized plant endophytes was significantly higher than unsterilized soil applied with sterilized endophytes, indicating a close relationship between endophytes and microorganisms on INO3.MNrec was stimulated by the photosynthetic substrate supply, and the increasing microbial N immobilization induced by plant significantly increased the ratio of I to M in the presence of maize, which was beneficial to soil N retention and reduced N loss. Our results provide new insights of the MIT for better understanding the productivity of agricultural systems and its driving factors.Methods: Soil nitrogen (N) mineralization-immobilization turnover (MIT) regulates the inorganic N supply in terrestrial ecosystems. Much research has been done to understand the factors that control the MIT in soils, but how plants-soil-microbe interactions influence the MIT remains to be further explored.A series of 15N tracing experiments, including with and without maize (Zea mays L.) planting, maize shading, and maize root endophytes inoculation, were conducted to investigate the drivers of the change in MIT by the presence of plant.Soil gross N mineralization (M), especially the mineralization of recalcitrant organic-N to NH4+ (MNrec, 0.51 mg N kg−1 d−1) was significantly improved by maize compared with the control treatment (without maize) (0.07 mg N kg−1 d−1). MNrec significantly decreased after removing maize or covering maize with a black box (preventing photosynthesis). Soil dissolved organic carbon (DOC) concentration significantly decreased after preventing photosynthesis, and showed a significant positive relationship with MNrec, confirming that photosynthetic substrate supply was the dominating factor in stimulating MNrec. Simultaneously, the release of absorbed NH4+ on the cation exchange sites increased with decreasing MNrec when photosynthesis was prevented. Microbial N immobilization (I), especially NO3− immobilization rate (INO3), was significantly stimulated in all maize treatments compared to the control. The INO3 of unsterilized soil applied with unsterilized plant endophytes was significantly higher than unsterilized soil applied with sterilized endophytes, indicating a close relationship between endophytes and microorganisms on INO3.MNrec was stimulated by the photosynthetic substrate supply, and the increasing microbial N immobilization induced by plant significantly increased the ratio of I to M in the presence of maize, which was beneficial to soil N retention and reduced N loss. Our results provide new insights of the MIT for better understanding the productivity of agricultural systems and its driving factors.Results: Soil nitrogen (N) mineralization-immobilization turnover (MIT) regulates the inorganic N supply in terrestrial ecosystems. Much research has been done to understand the factors that control the MIT in soils, but how plants-soil-microbe interactions influence the MIT remains to be further explored.A series of 15N tracing experiments, including with and without maize (Zea mays L.) planting, maize shading, and maize root endophytes inoculation, were conducted to investigate the drivers of the change in MIT by the presence of plant.Soil gross N mineralization (M), especially the mineralization of recalcitrant organic-N to NH4+ (MNrec, 0.51 mg N kg−1 d−1) was significantly improved by maize compared with the control treatment (without maize) (0.07 mg N kg−1 d−1). MNrec significantly decreased after removing maize or covering maize with a black box (preventing photosynthesis). Soil dissolved organic carbon (DOC) concentration significantly decreased after preventing photosynthesis, and showed a significant positive relationship with MNrec, confirming that photosynthetic substrate supply was the dominating factor in stimulating MNrec. Simultaneously, the release of absorbed NH4+ on the cation exchange sites increased with decreasing MNrec when photosynthesis was prevented. Microbial N immobilization (I), especially NO3− immobilization rate (INO3), was significantly stimulated in all maize treatments compared to the control. The INO3 of unsterilized soil applied with unsterilized plant endophytes was significantly higher than unsterilized soil applied with sterilized endophytes, indicating a close relationship between endophytes and microorganisms on INO3.MNrec was stimulated by the photosynthetic substrate supply, and the increasing microbial N immobilization induced by plant significantly increased the ratio of I to M in the presence of maize, which was beneficial to soil N retention and reduced N loss. Our results provide new insights of the MIT for better understanding the productivity of agricultural systems and its driving factors.Conclusions: Soil nitrogen (N) mineralization-immobilization turnover (MIT) regulates the inorganic N supply in terrestrial ecosystems. Much research has been done to understand the factors that control the MIT in soils, but how plants-soil-microbe interactions influence the MIT remains to be further explored.A series of 15N tracing experiments, including with and without maize (Zea mays L.) planting, maize shading, and maize root endophytes inoculation, were conducted to investigate the drivers of the change in MIT by the presence of plant.Soil gross N mineralization (M), especially the mineralization of recalcitrant organic-N to NH4+ (MNrec, 0.51 mg N kg−1 d−1) was significantly improved by maize compared with the control treatment (without maize) (0.07 mg N kg−1 d−1). MNrec significantly decreased after removing maize or covering maize with a black box (preventing photosynthesis). Soil dissolved organic carbon (DOC) concentration significantly decreased after preventing photosynthesis, and showed a significant positive relationship with MNrec, confirming that photosynthetic substrate supply was the dominating factor in stimulating MNrec. Simultaneously, the release of absorbed NH4+ on the cation exchange sites increased with decreasing MNrec when photosynthesis was prevented. Microbial N immobilization (I), especially NO3− immobilization rate (INO3), was significantly stimulated in all maize treatments compared to the control. The INO3 of unsterilized soil applied with unsterilized plant endophytes was significantly higher than unsterilized soil applied with sterilized endophytes, indicating a close relationship between endophytes and microorganisms on INO3.MNrec was stimulated by the photosynthetic substrate supply, and the increasing microbial N immobilization induced by plant significantly increased the ratio of I to M in the presence of maize, which was beneficial to soil N retention and reduced N loss. Our results provide new insights of the MIT for better understanding the productivity of agricultural systems and its driving factors. [ABSTRACT FROM AUTHOR]

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 Ntrace plant tool to identify feedbacks between maize genotypes N uptake and soil N transformations and their mechanisms. Maize total N uptake rates (U TN) varied among genotypes and soils. U TN in alkaline soil was 1.1–2.1 times higher than that in acidic soil. NH 4 + and NO 3 − uptake rates (U NH4 and U NO3 , 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. U TN , and U NO3 were positively correlated to soil gross N mineralization (M), and autotrophic nitrification (O NH4) 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 O NH4 to increase NH 4 + residence time, which in turn was responsible for the positive relationship between U NH4 and O NH4. In contrast, the stimulation of heterotrophic nitrification (O Nrec) by plants improved the NO 3 − 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. • Maize genotypes show the significant differences in N uptake rates. • Maize N uptake rate is regulated by NH 4 + and NO 3 − transport genes in acidic soil. • Feedbacks between maize genotypes and soil N transformation affect maize N uptake. • Genotype with strong ability regulating plant-soil feedbacks can largely enhance maize N uptake. [ABSTRACT FROM AUTHOR]

Abstract Long-term application of organic fertilizers in acidified agricultural soils could alter soil nitrogen (N) immobilization by providing carbon (C) source and alleviating soil acidification. However, an understanding of the relative importance of abiotic and biotic N immobilization in acidified agricultural soils following long-term organic fertilizer application is largely lacking. Generally, the application of crop straw, which has a higher C/N ratio than animal manure, results in higher biotic N immobilization in soils. In contrast, the application of animal manure can result in greater stimulation of soil microbial activity due to a higher capacity to alleviate acidification. Resolving this contradiction is critical for predicting microbial N retention capacity and the effects of different types of organic fertilizer in acidified agricultural soils. A laboratory 15N tracer experiment was carried out to examine the effects of long-term animal manure and crop straw application on abiotic and biotic N immobilization in an acidified agricultural soil in China. Animal manure application had a higher stimulation effect on abiotic and biotic N immobilization than crop straw. Microbial NH 4 +-N immobilization, as opposed to abiotic NH 4 +-N immobilization, largely contributed to the retention of NH 4 +-N. In contrast, abiotic NO 3 −-N immobilization had a more significant role in NO 3 −-N retention than microbial NO 3 −-N immobilization under elevated C conditions. Animal manure-induced increases in microbial NH 4 +-N and NO 3 −-N immobilization could be attributed to both increased C availability and enhanced soil microbial activity due to an increase in soil pH. The results suggest that long-term inputs of organic material to acidified agricultural soils could enhance abiotic and biotic N immobilization capacity. Highlights • Animal manure stimulated N immobilization more than straw. • High microbial N immobilization in animal manure due to high C availability and soil pH. • Microbial immobilization largely contributed to NH 4 +-N retention. • Abiotic immobilization had a more significant role in NO 3 −-N retention. [ABSTRACT FROM AUTHOR]

Abstract In acidic soils heterotrophic nitrification is considered to be an important NO 3 − production process. Previous studies have demonstrated that the content of soil C and soil C:N ratio was positively correlated with heterotrophic nitrification. However, relatively high heterotrophic nitrification rates also observed in cultivated black soils, with relatively low soil organic C content and soil C:N ratio compared to forest soils. To obtain a mechanistic understanding of heterotrophic nitrification the simultaneously occurring gross N transformations were quantified in five cultivated soils (SC) and six forest soils (SF) in subtropical region, ten cultivated soils (TC) and seven forest soils (TF) in temperate region subsequently analysed with respect to the soil physicochemical properties (pH, C:N ratio, SOC, TN), the chemical compositions of soil organic C (recalcitrant and labile organic C) and microbial communities (abundance of fungi, bacteria, amoA gene of AOB and AOA). The results showed that a significantly higher heterotrophic nitrification rate was observed in TC (0.52 mg N kg−1 d−1) compared to the other soils. The total content of soil C and soil C:N ratio could not explain the occurrence of heterotrophic nitrification but recalcitrant C fractions of carbonyl C and aromatic C, and the ratio of recalcitrant C to labile C (R:L ratio) was significantly positively correlated with heterotrophic nitrification rates (P < 0.01). This was further confirmed by structural equation modeling (SEM) that the R:L ratio was the most important indicator, suggesting that chemical composition of SOC is likely to be key to explain and predict heterotrophic nitrification rates across a large range of soils including soils high in black carbon such as chernozems, old grassland soils and/or anthropogenic soils such as anthrosols (Terra preta). In addition, heterotrophic nitrification rates were significantly positively correlated with fungal gene copy numbers (P < 0.05), providing a strong indication that fungi might be the predominant organisms carrying out heterotrophic nitrification. Highlights • SOC content and C/N are not enough to explain occurrence of heterotrophic nitrification. • Heterotrophic nitrification rates are associated with the composition of SOC. • Soil recalcitrant C plays an important role in regulating heterotrophic nitrification. [ABSTRACT FROM AUTHOR]

Conservation tillage practices, such as no tillage (NT) and ridge tillage (RT), have been widely proposed for use in many countries to increase carbon sequestration. However, understanding of how conservation tillage affects soil gross N transformation is still very limited. In this study, a 15 N-tracing experiment was carried out to investigate the effects of 14 years of RT and NT practices on the gross N transformation rates in black soils of northeast China. Based on the results for soil N transformations, soil organic carbon (SOC) sequestration, and crop yields, we propose suitable conservation tillage practices. The results showed that the NH 4 + immobilization rates were significantly higher under RT than NT. The heterotrophic nitrification rate in RT was significantly higher than in either conventional tillage (CT) or NT, contributing more than 89% of the total nitrate production in RT. NO 3 − immobilization was negligible under NT and CT, but this process was significantly stimulated by RT. Consequently, relative to NT and CT, a long-term RT treatment can stimulate inorganic N retention capacity and thus provides greater capacity to supply available N for uptake by crop plants. In addition, the average concentrations of SOC and total nitrogen (TN) in the 0–15 cm soil layer under RT were significantly higher than those under NT and CT. The average maize yields (2011–2015) under RT were similar to NT, both of which were significantly higher than under CT. Therefore, RT is arguably a more suitable tillage practice than NT and CT for enhancing the N retention, maize yield, and C sequestration for the studied black soils. [ABSTRACT FROM AUTHOR]

Nitrite (NO 2 − ) is an intermediate in a number of soil N transformations, and is a precursor for NO and N 2 O emissions. Due to the transient nature of NO 2 − it is often neglected in N cycling research. Thus, the full suite of production and consumption pathways of NO 2 − in terrestrial ecosystems is poorly understood. The primary objectives of this study were to clarify the production and consumption pathways of NO 2 − in the acidic forest soils and compare the NO 2 − dynamics between temperate and subtropical forest soils. A series of 15 N tracing studies were performed, where 15 N was added as NH 4 + , NO 3 − , or NO 2 − to temperate and subtropical forest soils, and NO 2 − transformations were investigated in these acidic forest soils. The results showed that the average NO 2 − production rates ranged from 4.42 to 5.90 mg kg − 1 d − 1 and from 1.29 to 2.77 mg kg − 1 d − 1 in the temperate and subtropical forest soils, respectively, within the 1–6 h incubations. Oxidation of soil organic N was the dominant NO 2 − production pathway in the acidic forest soils, which was negatively related to soil pH (p < 0.01). The average consumption rates of NO 2 − varied during the incubation period from 12.46 to 14.10 mg kg − 1 d − 1 and from 5.84 to 6.74 mg kg − 1 d − 1 in the temperate and subtropical forest soils, respectively. The NO 3 − pool recovered 6–44% of added 15 N-NO 2 − in the studied forest soils, which was positively related to soil pH (p < 0.05). The 32–36% of added 15 N-NO 2 − was incorporated into insoluble soil organic N pool (SON) associated with soil organic C concentration. Nitrite incorporation into DON was also important for soil N retention in the subtropical strongly acidic forest soils (pH 4.7), however, in the temperate acidic forest soils, 15 N was not detected in the DON pool. Self-decomposition of NO 2 − to NO and NO 2 was a main pathway under subtropical strongly acidic conditions. The NO 2 − turnover was most likely the key driving force for N transformations in the acidic forest soils. Further work regarding the dynamics of NO 2 − and their mechanisms in soils of the different terrestrial ecosystems is important to gain an improved understanding of the global N cycle. [ABSTRACT FROM AUTHOR]

No tillage (NT) is a common conservation tillage practice used to enhance soil C sequestration. However, to date, the understanding regarding the impact of NT on soil nitrogen (N) transformations remains limited. Here, we used 15 N tracing to investigate the effects of 14 years of NT on the gross N transformation rates in black soil in China. We also evaluated the risk of N loss from soil in NT systems compared to that treated with conventional tillage (CT). The gross N mineralization rates in the top 5 cm of NT-treated soil was more than three times higher than the rates in the 5–15 cm layer ( P < 0.01), which was significantly higher than those in the same layer of CT soils ( P < 0.05). However, the total gross NH 4 + immobilization rates were almost negligible in NT soil than in CT soil, which led to significantly higher net mineralization rates. The highest gross autotrophic nitrification rates were observed in the top 5 cm of NT soil, which was significantly higher than those in CT ( P < 0.05). Heterotrophic nitrification rates were very low in NT soil, and ranged from 0.02–0.11 mg N·kg − 1 d − 1 , which were significantly lower than those in CT (average rate of 0.80 mg N·kg − 1 d − 1 ). The ratio of autotrophic nitrification to NH 4 + immobilization, and the N 2 O emission rate were significantly higher in NT than in CT. Soil N 2 O emission rates were positively correlated with autotrophic nitrification rates. Thus, long-term NT treatment was more likely than CT treatment to increase the risk of NO 3 − leaching and N 2 O emission. [ABSTRACT FROM AUTHOR]

In this study, a 15 N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a long-term repeated N application experiment. The field study was initiated in 2003 under a wheat-maize rotation system in the subtropical rain-fed purple soil region of China. The experiment included six fertilization treatments applied on an equivalent N basis (280 kg N ha −1 ), except for the residue only treatment which received 112 kg N ha −1 : (1) UC, unfertilized control; (2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure (40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6) RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The results showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following long-term applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH 4 + ) immobilization rates and caused a strong competition with nitrifiers for NH 4 + , thus preventing a build-up of nitrate (NO 3 − ). Overall, in this study, we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM, OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China. [ABSTRACT FROM AUTHOR]

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 (N 2 O) 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, N 2 O 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 N 2 O emissions under plastic-shed cultivation. Under long-term plastic-shed cultivation (>2 years), negligible gross nitrate-N (NO 3 −-N) immobilization rates were observed due to the preferential use of ammonium-N by soil microbes, which was probably responsible for rapid NO 3 −-N accumulation in soils. Overall, these results could be of help to elucidate the mechanisms of soil N 2 O emission and NO 3 −-N accumulation in plastic-shed vegetable fields. • Plastic-shed vegetable cultivation altered soil gross N transformation rates. • Gross N mineralization rate decreased with decreasing soil total phosphorus. • Plastic-shed vegetable cultivation decreased gross autotrophic nitrification rates. • Decreased gross autotrophic nitrification rate accounted for reduced N 2 O emissions. [ABSTRACT FROM AUTHOR]

Abstract: To date, very few studies have been conducted to investigate the characteristics of gross nitrogen (N) transformations in subtropical agricultural soils. In this study, 12 natural woodland and 10 agricultural soils were collected to investigate the effects of land use on soil gross N transformations in the humid subtropical zones in China. The results showed that gross autotrophic nitrification rates (average 0.19 mg N kg−1 d−1) in the woodland soils were significantly lower than those determined in the agricultural soils (average 1.81 mg N kg−1 d−1) (p < 0.01). However, the NO3 − immobilization rates (average 0.10 mg N kg−1 d−1) in the agricultural soils were significantly lower than in the woodland soils (average 0.47 mg N kg−1 d−1) (p < 0.01). On average, 98% of the total NO3 − produced could be immobilized into organic-N in the woodland soils, while, it accounted for only 10% in the agricultural soils. These differences in gross N transformations resulted in the inorganic N being dominated by NH4 + in the woodland soils; however, NO3 − dominated the inorganic N in the agricultural soils. The risk of N leaching and runoff from soil sharply increases after woodland soils are converted to agricultural soils. Application of organic fertilizers with high C/N ratios to agricultural soils in subtropical regions to increase soil organic C content and the C/N ratio is expected to improve NO3 − immobilization capacity and reduce the risk of N leaching and runoff from soil. [Copyright &y& Elsevier]

Reactive nitrogen (N) input can profoundly alter soil N transformations and long-term productivity of forest ecosystems. However, critical knowledge gaps exist in our understanding of N deposition effects on internal soil N cycling in forest ecosystems. It is well established that N addition enhances soil N availability based on traditional net mineralization rate assays. Yet, experimental additions of inorganic N to soils broadly show a suppression of microbial activity and protein depolymerization. Here we show, from a global meta-analysis of 15N-labelled studies that gross N transformation rates in forest soil organic and mineral horizons differentially respond to N addition. In carbon (C)-rich organic horizons, N addition significantly enhanced soil gross rates of N mineralization, nitrification and microbial NO 3 ¯ immobilization rates, but decreased gross microbial NH 4 + immobilization rates. In C-poor mineral soils, in contrast, N addition did not change gross N transformation rates except for increasing gross nitrification rates. An initial soil C/N threshold of approx. 14.6, above which N addition enhanced gross N mineralization rates, could explain why gross N mineralization was increased by N deposition in organic horizons alone. Enhancement of gross N mineralization by N deposition was also largely attributed to enhanced N mineralization activity per unit microbial biomass. Our results indicate that the net effect of N input on forest soil gross N transformations are highly stratified by soil C distribution along the soil profile, and thus challenge the perception that N availability ubiquitously limits N mineralization. These findings suggest that these differences should be integrated into models to better predict forest ecosystem N cycle and C sequestration potential under future N deposition scenarios. [ABSTRACT FROM AUTHOR]