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An aerobic 15N microcosmic experiment was conducted to compare the inhibitory effects of the biological nitrification inhibitor (BNI), methyl 3-(4-hydroxyphenyl) propionate (MHPP) at rates of 500 and 1000 mg kg−1 with the synthetic nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) at 1% of applied NH4+, on the gross nitrification rate (n_gross) and on the abundance and community composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) of two contrasting soils (pH: 5.10 vs. 8.15, clay content 17.8 vs. 30.8). DMPP inhibited 56.6% of n_gross in the acidic soil and 50.3% in the calcareous soil, whereas MHPP inhibited 18.3–55.5% of n_gross in the acidic soil and 14.1–20.2% in the calcareous soil. MHPP used at the high rate showed the same inhibition on n_gross as DMPP in the acidic soil but not in the calcareous soil. DMPP and MHPP likely regulated n_gross by causing niche differentiation between AOA and AOB. Moreover, the community composition of AOB was more sensitive to nitrification inhibitor application than that of AOA, particularly in the acidic soil. However, the response of AOB community composition was less sensitive to the application of MHPP than to that of DMPP. MHPP mainly targeted Nitrosospira clusters 3a.2, 3b.2, and 9 of the AOB in the acidic soil.

Microbial nitrification in soils is a major contributor to nitrogen (N) loss in agricultural systems. Some plants can secrete organic substances that act as biological nitrification inhibitors (BNIs), and a small number of BNIs have been identified and characterized. However, virtually no research has focused on the important food crop, rice (Oryza sativa). Here, 19 rice varieties were explored for BNI potential on the key nitrifying bacterium Nitrosomonas europaea. Exudates from both indica and japonica genotypes were found to possess strong BNI potential. Older seedlings had higher BNI abilities than younger ones; Zhongjiu25 (ZJ25) and Wuyunjing7 (WYJ7) were the most effective genotypes among indica and japonica varieties, respectively. A new nitrification inhibitor, 1,9-decanediol, was identified, shown to block the ammonia monooxygenase (AMO) pathway of ammonia oxidation and to possess an 80% effective dose (ED80) of 90μl−1. Plant N-use efficiency (NUE) was determined using a 15N-labeling method. Correlation analyses indicated that both BNI abilities and 1,9-decanediol amounts of root exudates were positively correlated with plant ammonium-use efficiency and ammonium preference. These findings provide important new insights into the plant–bacterial interactions involved in the soil N cycle, and improve our understanding of the BNI capacity of rice in the context of NUE.

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood [delta].sup.15N in 2014, and tested the strength of correlation between tree ring [delta].sup.15N of four species and stream water NO.sub.3.sup.- loss from 1971 to 2000. Higher soil NO.sub.3.sup.- was weakly associated with higher wood [delta].sup.15N across species, and higher soil net nitrification rates were associated with higher [delta].sup.15N for Quercus rubra only. The [delta].sup.15N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO.sub.3.sup.-. L. tulipifera tree ring [delta].sup.15N had a stronger association with stream water NO.sub.3.sup.- than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring [delta].sup.15N, with a strong correlation between the wood [delta].sup.15N and NO.sub.3.sup.- leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring [delta].sup.15N, and given the weak relationships between contemporary wood [delta].sup.15N and soil N cycle measurements, caution is warranted when using wood [delta].sup.15N to infer changes in the N cycle.

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3−), and in fertilized soils it can lead to substantial N losses via NO3− leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the ‘where’ and ‘how’ of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3− retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Inhibitors of urease and ammonia monooxygenase can limit the rate of conversion of urea to ammonia and ammonia to nitrate, respectively, potentially improving N fertilizer use efficiency and reducing gaseous losses. Winter wheat grown on a sandy soil in the UK was treated with urea fertilizer with the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT), the nitrification inhibitor dicyandiamide (DCD) or a combination of both. The effects on soil microbial community diversity, the abundance of genes involved in nitrification and crop yields and net N recovery were compared. The only significant effect on N-cycle genes was a transient reduction in bacterial ammonia monooxygenase abundance following DCD application. However, overall crop yields and net N recovery were significantly lower in the urea treatments compared with an equivalent application of ammonium nitrate fertilizer, and significantly less for urea with DCD than the other urea treatments.

Agriculture is the single largest geo-engineering initiative that humans have initiated on planet Earth, largely through the introduction of unprecedented amounts of reactive nitrogen (N) into ecosystems. A major portion of this reactive N applied as fertilizer leaks into the environment in massive amounts, with cascading negative effects on ecosystem health and function. Natural ecosystems utilize many of the multiple pathways in the N cycle to regulate N flow. In contrast, the massive amounts of N currently applied to agricultural systems cycle primarily through the nitrification pathway, a single inefficient route that channels much of this reactive N into the environment. This is largely due to the rapid nitrifying soil environment of present-day agricultural systems...

Nitrification is a key process of the nitrogen (N) cycle in soil with major environmental implications. The recent discovery of ammonia-oxidizing archaea (AOA) questions the traditional assumption of the dominant role of ammonia-oxidizing bacteria (AOB) in nitrification. We investigated AOB and AOA growth and nitrification rate in two different layers of three grassland soils treated with animal urine substrate and a nitrification inhibitor [dicyandiamide (DCD)]. We show that AOB were more abundant in the topsoils than in the subsoils, whereas AOA were more abundant in one of the subsoils. AOB grew substantially when supplied with a high dose of urine substrate, whereas AOA only grew in the Controls without the urine-N substrate. AOB growth and the amoA gene transcription activity were significantly inhibited by DCD. Nitrification rates were much higher in the topsoils than in the subsoils and were significantly related to AOB abundance, but not to AOA abundance. These results suggest that AOB and AOA prefer different soil N conditions to grow: AOB under high ammonia (NH₃) substrate and AOA under low NH₃ substrate conditions.

Aims Paddy soil is one of the main sources of global nitrous oxide (N 2 O) emissions via multiple pathways regulated by different microbes. However, the relative contributions of N 2 O production pathways with the addition of organic carbon (C) in different paddy soils are poorly understood. Methods 15 N-stable isotope and acetylene (C 2 H 2 ) inhibition were used to differentiate the relative contributions of autotrophic and heterotrophic nitrification (ANF and HNF) and denitrification (DNF) to N 2 O emissions in two paddy soils (acid vs. neutral soil) with glucose addition. Results HNF and DNF were the main N 2 O pathways which contributed between 85% to 100% of the total N 2 O production at 70% water filled pore space. Low soil pH inhibited soil nitrification and the activity of ammonia oxidizers compared with neutral paddy soil. Glucose reduced nitrification rate and stimulated N 2 O production significantly, mainly via DNF in the two paddy soils. Moreover, glucose increased the relative contribution of DNF to total N 2 O production in the first 7 days and total N 2 O amounts from HNF over the 14-day incubation. Conclusions HNF and DNF rather than ANF dominated the N 2 O emissions regardless of soil pH. Glucose had a positive effect on N 2 O emissions by influencing HNF and DNF.

Chemoautotrophic canonical ammonia oxidizers (ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB)) and complete ammonia oxidizers (comammox Nitrospira) are accountable for ammonia oxidation, which is a fundamental process of nitrification in terrestrial ecosystems. However, the relationship between autotrophic nitrification and the active nitrifying populations during .sup.15N-urea incubation has not been totally clarified. The .sup.15N-labeled DNA stable isotope probing (DNA-SIP) technique was utilized in order to study the response from the soil nitrification process and the active nitrifying populations, in both acidic and neutral paddy soils, to the application of urea. The presence of C.sub.2H.sub.2 almost completely inhibited NO.sub.3.sup.--N production, indicating that autotrophic ammonia oxidation was dominant in both paddy soils. .sup.15N-DNA-SIP technology could effectively distinguish active nitrifying populations in both soils. The active ammonia oxidation groups in both soils were significantly different, AOA (NS (Nitrososphaerales)-Alpha, NS-Gamma, NS-Beta, NS-Delta, NS-Zeta and NT (Ca. Nitrosotaleales)-Alpha), and AOB (Nitrosospira) were functionally active in the acidic paddy soil, whereas comammox Nitrospira clade A and Nitrosospira AOB were functionally active in the neutral paddy soil. This study highlights the effective discriminative effect of .sup.15N-DNA-SIP and niche differentiation of nitrifying populations in these paddy soils.

Many studies have shown the efficiency of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) in suppressing nitrification and nitrous oxide (N2O) emissions. However, the effect of DMPP on soil gross nitrogen transformations and the mechanism of its inhibitory effects on N2O production pathways remain unknown. A 15N tracing experiment was conducted to investigate the effect of DMPP on gross N transformation rates and pathways of N2O production in two typical Chinese and UK agricultural soils. The soils differed in organic carbon (C) and clay content but otherwise had similar properties. The results showed that the application of DMPP decreased the gross autotrophic nitrification rate (p < 0.05) by 21.6% in the Chinese soil and 9.4% in the UK soil. The lower inhibitory efficiency of DMPP in the UK soil was likely to have been due to high rates of adsorption by soil organic C and clay. The total gross rate of mineralization was lower in the presence of DMPP in both soils, likely because there was a regulatory feedback when ammonium concentrations were high. DMPP also significantly reduced cumulative N2O emissions (p < 0.05) in both soils (by between 15.8 and 68.4%), which might be attributed to the dual inhibitory effect of the DMPP on autotrophic nitrification rate and the proportion of N2O produced by autotrophic nitrification processes. This finding will help to predict the sites where DMPP is likely to be most effective and allow the user to target DMPP application to soils with particular properties.