Inhibition Of Nitrification In Soil Research Articles

In-vitro assessment of 3,5-diaryl-1h-pyrazoles for nitrification inhibition in soil

Nitrification process involves conversion of nitrogen into nitrates by soil microbes which are then taken up by plants. But due to denitrification, ammonia volatilization, nitrous oxide emission, leaching, run off, etc., the plant available nitrogen is lost from the soil. Pyrazoles are good nitrification inhibitors and DMPP (3,4-dimethylpyrazole phosphate) is a known commercial nitrification inhibitor in agricultural crops. The study explores the potential of a group of diarylpyrazoles for their nitrification inhibitory properties under in-vitro conditions. The test pyrazole compounds were applied at recommended dose of 1% of urea nitrogen-N. Four of the pyrazole compounds showed promising results with several compounds having more than 50% nitrification inhibition. Some compounds showed better performance than the standard DCD (dicyandiamide) at 1% and 5%. There were few compounds which showed no inhibition. Results suggest that some of the compounds proved to be good inhibitors of nitrification under laboratory conditions.

Linking soil nutrient cycling and microbial community with vegetation cover in riparian zone

The riparian zone is a spatially fluctuating ecotone between terrestrial and aquatic ecosystems and has key ecological functions, due to specific geographic conditions and seasonal environmental changes. The comprehensive functions of vegetation cover and microbial community in riparian zones might generate positive consequences for nutrient cycling. In this study, four different vegetation-covered soils in a riparian zone on the Yangtze River was selected. Soil nutrient concentration and microbial properties, including nitrogen (N) and phosphorus (P) cycling and microbial community structure, were analyzed, and the relationships among soil nutrient concentrations, microbial properties and vegetation covers were also linked. Vegetation covers increased soil NH4+-N supply by enhancing N mineralization and positively affected available N retention through inhibition of soil nitrification. Plant species outweighed plant biomass in determining soil microbial properties related to P mineralization, including phosphatase activities and functional gene abundances. Vegetation covers significantly increased Acidobacteria and Mortierellomycota relative abundances. Soil bacterial and fungal communities had opposing responses to the impacts of plant species richness and biomass. In the riparian zone, plant species outweighed plant biomass in promoting soil N and P mineralization, and vegetation covers, regardless of plant species, improved available nutrient supplies and retentions and shaped soil microbial community.

Potential allelopathic effect of Brachiaria decumbens root exudates on neotropical tree seedlings

The reforestation of degraded areas in the neotropics is hampered by the invasion of Brachiaria spp. The aggressiveness of these grasses may be related to the release of allelochemicals in the soil, and the sensitivity of tree seedlings of distinct successional classes to exudates of Brachiaria roots has not been addressed. Here, we evaluated the effects of exudates from Brachiaria decumbens roots on the growth, physiology and nitrogen metabolism of seedlings of six tree species native to Brazilian Atlantic Forest: Trema micrantha, Heliocarpus popayanensis and Cecropia pachystachya (pioneers), Cariniana estrellensis, Eugenia brasiliensis and Guarea kunthiana (non-pioneers). The seedlings were cultivated in fertile soil without (control) and with B. decumbens root exudates (AL treatment). Total leaf area and root and shoot dry weights of seedlings of pioneer species were drastically reduced by AL, whereas non-pioneers were less negatively affected. The gas exchange parameters and chlorophyll content of seedlings of both successional groups were altered by AL. AL decreased endogenous nitrate contents in the seedlings, in accordance with the inhibition of soil nitrification by this treatment. As consequence, AL reduced the endogenous protein contents of pioneer seedlings. Overall, these results demonstrate the negative allelopathic potential of exudates from B. decumbens roots on neotropical tree seedlings. Pioneer species, which are highly dependent on nitrate, were more negatively affected by the grass exudates, suggesting an indirect allelopathic effect mediated by the inhibition of soil nitrification, which may hamper the restoration of degraded areas. However, direct allelopathic mechanisms cannot be ruled out.

A composite guanyl thiourea (GTU), dicyandiamide (DCD) inhibitor improves the efficacy of nitrification inhibition in soil

This study investigated whether applying dicyandiamide (DCD) and guanyl thiourea (GTU) in conjunction with urea improves the efficacy of nitrification inhibition relative to traditional fertiliser application of urea or urea + DCD. Urea at a rate of 100 mg N kg−1 soil was applied to soil microcosms (high nutrient tenosol and low nutrient hydrosol) which were treated with either no inhibitor (urea-only); 15 mg DCD kg−1 soil or 15 mg DCD kg−1 soil plus 21 mg GTU kg soil−1. Mineral N (NH4+ & NO3−) concentrations, potential nitrification rates (PNR) and abundances of ammonia oxidising bacteria (AOB) were measured over time. After 100-days incubation, ∼73 mg N kg−1 soil was found as NH4+ when urea + DCD + GTU were applied to the tenosol. NH4+ concentrations were lower (11–32 mg N kg−1 soil) when urea or urea + DCD were applied. This suggests that the application of GTU in conjunction with DCD elongated the effects of nitrification inhibition. In both soils, PNRs were faster and AOB abundances (gene copies g−1 soil) were higher when urea was applied without nitrification inhibitors. There were, however, no differences in PNR or AOB abundances in either soil type when ‘urea + DCD’ or ‘urea + DCD + GTU’ were applied. The results indicate that the application of GTU with DCD may extend nitrification inhibition in certain soil types. This finding has the potential to improve the efficacy of commercially available and widely used inhibitors such as DCD.

Ameliorating Effects of Fungus Chaff and Its Biochar on Soil Acidity

A biochar was generated from fungus chaff at 300 °C, and the ameliorating effects of fungus chaff and its biochar on an acidic Ultisol were compared using incubation experiments. Incorporation of fungus chaff and its biochar significantly increased soil pH and soil exchangeable base cations but decreased soil exchangeable acidity. The ameliorating effect was greater for the biochar than the fungus chaff, and thus the biochar was a better amendment for acidic soil than its feedstock of fungus chaff. The biochar ameliorated soil acidity mainly through the release of its contained alkaline substances, while fungus chaff increased pH of acidic soils through two mechanisms: release of alkaline substances and inhibition of soil nitrification. The incorporation of fungus chaff increased soil-available organic carbon and thus accelerated the microbial assimilation of inorganic nitrogen, while incorporation of fungus chaff biochar enhanced nitrification due to increased soil pH.

Acidification in Rhizospheric Soil of Field-Grown Sorghum Decreases Nitrification Activity

To date, most studies on biological nitrification inhibition (BNI) in sorghum have been performed with plants grown in hydroponic systems. However, the current study was conducted to determine whether or not sorghum inhibits nitrification in fields of Alfisols, and clarify the mechanism that results in inhibition of soil nitrification in the field. Nitrification activity in the rhizosphere of sorghum (Sorghum bicolor (L.) Moench) i.e. soil attached to its roots within a few millimeters was measured and compared with those in adjacent bulk soil. Sweet sorghum (6 varieties) and grain sorghum (3 varieties) were cultivated in 4 Alfisol fields in a semi-arid tropical region of India during the 2010 or 2011 rainy seasons. Soil samples were collected three times during the growing season. Nitrification activity in the rhizospheric soil was significantly lower than that in the bulk soil during 8 out of 12 samplings while the pH (H2O, 1:2) of the rhizospheric soil was significantly lower than that of the bulk soil in 10 out of 12 samplings. Acidification of the soil by sulfuric acid decreased the nitrification activity to a comparable extent, as emerged in the rhizospheric soils. Our results indicate that acidification of soil around roots would be one of the main causes of nitrification inhibition by sorghum in the field.

Chloride ion as nitrification inhibitor and its biocidal potential in soils

The accumulation of chlorine ion in soils and plants due to the continued use of potassium chloride triggers a series of physiological disorders in plants and microorganisms deserving special attention. This study aimed to evaluate the inhibition potential of chloride ions added to soils as ammonium chloride (AC) and/or KCl in the reaction of soil nitrification. The experiment was conducted under laboratory conditions. The soil was incubated for 21 days at 70% of maximum water retention capacity at 25 °C. The addition of AC, combined with KCl, decreased nitrate concentration in the soil. The increase of Cl content in the soil with the use of AC resulted in an inhibition of soil nitrification. The agricultural potential of using AC as N source was shown in many crops, although, caution regarding excess is recommended, which results in a build-up of salt in the soil and causes injury to the root system and microorganisms.

Effects of prolonged oral administration of dicyandiamide to dairy heifers on excretion in urine and efficacy in soil

Oral administration of the nitrification inhibitor dicyandiamide (DCD) to grazing ruminants for excretion in urine represents a targeted mitigation strategy to reduce nitrogen (N) losses from grazed pastures. A field trial and allied laboratory incubation study were conducted to examine the effects of oral administration of DCD to non-lactating Friesian dairy heifers on excretion of DCD in urine and efficacy in soil. Dairy heifers were orally administered DCD daily at three treatment levels (low, medium and high; 12, 24 and 36gDCDheifer−1day−1, respectively) and compared to a nil-DCD control group over a 90-day continuous dosing period. There were no adverse effects of DCD administration on heifer health or growth, as inferred by live-weight gain and measured blood metabolite levels. Prolonged administration of DCD to dairy heifers resulted in the sustained excretion of DCD in the urine over 90 days and inhibition of nitrification of urinary-N in the silty peat soil for up to 56 days (incubated at 20°C; P<0.001). Field soil sampling (0–75mm depth) of individual urine patches for DCD analysis revealed that a 3-fold increase in the rate of DCD administered resulted in a similar increase in the concentration of DCD voided in the urine and subsequently deposited in urine patches (median equivalent DCD application rates of 22, 36 and 59kgha−1 for the low, medium and high DCD treatment levels, respectively; P<0.001). However, large differences (up to 40-fold) existed between individual urine patches in the rate of DCD deposited at each treatment level, which showed a positively skewed distribution. This study highlights the viability of prolonged daily administration of DCD to ruminants for sustained excretion in urine and effective inhibition of nitrification in soil as a practical targeted mitigation technology to reduce urinary-N losses from grazed pastures.

Effect of Azadirachta indica (neem), sodium thiosulphate and calcium chloride on changes in nitrogen transformations and inhibition of nitrification in soil incubated under laboratory conditions

A laboratory experiment was conducted to examine the effects of nitrification inhibitors (NIs) neem seedcake (Azadirachta indica) (NSC), sodium thiosulphate (Na2S2O3) and calcium chloride (CaCl2) on changes in NH4+–N, inhibition of nitrification and recovery of applied nitrogen (N) in soil. Surface soil samples of 0–15cm were collected from an arable field, amended with urea N (UN) at the rate 200mgNkg−1, UN+NSC, UN+Na2S2O3 and UN+CaCl2 and incubated at 22°C periodically over 50d. Soil without any amendment was used as check (control). Results indicated that more than 58% of N applied as NH4- disappeared over a period of 50d from the soil mineral-N pool. Some of this N (21%) was accumulated as NO3-–N while the remaining N was unaccounted for. Addition of nitrification inhibitors NSC, Na2S2O3, and CaCl2 resulted in a decrease in the extent of NH4+ disappearance by 35%, 44% and 30%, respectively. In the treatment receiving UN alone, 56mg NO3-–Nkg-1 was accumulated over 50d (maximum 93mgkg−1) indicated an active nitrification. Application of nitrification inhibitors NSC, Na2S2O3, and CaCl2 with UN inhibited nitrification by 54%, 64%, and 59%, respectively. Apparent N recovery (ANR) in the treatment receiving UN alone was 63% that substantially increased to 83%, 89% and 76% in the treatments receiving UN+NSC, UN+Na2S2O3, and UN+CaCl2, respectively indicating 32%, 41% and 20% increase in N recovery. Among three NIs tested, Na2S2O3 proved superior in inhibiting nitrification and increasing ANR. The study demonstrated that application of NSC, Na2S2O3, and CaCl2 which are cheap and easily available NIs inhibited nitrification and improved N recovery efficiency of applied N in an arable soil very effectively. It is suggested that these inhibitors should be tested under field conditions for increasing NUE and improving crop productivity.

Effectiveness of Chloride Compared to 3,4-Dimethylpyrazole Phosphate on Nitrification Inhibition in Soil

Microbial oxidation of ammonium to nitrate may impose dangers to ecosystem functioning through soil and atmosphere contamination with end products or intermediate gases. A wide range of chemicals can inhibit nitrification under soil and laboratory conditions. In the present study, the effectiveness of chloride compared to 3,4-dimethylpyrazole phosphate (DMPP) as a standard nitrification inhibitor was evaluated. The results showed that DMPP (especially with double concentrations) inhibited nitrification for a longer time, until the end of incubation period. Chloride in the form of ammonium chloride (NH4Cl) or potassium chloride (KCl) also significantly inhibited nitrification compared to the control during the 7-week incubation period. This inhibition was positively correlated with applied chloride concentrations in soil. During a 5-week incubation period, the strongest concentration (500 mg/kg soil) showed more inhibition than concentrations of 250 or 100 mg/kg soil, particularly when compared to control. The results suggest that beside commercial nitrification inhibitors, chloride can significantly inhibit microbial nitrification in soil. Therefore, when chloride is not a soil problem, the chloride form of nitrogen fertilizers (e.g., ammonium chloride) could be a proper nitrogen fertilizer.

Heavy‐Metal Inhibition of Nitrification in Selected Iowa Soils Treated with Stay‐N 2000

Heavy‐metal inhibition of nitrification in soils treated with reformulated nitrapyrin was investigated. Clarion and Okoboji soils were treated with ammonium sulfate [(NH4)2SO4] and a nitrification inhibitor. Copper(II) (Cu), Zinc(II) (Zn), Cadmium(II) (Cd), or Lead(II) (Pb) were added to each soil. A first‐order equation was used to calculate the maximum nitrification rate (K max), duration of lag period (t′), period of maximum nitrification (Δt), and the termination period of nitrification (t s). In the Clarion soil, the K max decreased from 12 mg kg−1 d−1 without the nitrification inhibitor to 4, 0.25, 0.86, and 0.27 mg kg−1 d−1, respectively, when the inhibitor and Cu, Zn, Pb, or Cd were applied. In the Okoboji soil, K max decreased from 22 mg kg−1 d−1 with no inhibitor to 6, 3, 4, and 2 mg kg−1 d−1, respectively, when an inhibitor and Cu, Zn, Pb, or Cd were added. The t′ varied from 8 to 25 d in the Clarion soil and from 5 to 25 d in the Okoboji soil, due to addition of Cu, Zn, Pb, or Cd and the inhibitor.

Effects of Cr 3+, Cr 6+ and tannery sludge on C and N mineralization and microbial activity in semi-arid soils

The aim of this study was to evaluate the effect of two Cr species (Cr 3+ and Cr 6+) on N and C mineralization and dehydrogenase activity in semi-arid soils. The Cr species (250 mg kg −1 soil) were either added alone or mixed with tannery sludge (0.0125 g g −1) to three soils: cultivated soils, and outside and under the canopy of mesquite trees were then incubated for 180 days at 25 °C. Sole Cr 6+ addition had a higher inhibition of CO 2 production rate in cultivated soil (58–73%) than in soils under the canopy and outside the canopy. Soil outside the canopy amended with Cr 6+ showed the highest inhibition of dehydrogenase activity (40–100%) followed by cultivated and under the canopy soils. However, Cr 6+ added alone increased the inhibition of nitrification in soil outside the canopy (68–84%, from 30 to 120 days), followed by under the canopy and cultivated soils. The addition of tannery sludge to Cr 6+ significantly reduce the CO 2 production rate and dehydrogenase activity in all three soils, and increased the inhibition of nitrification in the following order: outside the canopy, cultivated and under the canopy soils. The addition of Cr 3+ or Cr 3+ plus tannery sludge either stimulated or inhibited CO 2 production rate, dehydrogenate activity and ammonification in the three soils in no clearly defined order. Measurement of dehydrogenase activity was the best tool for assessing the harmful effect of Cr 6+ on soil microbial activity in semi-arid soils exposed for an extended period.

Nitrification Inhibition in Soil and Ecosystem Functioning - An Overview

Nitrification Inhibition in Soil and Ecosystem Functioning - An Overview

Effect of halomethyl-1,3,5-triazines on nitrification of ammonia

Nitrification inhibitory activity of halomethyl-1,3,5-triazines was determined by measuring the inhibitory activities on ammonia-oxidation to nitrate (NO 3 - -N) in an upland soil and on ammonia-oxidation to nitrite (NO 2 - -N) by Nitrosomonas europaea ATCC 25978 (ATCC) and Nitrosomonas sp TK 794 (TK). Within the chlorinated trimethyl-1,3,5-triazines, those bearing at least one trichloromethyl group inhibited nitrification more strongly, both in soil and in cell suspension of ATCC, than other mono- or dichlorinated methyl-1,3,5-triazines. Introduction of an amino group to 2,4,6-tris(trichloromethyl)-1,3,5-triazine gave 10- and 100-fold increases of nitrification inhibitory activity in soil and ATCC cell culture, respectively. Within the trihalomethyl-1,3,5-triazines, those having tribromomethyl group(s) exhibited rather weaker nitrification inhibition in soil than the corresponding trichloromethyl-1,3,5-triazines, although they showed a strong inhibition in cell suspension. Ammonium oxidation in ATCC was inhibited more strongly than that in TK. In QSAR studies, the optimum log P values were calculated as c4.30. By using this value it will become possible to design highly active trichloromethyl-1,3,5-triazine nitrification inhibitors.

Dicyandiamide effects on nitrification and total inorganic soil nitrogen in sandy soils

Inhibition of nitrification in soil results in a decreased ratio of nitrate‐nitrogen (NO3‐N) to ammonium‐nitrogen (NH4‐N). If the conditions for NO3‐N loss by leaching or denitrification exist, nitrification inhibitors should increase concentrations of total inorganic soil nitrogen (N) (TISN) (NH4‐N + NO3‐N). This can then result in plants taking up more N and developing more crop yield or biomass. This study examined whether inhibition of nitrification by dicyandiamide (DCD) would result in increased concentrations of TISN under field conditions. The effects of DCD on soil N were evaluated in hyperthermic sandy soils planted to potato (Solanum tuberosum L., cv. Atlantic). Treatments were factorial combinations of N as ammonium nitrate (NH4NO3) at 67, 134, and 202 kg N ha‐1 and DCD at 0, 5.6, and 11.2 kg DCD ha‐1. Soil NH4‐N, NO3‐N, and TISN concentrations were determined for up to five potato growth stages at two locations for two years for a total of 16 determinations (cases), i.e., four were not determined. The N form ratio [NO3‐N/(NH4‐N + NO3‐N] x 100 was decreased in 10 of 16 cases, indicating that nitrification was inhibited by DCD. With two of these 10 cases, TISN concentration increased, but with four others, TISN concentration decreased with at least one N rate. With four of these 10 cases, inhibition of nitrification had no effect on TISN concentration. Under the conditions of these field studies, DCD inhibited nitrification more often than not. Inhibition of nitrification was, however, more likely to reduce TISN concentration than to increase it. This may have been due to DCD effects on immobization of applied NH4‐N.

Inhibition of nitrification in soil by gaseous hydrocarbons

Recent work has shown that gaseous hydrocarbons such as methane, ethane, and ethylene are competitive inhibitors of the monooxygenase enzyme responsible for oxidation of ammonia by chemoautotrophic nitrifying microorganisms such as Nitrosomonas europaea. Because methane, ethane, and ethylene are produced by microbial activity in soil, we studied the possibility that they may inhibit oxidation of ammonia by the nitrifying soil microorganisms. We found that all three of these gaseous hydrocarbons inhibited nitrification in soil and that their ability to inhibit nitrification decreased in the order: ethylene > ethane > methane. Ethylene was much more effective than ethane or methane for inhibiting nitrification of ammonium in soil, but it was much less effective than acetylene, and it seems unlikely that the amounts of ethylene produced in soils will be sufficient to cause significant inhibition of nitrification by soil microorganisms.

Evaluation of 2‐Ethynylpyridine as a Soil Nitrification Inhibitor

AbstractRecent work in our laboratory showed that 2‐ethynylpyridine is a potent inhibitor of nitrification in soil. To further evaluate this compound as a soil nitrification inhibitor, we compared the effects of 2‐ethynylpyridine and 16 compounds patented or proposed as fertilizer amendments for inhibition of nitrification in soil. We measured production of NO2 and NO3 in soils incubated under aerobic conditions at 20, 25, or 30 °C after treatment with (NH4)2SO4 or urea. The data obtained showed that 2‐ethynylpyridine compared favorably with nitrapyrin [N‐Serve; 2‐chloro‐6‐(trichloromethyl)pyridine] and etridiazole [Dwell; 5‐ethoxy‐3‐trichloromethyl‐1,2,4‐thiadiazole] for inhibition of nitrification in soil and was considerably more effective than the other nitrification inhibitors tested (dicyandiamide [DCD], thiourea [Tu], guanylthiourea [ASU], potassium azide, 4‐amino‐1,2,4‐triazole [ATC], 2‐amino‐4‐chloro‐6‐methylpyridine [AM], 2‐mercaptobenzothiazole [MBT], 2,4‐diamino‐6‐trichloromethyltriazine, sulfathiazole [ST], sodium thiocarbonate [Na2CS3], N‐2,5‐dichlorophenylsuccinamide [DCS], 4‐nitrobenzotrichloride, 4‐mesylbenzotrichloride, and ammonium thiosulfate [(NH4)2S2O3]). Other studies showed that 2‐ethynylpyridine is more effective for inhibiting nitrification of NH4‐N than of urea‐N, that it has very little, if any, effect on hydrolysis of urea or denitrification of NO3 in soil, and that its effectiveness as a soil nitrification inhibitor is markedly affected by soil type and temperature. These findings and other work reported indicate that 2‐ethynylpyridine deserves consideration as a fertilizer amendment for retarding nitrification of fertilizer N in soil.

Evaluation of 3-methylpyrazole-1-carboxamide as a soil nitrification inhibitor

Laboratory studies to evaluate 3-methylpyrazole-1-carboxamide (MPC) as a soil nitrification inhibitor showed that it was comparable to nitrapyrin (N-Serve) for inhibiting nitrification of ammonium in soil, but was not as effective as etridiazole (Dwell) or 2-ethynylpyridine. They also showed that the effectiveness of MPC as a soil nitrification inhibitor is markedly affected by soil type and soil temperature, that MPC is more effective for inhibiting nitrification of ammonium-N than of urea-N, and that MPC has little, if any, effect on hydrolysis of urea or denitrification of nitrate in soil. These observations and other work discussed indicate that MPC is one of the most promising compounds so far proposed for inhibition of nitrification in soil.

Inhibition of nitrification in soil by heterocyclic nitrogen compounds

The relationship between the structures of diverse heterocyclic nitrogen (N) compounds and the effectiveness of these compounds for the inhibition of nitrification in soil was studied by determining the effects of different amounts of 12 unsubstituted and 33 substituted heterocyclic N compounds on the production of (NO 2 − +NO 3 − )-N in soils incubated at 25 °C for 21 days after treatment with ammonium sulfate. The results showed that unsubstituted heterocyclic N compounds containing two adjacent ring N atoms inhibit nitrification in soil and that two of these compounds, pyrazole and 1,2,4-triazole, are potent inhibitors. They also showed that several substituted pyrazoles and thiadiazoles are good inhibitors of nitrification in soil (e.g., 3-methylpyrazole and 3,4-dichloro-1,2,5-thiadiazole).

Inhibition of Nitrification in Soil by Acetylenic Compounds

AbstractRecent work has shown that acetylene (ethyne) is a potent inhibitor of nitrification in soil. To discover if substituted acetylenes inhibit nitrification in soil, we determined the effects of seventeen monosubstituted (HC ≡ CR) acetylenes (propyne, 1‐butyne, 2‐ethynylpyridine, phenylacetylene, 3‐butyn‐2‐one, 1‐phenyl‐2‐propyn‐1‐ol, 1‐pentyne, 4‐phenyl‐1‐butyne, 5‐phenyl‐1‐pentyne, 1‐ethynylcycloh‐examine, 2‐propyn‐1‐ol, 2‐propyn‐1‐amine, 1‐hexyne, 3‐butyn‐1‐ol, 1‐heptyne, 1‐octyne, acetylene monocarboxylic acid) and of seven disubstituted (RC ≡ CR′) acetylenes (2‐butyne, 3‐phenyl‐2‐propyn‐1‐ol, diphenylacetylene, 2‐butyn‐1‐ol, 2‐butyn‐1,4‐diol, acetylene dicarboxylic acid, 2‐butynoic acid) on production of (NO‐3 + NO‐2)‐N in soils treated with (NH4)2SO4. The data obtained showed that most of the substituted acetylenes tested inhibited nitrification in soil and that some were potent inhibitors of nitrification. The results with the four gaseous compounds tested showed that acetylene, propyne, and 1‐butyne inhibited nitrification in soils at partial pressures as low as 0.1 or 1.0 Pa, whereas 2‐butyne had little or no effect on nitrification at partial pressures as high as 10 Pa. The data obtained with the 21 nongaseous compounds tested showed that 2‐ethynylpyridine, phenylacetylene, and 3‐butyn‐2‐one had the greatest inhibitory effect on nitrification and that acetylene monocarboxylic acid, acetylene dicarboxylic acid, and 2‐butynoic acid had the least effect. Studies to measure production of C2H2 in soils treated with the nongaseous compounds showed that the inhibition of nitrification observed with these compounds was not due to C2H2 formed by their decomposition in soil. Experiments reported indicate that 2‐ethynylpyridine and phenylacetylene compare favorably with nitrapyrin (N‐Serve) and etridiazole (Dwell) as soil nitrification inhibitors and have potential practical value for retarding nitrification of fertilizer N in soil.