Effects of successive soil freeze-thaw cycles on nitrification potential of soils

Abstract

In our previous report (Yanai et al. 2004: Soil Sci. Plant Nutr., 50, 821–829), we demonstrated that soil freeze-thaw cycles caused a partial sterilization of the soil microbial communities and exerted limited effects on the potential of organic matter decomposition of soils. In the present study, the effects of soil freeze-thaw cycles on the nitrification potential of soils were examined and the impacts of the freeze-thaw cycles on the nitrifying communities were analyzed. Samples of surface soils (0 to 10 cm depth) were collected, from tropical arable land sites, temperate forest, and arable land sites~ Nitrification potential was assayed by the incubation of soils with or without the addition of 200 fig N of ammonium sulfate per g soil to reach a moisture content adjusted to 60% of maximum water-holding capacity at 27~wC following four successive soil freeze-thaw cycles (-13 and 4°C at 12 h-intervals). Nitrification potential of the soils, in which the decrease in the microbial biomass following the freeze-thaw cycles was less appreciable, was not inhibited by the soil freeze-thaw cycles. On the other hand, the nitrification potential of the soils, in which the decrease in the microbial biomass following the soil freeze-thaw cycles was relatively more appreciable, was clearly inhibited by the freeze-thaw cycles or was undetectable even in the unfrozen control. Surprisingly, nitrate production in the samples of an arable soil collected from Vietnam was inhibited by the addition of ammonium sulfate, and thus the effects of counter-anions of ammonium salts on the nitrification potential of the soils were examined. Since a much larger amount of nitrate was produced in the Vietnam soil with the addition of ammonium acetate and ammonium hydrogen carbonate than that in the soil with the addition of ammonium sulfate, it was considered that ammonium sulfate inhibited nitrification in the soil. These results indicated that ammonium sulfate may not always be a suitable substrate for estimating the nitrification potential of soils. Relationship between soil physicochemical properties and the effect of the soil freeze-thaw cycles on the nitrification potential was evaluated and it was considered that the soil pH(KCI) was likely to be responsible for the difference in the responses among soils, assuming that the pH values changed in unfrozen water under the frozen conditions of soils.     

Nitrification in two coniferous forest soils after different fertilization treatments

The effects of seven different fertilization treatments on nitrification in the organic horizons of a Myrtillus-type (MT) and a Calluna-type pine forest in southern Finland were studied. No (NO^?₃ + NO^?₂)-N accumulated in unfertilized soils during 6 weeks at 14 or 20°C in the laboratory. Net nitrification was stimulated by urea in both soils (but more in the MT pine forest soil) and to a lesser degree by wood ash but not by ammonium nitrate or nitroform (ureaformaldehyde). Nitrification was not detected in nitroform fertilized soils although ammonium accumulation was high during incubation. In the MT pine forest soil, net nitrification appeared to be stimulated by apatite, biotite and micronutrients. Nitrapyrin inhibited nitrification indicating that it was carried out by autotrophic nitrifiers. In the urea-fertilized MT pine forest soil, nitrification took place at an incubation temperature of 0°C. Accumulation of (N0^?₃ + NO^?₂)-N was highest in soil sampled at < 10°C.     

Loss of nitrogen from ammoniacal fertilizers incubated under different soil and temperature conditions

Abstract

Ammonium sulfate and urea were added to three soils of widely different composition. After incubation for 28 days at 20°C, from 13 to 89% of the N from ammonium sulfate, and 8 to 71% of the N as urea were not recovered, and at 40°C, 44 to 95% of the N as ammonium sulfate, and 33 to 81% of the N as urea was not recovered as either ammonium‐N or nitrate‐N. Significantly more N was lost from a soil containing 3% calcium carbonate at pH 8.4 as compared to the two other soils containing 25 and 35% calcium carbonate, which have pH's of 7.7 and 7.5, respectively. An incubation temperature of 40°C appeared quite unfavourable for nitrification.     

Effect of rewetting air-dried soils on pH and accumulation of mineral nitrogen

The effect of rewetting a number of air-dried soils on pH and on accumulation of mineral-N was examined in a laboratory incubation study. When rewetted-soils were incubated at 25°C three patterns of change in soil _pH and in accumulation of mineral-N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil _pH initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the _pH decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant.
Such changes in soil _pH, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where _pH-dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air-dried soils.     

Einfluß von Wassergehalt und Salzen auf die Nitrifikation in Proben einer podsoligen Braunerde

Influence of watercontent and salts on the nitrification in samples of a Dystric Cambisol Samples of a Dystric Cambisol from a beech site produced nitrate but autotrophic nitrifying microorganisms could not be detected. Net nitrification of the humic layer and the upper 5 cm of the mineral soil during incubation at 22°C was investigated. Nitrification rate increased with increasing water content of the soil. Additions of ammonium or peptone did not increase the nitrification in the humic layer. Supply of (ammonium-)sulphates and chlorides with concentrations higher than 2 mMol per kg soil inhibited nitrification totally. This could not be ascribed to pH-changes. Additions of phosphates, lime or alkali to the soil samples increased nitrification.     

Urea transformation in some tropical soils

The effects of incubation at 20°, 30° and 40°C and urea concentrations of 0, 50, 100 and 200 μg N/g soil on urea hydrolysis and nitrification were investigated in three Nigerian soils. At constant temperature urea hydrolysis and rate of NO₃^? accumulation increased with increasing rate of urea addition. Urea was rapidly hydrolyzed within 1 week of incubation. Nitrification in Apomu soil increased with increasing incubation temperature. Nitrification was slow in acid Nkpologu soil (pH 4.7). Texture, cation exchange capacity and C:N ratios of the soils were not related to urea hydrolysis or nitrification. Nitrite accumulation in these soils was insignificant. Soil pH was decreased by nitrification of hydrolyzed urea-N.     

MEASUREMENT OF LOSSES FROM FERTILIZER NITROGEN DURING INCUBATION IN ACID SANDY SOILS AND DURING SUBSEQUENT GROWTH OF RYEGRASS, USING 15N-LABELLED FERTILIZERS

Ammonium sulphate and calcium nitrate both containing excess ¹⁵N were applied to four acid sandy soils; two were from old arable fields and two from grassland, selected so that one of each pair was about pH 5 and the other about pH 6 (in water). The soils were incubated for 6 weeks at 21°C in large glazed earthenware pots, one set with the nitrification inhibitor 2-chloro-6-(trichloromethyl)-pyridine added and another without inhibitor. Ammonium and nitrate N were determined at intervals, and the total-N at the start and after 6 weeks. The atom per cent ¹⁵N in the mineral-N extracted from soils treated with ammonium sulphate was determined after 0, 3, and 6 weeks, and in the total-N of all the soils given N-fertilizer at 0 and 6 weeks. Much added N was immobilized at first, but some was re-mineralized during the second half of the incubation. Mineral-N extracted from soils treated with ammonium sulphate contained less ¹⁵N than the fertilizer added, showing that part of the apparent re-mineralization during the second half was from unlabelled soil organic matter. After incubating for 6 weeks less than 5 per cent of the N added as nitrate was lost but about 5 per cent of the labelled-N added as ammonium sulphate was lost from the two grassland soils. Adding the inhibitor prevented this loss. After incubating, the soil remaining in each jar was halved to provide duplicate pots and sown with ryegrass. A similar series of pots with the same treatments (but with unlabelled fertilizer) was also prepared from the soils that had been stored slightly moist and at 21°C; these were sown with ryegrass. All pots were harvested after 42 days and again after 70 days. More than 93 per cent of the labelled-N was recovered in plants and soil, except from the two grassland soils to which calcium nitrate was added. It is concluded that while a little nitrogen may be lost during nitrification in some of these soils, more nitrogen may be lost during the growth of grass, when nitrate is present in relatively large amounts. The nitrification inhibitor decreased yields of grass at the first cutting on grassland soils treated with ammonium, but increased them on soil treated with nitrate, suggesting that changing the proportions of nitrate to ammonium by adding the inhibitor alters the growth rate and yield of grass.     

氮素浓度和水分对水稻土硝化作用和微生物特性的影响

为了明确不同氮素浓度和水分对土壤硝化作用和微生物特性的影响,特别是高氮素浓度下的响应特异性,以红壤水稻土为供试土壤,设置4个硫铵用量水平[0(CK)、120 mg(N).kg-1(A1)、600 mg(N).kg-1(A2)、1 200 mg(N).kg-1(A3)],调节土壤水分为饱和持水量(WHC)的40%、60%和80%,研究了短期内不同氮素浓度和不同水分条件下土壤硝化作用、微生物生物量碳和微生物功能多样性的变化。结果表明:在40%、60%和80%WHC水分条件时,硫铵A2、A3浓度处理土壤硝化率和硝化速率普遍较低,硫铵A1浓度处理硝化率和硝化速率随土壤含水量的升高而升高;同含水量时随硫铵用量的升高而显著降低。在40%、60%和80%WHC水分条件时,微生物生物量碳随硫铵浓度的升高而降低;同浓度硫铵用量水平时,微生物生物量碳的变化基本表现为:60%WHC80%WHC40%WHC。分析发现不同水分和硫铵处理之间存在交互作用。BIOLOG分析显示:不同氮素浓度和不同水分处理,60%WHC下A1处理的平均吸光值(AWCD)和Shannon、Simpson、McIntosh指数最大,其次为60%WHC的硫铵CK处理,而不同水分下硫铵A2、A3处理,其AWCD值和Shannon、Simpson、McIntosh多样性指数都较低,进一步说明过量施肥导致微生物活性降低。不同氮素浓度和水分条件下土壤微生物和生化性状不同,过量施用化肥后将有可能造成土壤微生物性状和生化功能衰减。     

Potential use of karanjin (3-methoxy furano-2′,3′,7,8-flavone) as a nitrification inhibitor in different soil types

Karanjin, a furanoflavonoid (3-methoxy furano –?2 ′ , 3 ′ , 7, 8-flavone), is obtained from the seeds of karanja tree (Pongamia glabra Vent.), which is reported to have nitrification inhibitory properties but has been tested in few soil types. Efficiency of karanjin as a nitrification inhibitor in seven different soils of India was tested in a laboratory incubation study. The soils (800?g) were adjusted to field capacity moisture content, fertilized with urea and urea combined with karanjin at a rate of 20% of applied urea-N (100?mg?kg^???1 soil) and incubated at 35°C for a period of 7 weeks, during which urea [CO(NH₂)₂], ammonium (NH₄ ^?+?), nitrite (NO₂ ^???) and nitrate (NO₃ ^???) content in the soils was measured periodically and nitrification inhibition at different stages was calculated. Urea hydrolysis was almost complete within 72?h of application in all the soils and was not affected by karanjin. Karanjin had conserved ammonium in all the soils at all stages and nitrate formation was effectively minimized. Nitrite in soils was short-lived and low. Nitrification inhibition by karanjin remained high for a period of approximately 6 weeks, decreased with time and ranged from 9?–?76% for all the soils. The study shows that this plant product can be an effective nitrification inhibitor in several types of soil.     

Umsetzung von Cyanamid,Harnstoff und Ammonsulfat in Abhängigkeit von Temperatur und Bodenfeuchtigkeit

Transformation of cyanamide, urea and ammonium sulfate as influenced by temperature and moisture of soil The conversion of cyanamide, urea and ammonium sulfate solutions to nitrate was investigated in a sandy silt loam (pH 6.2) in relation to temperature and soil moisture conditions. 1. Cyanamide was transformed to urea within 1–5 days. Increasing temperature (2°–100°C) accelerated the breakdown, whereas high moisture conditions (120 % of total water capacity) decreased transformation. 2. The hydrolysis of urea to ammonia took place within 5–10 days even at 2°C regardless of whether cyanamide or urea was added. Low soil moisture (40 % of total water capacity) and high temperature (up to 50°) accelerated the breakdown. 3. Following urea application (20 mg N) there was a transient formation of up to five times more nitrite (0.5 mg NO₂-N) as compared with cyanamide or ammonium sulfate treatments. 4. Clear differences were observed in the rates of nitrification. The rate was greater for urea than for cyanamide and ammonium sulfate. The formation of nitrate began at 2°C, with an optimum between 20° and 30°C. Under flooded conditions (120 % of total water capacity) and low temperature the rate of nitrification was slow. At higher temperatures rapid denitrification took place.     

Factors Affecting Nitrification in Soils

Abstract

Nitrification in soil converts relatively immobile ammonium‐nitrogen (N) to highly mobile nitrate‐N (via nitrite), and this has implications for N‐use efficiency by agricultural systems as well as for environmental quality, especially in situations where the potential for loss of soil or added N is high following nitrate formation. The literature on various physical, environmental, and chemical factors and their interactions on nitrification in soil is reviewed and discussed with examples from natural and agro‐ecosystems. Among the various factors, soil matrix, water status, aeration, temperature, and pH have strong influence on nitrification. The information on factors that influence nitrification is useful when developing strategies for regulating nitrification in soils by employing chemical or biological nitrification inhibitors.     

Temperature effects on ammonification and nitrification in a tropical soil

Ammonification of soil organic N and nitrification of ammonium-N was studied in Tindall clay loam over a range of temperatures from 20–60 C. Nitrification rates at each temperature were constant throughout the 28 day incubation, whereas most of the ammonification occurred in the first 7 days. The optimum for nitrification was close to 35 C. exhibiting a sharp peak at this temperature at which the potential rate was 4.8 μg N/g day^?1, compared with 0.5 μg N/g day^?1 at 20°C and 0.25 μg N/g day^?1 at 60°C. The optimum temperature for ammonification was approximately 50°C at which the rate was 2.8 μg N/g day^?1 in the first 7 days but only 0.5 μg N/g day^?1 between 14 and 28 days.The temperature responses could be described mathematically with functions of the type log_oN = k × 1/T.The results are discussed in relation to daily patterns of N mineralization in the field where temperatures show diurnal fluctuation.     

Effect of soil solarization on subsequent nitrification activity at elevated temperatures

Soil solarization is a nonchemical method of soil disinfection achieved by covering the soil surface with sheets of vinyl plastic to generate elevated soil temperature, generally over 45°C. Such elevated temperatures may be detrimental to some nitrifying microorganisms and favorable to others. However, little information exists to indicate how nitrification activity in soil is affected after solarization. We performed several experiments to investigate the effects of soil solarization on nitrification activity. We found that: (1) if a soil was subjected to pretreatment of 45 or 50°C for as little as 1 d, nitrification activity in a subsequent incubation at 30°C was less than that of a soil that did not receive any high-temperature pretreatment. However, if a soil received pretreatments of 45 or 50°C for more than 7 d, nitrification activity in a subsequent incubation at 45 or 50°C was greater than that of soil that did not receive high temperature pretreatment. (2) Nitrification activity in three kinds of soil taken from 0–5 cm depth after solarization treatment was greater at 45°C than 30°C. (3) Nitrification activity at 45°C in soil that had received solarization in the preceding year was greater than that in soil that had not been subjected to solarization. This was consistent with the fact that the population densities of ammonia oxidizers were greater in soils that had been subjected to solarization. These results suggest that soil solarization induces nitrifying microorganisms that are more active at 45–50°C than they are at 30°C, and that the effect of solarization on nitrification persists until the next crop season.     

NON-EXCHANGEABLE AMMONIUM IN SOILS OF ISRAEL AND ITS RELATION TO CLAY AND PARENT MATERIALS

The occurrence of non-exchangeable ammonium in the main soil groups of Israel and its relation to various factors were determined in 129 samples from thirty-one profiles. In arid and semiarid soils the level ranged from 0·18-0·53 me/100 g, with most values falling between 0·25 and 0·40 me/100 g. In the sub-humid Mediterranean region the values ranged between 0·04 and 1·0 me/100 g, with most of them falling between 0·14 and 0·65 me/100 g. In most cases the percentage of non-exchangeable ammonium-N in the total soil nitrogen was in the range of 2–25 per cent. The clay fraction is a significant factor influencing the amount of non-exchangeable ammonium in the soils tested. The relatively low amounts of non-exchangeable ammonium found are consistent with the predominant clay mineral in most of the soils being montmorillonite. A good correlation was obtained with the content of illite, which is the main carrier of the non-exchangeable ammonium. The mean non-exchangeable K/non-exchangeable NH, ratio in the Israeli soil clays was 17. In soils derived from or affected by aeolian dust, the level of non-exchangeable ammonium calculated for the clay fraction spans that of aeolian dust and is, in general, higher than in soils developed from limestone, dolomite, marl, tuff, or basalt.     

Nitrapyrin decreased nitrification of nitrogen released from soil organic matter but not amoA gene abundance at high soil temperature

Water pulses have a significant impact on nitrogen (N) cycling, making management of N challenging in agricultural soils that are exposed to episodic rainfall. In hot, dry environments, wetting of dry soil during summer fallow causes a rapid flush of organic matter mineralisation and subsequent nitrification, which may lead to N loss via nitrous oxide emission and nitrate leaching. Here we examined the potential for the nitrification inhibitor nitrapyrin to decrease gross nitrification at elevated temperature in soils with contrasting soil organic matter contents, and the consequent effects on ammonia oxidiser populations. Soil was collected during summer fallow while dry (water content 0.01 g g⁻¹ soil) from a research site with two management treatments (tilled soil and tilled soil with long-term additional crop residues) by three field replicates. The field dry soil (0–10 cm) was wet with or without nitrapyrin, and incubated (20 or 40 °C) at either constant soil water content or allowed to dry (to simulate summer drying after a rainfall event). Gross N transformation rates and inorganic N pools sizes were determined on six occasions during the 14 day incubation. Bacterial and archaeal amoA gene abundance was determined on days 0, 1, 7 and 14. Nitrapyrin increased ammonium retention and decreased gross nitrification rates even with soil drying at 40 °C. Nitrification was likely driven by bacterial ammonia oxidisers, as the archaeal amoA gene was below detection in the surface soil layer. Bacterial ammonia oxidiser gene abundances were not affected by nitrapyrin, despite the decrease in nitrifier activity. Increased soil organic matter from long-term additional crop residues diminished the effectiveness of nitrapyrin. The present study highlights the potential for nitrapyrin to decrease nitrification and the risk of N loss due to mineralisation of soil organic matter under summer fallow conditions.     

4种熏蒸剂对土壤氮素转化的影响

采用室内恒温通气培养法,以北京大棚蔬菜地土壤为对象,研究熏蒸剂氯化苦(Pic)、碘甲烷(MeI)、1,3-二氯丙烯(1,3-D)和二甲基二硫(DMDS)对土壤氮素矿化和硝化的影响。结果表明,4种熏蒸剂处理后短期内均能显著增加土壤中氮累积矿化量,在处理后第0d,1,3-D、MeI、DMDS、Pic处理的氮累积矿化量分别为320.62mg·kg-1、317.25mg·kg-1、287.87mg·kg-1、278.73mg·kg-1,较对照(189.89mg·kg-1)分别增加68.85%、67.07%、51.60%、46.78%。4种熏蒸剂处理后土壤硝化作用过程受到显著抑制,在药剂熏蒸处理第0d,各熏蒸处理土壤中铵态氮含量均高于对照组,其中MeI处理组铵态氮含量最高,为194.97mg·kg-1,对照组铵态氮含量最低,为28.82mg·kg-1。Pic、1,3-D、DMDS、MeI处理后第0d硝化抑制率分别为40.8%、20.8%、26.9%、24.1%。Pic、1,3-D、MeI对硝化作用的抑制至少维持两周,DMDS的抑制作用至少维持1周。在后期培养过程中,各处理矿化作用和硝化作用都逐渐恢复至对照水平。     

Effects of nitrification inhibitors on transformations of urea nitrogen in soils

The effects of three patented nitrification inhibitors on transformations of urea N in soils were studied by determining the effects of these compounds (10 μg/g of soil) on urea hydrolysis, ammonia volatilization. and production of ammonium, nitrite, and nitrate in soils incubated under aerobic conditions (30°C, 60% WHC) after treatment with urea (400 μg of urea N/g of soil). The inhibitors used (N-Serve, ATC, and CL-1580) had little, if any, effect on urea hydrolysis, but they retarded nitrification of the ammonium formed by urea hydrolysis and increased gaseous loss of urea N as ammonia. They also decreased the amount of (urea + exchangeable ammonium + nitrite + nitrate) — N found in urea-treated soils after various times.Two of the soils used accumulated substantial amounts of nitrite(> 160 μg of nitrite N/g of soil) when incubated under aerobic conditions after treatment with urea. Addition of nitrification inhibitors to these soils eliminated or substantially reduced nitrite accumulation and greatly retarded nitrate formation, but had little, if any, effect on the recovery of urea N as (urea + exchangeable ammonium + nitrite + nitrate + ammonia) — N after various times. This finding and other observations reported indicate that the “nitrogen deficits” observed in studies of urea N transformations in soils may not largely be due to gaseous loss of urea N through chemodenitrification and are at least partly due to volatilization and fixation of the ammonium formed by urea hydrolysis in soils. The work reported also indicates that N-Serve and other nitrification inhibitors may prove useful for reduction of the nitrite toxicity problems associated with the use of urea as a fertilizer but that application of such inhibitors in conjunction with fertilizer urea, when surface applied, may promote gaseous loss of urea N as ammonia.     

DCD 在不同质地土壤上的硝化抑制效果和剂量效应研究

通过硝化抑制剂抑制土壤硝化作用是实现作物铵硝混合营养和提高氮肥利用率的重要途径之一。本试验采用室内模拟的方法, 在人工气候室(25 ℃)黑暗培养条件下, 应用新疆石灰性土壤研究了不同剂量的双氰胺(dicyandiamide, DCD)在砂土、壤土、黏土3 种不同质地土壤中对土壤硝态氮、铵态氮转化的影响及DCD 的剂量效应和硝化抑制效果。处理30 d 内, 各剂量DCD 处理对砂土的硝化抑制率为96.5%~99.4%(平均值为98.3%), 在黏土上为66.9%~85.6%(平均值为77.6%), 在壤土上为49.3%~79.4%(平均值为67.7%), 总体硝化抑制率表现为砂土>黏土>壤土。在砂土上DCD 的剂量效应不明显, DCD 用量从纯氮的1.0%增加到7.0%时, 土壤中硝态氮含量仅增加1.9~10.7 mg·kg^-1(培养30 d 时); 而在壤土和黏土中, 土壤硝态氮含量随DCD 浓度的增加而显著下降, 存在明显剂量效应。这说明施用DCD 可显著抑制新疆石灰性土壤的硝化作用过程, 在砂土、壤土、黏土中DCD 的最佳浓度分别为纯氮用量的6.0%、7.0%和7.0%, 并在培养30 d 内发挥显著作用。     

Effect of External Carbon Source on Municipal Wastewater at Low Temperatures

Nitrogen removal from wastewater is usually severely inhibited under low temperatures. The wastewater enrichment using an external carbon source has the influence on the stability and efficiency of the nitrification and denitrification processes during the biological wastewater treatment. This paper reports the results of the study where the effect of temperature and addition of an external carbon source on the efficiency of wastewater treatment process were investigated. Nitrification and denitrification rates were determined in the laboratory-scale treatment system, operating under low-temperature conditions, ranging from 6 up to 15 °C. Ethanol was used as an external carbon source. The addition of ethanol resulted in the increase during the nitrification rate at lower temperature (up to 71% at 6 °C and up to 11% at 15 °C). Similar tendency was observed during the denitrification process. Denitrification rate increased up to 81% at 6 °C and up to 10% at 15 °C, respectively. Nitrification rate was slightly higher compared to the denitrification rate. Two-variable model equations for calculation of an external carbon amount required were based on the experimental data and in order to reach desirable process rate at particular wastewater temperature were developed. The independency from wastewater temperature and the amount of loaded carbon explicit interdependence between nitrification and denitrification rates were observed.     

Effect of cropping systems on nitrification in soils

Abstract

Limited information is available about the effect of cropping systems and N application on nitrification potential of soils. This study was conducted to evaluate nitrification rates of soils that have been under long‐term cropping systems at three sites in Iowa. Each experiment consisted of three cropping systems (continuous corn, corn‐soybean‐corn‐soybean, and corn‐oats‐meadow‐meadow) and two fertilizer treatments: untreated (0 N) and treated (+ N) with ammonium or ammonium‐forming fertilizers (180 or 200 kg ha/yr) before corn. The rate of nitrification was studied at 30°C. Results showed that, although soil pH decreased in the plots treated with ammoniacal fertilizers before corn in the cropping system, the rate of nitrification was significantly greater in N‐treated than in untreated plots, suggesting that fertilization with ammonium or ammonium‐forming fertilizers either increased the microbial populations responsible for nitrification in soils and/or that such treatments increased the efficiency of the nitrifiers by inducing the enzymes responsible for conversion of NH⁴⁺ to NO₃‐. The results suggest that continuous application of ammonium or ammonium‐forming fertilizer could enhance the nitrification rate and increase the potential of contamination of groundwater with nitrate.