Influence of Willow Biochar Amendment on Soil Nitrogen Availability and Greenhouse Gas Production in Two Fertilized Temperate Prairie Soils

The potential of biochar to improve numerous soil physical, chemical and biological properties is well known. However, previous research has concentrated on old and highly weathered tropical soils with poor fertility, while reports regarding the influence of biochar application on relatively young and fertile temperate prairie soils are limited. Furthermore, the mechanism(s) underlying biochar-induced effects on the plant availability of inorganic nitrogen (N) fertilizers and their relationship to greenhouse gas production is not well understood. The objective of this study was to determine the effect of a biochar soil amendment, produced by slow pyrolysis using shrub willow (Salix spp.) bioenergy feedstock, on CO₂, N₂O and CH₄ fluxes by two contrasting marginal soils from Saskatchewan, Canada with and without added urea, over a 6-week incubation period. Biochar decreased soil N availability after 6 weeks only in the lower organic matter (Brown) soil, with no effect on the Black soil, regardless of fertilizer N addition, which was attributed to soil N immobilization by heterotrophs mineralizing the labile biochar-carbon. There appeared to be a synergistic effect when combining biochar and urea, evidenced by enhanced urease activity and higher initial nitrification rates compared to biochar or fertilization alone. The accelerated urea hydrolysis in the presence of biochar may increase NH₃ volatilization losses associated with urea fertilization and, therefore, warrants further investigation. The decreased N₂O emissions following biochar addition, with (both soils) or without (Black soil) fertilizer N, could be due to decreased ammonium and nitrate availability, along with changes in denitrification potential as related to improved aeration. Biochar significantly reduced the water-filled pore space, which concurrently increased CH₄ consumption in both soils. The lack of biochar effect on CO₂ emissions from either soil, with or without fertilizer N, suggests enhanced CO₂ consumption by autotrophic nitrifiers. Biochar application appears to be an effective management approach for improving N₂O and CH₄ fluxes in temperate prairie soils.     

Rapid Cycling of Organic Nitrogen in Taiga Forest Ecosystems

ABSTRACT We examined the dynamics of organic nitrogen (N) turnover in situ across a primary successional sequence in interior Alaska, USA, in an attempt to understand the magnitude of these fluxes in cold, seasonally frozen soils. Through a combination of soil extraction procedures and measurements of ¹³C-enriched CO₂ efflux from soils amended in the field with ¹³C-labeled amino acids, we were able to trace the fate of this N form. Amino acid turnover in situ at soil temperatures of 10°C or below show that amino acids represent a highly dynamic soil N pool with turnover times of approximately 3–6 h. The rapid turnover of free amino acids is associated with high soil proteolytic activity, which in turn is tightly correlated with soil protein concentration. Moreover, these estimates of soil amino acid turnover in the field correspond well with measurements of amino acid turnover under equivalent temperatures in the laboratory. The gross flux of amino acid-N over the growing season greatly exceeded the annual vegetation N requirement, suggesting that microbial biomass represent a significant sink for this organic N. Depending on the strength of this sink, N flow via free soil amino acids can potentially account for the entire N demand of vegetation in the absence of net N mineralization. These relationships underscore the important biogeochemical role of labile DON fractions in high-latitude forest ecosystems.     

Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests

Nitrogen (N) inputs from atmospheric deposition can increase soil organic carbon (SOC) storage in temperate and boreal forests, thereby mitigating the adverse effects of anthropogenic CO₂ emissions on global climate. However, direct evidence of N-induced SOC sequestration from low-dose, long-term N addition experiments (that is, addition of < 50 kg N ha⁻¹ y⁻¹ for > 10 years) is scarce worldwide and virtually absent for European temperate forests. Here, we examine how tree growth, fine roots, physicochemical soil properties as well as pools of SOC and soil total N responded to 20 years of regular, low-dose N addition in two European coniferous forests in Switzerland and Denmark. At the Swiss site, the addition of 22 kg N ha⁻¹ y⁻¹ (or 1.3 times throughfall deposition) stimulated tree growth, but decreased soil pH and exchangeable calcium. At the Danish site, the addition of 35 kg N ha⁻¹ y⁻¹ (1.5 times throughfall deposition) impaired tree growth, increased fine root biomass and led to an accumulation of N in several belowground pools. At both sites, elevated N inputs increased SOC pools in the moderately decomposed organic horizons, but decreased them in the mineral topsoil. Hence, long-term N addition led to a vertical redistribution of SOC pools, whereas overall SOC storage within 30 cm depth was unaffected. Our results imply that an N-induced shift of SOC from older, mineral-associated pools to younger, unprotected pools might foster the vulnerability of SOC in temperate coniferous forest soils.

     

Effects of Exotic Earthworms on Soil Phosphorus Cycling in Two Broadleaf Temperate Forests

We compared the biogeochemical cycling of phosphorus (P) in northern hardwood forest plots invaded by exotic earthworms versus adjacent uninvaded reference plots. In three of the six pairs of plots, earthworm invasion resulted in significantly more total P in the upper 12 cm of soil. The finding of increased amounts of unavailable and occluded inorganic P forms in the invaded plots suggests that earthworm activity mobilized unweathered soil particles from deeper layers of the soil, increasing the stocks of total P in surface soils. In two pairs of plots, the earthworm-invaded soils had less total P than the reference soils. In these plots, earthworm activity resulted in augmented rates of P cycling and alteration of the physical structure of the soil that increased loss of P in leaching water, reducing the total amount of P. We hypothesize that the different effects of earthworm invasion on the soil P cycle result from unique characteristics of the ecological groups of earthworms dominating each site. The invaded plots with increased total P were dominated by the anecic species Lumbricus terrestris, a large earthworm that constructs deep, vertical burrows and is effective at moving soil materials from and to deeper layers of the profile. In contrast, the earthworm-invaded plots where the total P in the surface soil decreased were dominated by the epi-endogeic species L. rubellus, which feeds and lives in the upper organic layers of the soil. In these plots, earthworms significantly increased the amount of readily exchangeable P in the soil, increasing the loss of this element in leaching water.     

Long-Term Nitrogen Addition Does Not Increase Soil Carbon Storage or Cycling Across Eight Temperate Forest and Grassland Sites on a Sandy Outwash Plain

Ecosystems - Experimental nitrogen (N) deposition generally inhibits decomposition and promotes carbon (C) accumulation in soils, but with substantial variation among studies. Differences in...     

Precipitation Pattern Regulates Soil Carbon Flux Responses to Nitrogen Addition in a Temperate Forest

Ecosystems - Changes in precipitation frequency and intensity are predicted to be more intense and frequent accompanying climate change and may have immediate or potentially prolonged effects on...     

Nitrogen Fixation and Nitrogen Assimilation in a Temperate Saline Ecosystem

As nitrogen is known to be a limiting factor for plant growth, we were interested in the relationship between soil microbial activity and the nitrogen assimilation of 5 different halophytes from 4 saline sites near the lake “Neusiedlersee”, Austria. The following were studied between May and October 1985: nitrogen fixation (¹⁵N₂ and acetylene reduction): N-mineralization; several soil characteristics and in vivo nitrate reductase activity of roots and shoots of these plants. NO^?₃, org. N- and carboxylate contents of both roots and shoots, as well as the effect of NO^?₃-fertilization on the amounts of these substances, were determined on plants growing in the field during a 3-day period in September 1985. Fertilization led to a decrease in acetylene reduction activity at most sites, and an increase in the nitrate reductase activity of the shoots of all plants. Overall, carboxylate and organic nitrogen contents of these halophytes did not change in response to fertilization. Only in the roots of Aster tripolium and Atriplex hastata was there a marked increase in the nitrate reductase activity in response to fertilization. Species growing at the same site, such as Plantago maritima and Lepidium crassifolium showed contrasting levels of assimilatory activity. Apparent low rates of ammonification and nitrification were detected in soils from the 4 sites. The results are discussed in relation to the nitrogen and carbon economies of the microorganisms and plants.     

Climate Variation and Soil Carbon and Nitrogen Cycling Processes in a Northern Hardwood Forest

We exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF) to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50–100 years for northeastern North America on soil carbon (C) and nitrogen (N) cycle processes. Our objectives were to (1) characterize differences in soil temperature, moisture and frost associated with elevation at the HBEF and (2) evaluate variation in total soil (TSR) and microbial respiration, N mineralization, nitrification, denitrification, nitrous oxide (N₂O) flux, and methane (CH₄) uptake along this gradient. Low elevation sites were consistently warmer (1.5–2.5°C) and drier than high elevation sites. Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots. Net N mineralization and nitrification were slower in warmer, low elevation plots, in both summer and winter. In summer, this pattern was driven by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer soils. These data suggest that N cycling and supply to plants in northern hardwood ecosystems will be reduced in a warmer climate due to changes in both winter and summer conditions. TSR was consistently faster in the warmer, low elevation plots. N cycling processes appeared to be more sensitive to variation in soil moisture induced by climate variation, whereas C cycling processes appeared to be more strongly influenced by temperature.     

Correction to: Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests

Ecosystems - In the article by Forstner et al. (2018), the surnames of co-authors Katharina M. Keiblinger and Patrick Schleppi were misspelled. We apologize and ask readers to cite the corrected...     

Age-Related Modulation of the Nitrogen Resorption Efficiency Response to Growth Requirements and Soil Nitrogen Availability in a Temperate Pine Plantation

Nitrogen (N) resorption is a key strategy for conserving N in forests, and is often affected by soil nutrient condition and N sink strength within the plant. However, our understanding of the age-related pattern of N resorption and how increasing N deposition will affect this pattern is limited. Here, we investigated N resorption along a chronosequence of stands ranging in age from 2 to 100 years old, and conducted a 4-year exogenous N input experiment in stands at age class 11, 20, and 45 in a Larix Principis-rupprechtii plantation in north China. We found a logarithmic increase in leaf N resorption efficiency (NRE) and green leaf N concentration, and a logarithmic decrease in senesced-leaf N concentration along the stand-age chronosequence. Leaf NRE was negatively correlated with plant-available N concentration. Stand-level N resorption was positively correlated with the annual N requirement for tree growth. N resorption contributed to 45, 62, and 68% of the annual N supply in the 11-, 20-, and 45-year-old stands, respectively. Our exogenous N input experiment showed that leaf NRE in the 11- and 20-year-old stands decreased 17 and 12% following a 50-kg N ha^?1 y^?1 input. However, leaf NRE was not affected in the 45-year-old stand. The increases in leaf NRE and the contribution of N resorption to annual N supply along stand ages suggested that, with stand development, tree growth depends more on N resorption to supply its N need. Furthermore, the leaf NRE of mature stand was not decreased under exogenous N input, suggesting that mature stands can be stronger sinks for N deposition than young stands due to their higher capacity to retain the deposited N within plants via internal cycle. Ignoring age-related N use strategies can lead to a bias in N cycle models when evaluating forest net primary production under increasing global N deposition.     

Nitrogen Fixation and Leaching of Biological Soil Crust Communities in Mesic Temperate Soils

Biological soil crust is composed of lichens, cyanobacteria, green algae, mosses, and fungi. Although crusts are a dominant source of nitrogen (N) in arid ecosystems, this study is among the first to demonstrate their contribution to N availability in xeric temperate habitats. The study site is located in Lucas County of Northwest Ohio. Using an acetylene reduction technique, we demonstrated potential N fixation for these crusts covering sandy, acidic, low N soil. Similar fixation rates were observed for crust whether dominated by moss, lichen, or bare soil. N inputs from biological crusts in northwestern Ohio are comparable to those in arid regions, but contribute substantially less N than by atmospheric deposition. Nitrate and ammonium leaching from the crust layer were quantified using ion exchange resin bags inserted within intact soil cores at 4 cm depth. Leaching of ammonium was greater and nitrate less in lichen than moss crusts or bare soil, and was less than that deposited from atmospheric sources. Therefore, biological crusts in these mesic, temperate soils may be immobilizing excess ammonium and nitrate that would otherwise be leached through the sandy soil. Moreover, automated monitoring of microclimate in the surface 7 cm of soil suggests that moisture and temperature fluctuations in soil are moderated under crust compared to bare soil without crust. We conclude that biological crusts in northwestern Ohio contribute potential N fixation, reduce N leaching, and moderate soil microclimate.     

Distinguishing between Nitrification and Denitrification as Sources of Gaseous Nitrogen Production in Soil

The source of N₂O produced in soil is often uncertain because denitrification and nitrification can occur simultaneously in the same soil aggregate. A technique which exploits the differential sensitivity of these processes to C₂H₂ inhibition is proposed for distinguishing among gaseous N losses from soils. Denitrification N₂O was estimated from 24-h laboratory incubations in which nitrification was inhibited by 10-Pa C₂H₂. Nitrification N₂O was estimated from the difference between N₂O production under no C₂H₂ and that determined for denitrification. Denitrification N₂ was estimated from the difference between N₂O production under 10-kPa C₂H₂ and that under 10 Pa. Laboratory estimates of N₂O production were significantly correlated with in situ N₂O diffusion measurements made during a 10-month period in two forested watersheds. Nitrous oxide production from nitrification was most important on well-drained sites of a disturbed watershed where ambient NO₃⁻ was high. In contrast, denitrification N₂O was most important on poorly drained sites near the stream of the same watershed. Distinction between N₂O production from nitrification and denitrification was corroborated by correlations between denitrification N₂O and water-filled pore space and between nitrification N₂O and ambient NO₃⁻. This technique permits qualitative study of environmental parameters that regulate gaseous N losses via denitrification and nitrification.     

Enhancing Soil Organic Matter as a Route to the Ecological Intensification of European Arable Systems

Soil organic matter (SOM) is declining in most agricultural ecosystems, impacting multiple ecosystem services including erosion and flood prevention, climate and greenhouse gas regulation as well as other services that underpin crop production, such as nutrient cycling and pest control. Ecological intensification aims to enhance crop productivity by including regulating and supporting ecosystem service management into agricultural practices. We investigate the potential for increased SOM to support the ecological intensification of arable systems by reducing the need for nitrogen fertiliser application and pest control. Using a large-scale European field trial implemented across 84 fields in 5 countries, we tested whether increased SOM (using soil organic carbon as a proxy) helps recover yield in the absence of conventional nitrogen fertiliser and whether this also supports crops less favourable to key aphid pests. Greater SOM increased yield by 10%, but did not offset nitrogen fertiliser application entirely, which improved yield by 30%. Crop pest responses depended on species: Metopolophium dirhodum were more abundant in fertilised plots with high crop biomass, and although population growth rates of Sitobion avenae were enhanced by nitrogen fertiliser application in a cage trial, field populations were not affected. We conclude that under increased SOM and reduced fertiliser application, pest pressure can be reduced, while partially compensating for yield deficits linked to fertiliser reduction. If the benefits of reduced fertiliser application and increased SOM are considered in a wider environmental context, then a yield cost may become acceptable. Maintaining or increasing SOM is critical for achieving ecological intensification of European cereal production.     

Soil Respiration and Organic Carbon Dynamics with Grassland Conversions to Woodlands in Temperate China

Soils are the largest terrestrial carbon store and soil respiration is the second-largest flux in ecosystem carbon cycling. Across China''s temperate region, climatic changes and human activities have frequently caused the transformation of grasslands to woodlands. However, the effect of this transition on soil respiration and soil organic carbon (SOC) dynamics remains uncertain in this area. In this study, we measured in situ soil respiration and SOC storage over a two-year period (Jan. 2007–Dec. 2008) from five characteristic vegetation types in a forest-steppe ecotone of temperate China, including grassland (GR), shrubland (SH), as well as in evergreen coniferous (EC), deciduous coniferous (DC) and deciduous broadleaved forest (DB), to evaluate the changes of soil respiration and SOC storage with grassland conversions to diverse types of woodlands. Annual soil respiration increased by 3%, 6%, 14%, and 22% after the conversion from GR to EC, SH, DC, and DB, respectively. The variation in soil respiration among different vegetation types could be well explained by SOC and soil total nitrogen content. Despite higher soil respiration in woodlands, SOC storage and residence time increased in the upper 20 cm of soil. Our results suggest that the differences in soil environmental conditions, especially soil substrate availability, influenced the level of annual soil respiration produced by different vegetation types. Moreover, shifts from grassland to woody plant dominance resulted in increased SOC storage. Given the widespread increase in woody plant abundance caused by climate change and large-scale afforestation programs, the soils are expected to accumulate and store increased amounts of organic carbon in temperate areas of China.     

The Role of Biological Soil Crusts in Nitrogen Cycling and Soil Stabilization in Kangerlussuaq,West Greenland

Ecosystems - Biological soil crusts (biocrusts) naturally coexist with vascular plants in many dryland ecosystems. Although most studies of dryland biocrusts have been conducted in warm deserts,...     

Microbial Nitrogen Cycling Processes in a Sulfidic Coastal Marsh

Sulfide distribution is a key controller of vegetation zonation in coastal ecosystems, but data are limited regarding its impact on the spatial distribution of important N cycling processes. We assessed vegetation distribution and density and, mineral N pool sizes, composition and transformations in a sulfidic coastal marsh in relation to distance from sulfur springs. We observed strong relationships between vegetation attributes (species and density) and mineral N status with greater total inorganic N, NO₃⁻ and denitrification enzyme activity (DEA) in sediment samples from areas populated by Crithmum maritimum (mid-way between S springs and sea shore) than in sediments from areas colonized by either Agropyron repens (closest to the S springs) or mangrove (Rhizophora mangleL., farthest from the springs). Our data also suggest close links between N cycling and SO₄⁻² reduction. The latter resulted in net release of NH₄⁺ ranging from 0.9 mg N kg⁻¹ in the low density C. maritimum to 3.2 mg N kg⁻¹ in the high-density A. repens, during a 3-day incubation. We also tested for microbial adaptation to long-term high sulfide exposure by measuring DEA using the C₂H₂ block method (which has been found to be strongly affected by the presence of sulfide) and amendment of marsh sediment samples with NaMoO₄ to suppress reduced S production. In sediments extracted from sites near the sulfur springs (A. repens and C. maritimum), the C₂H₂ blockage assay yielded similar results without and with NaMoO₄ addition. However, in samples from a mangrove located further downstream from the springs, DEA was substantially lower (2.3 vs. 6.8 mg N₂O-N kg⁻¹ sediment d⁻¹) when production of reduced S was not inhibited by NaMoO₄. These results suggest that denitrifying microbes in the high sulfide areas may have adapted to the presence of sulfide, allowing for high rates of N and S cycling to occur simultaneously in these marshes.     

Coarse Woody Debris Stimulates Soil Enzymatic Activity and Litter Decomposition in an Old-Growth Temperate Forest of Patagonia, Argentina

In most temperate forest ecosystems, tree mortality over time generates downed logs that accumulate as coarse woody debris (CWD) on the forest floor. These downed logs and trunks have important recognized ecosystem functions including habitat for different organisms and long-term organic C storage. Due to its recalcitrant chemical composition and slow decomposition, CWD can also have direct effects on ecosystem carbon and nutrient turnover. CWD could also cause changes indirectly through the physical and chemical alterations that it generates, although it is not well-understood how important these indirect effects could be for ecosystem processes and soil biogeochemistry. We hypothesized that in an old-growth mature forest, CWD affects carbon and nutrient cycles through its “proximity effects”, meaning that the forest floor near CWD would have altered soil biotic activity due to the environmental and biogeochemical effects of the presence of CWD. We conducted our study in an old-growth southern beech temperate forest in Patagonia, Argentina, where we estimated and classified the distribution and mass, nutrient pools and decay stage of CWD on the forest floor, and evaluated its impact on litter decomposition, soil mites and soil enzymatic activity of carbon and phosphorus-degrading enzymes. We demonstrate here that CWD in this ecosystem represents an important organic carbon reservoir (85 Mg ha^?1) and nitrogen pool (0.42 Mg ha^?1), similar in magnitude to other old-growth forests of the Northern Hemisphere. In addition, we found significant proximity effects of CWD, with increased C-degrading soil enzyme activity, decreased mite abundance, and more rapid litter decomposition beneath highly decayed CWD. Considered at the ecosystem scale in this forest, the removal of CWD could cause a decrease of 6% in soil enzyme activity, particularly in the summer dry season, and nearly 15% in annual litter decomposition. We conclude that beyond the established importance of CWD as a long-term carbon reservoir and habitat, CWD contributes functionally to the forest floor by influencing the spatial heterogeneity of microbial activity and carbon and nutrient turnover. These proximity effects demonstrate the importance of maintenance of this ecosystem component and should be taken into consideration for management decisions pertaining to carbon sequestration and functional diversity in natural forest ecosystems.     

Uptake of Allochthonous Dissolved Organic Matter from Soil and Salmon in Coastal Temperate Rainforest Streams

Dissolved organic matter (DOM) is an important component of aquatic food webs. We compare the uptake kinetics for NH₄–N and different fractions of DOM during soil and salmon leachate additions by evaluating the uptake of organic forms of carbon (DOC) and nitrogen (DON), and proteinaceous DOM, as measured by parallel factor (PARAFAC) modeling of DOM fluorescence. Seasonal DOM slug additions were conducted in three headwater streams draining a bog, forested wetland, and upland forest using DOM collected by leaching watershed soils. We also used DOM collected from bog soil and salmon carcasses to perform additions in the upland forest stream. DOC uptake velocity ranged from 0.010 to 0.063 mm s⁻¹ and DON uptake velocity ranged from 0.015 to 0.086 mm s⁻¹, which provides evidence for the whole-stream uptake of allochthonous DOM. These findings imply that wetlands could potentially be an important source of DOM to support stream heterotrophic production. There was no significant difference in the uptake of DOC and DON across the soil leachate additions (P > 0.05), although differential uptake of DOM fractions was observed as protein-like fluorescence was removed from the water column more efficiently than bulk DOC and DON (P < 0.05). Moreover, PARAFAC analysis of DOM fluorescence showed that protein-like fluorescence decreased downstream during all DOM additions, whereas humic-like fluorescence did not change. This differential processing in added DOM suggests slow and fast turnover pools exist for aquatic DOM. Taken together, our findings argue that DON could potentially fill a larger role in satisfying biotic N demand in oligotrophic headwater streams than previously thought. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions  J.B.F. conceived of or designed study, performed research, analyzed data, contributed new methods or models, and wrote the paper. E.H. conceived of or designed study and analyzed data. R.T.E. conceived of or designed study and analyzed data. J.B.J. contributed new methods or models and analyzed data.     

Identification of Heterotrophic Nitrification in a Sierran Forest Soil

A potential for heterotrophic nitrification was identified in soil from a mature conifer forest and from a clear-cut site. Potential rates of NO₂⁻ production were determined separately from those of NO₃⁻ by using acetylene to block autotrophic NH₄⁺ oxidation and chlorate to block NO₂⁻ oxidation to NO₃⁻ in soil slurries. Rates of NO₂⁻ production were similar in soil from the forest and the clear-cut site and were strongly inhibited by acetylene. The rate of NO₃⁻ production was much greater than that of NO₂⁻ production, and NO₃⁻ production was not significantly affected by acetylene or chlorate. Nitrate production was partially inhibited by cycloheximide, but was not significantly reduced by streptomycin. Neither the addition of ammonium nor the addition of peptone stimulated NO₃⁻ production. ¹⁵N labeling of the NH₄⁺ pool demonstrated that NO₃⁻ was not coming from NH₄⁺. The potential for heterotrophic nitrification in these forest soils was greater than that for autotrophic nitrification.     

Nitrification and Autotrophic Nitrifying Bacteria in a Hydrocarbon-Polluted Soil

In vitro ammonia-oxidizing bacteria are capable of oxidizing hydrocarbons incompletely. This transformation is accompanied by competitive inhibition of ammonia monooxygenase, the first key enzyme in nitrification. The effect of hydrocarbon pollution on soil nitrification was examined in situ. In a microcosm study, adding diesel fuel hydrocarbon to an uncontaminated soil (agricultural unfertilized soil) treated with ammonium sulfate dramatically reduced the amount of KCl-extractable nitrate but stimulated ammonium consumption. In a soil with long history of pollution that was treated with ammonium sulfate, 90% of the ammonium was transformed into nitrate after 3 weeks of incubation. Nitrate production was twofold higher in the contaminated soil than in the agricultural soil to which hydrocarbon was not added. To assess if ammonia-oxidizing bacteria acquired resistance to inhibition by hydrocarbon, the contaminated soil was reexposed to diesel fuel. Ammonium consumption was not affected, but nitrate production was 30% lower than nitrate production in the absence of hydrocarbon. The apparent reduction in nitrification resulted from immobilization of ammonium by hydrocarbon-stimulated microbial activity. These results indicated that the hydrocarbon inhibited nitrification in the noncontaminated soil (agricultural soil) and that ammonia-oxidizing bacteria in the polluted soil acquired resistance to inhibition by the hydrocarbon, possibly by increasing the affinity of nitrifying bacteria for ammonium in the soil.