Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co-occurring environmental changes. We examined the effects of urbanization and fragmentation on oak-dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.

Phytoremediation efficacy of native vegetation for nutrients and heavy metals on soils amended with poultry litter and fertilizer

Poultry litter on agricultural lands could introduce nitrogen (N), phosphorus (P), heavy metals in soil and ground water. Native vegetations were identified to assess efficacy for phytoremediation of nutrients and metals from soil and water. Objective was to measure capability of multi-year native species to remove metals, nutrients, and prevent Nitrate-N leaching below the rooting zone. Treatments were distributed in four replicates with/without fertilization. Suction lysimeters were installed at 30, 60, and 90-cm depths in 3 of 4 replicates. Species were identified, recorded, five specified cuttings sampled. Plant, soil, water samples were prepared and analyzed by spectroscopy. Nitrate-N extraction, nitrates in water samples were determined using flow injection analysis. Fertilized plots (NVM) had 39% more biomass yield than unfertilized plots (NVN). In plants, nutrient and metal concentrations varied significantly with 14% increase in Zn, 36% and 26% in K and Mg over NVN for first and second year. Uneven between NVM and NVN, topsoil had higher values for most nutrients and metals. Largest P and (NO3-)-N in plant and water were observed from NVM. Cultivation of native vegetation appears to be an effective approach for remediation of excess nitrates-N, P, heavy metals from surface and sub-surface zones of the soil.

Microbial Biogeochemical Cycling of Nitrogen in Arid Ecosystems

Arid ecosystems cover ∼40% of the Earth's terrestrial surface and store a high proportion of the global nitrogen (N) pool. They are low-productivity, low-biomass, and polyextreme ecosystems, i.e., with (hyper)arid and (hyper)oligotrophic conditions and high surface UV irradiation and evapotranspiration. These polyextreme conditions severely limit the presence of macrofauna and -flora and, particularly, the growth and productivity of plant species. Therefore, it is generally recognized that much of the primary production (including N-input processes) and nutrient biogeochemical cycling (particularly N cycling) in these ecosystems are microbially mediated. Consequently, we present a comprehensive survey of the current state of knowledge of biotic and abiotic N-cycling processes of edaphic (i.e., open soil, biological soil crust, or plant-associated rhizosphere and rhizosheath) and hypo/endolithic refuge niches from drylands in general, including hot, cold, and polar desert ecosystems. We particularly focused on the microbially mediated biological nitrogen fixation, N mineralization, assimilatory and dissimilatory nitrate reduction, and nitrification N-input processes and the denitrification and anaerobic ammonium oxidation (anammox) N-loss processes. We note that the application of modern meta-omics and related methods has generated comprehensive data sets on the abundance, diversity, and ecology of the different N-cycling microbial guilds. However, it is worth mentioning that microbial N-cycling data from important deserts (e.g., Sahara) and quantitative rate data on N transformation processes from various desert niches are lacking or sparse. Filling this knowledge gap is particularly important, as climate change models often lack data on microbial activity and environmental microbial N-cycling communities can be key actors of climate change by producing or consuming nitrous oxide (N₂O), a potent greenhouse gas.

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 seed-cake (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 0-15 cm were collected from an arable field, amended with urea N (UN) at the rate 200 mg N kg⁻¹, UN+NSC, UN+Na₂S₂O₃ and UN+CaCl₂ and incubated at 22°C periodically over 50 d. 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 50 d 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, 56 mg NO₃⁻-N kg⁻¹ was accumulated over 50 d (maximum 93 mg kg⁻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.

Effects of Soil on Ammonia, Ethylene, Chloroethane, and 1,1,1-Trichloroethane Oxidation by Nitrosomonas europaea

Ammonia monooxygenase (AMO) from Nitrosomonas europaea catalyzes the oxidation of ammonia to hydroxylamine and has been shown to oxidize a variety of halogenated and nonhalogenated hydrocarbons. As part of a program focused upon extending these observations to natural systems, a study was conducted to examine the influence of soil upon the cooxidative abilities of N. europaea. Small quantities of Willamette silt loam (organic carbon content, 1.8%; cation-exchange capacity, 15 cmol/kg of soil) were suspended with N. europaea cells in a soil-slurry-type reaction mixture. The oxidations of ammonia and three different hydrocarbons (ethylene, chloroethane, and 1,1,1-trichloroethane) were compared to results for controls in which no soil was added. The soil significantly inhibited nitrite production from 10 mM ammonium by N. europaea. Inhibition resulted from a combination of ammonium adsorption onto soil colloids and the exchangeable acidity of the soil lowering the pH of the reaction mixture. These phenomena resulted in a substantial drop in the concentration of NH(4) in solution (10 to 4.5 mM) and, depending upon the pH, in a reduction in the amount of available NH(3) to concentrations (8 to 80 muM) similar to the K(s) value of AMO for NH(3) ( approximately 29 muM). At a fixed initial pH (7.8), the presence of soil also modified the rates of oxidation of ethylene and chloroethane and changed the concentrations at which their maximal rates of oxidation occurred. The modifying effects of soil on nitrite production and on the cooxidation of ethylene and chloroethane could be circumvented by raising the ammonium concentration in the reaction mixture from 10 to 50 mM. Soil had virtually no effect on the oxidation of 1,1,1-trichloroethane.

Effects of oxygenation on ammonia oxidation potential and microbial diversity in sediment from surface-flow wetland mesocosms

Addition of oxygen to surface-flow wetland mesocosms treating synthetic secondary effluent resulted in a significant increase in ammonia oxidation potential in sediment compared to non-oxygenated controls. Ammonia oxidation potential in oxygenated wetland sediment (1.2-3.5 mg N g dw(-1) d(-1)) was 2-3 orders of magnitude higher than those measured in sediment and soil systems reported in the literature. Phylogenic analysis of sediment from the two treatments revealed substantial differences in microbial diversity including the presence of ammonia-oxidizing bacteria (Nitrosomonas oligotropha) and denitrifying bacteria only in oxygenated sediment, and an increase in the diversity of aerobic phototrophs and methanotrophs in control sediment. These observations supported the contention by Palmer et al. (2009) that oxygenation 'activated' nitrifying bacteria in wetland sediment leading to high rates of biological ammonia oxidation.

Autotrophic ammonia oxidation at low pH through urea hydrolysis

Ammonia oxidation in laboratory liquid batch cultures of autotrophic ammonia oxidizers rarely occurs at pH values less than 7, due to ionization of ammonia and the requirement for ammonium transport rather than diffusion of ammonia. Nevertheless, there is strong evidence for autotrophic nitrification in acid soils, which may be carried out by ammonia oxidizers capable of using urea as a source of ammonia. To determine the mechanism of urea-linked ammonia oxidation, a ureolytic autotrophic ammonia oxidizer, Nitrosospira sp. strain NPAV, was grown in liquid batch culture at a range of pH values with either ammonium or urea as the sole nitrogen source. Growth and nitrite production from ammonium did not occur at pH values below 7. Growth on urea occurred at pH values in the range 4 to 7.5 but ceased when urea hydrolysis was complete, even though ammonia, released during urea hydrolysis, remained in the medium. The results support a mechanism whereby urea enters the cells by diffusion and intracellular urea hydrolysis and ammonia oxidation occur independently of extracellular pH in the range 4 to 7.5. A proportion of the ammonia produced during this process diffuses from the cell and is not subsequently available for growth if the extracellular pH is less than 7. Ureolysis therefore provides a mechanism for nitrification in acid soils, but a proportion of the ammonium produced is likely to be released from the cell and may be used by other soil organisms.

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.

Hyphal transport of 15 N-labelled nitrogen by a vesicular-arbuscular mycorrhizal fungus and its effect on depletion of inorganic soil N

Hyphal transport of nitrogen from a ¹⁵ N-labelled ammonium source by a VA-mycorrhizal fungus was studied under controlled experimental conditions. Cucumis sativus L. cv. Aminex (F1 hybrid) was grown alone or together with Glomus intraradices Schenck and Smith in containers with a hyphal compartment separated from the rooting medium by a fine nylon mesh. Lateral movement of the applied ¹⁵ N towards the roots was minimized by using a nitrification inhibitor (N-serve) and a hyphal buffer compartment. Recovery of ¹⁵ N by mycorrhizal and non-mycorrhizal plants was 6 and 0%, respectively, after a labelling period of 23 days. The corresponding figures, without N-serve added, were 4 and 7%. A prolongation of the labelling period by 8 days (N-serve applied) resulted in an increase in the ¹⁵ N recovery by mycorrhizal plants to 30% of the applied ¹⁵ N. Non-mycorrhizal plants contained only traces of ¹⁵ N. The external hyphae depleted the soil in the hyphal compartment efficiently for inorganic N. In contrast, hyphal compartments of control containers still contained considerable amounts of inorganic N. The ¹⁵ N assimilated by the external hyphae in one hyphal compartment was not translocated in significant amounts to the external hyphae in another hyphal compartment. The possible implication of this for inter-plant N transfer by VA hyphal connections is discussed.

An Acidophilic and a Neutrophilic Nitrobacter Strain Isolated from the Numerically Predominant Nitrite-Oxidizing Population of an Acid Forest Soil

Two physiologically and serologically distinct strains of chemoautotrophic nitrite-oxidizing bacteria were isolated as numerically predominant members of the nitrite-oxidizer population of an undisturbed forest soil with a pH range of 4.3 to 5.2. One isolate responded as a neutrophile, characteristic of the family Nitrobacteraceae, and cross-reacted strongly with fluorescent antibody to Nitrobacter strain Engel. The second isolate responded as an acidophile in pure culture, demonstrated maximal nitrite oxidation activity at pH 5.5, and had a pH tolerance range of pH 4.1 to 7.2. Nitrite oxidase in whole cells of the acidophile sustained activity to at least pH 3.5. Cell morphology of both strains typified the genus Nitrobacter in all respects when cultured at pH 7. However, under more acidic conditions the acidophile tended to elongate and at times appeared to branch. These data provide the first evidence for the existence of an acidophilic chemoautotrophic nitrifying bacterium. Isolation of the neutrophilic Nitrobacter strain reported here complements the earlier isolation of a neutrophilic Nitrosospira strain to provide further evidence of a prominent acid-intolerant population of chemoautotrophic nitrifiers in this acid forest soil.