Nitrate loss from a restored floodplain in the Lower Cosumnes River, California

Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO ₃ ⁻ ) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO ₃ ⁻ -N loss from the overlying water in both sandy and clayey soils and at three levels of NO ₃ ⁻ -N (ambient, +1 mg N l⁻¹, +5 mg N l⁻¹). In addition, we examined NO ₃ ⁻ -N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N₂O–N cm⁻³ h⁻¹. In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 μg N l⁻¹ h⁻¹ over all treatments and all 3 months examined. Significant loss of NO ₃ ⁻ -N (60–93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO ₃ ⁻ -N uptake by phytoplankton. When scaled up, potential NO ₃ ⁻ -N loss from the restored floodplain represented 0.6–4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and ~24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO ₃ ⁻ sink.     

Prevalence of nitrous oxide reducing capacity in denitrifiers from a variety of habitats

Summary A total of 81 strains isolated by T. N. Gamble from soils from eight countries, fresh water lake sediments and nitrified poultry manure were examined for their ability to grow on N₂O as their electron acceptor, as well as for their tendency to produce N₂O from NO ₃ ⁻ in the absence and presence of acetylene. Seventy-seven of the 81 strains were confirmed as denitrifiers. Fifty-nine of the 77 strains grew on N₂O, while 12 strains produced N₂O but could not utilize it. Six strains reduced NO ₃ ⁻ to N₂ but could not grow on N₂O, suggesting that even if N₂O is always an intermediate product of denitrification, it is not always a freely diffusible intermediate. The organisms, however, would consume N₂O that accumulated early in growth and accumulated N₂O in the presence of acetylene. Thus the total number of N₂O users was 65 strains or 83% of the total tested. This implies that the N₂O reducing capacity of denitrifiers occur widely in nature. A high proportion ofPseudomonas fluorescens biotype II reduced N₂O. The accumulation of N₂O from NO ₃ ⁻ in the presence of acetylene provides strong evidence that N₂O is generally an intermediate in denitrification as well as provides additional support for the usefulness of this chemical as a general inhibitor of N₂O reduction.     

Soil nitrogen cycling and nitrous oxide flux in a Rocky Mountain Douglas-fir forest: effects of fertilization,irrigation and carbon addition

Nitrous oxide fluxes and soil nitrogen transformations were measured in experimentally-treated high elevation Douglas-fir forests in northwestern New Mexico, USA. On an annual basis, forests that were fertilized with 200 kg N/ha emitted an average of 0.66 kg/ha of N₂O-N, with highest fluxes occurring in July and August when soils were both warm and wet. Control, irrigated, and woodchip treated plots were not different from each other, and annual average fluxes ranged from 0.03 to 0.23 kg/ha. Annual net nitrogen mineralization and nitrate production were estimated in soil and forest floor usingin situ incubations; fertilized soil mineralized 277 kg ha⁻¹ y⁻¹ in contrast to 18 kg ha⁻¹ y⁻¹ in control plots. Relative recovery of¹⁵NH₄-N applied to soil in laboratory incubations was principally in the form of NO³-N in the fertilized soils, while recovery was mostly in microbial biomass-N in the other treatments. Fertilization apparently added nitrogen that exceeded the heterotrophic microbial demand, resulting in higher rates of nitrate production and higher nitrous oxide fluxes. Despite the elevated nitrous oxide emission resulting from fertilization, we estimate that global inputs of nitrogen into forests are not currently contributing significantly to the increasing concentrations of nitrous oxide in the atmosphere.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Denitrification activity of Bradyrhizobium sp. isolated from Argentine soybean cultivated soils

Two hundred and fifty strains, all of them representatives of native Bradyrhizobium sp., isolated from soils cultivated with soybean have been characterized by their denitrification activity. In addition, the denitrification potential of those soils was also measured by evaluating the most-probable-number (MPN) of denitrifying bacteria and the denitrification enzyme assay (DEA). Of the 250 isolates tested, 73 were scored as probable denitrifiers by a preliminary screening method. Only 41 were considered denitrifiers because they produced gas bubbles in Durham tubes, cultures reached an absorbance of more than 0.1 and NO³⁻ and NO²⁻ were not present. Ten of these 41 were selected to confirm denitrification and to study denitrification genes. According to N₂O production and cell protein concentration with NO³⁻, the isolates could be differentiated in three categories of denitrifiers. The presence of the napA, nirK, norC and nosZ genes was detected by production of a diagnostic PCR product using specific primers. RFLP from the 16S-23S rDNA spacer region (IGS) revealed that denitrifiers strains could be characterized as Bradyrhizobium japonicum and strains which were non-respiratory denitrifiers as B. elkanii.     

Effect of nutrient loading on biogeochemical and microbial processes in a New England salt marsh

Coastal marshes represent an important transitional zone between uplands and estuaries. One important function of marshes is to assimilate nutrient inputs from uplands, thus providing a buffer for anthropogenic nutrient loads. We examined the effects of nitrogen (N) and phosphorus (P) fertilization on biogeochemical and microbial processes during the summer growing season in a Spartina patens (Aiton (Muhl.)) marsh in the Narragansett Bay National Estuarine Research Reserve on Prudence Island (RI). Quadruplicate 1 m² plots were fertilized with N and P additions, N-only, P-only, or no additions. N-only addition significantly stimulated bacterial production and increased pore water NH₄⁺ and NO₃⁻ concentrations. Denitrification rates ranged from 0 to 8 mmol m⁻² day⁻¹. Fertilization had no apparent effect on soil oxygen consumption or denitrification measured in the summer in intact cores due to high core-to-core variation. P fertilization led to increased pore water dissolved inorganic phosphorus (DIP) concentrations and increased DIP release from soils. In contrast the control and N-only treatments had significant DIP uptake across the soil-water interface. The results suggest that in the summer fertilization has no apparent effect on denitrification rates, stimulates bacterial productivity, enhances pore water nutrient concentrations and alters some nutrient fluxes across the marsh surface.     

Rapid Nitrate Loss and Denitrification in a Temperate River Floodplain

Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring, denitrification assays, and experimental nitrate (NO₃⁻ -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased over 75% from 1.41 mg l⁻¹, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l⁻¹ on 22 April 2001. Simultaneously NO₃⁻ -N was attenuated almost 100% from 1.09 to <0.002 mg l⁻¹. Unamended sediment denitrification rates were moderate (0–483 μg m⁻² h⁻¹) and seasonally variable, and activity was limited by the availability of NO ₃⁻ -N on all dates. Two experimental NO₃⁻ -N pulse additions to floodplain water bodies confirmed rapid NO₃⁻ -N depletion. Over 80% of the observed NO ₃⁻ -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant fraction (>60%) of NO₃⁻ -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification was likely responsible for most of the NO₃⁻ -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.     

Fungal control of nitrous oxide production in semiarid grassland

Fungi are capable of both nitrification and denitrification and dominate the microbial biomass in many soils. Recent work suggests that fungal rather than bacterial pathways dominate N transformation in desert soils. We evaluated this hypothesis by comparing the contributions of bacteria and fungi to N₂O production at control and N fertilized sites within a semiarid grassland in central New Mexico (USA). Soil samples were taken from the rhizosphere of blue grama (B. gracilus) and the microbiotic crusts that grow in open areas between the bunch grasses. Soils incubated at 30% or 70% water holding capacity, were exposed to one of three biocide treatments (control, cycloheximide or streptomycin). After 48 h, N₂O and CO₂ production were quantified along with the activities of several extracellular enzymes. N₂O production from N fertilized soils was higher than that of control soils (165 vs. 41 pmol h⁻¹ g⁻¹), was higher for crust soil than for rhizosphere soil (108 vs. 97 pmol h⁻¹ g⁻¹), and increased with soil water content (146 vs. 60 pmol h⁻¹ g⁻¹). On average, fungicide (cycloheximide) addition reduced N₂O production by 85% while increasing CO₂ production by 69%; bactericide (streptomycin) reduced N₂O by 53% with mixed effects on CO₂ production. N₂O production was significantly correlated with C and N mineralization potential as measured by assays for glycosidic and proteolytic enzymes, and with extractable nitrate and ammonium. Our data indicate that fungal nitrifier denitrification and bacterial autotrophic nitrification dominate N transformation in this ecosystem and that N₂O production is highly sensitive to soil cover, N deposition and moisture.     

Dynamics of Inorganic Nitrogen in Nitrate and Glucose-amended Alkaline–saline Soil

Induction of assimilatory NO ₃ ⁻ reduction through the application of an easily decomposable substrate in alkaline–saline soils of the former lake Texcoco (Mexico) resulted in a fast immobilization of NO ₃ ⁻ in excess of N required for metabolic activity and the release of large concentrations of NO ₂ ⁻ and smaller amounts of NH ₄ ⁺ . We postulated that this was regulated by the amounts of NO ₃ ⁻ and glucose added, and affected by the specific characteristics of soil from the former lake Texcoco. This was investigated by spiking soils of different electrolytic conductivity (EC) 56.0 dS m⁻¹ (soil A of Texcoco) and 11.6 dS m⁻¹ (soil B of Texcoco) with different concentrations of NO ₃ ⁻ and glucose while dynamics of CO₂, NH ₄ ⁺ , NO ₂ ⁻ and NO ₃ ⁻ were monitored in an aerobic incubation for 7 days. For comparison reasons (control) an agricultural soil with low EC (0.3 dS m⁻¹) was included as well. In the agricultural soil, 67% of the added glucose mineralized within 7 days, but only 15% in soil A of Texcoco and 20% in soil B of Texcoco. The application of NO ₃ ⁻ to the agricultural soil added with glucose increased cumulative production of CO₂ 1.2 times, 1.5 times in soil A of Texcoco and 1.8 times in soil B of Texcoco. Concentration of NO ₂ ⁻ increased to > 100 mg NO ₂ ⁻ -N kg⁻¹ when 1000 mg glucose-C kg⁻¹ and 500 mg NO ₃ ⁻ -N kg⁻¹ were added to soil A and B of Texcoco, but remained < 3 mg NO ₂ ⁻ -N kg⁻¹ in the agricultural soil. The ratio between the cumulative production of CO₂ and the decrease in concentration of NO ₃ ⁻ was approximately one in soil A and B of Texcoco, but 10 in the agricultural soil after 3 days. It was found that micro-organisms in the alkaline–saline soil of the former lake Texcoco were capable of immobilizing large quantities of NO ₃ ⁻ when an easy decomposable substrate was available in excess of what might be required for metabolic activity while producing large concentrations of NO ₂ ⁻ , but these phenomena were absent in an agricultural soil. In soil of Texcoco, concentrations of NO ₂ ⁻ and NH ₄ ⁺ increased with increased salinity and availability of NO ₃ ⁻ . This ability to remove large quantities of NO ₃ ⁻ under these conditions and then utilize it at a later time might benefit micro-organisms of the N limited alkaline–saline soils of Texcoco.     

Gaseous nitrogen losses and ammonia volatilization measurement following land application of cattle slurry in the mid-Atlantic region of the USA

To provide locally-determined field data for extension and environmental management purposes, gaseous N losses were measured following cattle slurry application to an arable silty-loam soil in the mid-Atlantic region of the USA. The field had been cropped to no-till maize. NH₃ volatilization was measured with the micro-meteorological, integrated horizontal flux (IHF) method, and denitrification with a core incubation method using acetylene inhibition. An early-winter surface application (5 December 1996; 88 m³ ha⁻¹ supplying 91 kg NH ₄ ⁺ -N ha⁻¹) was either unincorporated or immediately incorporated. NH₃ volatilization was measured from the unincorporated application, and denitrification from both slurry treatments and appropriate control soils. Total NH₃ loss from the unincorporated slurry application was 19% of applied NH ₄ ⁺ -N; temperatures were cool (4–6 °C), and 25 mm of rain fell within 24 h of application. For 3 months, enhanced denitrification occurred from the two slurry treatments, with generally higher rates from the incorporated slurry. Total net denitrification loss from the surface-applied and incorporated slurry treatments was, respectively, 11 and 17% of applied NH ₄ ⁺ -N. Denitrification loss over the winter/early-spring period was appreciable but not substantial, even where NH₃ volatilization was restricted by immediate incorporation. From the spring application (30 April 1997, 39 m³ ha⁻¹ supplying 51 kg NH ₄ ⁺ -N ha⁻¹), total NH₃ loss was 71% of applied NH ₄ ⁺ -N. These NH₃ volatilization loss data and the similarity of climate suggest that NH₃ loss factors from recent NW European work are likely to be generally applicable in the mid-Atlantic region. NH₃ volatilization from the spring application was also measured using the Z-instrument (Z_INST) approach, and with a system of small wind tunnels. A comparative assessment of the three methods is reported.     

Suppression of nitrate formation within an exotic conifer plantation

Summary Nitrate-N losses to stream waters and soil inorganic N pools, nitrifying potentials and NO₃-N production rates were measured in 2 adjacent watersheds, one used as pasture and the other planted in exotic conifer forest (Pinus radiata D. Don). Estimated NO₃-N loss to stream waters draining the pine and pasture watersheds were 0.6kg ha⁻¹ y⁻¹ and 7.6 kg ha⁻¹ y⁻¹ respectively. Ammonium-N pool sizes were not significantly different between soils in the two watersheds but NO₃−N pools and nitrifying potentials were always lower in the pine watershed soil samples. Laboratory incubation experiments indicated that suppression of NO₃−N formation in pine watershed soils required the presence of live tree roots and was not due to the direct action of allelopathic chemicals on nitrifiers.     

Field measurement of nitrogen mineralization using soil core incubation and acetylene inhibition of nitrification

A technique for measuring net rates of mineralization under field conditions is described. Soil cores were incubated in the field in sealed containers with acetylene to inhibit nitrification and thereby minimize losses of N through denitrification. Mineralization was estimated as the difference between the mineral N content after a 14-d incubation and that determined from soil samples taken at the start of incubation. Mineralization in the spring and summer in unfertilized plots in the field amounted to 90 and 70 kg N ha⁻¹ in S.E. England under grass and grass/clover swards, respectively, and 40 kg N ha⁻¹ under a grass sward in S.W. England. Daily rates of mineralization ranged from 0.02 to 1.90 kg N ha⁻¹, with peak values related to re-wetting of the soil after dry weather. Laboratory incubation of soil showed that neither the low concentration of acetylene (2% v/v) adopted for field incubation, nor the accumulation of mineral N during incubation was likely to affect the total measurement, but that frequent and regular soil sampling was necessary to minimize the effects of changes in soil water content. Estimates for mineralization over the whole growing season (180 d) were obtained for two years from extrapolation of the early season field measurements and were, on average, 50% higher than predictions based on a chemical extraction index of potentially mineralizable N.     

Denitrification in the vadose zone and in surficial groundwater of a sandy entisol with citrus production

A portion of nitrate (NO ₃ ⁻ ), a final breakdown product of nitrogen (N) fertilizers, applied to soils and/or that produced upon decomposition of organic residues in soils may leach into groundwater. Nitrate levels in water excess of 10 mg L⁻¹ (NO₃–N) are undesirable as per drinking water quality standards. Nitrate concentrations in surficial groundwater can vary substantially within an area of citrus grove which receives uniform N rate and irrigation management practice. Therefore, differences in localized conditions which can contribute to variations in gaseous loss of NO ₃ ⁻ in the vadose zone and in the surficial aquifer can affect differential concentrations of NO₃–N in the groundwater at different points of sampling. The denitrification capacity and potential in a shallow vadose zone soil and in surficial groundwater were studied in two large blocks of a citrus grove of ‘Valencia’ orange trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jambhiri (L.)) under a uniform N rate and irrigation program. The NO₃–N concentration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collected from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundwater interface (SGWI). Groundwater samples from the monitoring wells (MW) were collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capacity ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N₂O–N kg⁻¹ soil at the surface soil and at the soil/groundwater interface, respectively. The denitrification potential increased by 100-fold with the addition of 200 mg kg⁻¹ each of N and C. The denitrification potential in the groundwater also followed a pattern similar to that for the soil samples. Denitrification potential in the soil or in the groundwater was greatest near the monitor well with shallow depth of vadose zone (MW3). Cumulative N₂O–N emission (denitrification capacity) from the SGWI soil samples and from stagnant water samples strongly correlated to microbial most probable number (MPN) counts (r² = 0.84 – 0.89), and dissolved organic C (DOC) (r² = 0.96 – 0.97). Denitrification capacity of the SGWI samples moderately correlated to water-filled pore space (WFPS) (r² = 0.52). However, extractable NO₃-N content of the SGWI soil samples poorly (negative) correlated to denitrification capacity (r² = 0.35). However, addition C, N or both to the soil or water samples resulted in significant increase in cumulative N₂O emission. This study demonstrated that variation in denitrification capacity, as a result of differences in denitrifier population, and the amount of readily available carbon source significantly (at 95% probability level) influenced the variation in NO₃–N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Belowground responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to warming and nitrogen fertilization in the eastern Tibetan Plateau

The impacts of global climatic change on belowground ecological processes of terrestrial ecosystems are still not clear. We therefore conducted an experiment in the subalpine coniferous forest ecosystem of the eastern edges of the Tibetan Plateau to study roots of Picea asperata seedlings and rhizosphere soil responses to soil warming and nitrogen availability from April 2007 to December 2008. The seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0 or 25 g m⁻²year⁻¹ N). We used a free air temperature increase from an overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The results showed that warming alone significantly increased total biomass, coarse root biomass and fine root biomass of P. asperata seedlings. Both total biomass and fine root biomass were increased, but coarse root biomass was significantly decreased by nitrogen fertilization and warming combined with nitrogen fertilization. Warming induced a prominent increase in soil organic carbon (SOC) and NO₃ ⁻-N of rhizosphere soil, while nitrogen fertilization significantly decreased SOC and NH₄ ⁺-N of rhizosphere soil. The warming, fertilization and warming × N fertilization interaction decreased soil microbial C significantly, but substantially increased soil microbial N. These results suggest that nitrogen deposition combined with warmer temperatures under future climatic change possibly will have no effect on fine root production of P. asperata seedlings, but could enhance the nitrification process of their rhizosphere soils in subalpine coniferous forests.     

Spatial and Temporal Variability in Sediment Denitrification Within an Agriculturally Influenced Reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 μg N₂O g ash free dry mass of sediments (AFDM)⁻¹ h⁻¹ or 0–2.7 μg N₂O g dry mass of sediments (DM)⁻¹ h⁻¹ at reservoir sites, vs. 0–12 μg N₂O gAFDM⁻¹ h⁻¹ or 0–0.27 μg N₂O gDM⁻¹ h⁻¹ at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO₃⁻-N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO₃⁻-N concentration up to a threshold of 0.88 mg NO₃⁻ -N l⁻¹, above which it appeared NO₃⁻-N was no longer limiting. Denitrification assays were amended seasonally with NO₃⁻-N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO₃⁻-N during fall and winter when ambient NO ₃⁻-N was low.     

Microbial populations, ammonification and nitrification in soil treated with urea and inorganic salts

Soil fertilization with urea at rates of 0.2 and 0.5 mg N/g soil was toxic for total counts of bacteria and fungi except with cellulolytic fungi where growth was promoted by addition of urea after 90-d incubation. Also, the population numbers of both bacteria and fungi were significantly decreased when soil was amended with CaCl₂ and K₂SO₄ applied at two levels (50 and 100 μmol/g soil). Some alleviation of the toxic effect of either urea or the inorganic salts was observed when they were applied in combination. Fungal species composition was found to be affected in the treated soil. The most tolerant fungi wereAlternaria alternata, Aspergillus flavus, A. fumigatus, A. niger, A. terreus, Eurotium amstelodami, E. chevalieri, Nectria hœmatococca, Pœcilomyces variotii andStachybotrys chartarum. Other species of fungi were either sensitive to all treatments or sensitive to some and tolerant to others. Ammonium nitrogen was found to be more accumulated in soil treated with urea mixed with inorganic salts compared with soil treated only with urea whereas NO ₃ ⁻ -N (and NO ₂ ⁻ -N) was decreased. The overall effect of the addition of inorganic salts on total mineralized nitrogen, however was promotive. Soil pH was increased in ureatreated soil but was depressed by application of CaCl₂ or K₂SO₄ to values lower than that of untreated soil. The results may be of agronomical interest in overcoming problems encountered with urea application as N fertilizer.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha⁻¹ year⁻¹. Locally estimated NO ₋ ³ deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the δ¹⁵N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO ₋ ³ , measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The δ¹⁵N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO ₋ ³ in these soils. Only a few sites had NO ₋ ³ concentrations exceeding 1 mg N l⁻¹ in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The δ¹⁵N enrichment factor was the variable best correlated with NO ₋ ³ concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO ₋ ³ from forest soils.     

Ponderosa Pine Responses to Elevated CO2and Nitrogen Fertilization

The effects of elevated CO₂ (ambient, +175, and +350 μl l⁻¹) and nitrogen fertilization (0, 100, and 200 kg N ha⁻¹ yr⁻¹ as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO₂ caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO₂ decreased in the +175 μl l⁻¹ and increased in the +350 μl l⁻¹ after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO₂ in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO₂. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO₂, was likely the soil organic N pool. There were no effects of elevated CO₂ on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO₂ and N treatments, indicating that N fertilization had no effect on responses to CO₂ and that CO₂ treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO₂.     

Effect of nitrate and glucose addition on denitrification and nitric oxide reductase (<Emphasis Type="Italic">cnorB</Emphasis>) gene abundance and mRNA levels in <Emphasis Type="Italic">Pseudomonas mandelii</Emphasis> inoculated into anoxic soil

The effect of glucose addition (0 and 500 μg C g⁻¹ soil) and nitrate (NO₃) addition (0, 10, 50 and 500 μg NO₃–N g⁻¹ soil) on nitric oxide reductase (cnorB) gene abundance and mRNA levels, and cumulative denitrification were quantified over 48 h in anoxic soils inoculated with Pseudomonas mandelii. Addition of glucose-C significantly increased cnorB _p (P. mandelii and related species) mRNA levels and abundance compared with soil with no glucose added, averaged over time and NO₃ addition treatments. Without glucose addition, cnorB _p mRNA levels were higher when 500 μg NO₃–N g⁻¹ soil was added compared with other NO₃ additions. In treatments with glucose added, addition of 50 μg NO₃–N g⁻¹ soil resulted in higher cnorB _p mRNA levels than soil without NO₃ but was not different from the 10 and 500 μg NO₃–N g⁻¹ treatments. cnorB _p abundance in soils without glucose addition was significantly higher in soils with 500 μg NO₃–N g⁻¹ soil compared to lower N-treated soils. Conversely, addition of 500 μg NO₃–N g⁻¹ soil resulted in lower cnorB _p abundance compared with soil without N-addition. Over 48 h, cumulative denitrification in soils with 500 μg glucose-C g⁻¹ soil, and 50 or 500 μg NO₃–N g⁻¹ was higher than all other treatments. There was a positive correlation between cnorB _p abundance and cumulative denitrification, but only in soils without glucose addition. Glucose-treated soils generally had higher cnorB _p abundance and mRNA levels than soils without glucose added, however response of cnorB _p abundance and mRNA levels to NO₃ supply depended on carbon availability.