Mean NO3 Research Articles

Detection of nitrate leaching through bypass flow using pan lysimeter, suction cup, and resin capsule

We compared the use of mixed-bed ion exchange resin capsules (RC), suction cups (SC), pan lysimeters (PL), and subsurface drainage (DR) for the detection of nitrate movement through a clayey soil where onion (Allium cepa L.) had been cultivated over a period of seven months. At the topsoil level, solutions collected with SC showed higher concentrations of NO3 − than the PL-collected samples. At 80-cm depth, however, the concentrations of NO3 − were higher for the DR and PL samples than for the SC samples, suggesting that bypass or macropore flow was the primary mechanism of NO3 − transport to subsurface drainage or groundwater, while solutions collected by SC mostly represented solutions inside soil aggregates. The use of the resin capsule method resulted in higher values of NO3 − at 15- than at 50-cm depth initially but the trend was reversed after sufficient leaching and plant uptake. High and significant correlations were obtained between the amount of NO3 − adsorbed on RC at 15-cm depth and the mean concentration of NO3 − in the DR samples during the RC installation period and between the NO3 − adsorbed on RC at 50-cm depth and the mean NO3 − concentration of PL samples at 80-cm depth. Such results indicate that the RC method which enables the detection of nitrate transport via macropore flow is a promising technique for nitrate leaching measurements.

Land use affects the distribution of soil inorganic nitrogen in smallholder production systems in Kenya

We hypothesized that the integration of trees and shrubs in agricultural landscapes can reduce NO3– leaching and increase utilization of subsoil N. A field survey was conducted on 14 farms on acid soils in the subhumid highlands of Kenya, where there is little use of fertilizers, to determine the effect of vegetation types (VT) on soil NH4+ and NO3– to 4 m depth. The VT included maize (Zea mays) with poor growth and good growth, Markhamia lutea trees scattered in maize, natural weed fallow, banana (Musa spp.), hedgerow, and eucalyptus woodlot. The effect of VT on NH4+ was small (<1 mg N kg–1). NO3– within a VT was about constant with depth below 0.25 m, but subsoil NO3– varied greatly among VT. Mean NO3–-N concentrations at 0.5–4 m depth were low beneath hedgerow and woodlot (<0.2 mg kg–1), intermediate beneath weed fallow (0.2–0.7 mg kg–1), banana (0.5–1.0 mg kg–1) and markhamia (0.5–1.6 mg kg–1), and high beneath both poor (1.0–2.1 mg kg–1) and good (1.9–3.1 mg kg–1) maize. Subsoil NO3– (0.5–4 m) was agronomically significant after maize harvest with 37 kg N ha–1 m–1 depth of subsoil beneath good maize and 27 kg N ha–1 m–1 depth beneath poor maize. In contrast, subsoil NO3– was only 2 kg N ha–1 m–1 depth beneath woodlot and hedgerow. These results demonstrate that the integration of perennial vegetation and the rotation of annual and perennial crops can tighten N cycling in agricultural landscapes.

Mineralisation of nitrogen in a chronosequence of rehabilitated bauxite mines

Mine site rehabilitation should aim to establish quickly and maintain the processes of nutrient cycling at rates comparable with, or approaching, those of native forests. Current management strategies for rehabilitating bauxite mines and other mine sites in Australia usually include planting fast-growing understorey species at high densities and applying fertiliser. We provide the first detailed study of nitrogen (N) availability and N transformations (mineralisation/immobilisation) in such rehabilitated mine sites. Mean concentrations of NO3– (0–5 cm) in a chronosequence (7, 13, 22, and 27 years old) of rehabilitated mine sites ranged from 0.5 to 1.3 kg/ha, and NH4+ from 4.0 to 9.5 kg/ha. In burnt and unburnt native jarrah (E. marginata Donn ex. Smith) forests adjacent to the mine site, mean NO3– concentrations in surface soil (0–5 cm) were 0.8 kg/ha (burnt) and 1.1 kg/ha (unburnt), and mean NH4+ concentrations were 6.8 kg/ha (burnt) and 7.8 kg/ha (unburnt). Concentration of NH4+ at 0–5 cm was strongly related to soil water content (R2 = 0.69, P &lt; 0.05) in rehabilitation sites, but not at 5–10 cm depth. Rates of N mineralisation (0–5 cm) in rehabilitation sites ranged from 34 to 52 kg/ha.year, of the same order as rates in native forest soil. In all rehabilitation and native forest sites, rates of N mineralisation were significantly related to rates of N-uptake at both 0–5 and 5–10 cm depth (R2 &gt; 0.63, P &lt; 0.05). Soil C/N ratios (0–5 cm) in rehabilitation sites ranged from 22.4 to 38.8, and in native forests from 35.6 (burnt) to 40.3 (unburnt). Soil C/N ratios increased with depth in both rehabilitation and native forest sites (ranged from 31.2 to 51.6). Availability of water was the major determinant of nitrogen availability in this strongly Mediterranean climate.

Effect of groundwater springs on NO3− concentrations during summer in Catskill Mountain streams

Groundwater and stream water data collected at three headwater catchments in the Neversink River watershed indicate that base flow is sustained by groundwater from two sources: a shallow flow system within the till and soil and a deep flow system within bedrock fractures and bedding planes that discharges as perennial springs. Data from eight wells finished near the till/bedrock interface indicate that saturated conditions are not maintained in the shallow flow system during most summers. In contrast, the discharge of a perennial spring remained constant during two summer rainstorms, providing evidence that the deep flow system is disconnected from the shallow flow system in summer. Discharge from perennial springs was the principal source of streamflow in a headwater reach during low flow. Mean NO3− concentrations were 20–25 μmol L−1 in five perennial springs during the summer but only 5–10 μmol L−1 in shallow groundwater. Thus the deep flow system does not reflect typical NO3− concentrations in the soil during summer. A hydrologic budget at a headwater drainage reveals that March and late fall are the principal groundwater recharge periods. Residence time modeling based on analyses of 18O and 35S indicates that groundwater in the deep flow system is 6–22 months old. These data indicate that summer base flow largely originates from previous dormant seasons when available soil NO3− is greater. In these Catskill watersheds, high base flow concentrations of NO3− during summer do not provide sufficient evidence that the atmospheric N deposition rate exceeds the demand of terrestrial vegetation.

Improved Water Quality through Water Table Management in Eastern Canada

A two-year field study was conducted in eastern Ontario to evaluate the effects of water table management on the quality of subsurface drain flows. Drainage discharge volumes and NO3--N concentrations in drainage effluent were monitored during the growing seasons. The 3.5 ha experimental field was strip-cropped with corn (Zea mays L.) and soybean (Glycine max Merr.). Controlled-drainage subirrigation was used to manage water from precipitation and subirrigation. The two controlled water table treatments were 50 cm and 75 cm from the surface. There was also a conventional free-drainage treatment (FD). In 1995, mean NO3--N concentrations in drainage water were reduced by 84% and 75% by the 50 and 75 cm controlled water table depths, respectively. In 1996, drain flow volumes and NO3--N concentrations were significantly reduced relative to free drainage. Total drain flow was reduced by 42% by the 50 cm controlled water table, while NO3--N concentrations were reduced by 61% and 52% by the 50 cm and 75 cm contr...

In vivo quantification of endotoxin-induced nitric oxide production in pigs from Na15NO3-infusion.

1. In this investigation the NO production rate is quantified in the pig during normotensive endotoxin-induced shock with increased cardiac output and during subsequent treatment with the NO synthase inhibitor N omega-monomethy-L-arginine (L-NMMA). NO production rate was derived from the plasma isotope-enrichment of 15N-labelled nitrate (15NO3-). 2. Three groups of animals (control, n = 5; endotoxin, n = 6; endotoxin + L-NMMA, n = 6) were anaesthetized and instrumented for the measurement of systemic and pulmonary haemodynamics. Each animal received a primed-continuous infusion of stable, non-radioactively labelled Na15 NO3 (bolus 30 mg, infusion rate 2.1 mg h-1). Arterial blood samples were taken 5, 10, 15, 30, 60 and 90 min later and every 90 minutes until the end of the experiment. 3. Continuous i.v. infusion of endotoxin was incrementally adjusted until mean pulmonary artery pressure (PAP) reached 50 mmHg and subsequently titrated to keep mean PAP approximately 35 mmHg. Hydroxyethylstarch was administered as required to maintain mean arterial pressure (MAP) > 60 mmHg. Six hours after the start of the endotoxin continuous i.v. L-NMMA (1 mg kg-1 h-1) was administered to the endotoxin + L-NMMA group. Haemodynamic data were measured before as well as 9 h after the start of the endotoxin. 4. After conversion of NO3- to nitro-trimethoxybenzene and gas chromatography-mass spectrometry analysis the total NO3- pool, basal NO3- production rate and the increase per unit time in NO3- production rate were calculated from the time-course of the 15NO3- plasma isotope-enrichment. A two compartment model was assumed for the NO3- kinetics, one being an active pool in which newly generated NO3- appears and from which it is eliminated, the other being an inactive volume of distribution in which only passive exchange takes place with the active compartment. 5. Although MAP did not change during endotoxin infusion alone, cardiac output (CO) increased by 42 +/- 40% (P < 0.05 versus baseline) by the end of the experiment due to a significant (P < 0.05 versus baseline) fall in systemic vascular resistance (SVR) to 65 +/- 25% of the baseline value. L-NMMA given with endotoxin did not change MAP, and both CO and SVR were maintained close to the pre-shock levels. 6. Baseline plasma NO3- concentrations were 43 +/- 13 and 40 +/- 10 mumol l-1 in the control and endotoxin animals, respectively, and did not differ at the end of the experiment (39 +/- 8 and 44 +/- 15 mumol l-1, respectively). The mean NO3- pool and basal NO3- production rate were 1155 +/- 294 mumol and 140 +/- 32 mumol h-1, respectively, without any intergroup difference. Endotoxin significantly increased NO3- production rate (23 +/- 10 mumol h-2, P < 0.05 versus control (6 +/- 7 mumol h-2) and endotoxin + L-NMMA groups). L-NMMA given with endotoxin (-1 +/- 2 mumol h-2, P < 0.05 versus control and endotoxin groups) had no effect. 7. Analysis of the time course of the 15NO3- plasma isotope enrichment during primed-continuous infusion of Na15NO3 allowed us to quantify the endotoxin-induced increase in NO3- production rate independently of total NO3- plasma concentrations. Low-dose L-NMMA blunted the increase in NO3- production rate while maintaining basal NO3- formation.

Effect of clear cutting on nutrient fluxes in a subalpine forest at Fraser, Colorado

Abstract. Nutrient fluxes were investigated on a forested and a clearcut plot in a mixed conifer high elevation (2900 m) forest at the Fraser Experimental Forest in Fraser, Colorado, USA. Plots were located on a coarse loamy mixed Dystric Cryochrept with relatively high base saturation (30-90%) and underlain by an impermeable clay subsoil. Following harvest in late 1984, annual mean NO3 concentrations of 195 to 198 μmol l-1 were observed from 1988 through 1990 and concentrations were still above reference levels in 1993. Total nitrogen loss attributable to leaching following harvest was estimated at 48kg ha-1 over 8 years. Over this same period, atmospheric nitrogen inputs exceeded annual outflow of NH4 plus NO3 from the control plots by approximately 11 kg N ha-1. A slight enrichment Of SO4 and Cl was observed from the harvested plot in 1986 but concentrations later fell below control plot levels, apparently due to dilution by the increased discharge from the harvested plot which was three to four times that from the control plot. Elevated Ca, Mg, and Na concentrations followed a similar pattern to NO3 due to exchange reactions, while a depression in alkalinity of about one-third the amount of NO3 found was also observed. Enrichment of K occurred primarily in water collected at less than 1 m depth. Increases in base cation loss due to leaching after harvest were about twice the amount that can be accounted for by the increased flux of NO3, SO4, and Cl anions. The excess reflects the increased water flux and consequent leaching of base cations in association with HCO3 and organic anions.

Nitrogen movement and water quality at a poorly-drained agricultural and riparian site in the Pacific Northwest

Grass seed cropping systems in the Pacific Northwest account for about half of the cool-season forage and turf grass seed production in the world. Grass seed cropping systems are intensely managed with inorganic fertilizers to sustain production. Much of the land where grass seed production occurs in western Oregon is marginally productive for most other crops because of the poorly drained soil conditions. The role of riparian areas bordering grass seed fields in moderating surface water and groundwater quality is not well understood. A study site in western Oregon was instrumented to determine how riparian areas bordering grass seed fields process N and thereby influence water quality. The site consisted of a perennial ryegrass (Lolium perenne L.) seed field, a grass riparian area, and an intermittent creek. Groundwater levels were monitored during the fall of 1995 to the late spring of 1996 and a complete set of samples were collected every two weeks from wells with water and analyzed for NO3− and NH4+. Total fertilizer applied to the crop was 200 kg N ha−1 yr−1. Data indicated a lateral subsurface hydrologic flow path from the cultivated field through the riparian zone, to the creek. Averaged over the season (November 1995—June 1996). riparian zone A and C horizon groundwater mean NO3− concentration was mostly undetectable and less than 3.8 mg L−1 in the agricultural field. At all sampling dates, the NO3− concentration was higher in the field than in the riparian zone. Seasonal mean groundwater NH+ concentrations for the riparian and field A and C horizons were ≤0.2 mg L−. Annual crop net N accumulation was 144 kg N ha−1. Riparian soil electrode potentials (Eh) at 25 and 45 cm averaged-200 to -100 mV during the wet cycle, whereas the field ranged between 0 to +50 mV. Collectively, findings indicate that both crop and riparian processes at the Lake Creek site are responsible for reducing shallow groundwater NO3− to low levels.

Elemental concentrations in fresh snowfall across a regional transect in the northeastern U.S.: Apparent sources and contribution to acidity

Fresh snowfall was collected on the surface of 8 lakes across a 350 km west-east transect from northeastern New York state to the coast of Maine after a single storm. In addition, every snowfall event during the winter of 1993 was collected on a single lake near the center of the transect. Across the transect, midwestern sources appear to dominate Pb and Cd concentrations, while Sb appears to be derived from midwestern sources as well as local and/or industrial East Coast sources. In all samples, the highest Na, Cl and Mg concentrations reflect a marine influence, but at some transect sites roadspray aerosol appears to contribute to Na and Cl concentrations. The regional pattern of Ca, K, Mn and Sr concentrations and Mn/Sr ratios indicate that woodsmoke may be an important winter source of these elements at some sites. In all samples, H+ is strongly correlated with NO3− (R2 = 0.97) and mean NO3−/SO42− molar ratios of 6.4 for transect samples, and 4.7 for temporal samples, are higher than mean NO3−/SO42− reported for other precipitation studies in the same region. The contribution of NO3− to the snowpack greatly exceeds that of SO42−, and may be a major source of acidity in aquatic ecosystems during snowmelt.

Increased soil nitrate losses under mature sugar maple trees affected by experimentally induced deep frost

This work reports and discusses data gathered during soil solution monitoring that was part of an experiment conducted in the Duchesnay Experimental Forest (Quebec, Canada) to study the effects of induced deep frost, superficial frost, and superficial frost plus drought on mature sugar maple trees (Acersaccharum Marsh.). Frost treatment was applied by preventing snow from accumulating under the canopy. Soil solution chemistry was modified when mature sugar maple trees declined after exposure to a severe deep frost. The first vegetation period after treatment showed that losses of NO3− below the rooting zone were greatly increased under affected trees. The leaching rate of NO3− and basic cations was directly related to the level of change in canopy dieback and transparency. A mean NO3− concentration of 630 μmolc•L−1 (53 times the controls; max. 4500 μmolc•L−1) was measured in soil solution under the deep frost treated trees. The leaching rate of K+ (18×, relative to the control) and Mn2+ (11×) was higher than that of Mg2+ (5×) and Ca2+ (2.6×). Acidification of the soil solution (50% more H+) as a result of intense nitrification caused an increase in aluminum concentration (5×) and a decrease in SO42−. The acidification during the year after treatment was equivalent to decades of atmospheric acid deposition. The seasonal mean of SO42− did not differ between treatments, but there was evidence of a significant correlation between pH and SO42− in soil solution. Concentration of NH4+ was also enhanced but to a lesser degree (10×) than that of NO3−. Specific conductivity was a good predictor of NO3−, Ca2+, Mg2+, and total Al in soil solution. The ion balance shifted from an anion deficit to a strong cation deficit when NO3− concentrations were high. Superficial frost with or without induced summer drought did not cause any significant change in soil solution chemistry compared with the controls. These results indicated the necessity to consider perturbations induced by extreme climate conditions, like deep soil frost, for the interpretation of soil solution chemistry data in the context of acid deposition studies and forest health.

Sources of aerosol nitrate and non-sea-salt sulfate in the Iceland region

Daily aerosol filter samples were collected on Heimaey, Iceland (63.40° N, 20.30° W), beginning in July 1991. Samples were analyzed for NO3−, non-sea-salt (nss) SO42−, and methanesulfonate (MSA). Along with SO2 and nss-SO42−, MSA is a product of the atmospheric oxidation of dimethyl sulfide (DMS) that is produced by marine organisms. For much of the time, concentrations of aerosol nss-SO42− and NO3− were relatively low. Occasionally, however, concentrations increased sharply, by an order of magnitude or more, often for periods of several days. These concentration peaks were usually associated with the presence of a high-pressure field over western Europe; the large-scale wind fields associated with the high pressure subsequently transported pollutants to the Iceland region. The 2-year mean NO3− aerosol concentration was 0.239 μg/m3, while that for nss-SO42− was 0.642 μg/m3; the median values were, respectively, 0.113 μg/m3 and 0.367 μg/m3. Excluding the high-aerosol events (i.e. about 10% of the samples), the NO3− average was 0.131 μg/m3 and that for nss SO42− was 0.385 μg/m3; these values are similar to those measured in the pristine South Pacific. Thus, although pollution events were relatively infrequent, they had a substantial impact on atmospheric chemistry in this region, in effect doubling the annual mean concentrations. There was a very strong seasonal cycle in MSA concentrations, with a summer maximum of about 500 ng/m3, which decreased to a few ng/m3 in December. The seasonal cycle of MSA matches that ofPhaeocystis pouchetii andEmiliania huxleyi, both of which are strong DMS producers; intense and widespread blooms of these organisms are often found around Iceland in the late spring and summer. During the summer, the nss-SO42−MSA ratio was very low much of the time, suggesting that biogenic DMS was the dominant source of aerosol nss-SO42− in this region in this season.

Nitrogen and sulfur species in Antarctic aerosols at Mawson, Palmer Station, and Marsh (King George Island)

High volume bulk aerosol samples were collected continuously at three Antarctic sites: Mawson (67.60° S, 62.50° E) from 20 February 1987 to 6 January 1992; Palmer Station (64.77° S, 64.06° W) from 3 April 1990 to 15 June 1991; and Marsh (62.18° S, 58.30° W) from 28 March 1990, to 1 May 1991. All samples were analyzed for Na+, SO 4 2− , NO 3 − , methanesulfonate (MSA), NH 4 + ,210Pb, and7Be. At Mawson for which we have a multiple year data set, the annual mean concentration of each species sometimes vary significantly from one year to the next: Na+, 68–151 ng m−3; NO 3 − , 25–30 ng m−3; nss SO 4 2− , 81–97 ng m−3; MSA, 19–28 ng m−3; NH 4 + , 16–21 ng m−3;210Pb, 0.75–0.86 fCi m−3. Results from multiple variable regression of non-sea-salt (nss) SO 4 2− with MSA and NO 3 − as the independent variables indicates that, at Mawson, the nss SO 4 2− /MSA ratio resulting from the oxidation of dimethylsulfide (DMS) is 2.80±0.13, about 13% lower than our earlier estimate (3.22) that was based on 2.5 years of data. A similar analysis indicates that the ratio at Palmer is about 40% lower, 1.71±0.10, and more comparable to previous results over the southern oceans. These results when combined with previously published data suggest that the differences in the ratio may reflect a more rapid loss of MSA relative to nss SO 4 2− during transport over Antarctica from the oceanic source region. The mean210Pb concentrations at Palmer and Marsh and the mean NO 3 − concentration at Palmer are about a factor of two lower than those at Mawson. The210Pb distributions are consistent with a210Pb minimum in the marine boundary layer in the region of 40°–60° S. These features and the similar seasonalities of NO 3 − and210Pb at Mawson support the conclusion that the primary source regions for NO 3 − are continental. In contrast, the mean concentrations of MSA, nss SO 4 2− , and NH 4 + at Palmer are all higher than those at Mawson: MSA by a factor of 2; nss SO 4 2− by 10%; and NH 4 + by more than 50%. However, the factor differences exhibit substantial seasonal variability; the largest differences generally occur during the austral summer when the concentrations of most of the species are highest. NH 4 + /(nss SO 4 2− +MSA) equivalent ratios indicate that NH3 neutralizes about 60% of the sulfur acids during December at both Mawson and Palmer, but only about 30% at Mawson during February and March.

Groundwater Nitrate and Denitrification in a Coastal Plain Riparian Forest

AbstractMechanisms of nitrate (NO3) removal from groundwater in riparian forests are poorly understood. This study was conducted in the Georgia coastal plain to: (i) determine changes in NO3 and Cl concentrations within shallow groundwater moving from a row‐crop field to a stream; (ii) determine the spatial and temporal distribution of denitrifiration potential relative to changes in NO3 concentrations; and (iii) determine whether NO3 or C supply was limiting denitrification potential. Nitrate and Cl concentrations in groundwater were measured biweekly or monthly for October 1988 through May 1990. Denitrification potentials, indicated by the denitrification enzyme assay, were measured bimonthly from October 1988 through October 1989. Modified potential measurements, lacking either NO3, C, or both, were also performed bimonthly. Both NO3 and NO3/Cl ratios in groundwater decreased by a factor of 7 to 9 in the first 10 m of forest. Within the next 40 m of forest, mean NO3 concentration decreased from 1.80 to 0.81 mg NO3‐N L−1. Denitrifiration potential was more than two orders of magnitude higher in the top 10 cm of soil than in the top 10 cm of the shallow aquifer. Denitrification potential was consistently highest in surface soil nearest the field and nearest the stream and was limited by NO3 availability in all surface soil samples. Denitrification potential was highest in October and August. Although NO3 is definitely being removed from shallow groundwater, it is apparently not due to direct denitrification from the saturated zone. High denitrification potential in surface soils, especially near the field/forest interface, may contribute to NO3 disappearance from shallow groundwater. Processes associated with intact riparian vegetation appear to play the primary role in N removal.

An analysis of variability in the activity of nitrifiers in a soil under pasture .I. Spatially dependent variability and optimum sampling strategy

Variability in the activity of nitrifying organisms in a Tokomaru silt loam (Typic Fragiaqualf) under a ryegrass-white clover pasture was measured using a short-term nitrification assay (SNA). Spatial dependence of the variability in SNA value and associated soil properties was examined for an area of 625 m2 using geostatistical analysis. The values of SNA, soil nitrate (NO3-N), and exchangeable ammonium (Ex-NH4+) were all highly variable and conformed to lognormal distributions. The range of spatial dependence in the variability of SNA, NO3-N, and the pH of the SNA incubation media, which was assumed to approximate the field soil pH, was 3.3, 12.0 and 13.8 m respectively. EX-NH4+ showed no spatial dependence. SNA, NO3-N and incubation pH varied isotropically within the sampling area, but soil moisture content was strongly anisotropic. These results are discussed in relation to the most efficient sampling strategy for estimation of the mean field NO3- concentration. It was concluded that sufficient small localized clusters of samples should be taken to give a low standard error of the mean, with each cluster separated by at least 12 m. In the case of the Tokomaru silt loam, 12 clusters, each comprising bulked subsamples, would be required for estimation of the mean field nitrate concentration with 95% probability of �, the sample mean, being within � 5% of �, the true mean. This represents a large sampling effort.

Drilling Method for Collection of Undisturbed Soil Monoliths

AbstractIn studies of water and solute movement in soils, use of undisturbed monoliths is usually preferred. A drilling technique to collect such monoliths (0.295 m i.d.; 1.0, 0.5, or 0.3 m in length) with minimal soil disturbance was developed. The drill consists of a steel cylinder with four mounted cutting teeth at the bottom, into which a plastic casing is inserted. The drill‐cylinder rotates around the casing and carves out a soil core that is gently pushed into the casing using only a minimum of hydraulic pressure. The technique has been used in a wide variety of research applications within soil science, including the measurement of hydraulic properties, soil evaporation, and soil aeration studies. Here, one application is described in which the monoliths were used as field lysimeters planted with either barley (Hordeum vulgare L.) or meadow fescue (Festuca pratensis Hudson). During a 6‐mo period, the grass lysimeters had a much lower total drainage (106 compared with 213 mm for barley) and also lower mean NO3 concentrations in the drainage water (&lt;0.1 compared with 9.3 mg N L−1). The uniformity of response among the replicates and large differences between the two treatments was taken as indirect evidence that sidewall flows were of little significance when using this technique.

Spatial variability of soil nitrate nitrogen after potatoes and its change during winter

Knowledge of the frequency distribution and spatial structure of the soil NO3-N is required to develop an efficient sampling strategy. A 1 ha polder field was sampled after the harvest of potatoes in October 1987, and in February and April 1988, without being fertilitzed since March 1987. These data sets were examined by a classical statistical as well as a spatial structure analysis. The October and February data sets were found to be lognormally distributed, the April data showed a normal frequency distribution. All three data sets had a spatial structure, although the October data were anisotropic and needed removal of a trend. The spatial variability of soil NO3−N decreased, became isotropic and evolved towards a larger range of spatial dependence during the winter. Knowledge of this structure permitted to krige or cokrige the data. The number of samples required to estimate the mean NO3−N content with an acceptable precision was found to be 39, 43 and 17 in October, February and April respectively.

The extent of snowpack influence on water chemistry in a North Cascades Lake

Integrated snowpack samples and lakewater samples were collected from a low‐alkalinity watershed in the North Cascades in 1984 and 1985 and analyzed for inorganic chemical parameters. Mean concentrations of NO3− and SO4−2 from North Cascades snowpack were near 4.5 and 5.5 μeq L−1, respectively. Mean pH was 5.2–5.3. During melting, anion levels in the snowpack decreased and pH increased. By July, mean NO3− and SO4−2 concentrations were 0.32 and 1.73 μeq L−1, respectively, and mean pH was 5.48. Acid neutralizing capacity at the lake outlet during snowmelt was diluted to 50% of April premelt values. During snowmelt runoff there was no apparent neutralization of bicarbonate by strong acids. By comparing lakewater NO3− concentrations to snowpack concentrations, over 75% of the NO3− in the lake could be attributed to a snowpack source. An estimated 30% of the SO4−2 in the lake was derived from the snowpack.

Particulate sulfate and nitrate in the boundary layer over the North Pacific Ocean

Concentrations of aerosol NO3− and non‐sea‐salt (nss) SO4= have been measured in samples collected weekly between January 1981 and March 1982 at seven stations in the Sea‐Air Exchange Program Asian Dust Network in the North Pacific between 4°N and 53°N. Mean NO3− concentrations were relatively uniform, ranging between 0.18 and 0.35 μg/m3 STP; mean nss SO4= ranged from 0.37 to 1.20 μg/m3 STP. At most stations there was no consistent correlation between NO3− and nss SO4=or between these species and mineral aerosol. At Midway these species did covary in a clear and consistent manner; this correlation appears to be forced by large‐scale meteorological factors. In some cases, high nss SO4= concentrations were associated with high mineral aerosol concentrations; this suggests that the SO4= in these episodes was transported from the continents. A particularly impressive event occurred in May 1981, when much of the North Pacific region was affected by extremely high nss SO4= concentrations; evidence suggests that the SO4= was not derived from the soil material itself. At Fanning Island, NO3− concentrations showed a strong seasonal variation; there is evidence that the high NO3− concentrations are attributable to transport from the continents. In contrast, nss SO4= concentrations at Fanning were relatively constant throughout the year, and they were consistent with recent estimates of the oceanic emission rates of sulfur as dimethyl sulfide. The NO3− data from the tropics suggest that there is a relatively uniform background concentration of about 0.10 μg/m3 STP; this concentration is close to that recently predicted by models that assume only lightning and the stratosphere as sources for NOx.