Differences among maize cultivars in the utilization of soil nitrate and the related losses of nitrate through leaching

In a 2-year field experiment conducted on a Gleyic Luvisol in Stuttgart-Hohenheim one experimental and nine commercial maize cultivars were compared for their ability to utilize soil nitrate and to reduce related losses of nitrate through leaching. Soil nitrate was monitored periodically in CaCl₂ extracts and in suction cup water. Nitrate concentrations in suction water were generally higher than in CaCl₂ extracts. Both methods revealed that all cultivars examined were able to extract nitrate down to a soil depth of at least 120 cm (1988 season) or 150 cm (1987 season). Significant differences among the cultivars existed in nitrate depletion particularly in the subsoil. At harvest, residual nitrate in the upper 150 cm of the profile ranged from 73–110 kg N ha^–1 in 1987 and from 59–119 kg N ha^–1 in 1988. Residual nitrate was closely correlated with nitrate losses by leaching because water infiltration at 120 cm soil depth started 4 weeks after harvest (1987) or immediately after harvest (1988) and continued until early summer of the following year. The calculated amount of nitrate lost by leaching was strongly influenced by the method of calculation. During the winter of 1987/88 nitrate leaching ranged from 57–84 kg N ha^–1 (suction cups) and 40–55 kg N ha^–1 (CaCl₂ extracts), respectively. The corresponding values for the winter of 1988/89 were 47–79 and 20–39 kg N ha^–1, respectively. ei]Section editor: B E Clothier     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

Seasonal flooding,nitrogen mineralization and nitrogen utilization in a prairie marsh

Flooding can be an important control of nitrogen (N) biogeochemistry in wetland ecosystems. In North American prairie marshes, spring flooding is a dominant feature of the physical environment that increases emergent plant production and could influence N cycling. I investigated how spring flooding affects N availability and plant N utilization in whitetop (Scolochloa festucacea) marshes in Manitoba, Canada by comparing experimentally spring-flooded marsh inside an impoundment with adjacent nonflooded marsh. The spring-flooded marsh had net N mineralization rates up to 4 times greater than nonflooded marsh. Total growing season net N mineralization was 124 kg N ha^–1 in the spring-flooded marsh compared with 62 kg N ha^–1 in the nonflooded marsh. Summer water level drawdown in the spring-flooded marsh decreased net N mineralization rates. Net nitrification rates increased in the nonflooded marsh following a lowering of the water table during mid summer. Growing season net nitrification was 33 kg N ha^–1 in the nonflooded marsh but < 1 kg N ha^–1 in the spring-flooded marsh. Added NO₃ ^–1 induced nitrate reductase (NRA) activity in whitetop grown in pot culture. Field-collected plants showed higher NRA in the nonflooded marsh. Nitrate comprised 40% of total plant N uptake in the nonflooded marsh but <1% of total N uptake in the spring-flooded marsh. Higher plant N demand caused by higher whitetop production in the spring-flooded marsh approximately balanced greater net N mineralization. A close association between the presence of spring flooding and net N mineralization and net nitrification rates indicated that modifications to prairie marshes that change the pattern of spring inundation will lead to rapid and significant changes in marsh N cycling patterns.     

N deposition, N transformation and N leaching in acid forest soils

Nitrogen deposition, mineralisation, uptake and leaching were measured on a monthly basis in the field during 2 years in six forested stands on acidic soils under mountainous climate. Studies were conducted in three Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantations (D20: 20 year; D40: 40 yr; D60: 60 yr) on abandoned croplands in the Beaujolais Mounts; and two spruce (Picea abies Karst.) plantations (S45: 45 yr; S90: 90 yr) and an old beech (Fagus sylvatica L.) stand (B150: 150 yr) on ancient forest soils in a small catchment in the Vosges Mountains. N deposition in throughfall varied between 7–8 kg ha^–1 year^–1 (D20, B150, S45) and 15–21 kg ha^–1 yr^–1 (S90, D40, D60). N in annual litterfall varied between 20–29 kg ha^–1 (D40, D60, S90), and 36–43 kg ha^–1 (D20, S45, B150). N leaching below root depth varied among stands within a much larger range, between 1–9 kg ha^–1 yr^–1 (B150, S45, D60) and 28–66 kg ha^–1 yr^–1 (D40, S90, D20), with no simple relationship with N deposition, or N deposition minus N storage in stand biomass. N mineralisation was between 57–121 kg ha^–1 yr^–1 (S45, D40, S90) and between 176–209 kg ha^–1 yr^–1 in (B150, D60 and D20). The amounts of nitrogen annually mineralised and nitrified were positively related. Neither general soil parameters, such as pH, soil type, base saturation and C:N ratio, nor deposition in throughfall or litterfall were simply related to the intensity of mineralisation and/or nitrification. When root uptake was not allowed, nitrate leaching increased by 11 kg ha^–1 yr^–1 at S45, 36 kg ha^–1 yr^–1 at S90 and between 69 and 91 kg ha^–1 yr^–1 at D20, D40, B150 and D60, in relation to the nitrification rates of each plot. From this data set and recent data from the literature, we suggest that: high nitrification and nitrate leaching in Douglas-fir soils was likely related to the former agricultural land use. High nitrification rate but very low nitrate leaching in the old beech soil was related to intense recycling of mineralised N by beech roots. Medium nitrification and nitrate leaching in the old spruce stand was related to the average level of N deposition and to the deposition and declining health of the stand. Very low nitrification and N leaching in the young spruce stand were considered representative of fast growing spruce plantations receiving low N deposition on acidic soils of ancient coniferous forests. Consequently, we suggest that past land use and fine root cycling (which is dependent on to tree species and health) should be taken into account to explain the variability in the relation between N deposition and leaching in forests.     

Nitrate leaching from three afforestation chronosequences on former arable land in Denmark

In regions dominated by agricultural activities, nitrogen (N) is recognized as a major pollutant in aquatic environments. In north‐western Europe, afforestation of agricultural land is part of a strategy to improve water quality. In Denmark, former arable land has been afforested during the past 40–50 years. This study evaluated the effect of afforestation of former arable land on nitrate leaching, based on three afforestation chronosequences. Precipitation, canopy throughfall and soil water were collected and soil moisture was monitored at two Danish locations, Vestskoven (nutrient‐rich, medium deposition) and Gejlvang (nutrient‐poor, high deposition). Afforestation was performed using Norway spruce [Picea abies (Karst.) L.] and common oak (Quercus robur L.) at Vestskoven and Norway spruce at Gejlvang. The results suggest that afforestation of former arable land initially leads to lower nitrate leaching than that occurring under the former agricultural land use, and largely below the standard of 50 mg NO⁻₃ L⁻¹ for groundwater to be utilized as drinking water. Nitrate concentrations became almost negligible in forest stands of 5–20 years of age. However, after canopy closure (>20 years) nitrate concentrations below the root zone and nitrate leaching tended to increase. This was attributed to increased N deposition with increasing canopy development and decreased N demand once the most N‐rich biomass compartments had been built up. Nitrate leaching started to increase at a throughfall deposition level of about 10 kg N ha⁻¹ yr⁻¹. Compared with nutrient‐poor sandy soils, nutrient‐rich clayey soils appeared more vulnerable to disturbance of the N cycle and to increased N deposition, leading to N saturation and enhanced nitrate leaching. In approximately the first 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce.     

Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests

The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha^?1 y^?1 in northern Norway and Finland to greater than 60 kg N ha^?1 y^?1 in the Netherlands and Czech Republic. The amount of NO₃^– leached covers a smaller range, between 1 and 40 kg N ha^?1 y^?1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P < 0.05) are observed between N input and NO₃^– leached. The slope of the relationship for those sites where C/N ratio is ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is > 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO₃^– leaching also shows a relationship with soil pH, with high rates of NO₃^– leaching only observed at sites with a pH < 4.5 and N inputs > 30 kg N ha^?1 y^?1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.     

Seasonal dynamics of denitrification activity in two water meadows

Seasonal variation in denitrification activity was measured in twoflooded water meadows, one on peaty and one on sandy soil, over a three-yearperiod. Measurements were taken during flooded and drained periods, usingthe acetylene-blockage technique, and the rates were compared to massbalance estimates of nitrate removal in the percolating water.Denitrification activity was higher in sandy soil than in peaty soil. Higherwater infiltration rate and thereby higher nitrate load was considered to bethe cause of the higher denitrification in the sandy soil. Floodingsignificantly increased denitrification, and the rates were higher in autumnand winter than in spring. This was considered to be a result of highernitrogen concentration in inflowing stream water during winter. Annualdenitrification was estimated to 430–460 kg N ha^-1yr^-1 in the sandy soil meadow, and 220 kg N ha^-1yr^-1 in the peaty soil meadow. In the sandy soil there was alarge discrepancy between nitrate removal rates and denitrification rates,which can be explained by nitrification of ammonium released from the soil.In the peaty soil nitrate disappearance and denitrification correspondedfairly well.     

Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition

Nitrogen transformations were studied in the forest floor and mineral soil (0–5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Curren nitrogen depositions (40 and 56 kg N ha^-1 yr^-1, respectively) were reduced to natural background levels (1–2 kg N ha^-1 yr^-1) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.     

Changes in soil mineral nitrogen during and after 3-year and 5-year set-aside and nitrate leaching losses after ploughing out the 5-year plant covers in the UK

The management and effects of 3-year and 5-year set-aside covers on soil mineral nitrogen (SMN, 0.0–0.9 m) were studied at six sites in England. Soil mineral N was measured annually in autumn and spring during the period of set-aside cover, with more frequent SMN sampling over the first winter after ploughing out the covers. Spring SMN was measured in the second year after set-aside. Nitrate leaching losses were also measured at three sites in the first winter after destruction of the 5-year set-aside covers. Winter cereals were grown in both test years after each set-aside period.Amounts of both autumn and spring SMN in the perennial rye-grass (PRG), perennial rye-grass/white clover (PRG/WC) and natural regeneration (NR) covers were generally less than, or similar to those in the continuous arable treatment during each year of set-aside, indicating a slightly smaller nitrate leaching risk under set-aside management. Slight increases in autumn SMN, and hence leaching potential were, however, observed under PRG/WC in the fourth and fifth years, compared with continuous arable cropping.Ploughing out of both 3-year and 5-year covers increased soil N supply and potential nitrate leaching losses over winter, compared with continuous arable cropping. By the following spring, mean increases across all sites in amounts of SMN after 3-year covers of PRG, NR and PRG/WC were 14, 18 and 33 kg ha^–1 N, respectively, compared with the arable rotation. Equivalent increases in spring SMN following destruction of the 5-year set-aside covers were almost identical, at 17, 19 and 33 kg ha^–1, respectively, although only the ploughed-out PRG/WC covers increased SMN at the clay sites. Measured nitrate leaching losses in the first winter after 5-year set-aside were greatest after PRG/WC at two sites on shallow chalk but greatest after NR, which had a naturally large clover content, at the third site which was on a sandy soil. However, the leaching losses after set-aside were relatively small, relative to typical losses after ploughing out intensively managed grass or grass/clover swards, and would have been compensated for by potentially less leaching during set-aside.Spring SMN measurements in the second year after ploughing out the set-aside covers, showed negligible or, for PRG/WC, only slight increases (12 – 18 kg ha^–1) in residual soil N supply after both 3-year and 5-year covers, compared to continuous arable cropping. The extra N mineralisation after cover destruction justified small reductions in fertiliser N inputs for the first, but not second crop following either 3- or 5-year set-aside, unless the cover had contained a large clover content. Both 3-year and 5-year set-aside covers had minimal or no effect on either organic matter content, apart from a slight increase in the PRG/WC treatments, or extractable phosphorus, potassium and magnesium status in the topsoil.     

The use of mean pool abundances to interpret 15N tracer experiments

A pulse dilution ¹⁵N technique was used in the field to determine the effect of the ammonium to nitrate ratio in a fertilizer application on the uptake of ammonium and nitrate by ryegrass and on gross rates of mineralization and nitrification. Two experiments were performed, corresponding approximately to the first and second cuts of grass. Where no substantial recent immobilization of inorganic nitrogen had occurred, mineralization was insensitive to the form of nitrogen applied, ranging from 2.1–2.6 kg N ha^-1 d^-1. The immobilization of ammonium increased as the proportion of ammonium in the application increased. In the second experiment there was evidence that high rates of immobilization in the first experiment were associated with high rates of mineralization in the second. The implication was that some nitrogen immobilized in the first experiment was re-mineralized during the second. Whether this was nitrogen taken up, stored in roots and released following defoliation was not clear. Nitrification rates in this soil were low (0.1–0.63 kg N ha^-1 d^-1), and as a result, varying the ratio of ammonium to nitrate applied markedly altered the relative uptake of ammonium and nitrate. In the first experiment, where temperatures were low, preferential uptake of ammonium occurred, but where >90% of the uptake was as ammonium, a reduction in yield and nitrogen uptake was observed. In the second experiment, where temperatures and growth rates were higher, the proportion of ammonium to nitrate taken up had no effect on yield or nitrogen uptake.     

Quantitative effects of soil nitrate,growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha^–1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha^–1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha^–1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m^–2 degree-day^–1, equivalent to 7 kg N ha^–1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.     

Soil N mineralization and nitrification in relation to nitrogen solution chemistry in a small forested watershed

Spatial variations in soil processes regulating mineral N losses to streams were studied in a small watershed near Toronto, Ontario. Annual net N mineralization in the 0–8 cm soil was measured in adjacent upland and riparian forest stands using in situ soil incubations from April 1985 to 1987. Mean annual rates of soil N mineralization and nitrification were higher in a maple soil (93.8 and 87.0 kg.ha^–1) than in a pine soil (23.3 and 8.2 kg.ha^–1 ). Very low mean rates of mineralization (3.3 kg.ha^–1) and nitrification (3.4 kg.ha^–1) were found in a riparian hemlock stand. Average NO₃-N concentrations in soil solutions were 0.3–1.0 mg.L^–1 in the maple stand and >0.06mg.L^–1 in the pine stand. Concentrations of NO₃–N in shallow ground water and stream water were 3–4× greater in a maple subwatershed than in a pine subwatershed. Rapid N uptake by vegetation was an important mechanism reducing solution losses of NO₃–N in the maple stand. Low rates of nitrification were mainly responsible for negligible NO₃–N solution losses in the pine stand.     

Tracing the Sources of Exported Nitrate in the Turkey Lakes Watershed Using 15N/14N and 18O/16O isotopic ratios

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. ¹⁵N/¹⁴N and ¹⁸O/¹⁶O isotopic ratios of nitrate can therefore be used to distinguish between these two sources of nitrate in surface waters and groundwaters. Two forested catchments in the Turkey Lakes Watershed (TLW) near Sault Ste. Marie, Ontario, Canada were studied to determine the relative contributions of atmospheric and microbial nitrate to nitrate export. The TLW is reasonably undisturbed and receives a moderate amount of inorganic nitrogen bulk deposition (8.7 kg N · ha⁻¹· yr⁻¹) yet it exhibits unusually low inorganic nitrogen retention (average = 65% of deposition). The measured isotopic ratios for nitrate in precipitation ranged from +35 to +59‰ (VSMOW) for δ¹⁸O and −4 to +0.8‰ (AIR) for δ¹⁵N. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately −1.0‰ for δ¹⁸O and a value of about 0 to +6‰ for δ¹⁵N, thus, the isotopic separation between atmospheric and soil sources of nitrate is substantial. Nitrate produced by nitrification of ammonium appears to be the dominant source of the nitrate exported in both catchments, even during the snowmelt period. These whole catchment results are consistent with the results of small but intensive plot scale studies that have shown that the majority of the nitrate leached from these catchments is microbial in origin. The isotopic composition of stream nitrate provides information about N-cycling in the forested upland and riparian zones on a whole catchment basis. Received 5 October 1999; accepted 18 August 2000     

Nitrogen deposition,distribution and cycling in a subalpine spruce-fir forest in the Adirondacks,New York,USA

Nitrogen inputs, fluxes, internal generation and consumption, and outputs were monitored in a subalpine spruce-fir forest at approximately 1000-m elevation on Whiteface Mountain in the Adirondacks of New York, USA. Nitrogen in precipitation, cloudwater and dry deposition was collected on an event basis and quantified as an input. Throughfall, stemflow, litterfall and soil water were measured to determine fluxes within the forest. Nitrogen mineralization in the forest floor was estimated to determine internal sources of available N. Lower mineral horizon soil water was used to estimate output from the ecosystem. Vegetation and soil N pools were determined.During four years of continuous monitoring, an average of 16 kg N ha^–1 yr^–1 was delivered to the forest canopy as precipitation, cloudwater and dry deposition from the atmosphere. Approximately 30% of the input was retained by the canopy. Canopy retention is likely the result of both foliar uptake and immobilization by bark, foliage and microorganisms. Approximately 40 kg of N was made available within the forest floor from mineralization of organic matter. Virtually all the available ammonium (mineralized plus input from throughfall) is utilized in the forest floor, either by microorganisms or through uptake by vegetation. The most abundant N component of soil water solutions leaving the system was nitrate. Net ecosystem fluxes indicate accumulation of both ammonium and nitrate. There is a small net loss of organic N from the ecosystem. Some nitrate leaves the bottom of the B horizon throughout the year. Comparisons with other temperate coniferous sites and examination of the ecosystem N mass balance indicate that N use efficiency is less at our site, which suggests that the site is not severely limited by N.     

Assessing nitrate leaching during the three‐first years of Miscanthus × giganteus from on‐farm measurements and modeling

Miscanthus × giganteus is often regarded as one of the most promising crops to produce sustainable bioenergy. This perennial crop, renowned for its high productivity associated with low input requirements, in particular regarding fertilizers, is thought to have low environmental impacts, but few data are available to confirm this. Our study aimed at assessing nitrate leaching from Miscanthus × giganteus crops in farmers' fields, thus including a wide range of soil and cropping system conditions. We focused on the first years of growth after planting as experimental studies have suggested that Miscanthus × giganteus, once established, results in low nitrate leaching. We combined on‐farm measurements and modeling to estimate drainage, leached nitrogen, and nitrate concentration in drainage water in 38 fields located in Center‐East France during two winters (November 2010 to March 2011, November 2011 to March 2012). Nitrate leaching and nitrate concentration in drainage water were on average very low. Nitrate leaching averaged 6 kg N ha^?1 whereas nitrate concentration averaged 12 mg l^?1. These low values are attributable to the low estimates of drainage water (mean = 166 mm) but also to the low soil mineral nitrogen contents measured at the beginning of winter (mean = 37 kg N ha^?1). Our results were, however, very variable, mainly due to the crop age: nitrate leaching and nitrate concentration were critically higher during the winter following the first growth year of Miscanthus × giganteus, reflecting the low development of the crop. This variability was also explained by the range of soil and cropping conditions explored in the on‐farm design: shallow and/or sandy soils as well as fields where establishment failed had a higher risk of nitrate leaching.     

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N<Subscript>2</Subscript>-fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N₂-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r ² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha^–1 y^–1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

Comparison of initial wetting pattern,nutrient concentrations in soil solution and the fate of15N-labelled urine in sheep and cattle urine patch areas of pasture soil

Three field experiments were carried out to compare cattle and sheep urine patches in relation to (i) initial wetting pattern and volume of soil affected, (ii) soil solution ionic composition and (iii) the fate of¹⁵N-labelled urine in the soil over the winter period. The distribution of Br^– (used as a urine tracer) across the soil surface and down the profile was irregular in all the patches. The pasture area covered by Br^– in the sheep patches was 0.04–0.06 m² and Br^– was detected to a depth of 150 mm. Cattle patches were significantly larger covering a surface area of 0.38–0.42 m² and penetrating to a depth of 400 mm. The rapid downward movement of urine occurred through macropore flow but even so, over half of the applied Br^– was detected in the 0–50 mm soil layer in both sheep and cattle patches. Due to the larger volume of urine added to the cattle patches (2000 mL for cattle and 200 mL for sheep) the effective application rate was about 5 L m^–2 compared with 4 L m^–2 for sheep. Concentrations of extractable mineral N and ionic concentrations in soil solution were higher in cattle than sheep patches particularly near the soil surface. In both sheep and cattle patches, urea was rapidly hydrolysed to NH ₄ ⁺ and nitrification occurred between 14 and 29 days after urine application. Initially the major anions and cations in the soil solution were HCO ₃ ^– , SO ₄ ⁼ , Cl^–, NH ₄ ⁺ , Mg⁺⁺, K⁺ and Na⁺, which were derived from the urine application. Ionic concentrations in the soil solution decreased appreciably over time due to plant uptake and possibly some leaching. As nitrification proceeded, NO ₃ ^– became the dominant anion in soil solution and the major accompanying cation was Ca⁺⁺. The fate of¹⁵N-labelled urine-urea was followed during a 5 month period beginning in late autumn. Greater leaching losses of NO ₃ ^– occurred below cattle patches (equivalent to 60 kg N ha^–1 below 300 mm and 37 kg N ha^–1 below 600 mm) compared with sheep patches (10 kg N ha^–1 below 300 mm and 1 kg N ha^– below 600 mm). While 6% of the applied¹⁵N was leached the amount of N leached was equivalent to 11% of the applied urine-N in cattle patches. This suggests that there was significant immobilsation-mineralisation turnover in urine patch soil with the release of mineral N from native soil organic matter. In both sheep and cattle patches 60% of the¹⁵N was accounted for in plant uptake, remaining in the soil and leaching. About 40% of the applied N was therefore lost through gaseous emission.     

Throughfall Nitrogen Deposition Has Different Impacts on Soil Solution Nitrate Concentration in European Coniferous and Deciduous Forests

Increases in the deposition of atmospheric nitrogen (N) influence N cycling in forest ecosystems and can result in negative consequences due to the leaching of nitrate into groundwaters. From December 1995 to February 1998, the Pan-European Programme for the Intensive and Continuous Monitoring of Forest Ecosystems measured forest conditions at a plot scale for conifer and broadleaf forests, including the performance of time series of soil solution chemistry. The influence of various ecosystem conditions on soil solution nitrate concentrations at these forest plots (n = 104) was then analyzed with a statistical model. Soil solution nitrate concentrations varied by season, and summer concentrations were approximately 25% higher than winter ones. Soil solution nitrate concentrations increased dramatically with throughfall (and bulk precipitation) N input for both broadleaf and conifer forests. However, at elevated levels of throughfall N input (more than 10 kg N ha^–1 y^–1), nitrate concentrations were higher in broadleaf than coniferous stands. This tree-specific difference was not observed in response to increased bulk precipitation N input. In coniferous stands, throughfall N input, foliage N concentration, organic layer carbon–nitrogen (C:N) ratio, and nitrate concentrations covaried. Soil solution nitrate concentrations in conifer plots were best explained by a model with throughfall N and organic layer C:N as main factors, where C:N ratio could be replaced by foliage N. The organic layer C:N ratio classes of more than 30, 25–30, and less than 25, as well as the foliage N (mg N g^–1) classes of less than 13, 13–17, and more than 17, indicated low, intermediate, and high risks of nitrate leaching, respectively. In broadleaf forests, correlations between N characteristics were less pronounced, and soil solution nitrate concentrations were best explained by throughfall N and soil pH (0–10-cm depth). These results indicate that the responses of soil solution nitrate concentration to changes in N input are more pronounced in broadleaf than in coniferous forests, because in European forests broadleaf species grow on the more fertile soils.     

Inhibiting nitrification and increasing yield of barley by band placement of thiourea with fall-applied urea

Incubation and field experiments were conducted on the influence of thiourea in inhibiting nitrification of urea N, and subsequently on reducing over-winter losses of fallapplied N. Under incubation, most of the added urea placed in bands was nitritified within five or six weeks. However, thiourea when pelleted with urea (21 urea to thiourea by weight) reduced the amount of nitrification to less than one-half during the same period.In two uncropped field experiments in an early dry fall, the application of pelleted urea+thiourea (21) in bands resulted in almost complete inhibition of nitrification of urea for four weeks. In two other uncropped field experiments begun in June with the same fertilizer in bands, half or less of applied N appeared as nitrate after eight weeks. In 10 cropped field experiments with 56 kg N ha^–1, urea+thiourea placed in bands depressed nitrification of fall-applied urea over the winter. By early May, the urea mixed into the soil in the previous fall was nearly all nitrified, while only one-half of the banded urea+thiourea was nitrified. The loss of mineral N by early May was 38% with urea mixed into the soil, but only 18% with bands of urea+thiourea.The 10 sites were cropped to spring barley. The increase in yield of grain or the increase in %N uptake from fertilier N was approximately only one-half as much with fall-applied urea mixed into the soil as compared to spring-applied urea added in the same way. Specifically, fall-applied mixed urea produced 930 kg ha^–1 less grain yield and 32% less N uptake from fertilizer N than did mixed urea in spring. On fall-application there was some benefit from banding of urea or with mixing urea+thiourea pellets into the soil, but the banding of urea+thiourea pellets gave more benefit. Among the fall applications, banded urea+thiourea pellets produced 670 kg ha^–1 more grain yield and 26% more N uptake in grain from fertilizer N than did urea mixed into the soil.