Denitrification Losses Research Articles

Sunola response to nitrogen fertilization

The response of sunola (Helianthus annuus L. 'AC Sierra') to nitrogen (N) fertilization under conventional and direct seeding tillage systems was examined in field experiments at sites in the semiarid Dark Brown and subhumid Black soil climatic zones over a 2-yr period of normal to above-normal growing season precipitation. Urea-N fertilizer was banded at the time of seeding at varying rates to a maximum of 150 kg N ha−1. Tillage treatment did not influence sunola response to N fertilization. Sunola seed yield responded greater to fertilization at the Black soil site compared with the Dark Brown soil site in both years. Sunola harvest indices were comparable across sites and years, ranging from 0.10 to 0.18. Seed oil concentration averaged about 4% higher at the Black compared with the Dark Brown soil site, and responded to fertilization differently between locations. Sunola water use efficiency was markedly higher in 1994 than in 1993 at both sites, primarily due to lower water consumption by the crop. Fertilizer recommendations can be based on a desired yield goal or marginal rate of return. A minimum acceptable marginal rate of return of 1, 1.5 and 2 times the cost of the fertilizer, indicated that fertilizer N plus soil nitrate-N levels of greater than 94, 66 and 38 kg N ha−1, respectively, were uneconomical at Scott; respective rates at Melfort were 183, 174 and 154 kg N ha−1. However, fertilizer rates in excess of 100 kg N ha−1 in the Black soil climatic zone can result in potentially high levels of residual nitrate-N in the soil profile, and thus have greater potential for N leaching and denitrification losses. Key words:Helianthus annuus L., nitrogen

Nitrogen losses due to denitrification from cattle slurry injected into grassland soil with and without a nitrification inhibitor

Injection of cattle slurry into a grassland soil decreases NH3 volatilisation and increases N utilisation by the sward, but may also increase denitrification losses. Denitrification rates were measured using a soil core incubation technique involving acetylene inhibition, following injection of cattle slurry (67 t ha−1) into a grassland soil. The slurry was injected, either with or without a nitrification inhibitor (DCD), on 8 December 1989. Two-weekly measurements were carried out up to 18 weeks after injection. Compared to the control plot, denitrification rates were significantly higher after slurry injection. Addition of DCD to the slurry almost eliminated this effect. Estimated N-losses during 18 weeks after injection were 0.9 (control), 4.1 (+DCD), and 13.7 (-DCD) kg N ha−1. Denitrification losses were 7% of the injected NH4-N and decreased to 2% of the injected NH4-N when DCD was added. Denitrification could account for about 19% of the difference in apparent recovery of N from slurry injected with and without DCD. The results suggested that considerable amounts of NO3− were lost due to leaching.

Surface application and shallow injection of cattle slurry on grassland: nitrogen losses, herbage yields and nitrogen recoveries

AbstractAn experiment was carried out over 2 years on grass and grass/clover swards in SW England lo compare herbage yields and N recovery following surface application or shallow injection of cattle slurry at three different times of application. In the second year, losses of N via ammonia volatilization, denitrification and nitrate leaching were measured from applications to the grass sward. On the grass sward, there was no significant effect of time or method of application on dry‐matter (DM) yield in the first year, although shallow injection reduced apparent N recovery (ANR) in the herbage by 45% when compared with surface application. In the second year, shallow injection reduced DM yields by 26% and ANR by 48%. On the grass/clover sward, there were no significant effects of time or method of application on DM yields or ANR in either year Inclusion of dicyandiamide (DCD) in the October slurry applications had no significant effect in the first year, but in the second year on the grass sward increased DM yield by 31% and 14% and ANR by 156% and 42% for shallow injection and surface applications respectively. Measurements in the second year on the grass sward showed a reduction in N loss by ammonia volatilization using shallow injection of 40% and 79% for March and June applications respectively. Losses due to denitrification were greatest following October application. Shallow injection increased denitrification losses following March application, but there were no significant differences following October or June applications. N losses due to leaching were small, with no significant difference between treatments. Reasons for the reductions in DM yield and ANR following shallow injection, despite the large reduction in N loss by ammonia volatilization, are discussed.

Macrophytic primary production and nutrient concentrations in a deltaic floodplain marsh of the Lower Paraná River

Biomass changes across an annual cycle were followed at two sampling sites in the floodplain marsh of the Lower Parana River: close to the river-shore and 800 m inside the floodplain marsh, both dominated by Scirpus californicus and Cyperus giganteus. Tidal influence determines a daily exchange of water between the river and the floodplain marsh. Estimated net primary production was higher in the river (2820 against 1770 g m−2). Contents of nitrogen and phosphorus in plant tissue decreased from the river to the floodplain (0.62 to 0.45% N and 0.18 to 0.14% P). In spite of the important water exchange between the river and the floodplain, a decrease in nitrate, oxygen and suspended matter, and an increase in soluble reactive phosphorus in the water were observed from the river towards the floodplain marsh. A primary production gradient exists from the river to the inner floodplain marsh, where production is nitrogen-limited, sustained mainly on nutrients supplied by the river. Floodplain marshes are nitrate sinks, probably through denitrification losses and macrophyte uptake.

Studies on the relationship between slurry pH, volatilization processes and the influence of acidifying additives

The mutual influence of slurry pH and volatilization processes on one hand, and the possibility of N conservation by the use of acidifying additives on the other, were investigated in static incubation experiments. The influence of the NH3 and CO2 volatilizations on slurry pH was studied by selectively supporting one or both processes. The addition of Ca2+ to slurry was compared to that of K+ and H+. The effects of Cl−, SO 4 2− and NO 3 − as corresponding anions of Ca2+ on slurry pH as well as NH3 and N2O emissions were tested. The slurry pH (7.4) increased during incubation. When CO2 volatilization was suppressed, the pH increase was reduced, and NH3 volatilization was cut down by 50%. Ca2+ additions hardly influenced the initial slurry pH, but reduced the pH increases and NH3 losses. Proton addition, in contrast, decreased slurry pH but did not decrease the subsequent pH rise. K+ had no effect on slurry pH and N losses. As compared to CaCl2, CaSO4 showed less effect on slurry pH and N losses. Ca(NO3)2 was nearly as effective as CaCl2 in preventing NH3 volatilization, but caused denitrification losses and elevated N2O production. Titration curves of the different slurry treatments were used to interpret the results of the incubation experiments. In a microplot field experiment the NH3 volatilization and slurry pH after surface application of slurry was measured. The acidifying and N conserving effects of Ca2+ and H+ additions were confirmed.

THE EFFECTS OF SOIL PHYSICAL PROPERTIES AND IRRIGATION METHOD ON DENITRIFICATION

Concern for denitrification loss of plant available N after irrigation led to field and laboratory denitrification studies. The objectives of the studies were to evaluate the effect of sprinkle and flood irrigation on soil physical properties and denitrification losses. Acetylene-inhibition techniques were used to monitor the effects of soil physical properties, such as bulk density and crusting, on field denitrification losses under flood and sprinkle irrigation. In one flood irrigation treatment, polyacrylamide (PAM) was incorporated at the rate of 650 kg ha-1 into a clay loam soil to enhance soil aggregate stability. In additional flood and sprinkle irrigation treatments, PAM was not applied. All treatments were irrigated on a weekly basis with 5 cm of water. Bulk density, soil moisture, and soil crusting were 1.2, 1.5, and 19.2 times higher, respectively, under flood irrigation than under sprinkle irrigation, and infiltration rates were 4.7 times lower under flood irrigation. Soil physical properties of flood-irrigated soils treated with PAM were similar to sprinkle-irrigated soils. Penetrometer measurements of the surface of PAM-treated and sprinkle-treated soils were not significantly different, and bulk density differences were less than 6%. The poor physical properties and wetter conditions of the flood-irrigated plots produced a fourfold increase in total cumulative nitrate losses by denitrification compared with sprinkle-irrigated plots. In field experiments, the highest denitrification rates were from flood-irrigated soils, without PAM, initiated within 24 h after irrigation. Rates decreased rapidly beyond that time, whereas PAM-treated soils evolved high amounts of N2O throughout both field and laboratory experiments. The continuously elevated denitrification activity in PAM-treated soils is believed to be caused more by increased anaerobic microsite activity within PAM-stabilized aggregates than by irrigation method. The regression coefficients of the relationship between percent water-filled pore space (WFPS) and denitrification rate were significant. Soil moisture content was not the only contributing factor to denitrification activity, however, as denitrification from sprinkle-irrigated soils remained significantly lower than from flood-irrigated soils when held at 80% WFPS. These results suggest that soil physical properties, such as structure and aggregate stability, have a large influence on denitrification activity.

Availability to subsequent crops and leaching of nitrogen in 15N-labelled sugarbeet tops and oilseed rape residues

SUMMARYIn autumn 1991, sugarbeet tops (Beta vulgaris L.) and different components of oilseed rape residues (Brassica napus L.), both labelled with 15N, were incorporated into the soil under field conditions at Askov Experimental Station, Denmark, using stainless steel cylinders to contain the treatments. The availability of this labelled N to a subsequent crop was measured, using as test crops autumn-sown rye (Secale cereale L.), wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.). In spring 1992, cylinders with 15 N-residues received NH4NO3 and those without 15NH415NO3. In a parallel experiment, 15N-labelled beet tops were incorporated in lysimeters. A four-course rotation of sugarbeet, spring barley (undersown with perennial ryegrass Lolium perenne L.), perennial ryegrass and winter wheat at two rates of calcium ammonium nitrate (CAN) or animal slurry was grown in these lysimeters. Leaching and the availability of beet top N to successive crops were followed for 2 years. The soil in the cylinders and lysimeters was a light sandy loam (˜ 10% clay).Of the 7·10 g N/m2 added in beet tops, 10–15% was harvested in two subsequent crops of barley and ryegrass and 13–19% was lost by nitrate leaching. Beet top N accounted for 3–7% of the total N offtake in 1992. In 1993 < 1·5% of the total N offtake in ryegrass was from the beet tops applied in 1991. Combining results from mineral fertilized treatments, it was found that 9% of the beet top N was removed in the first cereal crop, 9% was lost by nitrate leaching and 68% remained in the 0–20 cm soil layer (including roots), suggesting that the denitrification loss was < 15%.Incorporation of oilseed rape stubble (1·35 g N/m2), two rates of pods (6·25 and 18·75 g N/m2) or mixed residues (12·25 g N/m2) contributed 0·5, 2·3, 7·4 and 4·6%, respectively, to the total N harvested in the following crop of winter wheat. The percentage of the added labelled N taken up by the wheat ranged from 4·9 to 6·1%, with 60–79% remaining in the 0–20 cm layer after harvest.For beet tops it was calculated that 100 kg N/ha in residues incorporated in the autumn could replace 18 kg N/ha given in the following spring as mineral fertilizer. For oilseed residues, the corresponding average value was 9 kg N/ha.In fertilized cropping systems, oilseed rape residues had minor effects on the subsequent crop, so that an uneven return of residues, as often occurs with combined crops, would do little harm. A considerable proportion of the N applied in sugarbeet tops was lost by leaching and the residual value of the sugarbeet tops to subsequent crops was low.

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N2O) emission from grassland. During cold and dry conditions in early spring, emission of N2O from both ammonium (NH4+) and nitrate (NO3−) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N2O and total denitrification losses from NO3− containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N2O and via denitrification, respectively. Emissions of N2O and total denitrification losses from NH4+ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N2O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N2O emission from AS was much smaller than the effect of using NH4+ fertilizer instead of NO3− fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N2O from NO3− fertilizers but not from NH4+ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N2O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha−1 to 3.1% for a dressing of 300 kg N ha−1. In another experiment, N2O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N2O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N2O emission from intensively managed grasslands.

Influence of source, method of placement and simulated rainfall on the recovery of 15N-labelled fertilizers under zero tillage

The availability of fertilizer N to crop under zero tillage (ZT) versus conventional tillage (CT) is affected by immobilization, ammonia volatilization, denitrification or leaching. This study examined the relative importance of those four factors in influencing the availability of N to the crop and the recovery of applied 15N-labelled N in soil and plants. Field experiments were conducted at two sites in central Alberta to determine the influence of tillage, N source, method of N placement and simulated rainfall on the recovery of 15N-labelled fertilizers in spring barley (Hordeum vulgare L.) plants and soil. A first experiment compared four soil surface broadcast N sources [urea, ammonium sulphate (AS), ammonium nitrate (AN) and potassium nitrate (KNO3)]. The N recovery in barley plants was KNO3> AN > AS > urea. With the exception of KNO3, plant N recovery was greater under CT than under ZT, particularly for urea. Immobilized N in soil was often less with KNO3 than the other N sources. There was apparently virtually no denitrification loss during the one crop season used in these experiments, and the recovery of KNO3 was complete. In a second experiment, methods of placement were evaluated. The recovery of N in plants was lowest for surface broadcast urea under ZT, particularly when no water was added after urea application. Banding urea beside and/or below the seedrow in comparison to broadcasting markedly increased N recovery in plants, especially under ZT. The differences in N recovery between ZT and CT were greater for broadcast application than for band placement. Method of placement had little effect on N immobilization under ZT, but under CT the amount of immobilized N was less with subsurface banding than incorporation. Simulated rainfall immediately after surface broadcast urea application under ZT increased the N recovery in plants. In conclusion, surface broadcast urea under ZT was less efficient than the other N sources and its recovery improved considerably when placed in bands below the soil surface near the seedrow. Key words: Ammonium nitrate, banding, fate of applied N, 15N-labelled fertilizers, N immobilization, N recovery, surface broadcasting, urea

Potential for biological denitrification of fertilizer nitrogen in sugarcane soils

Nitrogen (N) fertilizer is being lost from sugarcane soils following application to the crop. This study was conducted to estimate the quantity of N being lost from the soil through biological denitrification and to determine the proportion of gaseous N being emitted either as N2O or as N2. Field studies were conducted on four different soils (humic gley, alluvial massive earth, red earth and gleyed podzolic), and on different crop management systems, by installing plastic (PVC) cylinders (23.5 cm diam., 25 cm long) in the soil to a depth of 20 cm beside the plant row in a ratoon sugarcane crop. 15N-labelled KNO3 was applied as a band across each cylinder to a depth of 2.5 cm at a rate of 160 kg N/ha. After rainfall or irrigation, the cylinders were capped for 3 h intervals and gas in the headspace sampled in the morning and afternoon, for up to 4 days. Denitrification losses from the humic gley ranged from 247 g N/ha.day for cultivated plots to 1673 g N/ha.day for no-till plots. Over the sampling period, this was equivalent to 3.2% and 19.7% of the N applied, respectively. Nitrous oxide accounted for 46% to 78% of the total N lost. For the alluvial, massive earth and the red earth and gleyed podzolic, losses over the sampling period ranged from 25 to 117 g N/h.day and represented <1% of the N applied. Recovery of 15N in the soil ranged from 67% at the first sampling on the red earth soil to 4.9% at the third sampling on the alluvial, massive earth soil. In a glasshouse study, intact soil cores (23.5 cm diam., 20 cm long), taken from the humic gley and the alluvial, massive earth, were waterlogged after band application of 15N-labelled KNO3 at a rate of 160 kg N/ha. Gas samples from the headspace were taken after 3 h, and then morning and afternoon for the next 14 days. Denitrification losses ranged from 13.2 to 38.6% of N applied with the majority of gaseous N loss occurring as N2. Total recoveries after 14 days, including the evolved gases, ranged from 68.7 to 88.2%. We conclude that denitrification is a major cause of fertilizer N loss from fine-textured soils, with nitrous oxide the major gaseous N product when soil nitrate concentrations are high.

Fate of urine nitrogen on mineral and peat soils in New Zealand

A field lysimeter experiment was conducted over 150 days to examine the fate of synthetic urinary nitrogen (N) applied to peat and mineral soils, with and without a water table. At the start of the winter season, synthetic urine labelled with 15N, was applied at 500 kg N ha−1. Plant uptake, leaching losses and nitrous oxide (N2O) fluxes were monitored. Total plant uptake ranged from 11% to 35% of the urine-N applied depending on soil type and treatment. Plant uptake of applied N was greater in the presence of a water table in the mineral soil. Nitrate-N (NO3 --N) was only detected in leachates from the mineral soil, at concentrations up to 146 μg NO3 --N mL−1. Presence of a water table in the mineral soil reduced leaching losses (as inorganic-N) from 47% to 6%, incrased plant uptake and doubled apparent denitrification losses. In the peat soils leaching losses of applied urine-N as inorganic-N were low (<5%). Losses of N as N2O were greater in the mineral soil than in the peat soils, with losses of 3% and <1% of N applied respectively after 100 days. Apparent denitrification losses far exceeded N2O losses and it is postulated that the difference could be due to dinitrogen (N2) loss and soil entrapment of N2.

Denitrification Losses from Sandy and Sandy Loam Soils under Spring Barley Fertilized with Slurry and Ammonium Nitrate

The interaction between soil factors and denitrification loss from soil is not well understood. This study was conducted to determine whether soil air-filled porosity (AFP), nitrate concentration a...

Fate of nitrogen in pig effluent applied to a shallow stony pasture soil

Abstract A 2‐year lysimeter experiment was conducted to determine the fate of nitrogen (N) in pig effluent applied to a shallow stony pasture soil. In the first year (1991), pig effluent was applied in a single application at three rates of total N (0, 200, and 600 kg N/ha) to 12 monolith lysimeters (500 mm diameter × 450 mm deep) during autumn (March). In the second year of the experiment, the 200 kg N/ha lysimeters received a repeat application using 15N‐labelled effluent. Application at the 200 kg N/ha rate resulted in: 10% of the applied N being lost by ammonia volatilisation; 5% being leached (11 kg N/ha) in the first year and less than 2.5% of applied 15N being leached in the second year; 35% of the applied effluent 15N was recovered by the pasture; and 14% was recovered in the soil plus roots. Denitrification losses (estimated by difference) were high (39%) and were attributed to the soluble carbon content of the effluent, a supply of nitrate from nitrification of slurry N, and transient anaerobic ...

Denitrification in tillage and no tillage pampean soils: Relationships among soil water, available carbon, and nitrate and nitrous oxide production

Abstract The interactions among the denitrification (DN) regulating factors under no tillage (NT) and conventional tillage (CT) during the production of a wheat crop were studied. The usefulness of the water field pore space (WFPS) as an indicator of the magnitude of the nitrous oxide (N2O) losses under the two tillage methods was also analyzed. The N2O production, mineralizable carbon (MC), WFPS, and nitrate (NO3) content were measured in a typic argiudol. The NT system increased N2O losses by DN at the 0–10 cm sampling depth because of its action on the MC and WFPS. Denitrification correlated significantly with the WFPS (r = 0.6, p<0.01), MC (r = 0.69, p<0.01), and NO3 content (r = 0.37, p<0.05). The influence of these factors on denitrification explained 55% of the differences between tillage methods, and as a consequence, we would suggest another possible influence of other factors not considered in this investigation. The addition of fertilizer (urea) would result in an increase in DN losses with nei...

Denitrification losses of N from fresh and composted manures

Denitrification losses of N from fresh and composted manures

Nitrogen Dynamics in Irrigated Corn: Soil‐Plant Nitrogen and Atmospheric Ammonia Transport

AbstractAmmonia transport in cropping systems can affect atmospheric pollution, soil‐plant N relationships, and crop N‐use efficiency. The purpose of this study was to evaluate the N dynamics of a corn (Zea mays L., Pioneer hybrid 3379) cropping system and determine the effect of NH3 transport and crop NH3 compensation point on N relations in the crop. Soil and plant N measurements were made during the growing season along with plant‐atmosphere NH3 transport using flux‐gradient micrometeorological techniques. The crop generally emitted NH3 throughout the season when the atmospheric NH3 concentrations were near background levels. During periods of high atmospheric NH3 concentrations, the crop absorbed significant amounts of NH3 from the atmosphere. Net seasonal NH3 transport was determined to be a small loss, based on the number of measurement periods throughout the growing season; however, N isotope studies in a nearby field estimated significant NH3 losses. Nitrogen isotope studies cannot account for the substitution of 14NH3 absorbed from the atmosphere for volatilized 15NH3. These results indicate that care must be taken when using N isotopes to evaluate N losses from cropping systems when plants are actively growing. Nitrous oxide flux was measured late in the cropping season, but showed small denitrification losses during the measurement period. The crop NH3 compensation point varied during the time of day and with respect to soil‐plant‐atmosphere influence. Net NH3 transport accounted for little of the N lost from this cropping system.

The Use of Sequencing Batch Activated Sludge Reactors to Determine Nitrogen Balances and Optimum Periods of Pre-Aeration Denitrification

Four identical laboratory scale sequencing batch activated sludge plants were used to carry out comparative performance evaluations of nitrification and denitrification, and to obtain an accurate nitrogen balance for the system. In the first instance the plants were run under identical operational conditions to ensure that results were statistically valid. Ten performance parameters were compared and no significant differences at 95% confidence limits were found. A nitrogen mass balance, considering dissolved nitrogen species, waste biomass and denitrification losses during settlement, accounted for 87.4% of known inputs. On the introduction of denitrification periods, nitrate and total nitrogen removal increased with increasing anoxic period up to a maximum of 74% and 71% respectively. The inclusion of anoxic periods reduced total organic carbon (TOC) removal by as much as 19%. These losses are a consequence of maximising nitrate removal when the cycle duration is fixed. This is due to differences in efficiency between aerobic and denitrifying activity. Good linear relationships were shown between % nitrogen removal (r=0.93), effluent nitrate concentration (r=−0.94), T.O.C. removed (r=−0.99) and the ratio of anoxic to aerobic retention times; this was providing the anoxic period was taken to be the period of nitrate removal. These relationships may provide a guide for designing sequencing batch nitrification /denitrification systems. Some enhancement of nitrification was also evident at short denitrification phases.

Nitrogen Loss Inhibitors Evaluated for Humid-Region Wheat Production

Applied N fertilizer can be lost from the crop root zone by several pathways, reducing agronomic efficiency and cansing environmental problems. One approach that has been suggested to reduce N losses is to mix compounds with the N fertilizer that inhibit some steps in the loss pathway. Our objective was to evaluate the effectiveness of five potential inhibitors (ammonium polyphosphate [APP], ammonium thiosulfate [ATS], dicyandiamide [DCD], monoammonium phosphate [MAP], and KCl), singly and in combinations, on yield and N uptake of winter wheat (Triticum aestivum L.) in a humid climate. A total of four experiments were established over 2 yr in which spring N topdress applications were made either as urea-ammonium nitrate (UAN) solution or as UAN solution mixed with inhihitors. All treatments gave a total spring application rate of 60 lb N/acre. Yields exceeded 90 bu/acre in three of the experiments and reached 70 bu/acre in the fourth. Nitrogen rate treatments were also applied in each experiment to determine the N-responsiveness of each site. Large yield responses to applied N occurred at three of the experimental locations. Treatments that reduced fertilizer N losses would thus be expected to translate into increased yield at those three locations. However, none of the inhibitors tested increased yield or N uptake in any experiment despite ideal conditions for N loss (ammonia volatilization the first year, leaching and denitrification loss the second year), good uniformity in our experiments, and our use of eight replications for selected treatments in the second year. In one experiment with 100 bu/acre yields, we could have detected a 3.3 bu/acre yield increase, which would have required only 8 lb/acre additional N to achieve. This implies that no inhibitor reduced N loss by as much as 8 lb/acre in this experiment, which was conducted on a sandy soil and received 8 in. of rain within 4 wk of treatment application. While it is possible that some treatments gave yield increases below our limits of detection, even if this happened it would be cheaper in almost all cases to achieve this yield increase by applying a small amount of additional N than by using an inhibitor

The use of utilized metabolizable energy for grazing livestock to model the flux of nitrogen through Northern Ireland Grassland

A mass balance approach for nitrogen has been applied to Northern Ireland grassland. The nitrogen flux through grazing animals based on utilized metabolizable energy of forage (UME) was employed in regression analysis of historical data to give a measure of both fertilizer utilization and a background contribution which was equated with the net contribution of soil organic N to the inorganic N pool. This rate (52·0 kg N ha −1 ) is balanced by the input of plant dead matter and cattle excreta as dung leading to the conclusion that there is no net organic N accumulation in Northern Ireland grassland. Consideration of all known inputs and outputs of N suggests that 78% of the inputs of N are lost to the atmosphere by either denitrification, ammonia volatilization or unknown loss processes. Of the 22% remaining balance of N, only 9% is available as animal products and the remaining 13% is lost as leachate. There does appear to be scope for modifying present intensive systems to take into account the accumulations of N in the inorganic pool where animal products (14 kg N ha −1 represent only 5% of the available N pool (259 kg N ha −1 ).

Nutrient‐Loss Trends for Vegetable and Citrus Fields in West‐Central Florida: I. Nitrate

AbstractVegetable and citrus production in west‐central Florida is reportedly of considerable eutrophication hazard to local groundwater and surface‐water bodies, including a 33 000‐ha drinking‐water supply reservoir (Lake Manatee) near the coast of the Gulf of Mexico. Nitrate‐N levels were assessed for three vegetable‐production seasons during 1990 and 1991 using a combination of multilevel samplers into the shallow (surficial) aquifer beneath selected vegetable fields and citrus groves, coupled with piezometric wells around each field's periphery to assess direction and rate of groundwater flow. The NO3‐N concentrations beneath vegetable sites showed sizeable spikes (up to 130 mg L−1 NO3‐N), especially early in the season and at season's end, though almost exclusively at &lt;1‐m depth. These concentrations did not persist, however, with the overwhelming majority of samples from most vegetable sites evidencing NO3‐N concentrations below 1 mg L−1. At citrus sites from the same area, NO3‐N concentrations in the surficial water table (located 2–4 m below the soil surface in this case) have evidenced high (20–40 mg NO3‐N L−1) and persistent levels throughout the entire sampling period. This zone of NO‐N enrichment commonly extends 2 to 3 m into the surficial aquifer before NO3‐N concentrations decline to &lt;10 mg L−1 once more. Rates of lateral movement off‐site averaged 20 to 40 m yr−1 for the vegetable sites and 200 to 400 m yr−1 for the more undulating citrus sites. It is postulated that gaseous denitrification losses naturally remediate the periodic spikes in NO3‐N concentrations beneath the vegetable sites because of the high water tables maintained at such locations, whereas denitrification in the deep sands beneath typical citrus sites proceeds much more slowly due to such limitations as a soluble‐C energy source and/or a suitable microbial population. The results have implication with respect to shallow water table vs. deeply rooted crop‐production systems from the same geographic area, wherever such pairings may occur.