15N-labelled Ammonium Nitrate Research Articles

Fertilizer Application Method Provides an Environmental-Friendly Nitrogen Management Option for Sugarcane

PurposeNitrogen fertilizer management is an important agricultural tool that must be optimized to promote sustainable practices since the nitrogen-fertilizer recovery by plants (NRP) is low, leading to nitrogen losses to the environment. In sugarcane, N-fertilization has been investigated over the years but little attention has been given to N-fertilizer application methods. Sugarcane crop production and environmental impact regarding N-fertilizer application methods (i.e., applied onto the sugarcane straw layer and incorporated into the soil) were investigated in the present study aiming to achieve an environmental-friendly cropping system.MethodsSugarcane yield and NRP, N2O emissions, relevant components of the soil microbiological community and N-fertilizer retention in soil layers were quantified. The experiment was carried out in field conditions where N-fertilizer application methods using 15N-labelled ammonium nitrate (15NH415NO3) were compared to a control treatment with no N-fertilization.ResultsIncorporation of N-fertilizer into the soil increased the sugarcane yield by 17% (two-year average) compared to N-fertilizer applied onto the sugarcane straw layer, which was similar to control treatment. There was an increase in NRP-fertilizer of 79% due to the application of N-fertilizer incorporated into the soil. Furthermore, soil incorporation of N-fertilizer decreased N2O emission by 22% with the fertilizer N emission factor reduced four-fold. The N2O emissions were mostly associated with ammonium-oxidizing bacteria (AOB).ConclusionsOur results show that application of N-fertilizer incorporated into the soil is an environmental-friendly N-fertilization management which will improve agricultural sustainability and reduce environmental impacts.Graphical

Uptake of Fertilizer Nitrogen and Soil Nitrogen by Sorghum Sudangrass (Sorghum bicolor × Sorghum sudanense) in a Greenhouse Experiment with 15N-Labelled Ammonium Nitrate

A greenhouse experiment with sorghum sudangrass (Sorghum bicolor × Sorghum sudanense) and maize (Zea mays) was conducted to assess information on differences in their nitrogen and fertilizer utilization when used as energy crops. The aim was to contribute to the scarce data on sorghum sudangrass as an energy crop with regards to nitrogen derived from fertilizer (NdfF) in the plant’s biomass and fertilizer nitrogen utilization (FNU). Sorghum sudangrass and maize were each grown in eight bags of 45 L volume and harvested at maturity after 154 days. Each crop treatment was further divided in a control treatment (four bags each) that did not receive N fertilization and a fertilization treatment (four bags each) that received 1.76 g N, applying a 15N-labelled liquid ammonium nitrate fertilizer. Fertilization took place at the start of the experiment. After harvest, the whole plant was divided in the fractions “aboveground biomass” (ABM) and “stubble + rootstock” (S + R). Weight, N content and 15N content were recorded for each fraction. In addition, N content and 15N content were assessed in the soil before sowing and after harvest. The experiment showed that FNU of sorghum sudangrass (65%) was significantly higher than that of maize (49%). Both crops accumulated more soil N than fertilizer N. The share of fertilizer N on total N uptake was also higher with sorghum sudangrass (NdfF = 38%) compared to maize (NdfF = 34%). The observations made with our control plant (maize), showed that the results are plausible and comparable to other 15N studies on maize regarding yields, NdfF, and FNU, leading to the assumption that results on sorghum sudangrass are plausible as well. We therefore conclude that the results of our study can be used for the preliminary parametrization of sorghum sudangrass in soil organic matter (SOM) balance at field level.

Moisture effects on microbial protein biosynthesis from ammonium and nitrate in an unfertilised grassland

Incorporation of nitrogen (N) into soil microbial protein is central to the soil N cycle to mitigate N losses and support plant N supply. However, the effect of factors, such as water filled pore space (WFPS), which influence inorganic N transformations and losses, and thus microbial incorporation, are only poorly understood. This work aimed to bridge this gap, using compound-specific 15N-stable isotope probing to quantify microbial assimilation into the largest defined soil organic N pool, protein-N. This approach applied differentially 15N-labelled ammonium nitrate (NH4NO3) to an unfertilised UK grassland in a soil mesocosm study over 10 days. The soil microbial community showed a strong preference for NH4+ over NO3−, which varied with WFPS (85% > 55% > 70%). This preference decreased for amino acids further in biosynthetic proximity to the transamination step in amino acid biosynthesis. Combined incorporation of NH4+ and NO3− increased total hydrolysable amino acid-N concentration linked to WFPS (55% ∼ 85% > 70%). Incorporation rates of applied 15N showed the same trend as NH4+ preference with WFPS (85% > 55% > 70%), which is related to microbial activity and nutrient mobility. Despite differences in incorporation, when normalised to soil available N, incorporation was comparable in the short-term. Mechanistic control of WFPS via assimilation into the largest soil organic N pool is important to mitigate potential positive feedbacks to N losses and support N supply to plants.

Urea uptake by spruce tree roots in permafrost-affected soils

Biomass productivity of black spruce trees is strongly limited by soil nitrogen (N) in shallow active layer on permafrost. Trees and mycorrhizal roots are known to absorb amino acids to bypass slow N mineralization in N-limited boreal forest soils. However, amino acid uptake strategy of tree roots cannot fully explain their advantages in competition for soil N with other plants and microbes. We investigated the potentials of tree roots to absorb urea as well as amino acids and inorganic N, using tracer experiments of 13C, 15N-labelled glutamic acid and urea and 15N-labelled ammonium nitrate in black spruce forests growing on permafrost-affected soils with different permafrost table depths in the eastern edge of the Mackenzie river delta (northwest Canada). We demonstrated that black spruce roots have potentials to absorb intact urea but only in soils with shallow permafrost, where urea accumulates due to limited microbial mineralization activity. This contrasts with soils with deep permafrost table, where roots absorb amino acids and inorganic N. Allocation of fine roots to colder subsoil above permafrost provides advantages for trees monopolizing urea-N. Despite lower energy efficiency of urea utilization compared to inorganic N and amino acids, urea uptake is one of N acquisition strategies for spruce growing on N-limited subarctic soil.

Alfalfa Biomass Yield and Nitrogen Fixation in Response to Applied Mineral Nitrogen Under Saline Soil Conditions

Soil salinity is deleterious for the growth and biological N2 fixation of alfalfa (Medicago sativa L.), but nitrogen fertilization has been beneficial in some cases. This study was conducted to investigate the potential benefit of N fertilization on alfalfa growth and N2 fixation under saline conditions. Solutions of sodium chloride (NaCl) and hydrous calcium chloride (CaCl2.2H2O) were added to induce ECe (electrical conductivity of the saturated paste extract) targeted at 0.5, 5, 10, and 15 dS m−1 in a sandy loam soil. 15N-labeled ammonium nitrate (NH4NO3) was added to the soil at 30, 60, and 120 mg N kg−1 soil. Alfalfa shoot and root dry weights, root nodule counts, shoot total nitrogen (N), potassium (K) and sodium (Na+), and the amount of N derived from air (%Ndfa) were determined. Shoot Na concentration increased, and plant biomass, shoot N and K concentration, root nodules, and nitrogen Ndfa decreased as soil salinity increased. At all salinity levels, the highest plant biomass, shoot total N content, and K/Na ratio were recorded at 120 mg N kg−1 soil. Root nodulation and %Ndfa were highest at 60 mg N kg−1 soil under nonsaline conditions while under saline conditions, the highest nodulation and %Ndfa were recorded at 30 mg N kg−1 soil. Plant biomass, nodule number, and %Ndfa were greater in the third cut than in the first cut. Addition of nitrogen fertilizers seemed to have a positive effect in alleviating salt stress in alfalfa and maintaining shoot biomass production, but N fertilization may limit the plant’s ability to fix atmospheric N.

Nitrogen Rate Increase Not Required for No-Till Wheat in Cool and Humid Conditions

An increased nitrogen (N) supply was proposed to avoid grain yield (GY) reductions and successfully implement conservation agriculture (CA). We investigated interactions effects of tillage system and N supply on winter wheat (Triticum aestivum L.) at two sites in the Swiss midlands with no (0 kg N ha−1) and high (150–160 kg N ha−1) N supply using 15N-labelled ammonium nitrate in selected treatments. Wheat’s GY, yield components, N related traits and soil mineral N content (Nmin) under conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) were studied following two preceding crops: oilseed rape (Brassica napus L.) and maize (Zea mays L.). Wheat after oilseed rape had significantly higher GY and biomass than after maize while a yield decrease under NT compared with CT and MT was observed regardless of N supply level. Differences in soil Nmin among tillage systems were seldom found and were inconsistent. No differences in 15N fertilizer recovery were found between NT and CT while residual Nmin after harvest was lower under NT than CT or MT. In conclusion, we did not found consistent reductions in soil N availability and N uptake under NT that would justify an increased N supply for wheat under CA.

“Preferential” ammonium uptake by sugarcane does not increase the 15N recovery of fertilizer sources

As ammonium (NH4+) has been reported as the “preferred” mineral N source for sugarcane (Saccharum spp.), the predominant supply of NH4+ has been hypothesized as a means of increasing the 15N recovery and crop yield. We aimed to evaluate the 15N recovery by sugarcane as affected by 15N-NH4+ and 15N-NO3− addition. Under field conditions, 15N-labeled ammonium nitrate [labeling as 15NH4NO3 (15N-NH4+) and NH415NO3 (15N-NO3−)] and urea were surface-applied at a rate of 100 kg N ha−1. Under controlled conditions, sugarcane was grown in hydroponic medium (containing unlabeled N) until the maximum N demand period. Subsequently, plants received 15N-NH4+ or 15N-NO3− (as ammonium nitrate). Plants were then sampled at 1, 3, 7, and 15 d after fertilization (DAF) to estimate 15N recovery. Under field conditions, the 15N recovery for urea was lower than that for ammonium nitrate, partly explained by ammonia volatilization, but there was no difference in N use efficiency between the two fertilizer N sources, as well as between 15N-NH4+ and 15N-NO3− supply. Under controlled conditions, however, 15N-NH4+ resulted in higher 15N recovery than 15N-NO3− until 7 DAF. Nevertheless, as 15N-NH4+ became depleted in the nutrient solution, 15N-NO3− was taken up more intensely by the crop. At 15 DAF, highest 15N recovery was detected in the roots and whole plant under 15N-NO3− supply. Although uptake of NH4+ is faster than NO3−, the so-called “preference” for NH4+ by the crop did not necessarily translate into higher 15N recovery. Therefore, we suggest that the term “preference” should be used with caution to avoid misinterpretation.

Fate of 15N-labelled ammonium nitrate with or without the new nitrification inhibitor DMPSA in an irrigated maize crop

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N2O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using 15N single-labelled ammonium nitrate (15NH4NO3 or NH415NO3) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize (Zea mays L.) crop. Gaseous fluxes, i.e. N2O, 15N2O, 15N2, methane (CH4), and carbon dioxide (CO2) were measured, as well as soil mineral N, yield components and 15N recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N2O emissions (118 g N ha−1) compared to ammonium nitrate without inhibitor (231 g N ha−1). The 15N analyses revealed that most N2O losses (particularly during the emission peak) came from 15NH4NO3 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than NH415NO3 in this calcareous low C-content soil. As expected, DMPSA decreased N2O losses from 15NH4+ oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N2O losses coming from 15NO3−. No significant effect of DMPSA on CH4 oxidation or 15N2 fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15–30 cm and 30–45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N2O emissions from irrigated crops in semi-arid areas.

Periphyton support for littoral secondary production in a highly humic boreal lake

AbstractSteep stratification and poor light penetration in highly humic lakes typically restrict oxygenated littoral areas to narrow lake margins. However, in some instances, surrounding floating vegetation mats can sustain highly productive periphyton and more diverse invertebrate communities than pelagic areas. Little is known about how these littoral food webs function or the extent to which the pelagic and littoral food webs are coupled. We added 15N-labeled ammonium nitrate to the floating moss mat surrounding the littoral zone of Mekkojarvi, a small highly humic and fishless lake in southern Finland. Our goal was to increase the δ15N values of periphyton to investigate the diets of littoral invertebrates and possible pelagic–littoral coupling in the lake. We divided the lake in 2 with a plastic curtain and added European Perch (Perca fluviatilis) to 1 basin while the other remained fishless. δ15N of periphyton and most littoral invertebrates increased well above the natural abundance levels. δ15N of...

N use efficiencies and N2O emissions in two contrasting, biochar amended soils under winter wheat—cover crop—sorghum rotation

Biochar, a carbon-rich, porous pyrolysis product of organic residues, is evaluated as an option to tackle major problems of the global food system. Applied to soil, biochar can sequester carbon and have beneficial effects on nitrogen (N) cycling, thereby enhancing crop yields and reducing nitrous oxide (N2O) emissions. There is little understanding of the underlying mechanisms, but many experiments indicated increased yields and manifold changes in N transformation, suggesting an increase in N use efficiency. Biochar’s effects can be positive in extensively managed tropical agriculture, however less is known about its use in temperate soils with intensive fertilisation. We tested the effect of slow pyrolysis wood chip biochar on N use efficiency, crop yields and N2O emissions in a lysimeter system with two soil types (sandy loamy Cambisol and silty loamy Luvisol) in a winter wheat—cover crop—sorghum rotation. 15N-labelled ammonium nitrate fertiliser (170 kg N ha−1 in 3 doses, 10% 15N) was applied to the first crop to monitor its fate in three ecosystem components (plants, soil, leachate). Green rye was sown as cover crop to keep the first year’s fertiliser N for the second year’s sorghum crop (fertilised with 110 kg N ha−1 in two doses and natural abundance 15N). We observed no effects of biochar on N fertiliser use efficiency, yield or N uptake for any crop. Biochar reduced leaching by 43 ± 19% but only towards the end of the experiment with leaching losses being generally low. For both soils N2O emissions were reduced by 15 ± 4% with biochar compared to the control treatments. Our results indicate that application of the chosen biochar induces environmental benefits in terms of N2O emission and N leaching but does not substantially affect the overall N cycle and hence crop performance in the analyzed temperate crop rotation.

Recovery of15N Labeled Fertilizer byVitis viniferaL. cv. Cabernet Sauvignon: Effects of N Fertilizer Rates and Applied Water Amounts

A study was conducted to investigate the interaction of N fertilization rates and different irrigation amounts on N fertilizer recovery efficiency (REN) of Cabernet Sauvignon in a commercial vineyard near Oakville, California. The fertilizer treatments consisted of a control (no fertilizer) and two 15N labeled ammonium nitrate fertilizer applications (6.5 and 13.0 g N/vine). The N fertilizer was applied two weeks before anthesis. The irrigation treatments were various fractions (0.25, 0.5, and 1.0) of estimated vineyard water use (ETc), with the applied water amount for the 1.0 irrigation treatment from 7 Apr to 9 Sept equivalent to 312 mm. Midday leaf water potential (Ψl) was measured throughout the season to monitor vine water status. The labeled N fertilizer was detected in the petioles and leaf blades two weeks after application. Irrigation and fertilization treatments significantly affected midday Ψl. There were significant differences in aboveground vine biomass among the treatments. In general, those supplied with more water and/or N fertilizer had greater biomass compared with nonfertilized vines under deficit irrigation. Vine uptake of the 15N labeled fertilizer increased with increasing fertilization rates and irrigation amounts. The REN was significantly different between the two 15N fertilizer treatments (29% for 6.5 g N/vine and 24% for 13.0 g N/vine). REN at harvest for the 0.25, 0.5, and 1.0 ETc irrigation treatments was ~24, 28, and 27%, respectively, although not significantly different. The data indicate that fertilizer amount had a significant effect on REN under the conditions of this study and that irrigation rates at full ETc tended to increase REN in this vineyard when compared with deficit irrigation.

Effects of Irrigation Water Quality and Nitrogen Rate on the Recovery of 15N Fertilizer by Sorghum in Field Study

The effects of 15N-labeled ammonium nitrate on yield, uptake of nitrogen (N) by sorghum (Sorghum sudanense, Piper), and on N remaining in the soil were studied in a field experiment with different N rates (0, 50, and 100 kg N ha−1) and with two irrigation water qualities, well water (WW) and treated wastewater (TWW). Treated wastewater irrigation increased dry matter and N yield compared to WW. At equal N rates, recovery of 15N-labeled fertilizer by plants increased with TWW irrigation compared to WW (36% versus 23%). Neither fertilizer rate nor water quality had an effect on the 15N-labeled fertilizer remaining in the 0- to 60-cm layer of soil. On average 41% in the TWW treatment (49–33%) and 38% in WW treatment was mostly present in the surface 20-cm layer. Losses of 15N-labeled fertilizer were unaffected by irrigation water quality (35%) and increased with N application rate in TWW (4% versus 31%).

Use of the 15 N gas flux method to measure the source and level of N2O and N2 emissions from grazed grassland

Understanding the contribution of nitrification and denitrification to production of nitrous oxide (N2O), a potent greenhouse gas, is important in devising effective mitigation strategies to reduce emissions. In this study the 15N gas flux method was used to investigate N2O and N2 emissions following an application of 15N labelled ammonium nitrate (0.71 mol N m−2) to intensive grassland swards (grazed at 2.74 or 2.05 livestock units ha−1 year−1) at a site in Southern Ireland. The 15N labelled fertiliser (NO3 moiety 15N labelled at 60 at. %) was applied to designated soil areas in the field, enclosed by static chambers, in June 2009, September 2009 and March 2010. Fluxes of N2O and N2 were determined over 12 days on each occasion. N2O and N2 emissions were significantly (P < 0.001) lower in March 2010 than in June or September 2009. There was little difference between the two swards grazed at different stocking rates on N2O or N2 emissions. Mean cumulative N2O emissions over 103 h were 212.9, 279.5 and 62.06 mg m−2 for June 2009, September 2009 and March 2010, respectively. Mean cumulative N2 emissions for the three time periods were 818.8, 893.8 and 87 mg m−2, respectively. The N2O mole fraction averaged 0.21 and 0.23 in June 2009 and September 2009, respectively, but increased to 0.41 in March 2010 which may have been due to changes in denitrifier community composition or due to N2O reductase being sensitive to low soil temperatures. The results point to denitrification of nitrate as the major source of N2O at this site which may have implications for choice of fertiliser in moist temperate climates.

Fertilizer Nitrogen Timing and Uptake Efficiency of Hybrid Hazelnuts in the Upper Midwest, USA

Hybrids of Corylus avellana, C. americana and C. cornuta, are being developed as a potential crop for the Upper Midwest of the United States, but little is known about fertilizer nitrogen (N) management. We hypothesized that N application when the bushes were most fully leafed out would result in highest N uptake efficiency (NUE). We used 15N-labeled ammonium nitrate to measure NUE from soil applications in mid-April, late April, late May, early August, and mid-September. Nitrogen applied in either mid- or late April never comprised more than 5% of the total N in shoots or leaves, suggesting that N used for early leaf expansion came primarily from stored reserves. Applications made after April demonstrated that N was quickly translocated to rapidly growing plant parts: May applications comprised 9% of the N in leaves collected in July; August applications comprised 12% of the N in nut kernels collected in September; and September applications comprised 9% of N in catkins collected in October. Nitrogen applied in August and September appeared in new shoots the following April at higher levels than it did aboveground the previous October, showing that N applied late in the season may be stored belowground over the winter. NUE was highest for August and September applications at one site and August and mid-April applications at the other, implying that summer is generally the best time to apply N for most efficient uptake. However, overall NUE was low, only 5% for August applications, suggesting a need to develop other methods of improving NUE in hybrid hazelnuts.

Fertilizer 15N recovery in cereal crops and soil under shallow tillage

We examined the effect of tillage system on crop recovery of soil and fertilizer N. 15N-labelled ammonium nitrate was applied in the spring to small confined plots at five experimental sites where shallow tillage (ST) and mouldboard ploughing (MP) had been practised for up to 36 years. Winter wheat was grown on three sites and spring barley on the other two. Grain yields were similar under ST and MP at all sites, and total N uptake was similar under MP and ST at four of the five sites. Grain and straw recovered 59–64% of the applied 15N fertilizer except at the most sandy site where crop recovery was 37% (ST) and 44% (MP). At two sites, crop 15N recovery was lower under ST despite similar total N uptake for the two tillage systems. Recovery of 15N in crop and soil was 74–75% at the most sandy site and 86–92% at the other sites. Conversion from MP to ST tillage appeared to have little influence on the fertilizer N balance at each site.

Nutrient foraging via physiological and morphological plasticity in three plant species

Physiological and morphological plasticity of roots enhance plant nutrient uptake in spatiotemporally heterogeneous soil environments. We examined these two types of plasticity using three plant species (Solidago altissima (L.) Raf., Pinus taeda L., and Liquidambar styraciflua L.). We grew plants in pots (one plant per pot) with equal quantity of fertilizer applied either evenly over the pot surface (H) or on one-quarter of the pot surface (T). A high-concentration 15N-labeled ammonium nitrate solution was injected twice over 48 h before harvest at a random location in H pots, and in either unfertilized or fertilized portion of T pots. Physiological plasticity of N uptake was observed in S. altissima and L. styraciflua. The highest 15N uptake rate for L. styraciflua occurred in H pots (medium level), and that for S. altissima occurred in fertilized portions of T pots (rich level). When low-concentration 15N was added to S. altissima, no differences in uptake were noted among treatments, possibly because of interroot competition. In S. altissima and P. taeda, either morphological or physiological plasticity was strong. In L. styraciflua, both types of plasticity were strong. Total 15N uptake was enhanced when 15N was added to the fertilized patches. Physiological plasticity contributed &gt;70% of enhanced 15N uptake in S. altissima and L. styraciflua.

Plant‐mediated nitrous oxide emissions from beech (Fagus sylvatica) leaves

Nitrous oxide (N2O) emission estimates from forest ecosystems are based currently on emission measurements using soil enclosures. Such enclosures exclude emissions via tall plants and trees and may therefore underestimate the whole-ecosystem N2O emissions. Here, we measured plant-mediated N2O emissions from the leaves of potted beech (Fagus sylvatica) seedlings after fertilizing the soil with 15N-labelled ammonium nitrate (15NH4(15)NO3), and after exposing the roots to elevated concentrations of N2O. Ammonium nitrate fertilization induced N2O + 15N2O emissions from beech leaves. Likewise, the foliage emitted N2O after beech roots were exposed to elevated concentrations of N2O. The average N2O emissions from the fertilization and the root exposure experiments were 0.4 and 2.0 microg N m(-2) leaf area h(-1), respectively. Higher than ambient atmospheric concentrations of N2O in the leaves of the forest trees indicate a potential for canopy N2O emissions in the forest. Our experiments demonstrate the existence of a previously overlooked pathway of N2O to the atmosphere in forest ecosystems, and bring about a need to investigate the magnitude of this phenomenon at larger scales.

Accumulation of Pollutant Nitrogen in Calcareous and Acidic Grasslands: Evidence from N Flux and 15N Tracer Studies

Semi-natural calcareous and acidic grasslands are known to be sensitive to increased atmospheric N deposition. However, the fate of pollutant N within these systems is unknown. This paper reports on the first studies to determine the fate of added N within a calcareous and an acidic grassland subject to long-term simulated enhanced N deposition. Intact soil/turf cores were removed from field plots treated for six years with enhanced N deposition (ambient +0,+35 and +140 kg N ha−1 year−1). Cores were inserted into lysimeters and output fluxes of N were monitored in detail. Complete N budgets - calculated from the N flux data - showed considerable accumulation of N within the treated grasslands, up to 76% and 38% of pollutantNin the calcareous and acidic grasslands respectively. In the second study, the short-term (21 day) fate of pollutant N was determined by tracing 15N labelled ammonium nitrate (+35 kg N ha−1 year−1) though the acidic and calcareous lysimeters into plant, soil and leachate pools. Up to 91% and 59% of 15N was recovered in soils and vegetation of the calcareous and acidic grasslands respectively, with negligible amounts recovered in soil extractable ammonium and nitrate (<0.3%) and in leachate (<0.02%). This rapid short-term immobilisation of pollutant N supports the long-term accumulation of the element calculated from the N flux study.

Ammonium and nitrate uptake by leaves of the seagrass Thalassia testudinum: impact of hydrodynamic regime and epiphyte cover on uptake rates

Seagrasses rely on the uptake of dissolved inorganic nitrogen (DIN) from both sediment pore water and the water column for metabolic processes. Rates at which their leaves remove nutrients from the water column may be influenced by physiological factors, such as enzyme kinetics, and physical factors, including water flow and the presence of epiphytes on the leaf surface. While there is some evidence of the individual effects of these factors on uptake rates for individual plants, there is little information on the effects of these factors on seagrasses that are situated in their natural environment. In order to isolate the combined effects of water flow and epiphyte cover on uptake rates for Thalassia testudinum leaves while they were situated in a natural canopy we applied 15N-labeled ammonium and 15N-labeled nitrate in a series of field flume experiments. Hydrodynamic parameters related to thickness of diffusive boundary layers, including bottom shear stress and the rate of turbulent energy dissipation, were estimated from velocity profiles collected with an acoustic Doppler velocimeter. Rates of NH 4 + uptake for leaves with and without epiphyte cover were proportional to bottom shear stress and energy dissipation rate, while rates of NO 3 − uptake were not. For epiphytes, rates of both NH 4 + and NO 3 − uptake were dependent on hydrodynamic parameters. Epiphytes covering the leaf surface reduced rates of NH 4 + uptake for seagrass leaves by an amount proportional to the spatial area covered by the epiphytes (∼90%) and although epiphytes reduced NO 3 − uptake rates, the amount was not proportional to the extent of epiphyte cover. Results suggest that the rate at which seagrass leaves removed ammonium was limited by the rate of delivery to the surface of the leaves and was greatly reduced due to blockage of active uptake sites by epiphytes. Conversely, rates of nitrate uptake for the seagrass leaves were limited by the rate at which the leaves could process nitrate rather than the rate of delivery. Our findings quantitatively demonstrate the potential impact of hydrodynamic regime and epiphyte cover on rates of DIN uptake by T. testudinum leaves and how the importance of these factors in affecting uptake rates can vary depending on the form of DIN being assimilated.

Dry matter production and 15N recovery in spring wheat as affected by placement geometry of the fertilizer band

The growth and nitrogen (N) uptake of spring wheat as affected by placement geometry was studied in a pot experiment. The pots were covered by a mobile shelter to prevent disturbance from natural precipitation. The 15N-labeled ammonium nitrate was applied in a band parallel to a single row of the growing crop. The fertilizer was banded in 12 combinations of depth (1, 5, or 10 cm) and distance from the crop row (1, 5, 10, or 15 cm), or broadspread on the soil surface. Plants were sampled at the elongation phase (5 treatments only) and at ear emergence (all treatments) and dry matter, apparent N recovery and 15N recovery in the crop were measured. In addition, the number of shoots and a vegetation index were measured on the first sampling date, and the numbers of ears were counted on the second sampling date. Regression analysis by distance from the crop row and depth from soil surface were carried out. There was a negative effect of increased distance and a positive effect of increased depth for all parameters. The effects were more pronounced when the parameters were measured in the elongation phase. The distance by depth interaction was insignificant for parameters measured on the second sampling date, but the estimated negative interaction was significant for the number of shoots and the simple vegetation index measured on the first sampling date. Deeper placement of the N fertilizer band was not able to mitigate the negative effect of increased distance for the measured parameters. The distance from crop row should not exceed 6–7 cm and the depth below soil surface should exceed 4–5 cm to ensure a positive effect of N placement under adverse conditions, such as low precipitation.