15N abundance of soils and plants along an experimentally induced forest nitrogen supply gradient

¹⁵N abundances of soils and a grass species (Deschampsia flexuosa (L.) Trin.) were analysed in a forest fertilization experiment 10 years after the last fertilization. Nitrogen had been given as urea, at seven doses, ranging from 0 to 2400 kg N ha^-1. Previously, we have shown that plants in systems experiencing large losses of N become enriched with ¹⁵N. This was explained by the fact that processes leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrification and denitrification itself, tend to fractionate against ¹⁵N. In this experiment, ¹⁵N abundance increased with dose of N applied in both grass and soil total-N, but more so in the grass. This was interpreted to be due to the grass sampling small but active pools of N subject to losses. In contrast, soil total-N largely consists of inactive N that does not immediately exchange with pools of N from which fractionating losses occur. Hence, soil total-N shows a large pretreatment ¹⁵N memory effect, and is, therefore, and integrator of the long-term N balance. When short-term changes (years, decades) in N balances are monitored using variations in ¹⁵N abundance, plants are more suitable indicators of such change than is soil total-N.     

Denitrification and total N losses from an irrigated sandy-clay loam under maize–wheat cropping system

Denitrification and total N losses were quantified from an irrigated field cropped to maize and wheat, each receiving urea at 100 kg N ha^-1. During the maize growing season (60 days), the denitrification loss measured directly by acetylene inhibition-soil cover method amounted 2.72 kg N ha^-1 whereas total N loss measured by ¹⁵N balance was 39 kg ha^-1. Most (87%) of the denitrification loss under maize occurred during the first two irrigation cycles. During the wheat growing season (150 days), the denitrification loss directly measured by acetylene inhibition-soil cover and acetylene inhibition-soil core methods was 1.14 and 3.39 kg N ha^-1, respectively in contrast to 33 kg N ha^-1 loss measured by ¹⁵N balance. Most (70-88%) of the denitrification loss under wheat occurred during the first three irrigation cycles. Soil moisture and NO ₃ ^- -N were the major factors limiting denitrification under both crops. Higher N losses measured by ¹⁵N balance than C₂H₂ inhibition method were perhaps due to underestimation of denitrification by C₂H₂ inhibition method and losses other than denitrification, most probably NH₃ volatilization.     

Transfer of nitrogen from damaged roots of white clover (Trifolium repens L.) to closely associated roots of intact perennial ryegrass (Lolium perenne L)

An experiment is described in which the magnitude of N transferred from damaged white clover roots to perennial ryegrass was determined, using ¹⁵N labelling of the grass plant. There was no effect on the growth and N-fixation of the clover plants after removing part of the root system. The ¹⁵N data suggested that N had been acquired by all grass plants, even in plants grown alone with no further N supplied after labelling. However, after quantifying the mobile and stored N pools of the grass plants it was evident that significant transfer of N from clover to grass only took place from damaged clover roots. Dilution of the atom% ¹⁵N in the roots of the grass plants grown alone, and in association with undamaged clover roots, was explained by remobilisation of N within the plant.     

The significance of denitrification of applied nitrogen in fallow and cropped rice soils under different flooding regimes I. Greenhouse experimènts

The role of nitrification-denitrification in the loss of nitrogen from urea applied to puddled soils planted to rice and subjected to continuous and intermittent flooding was evaluated in three greenhouse pot studies. The loss of N via denitrification was estimated indirectly using the¹⁵N balance, after either first accounting for NH₃ volatilization or by analyzing the¹⁵N balance immediately before and after the soil was dried and reflooded. When urea was broadcast and incorporated the loss of¹⁵N from the soil-plant systems depended on the soil, being about 20%–25% for the silt loams and only 10%–12% for the clay. Ammonia volatilization accounted for an average 20% of the N applied in the silt loam. Denitrification losses could not account for more than 10% of the applied N in any of the continuously flooded soil-plant systems under study and were most likely less than 5%. Intermittent flooding of soil planted to rice did not increase the loss of N. Denitrification appeared to be an important loss mechanism in continuously flooded fallow soils, accounting for the loss of approximately 40% of the applied¹⁵N. Loss of¹⁵N was not appreciably enhanced in fallow soils undergoing intermittent flooding. Apparently, nitrate formed in oxidized zones in the soil was readily denitrified in the absence of plant roots. Extensive loss (66%) of¹⁵N-labeled nitrate was obtained when 100 mg/pot of nitrate-N was applied to the surface of nonflooded soil prior to reflooding. This result suggests that rice plants may not compete effectively with denitrifiers if large quantities of nitrate were to accumulate during intermittent dry periods.     

Nitrogen mineralization and denitrification as influenced by crop residue particle size

Managing the crop residue particle size has the potential to affect N conservation in agricultural systems. We investigated the influence of barley (Hordeum vulgare) and pea (Pisum sativum) crop residue particle size on N mineralization and denitrification in two laboratory experiments. Experiment 1: ¹⁵N-labelled ground (3 mm) and cut (25 mm) barley residue, and microcrystalline cellulose+glucose were mixed into a sandy loam soil with additional inorganic N. Experiment 2: inorganic¹⁵ N and C₂H₂ were added to soils with barley and pea material after 3, 26, and 109 days for measuring gross N mineralization and denitrification.Net N immobilization over 60 days in Experiment 1 cumulated to 63 mg N kg^-1 soil (ground barley), 42 (cut barley), and 122 (cellulose+glucose). More N was seemingly net mineralized from ground barley (3.3 mg N kg^-1 soil) than from cut barley (2.7 mg N kg^-1 soil). Microbial biomass peaked at day 4 with the barley treatments and at day 14 with the cellulose+glucose whereafter the biomass leveled out at values 79 mg C kg^-1 (ground), 104 (cut), and 242 (cellulose+glucose) higher than for the control soil. Microbial growth yields were similar for the two barley treatments, ca. 60 mg C g^-1 substrate C added, which was lower than the 142 mg C g^-1 C added with cellulose+glucose. This suggests that the 75% (w/w) holocelluloses and sugars contained with the barley material remained physically protected despite grinding. In Experiment 2 gross mineralization on day 3 was 4.8 mg N kg^-1 d^-1 with ground pea, twice as much as for all other treatments. On day 26 the treatment with ground barley had the greatest gross N mineralization. In static cores ground barley denitrified 11-fold more than did cut barley, whereas denitrification was similar for the two pea treatments. In suspensions denitrification was similar for the two treatments both with barley and pea residue.We conclude that the higher microbial activity associated with the initial decomposition of ground plant material is due to a more intimate plant residue-soil contact. On the long term, grinding the plant residues has no significant effect on N dynamics.     

Measurements of genetic variability for symbiotic dinitrogen fixation in fieldgrwon fababean (Vicia faba L.) using a low level15N-tracer technique

Before starting a breeding program aimed at improving the nitrogen nutrition ofVicia faba, the authors tried an alternative technique to the acetylene reduction assay, to measure some genetic variability in the plant material. The quantity of dinitrogen fixed by several cultivars ofVicia faba was estimated using a low enrichment¹⁵N tracer method and high precision¹⁵N mass spectrometry. The fababeans were cultivated for two years in two different soils. The percentage of fixed dinitrogen in the seed varied between genotypes from 40 to 83% of the total nitrogen and was positively correlated with the total seed nitrogen (r=0.64 to 0.86). A highly significant positive correlation was also found between the total seed nitrogen and the quantity of fixed dinitrogen in the seed (r=0.95 to 0.99). The technique used to measure dinitrogen fixation proved to be useful and reliable enough to discriminate between various genotypes, grown over a period of two years in two different soils. However, several non-fixing control plants showed significant differences in their¹⁵N enrichment and the problem of choosing a good reference plant was raised and discussed.     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH₄ ⁺-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from ¹⁵N balance measurements using forced air exchange (airflow rate: 20 L min^-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least ¹⁵N loss and vice-versa (r=0.99^**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83^**). The association between potential and actual N loss was closer for high-CEC soils ( 20 cmol [+] kg^-1 soil, r=0.91^**) than for low-CEC soils (<20 cmol [+] kg^-1 soil, r=0.85^**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.     

15N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.)

The ¹⁵N isotope dilution technique and the N difference method were used to estimate N₂ fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using ¹⁵N isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha^-1. The grass in the mixture had a lower atom %¹⁵N content and a higher N content than grass in a monoculture; therefore values for N₂ fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom %¹⁵N contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N₂ fixation.     

Turnover of residual 15N-labelled fertilizer N in soil following harvest of oilseed rape (t Brassica napus L.)

We studied the fate of ¹⁵N-labelled fertilizer nitrogen in a sandy loam soil after harvest of winter oilseed rape (Brassica napus L. cv. Ceres) given 100 or 200 kg N ha^-1 in spring, with or without irrigation. Our main objective was to quantify the temporal variations of the soil mineral N, the extractable soil organic N and soil microbial biomass N, and fertilizer derived N in these pools during autumn and winter. Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irrigated treatment. However, only in the latter treatment did we find significantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the first sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested plants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0–5 cm depth, but from the 5–15 cm depth a marked proportion of N derived from fertilizer was lost, probably by leaching. Negligible amounts of fertilizer derived extractable soil organic and mineral N (<1 kg N ha^-1, 0-15 cm) were found in all treatments after the first sampling.Soil microbial biomass N was not significantly affected by treatments and showed only small temporal variability (±11% of the mean 76 kg N ha^-1, 0- 15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was significantly affected by the treatments, with the extremes being 5.5 and 3.1 kg N ha^-1 in the 200N, non-irrigated and 100N, irrigated treatments, respectively. Also, the estimated exponential decay rate of microbial biomass N derived from fertilizer, differed greatly (2 fold) between these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In studies utilising ¹⁵N labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, because it may question the assumption that turnover rates are not affected by the insertion of the label.     

Variability in nitrogen fixation of common bean (Phaseolus vulgaris L.) intercropped with maize

Thirty one selected bean lines were evaluated in the field for ability to support N₂ fixation when intercropped with maize which received 0, 30 and 60 kg N ha^–1 as ammonium sulphate. The amount of fixed N₂ was estimated using the natural variation of ¹⁵N and wheat as the standard non-fixing crop. Nitrogen as low as 15 kg N ha^–1 at sowing suppressed nodule weight and activity (acetylene reduction activity) but not nodule number, suggesting that the main effect of mineral N was on nodule development and function. ¹⁵N data revealed a high potential of the bean genotypes to fix N₂, with the most promising ones averaging between 50–60% of seed N coming from fixation. Bean lines CNF-480, Puebla-152, Mexico-309, Negro Argel, CNF-178, Venezuela-350 and WBR22-3, WBR22-50 and WBR22-55 were ranked as good fixers.     

Measurement of N2-fixation in field-grown pigeonpea [Cajanus cajan (L.) Millsp.] using15N-labelled fertilizer

Two experiments were carried out from 1981 to 1983 in Vertisol field at ICRISAT Center, Patancheru, India to measure N₂-fixation of pigeonpea [Cajanus cajan (L.) Millsp.] using the¹⁵N isotope dilution technique. One experiment examined the effect of control of a nodule-eating insect on fixation while another in vestigated the effect of intercroping with cereals on fixation and the residual effect of pigeonpea on a succeeding cereal crop. Although both experiments indicated that at least 88% of the N in pigeonpea was fixed from the atmosphere, one result is considered fortuitous in view of the differential rates of growth of the legume and the control, sorghum [Sorghum bicolor (L.) Moench]. The difference method of calculation in dieated negative fixation and the results emphasized the problem of finding a suitable nonfixing control. In a second experiment, when all plants were confined to a known volume of soil to which¹⁵N fertilizer was added in the field, these problems were overcome, and isotope dilution and difference methods gave similar results of N₂-fixation of about 90%. In intercropped pigeonpea 96% of the total N was derived from the atmosphere. This estimate might be an artifact. There was no evidence of benefit from N fixed by pigeonpea to intercropped sorghum plants. Plant tissue¹⁵N enrichments of cereal crops grown after pigeonpea indicated that the cereal derived some N fixed by the previous pigeonpea. Thus residual benefits to cereals are not only an effect of ‘sparing’ of soil N.     

Denitrification measurements in aquatic sediments: A comparison of three methods

Measurements of denitrification using the acetylene inhibition,¹⁵N isotope tracer, and N₂ flux methods were carried out concurrently using sediment cores from Vilhelmsborg sø, Denmark, in an attempt to clarify some of the limitations of each technique. Three experimental treatments of overlying water were used: control, nitrate enriched, and ammonia enriched water. The N₂ flux and¹⁵N tracer experiments showed high rates of coupled nitrification/denitrification in the sediments. The acetylene inhibition method did not capture any coupled nitrification/denitrification. This could be explained by acetylene inhibition of nitrification. A combined¹⁵N tracer/acetylene inhibition experiment demonstrated that acetylene inhibition of N₂O reduction was incomplete and the method, therefore, only measured approximately 50% of the denitrification due to nitrate from the overlying water. Similar rates of denitrification due to nitrate in the overlying water were measured by the N₂ flux method and the acetylene inhibition method, after correcting for the 50% efficiency of acetylene inhibition. Rates of denitrification due to nitrate from the overlying water measured by the¹⁵N tracer method, however, were only approximately 35% or less of those measured by the acetylene inhibition or N₂ flux methods.     

Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N₂+N₂O)−¹⁵N from 98 atom %¹⁵N-labeled urea broadcast at 29 kg N ha⁻¹ into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N₂+N₂O)−¹⁵N during the 19 days following urea application never exceeded 28 g N ha⁻¹ day⁻¹. The total recovery of (N₂+N₂O)−¹⁵N evolved from the field was only 0.51% of the applied N, whereas total gaseous¹⁵N loss estimated from unrecovered¹⁵N in the¹⁵N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m⁻³ or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha⁻¹, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N₂+N₂O)−¹⁵N and total¹⁵N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N₂+N₂O)−¹⁵N and total¹⁵N loss from 27 kg applied nitrate-N ha⁻¹ were 20% and 53% during the same 19-day period. The failure of directly-recovered (N₂+N₂O)−¹⁵N to match total¹⁵N loss from added nitrate-¹⁵N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N₂+N₂O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g⁻¹ in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.     

Fate of nitrogen applied as Azolla and blue-green algae (Cyanobacteria) in waterlogged rice soils—a15N tracer study

Summary ¹⁵N tracer was used to detect the extent to which nitrogen of appliedAzolla caroliniana, Anabaena variabilis andNostoc muscorum was available for assimilation by the growing rice plants in pots under 4 cm flood water for 60 days. The rate of release of nitrogen from the above biofertilizers, the amount of nitrogen remaining in the soils and the amount that was lost from the soils during this period were also examined. Previously¹⁵N-labelled biomass of Azolla, Anabaena and Nostoc to provide 40 mg N was mixed thoroughly with 0.5 kg silt loam Bangladesh soil (Sonatola series) in each of three pots used for a single treatment. Each pot received four 16 days old IR8 rice seedlings. A parallet set of experiments was conducted without rice plants.It was found that nitrogen uptake in the rice plants was increased by 91, 176 and 215% on using Azolla, Anabaena and Nostoc which resulted in increased total dry matter yields (shoot plus root) of 74, 105 and 125%, respectively. Of the total¹⁵N applied at the start, 26, 49 and 53% was released from Azolla, Anabaena and Nostoc; about 7, 14 and 13% was lost by denitrification and 74, 51 and 47% remained in the soils as the undecomposed part of the biofertilizers, respeciively, after 60 days. Of 15.76, 22.72 and 25.92 mg N assimilated by the rice plants, 48, 61 and 62% was supplied by Azolla, Anabaena and Nostoc, respectively. The rest was obtained from the soil used.In the absence of the rice plants 30, 43 and 45% of applied¹⁵N of Azolla, Anabaena and Nostoc was released, respectively, in 60 days of which 93–96% was lost as N₂ through denitrification.     

Nitrogen uptake in needles of Scots pine (Pinus sylvestris L.) when exposed to gaseous ammonia and ammonium fertilizer in the soil

Young saplings of Pinus sylvestris L. were exposed to gaseous NH₃ at 53 or 105 g m^–3 for one year in open-top chambers. Saplings received ¹⁵N-labelled (NH₄)₂SO₄ via the soil. To examine the importance of foliar N uptake, changes in the concentration of total and labelled N in the needles were followed. Increase in needle biomass and N concentration were found in trees exposed to NH₃, confirming that atmospheric NH₃ acted as a N fertilizer. NH₃ had a greater and quicker effect than (NH₄)₂SO₄: compared with the growth in ambient air, the N concentration in the needles exposed to NH₃ had increased by 49% in four months, while the increase after highest N-fertilization (200 kg N ha^–1 y^–1) was only 8%. The small contribution of NH₄ ⁺ fertilization to the total N concentration was not due to a deficient N uptake: the ¹⁵N concentration in the needles increased significantly with time. On the other hand, NH₃ uptake in shoots may have a negative effect on the NH₄ ⁺ root uptake. The relation between plant N and atmospheric NH₃ concentration was non-linear and possible reasons for this observation are discussed. Fumigation with NH₃ significantly decreased the ratios of K/N and P/N, showing that fumigation disrupted the nutrient balance.     

Fertilizer and soil nitrogen accumulation by irrigated barley grown on a red-brown earth in south-eastern Australia

¹⁵N labelled (NH₄)₂SO₄ was applied to barley at 5 g N m⁻² (50 kg N ha⁻¹) in microplots at sowing to study the timing of the N losses and the contribution of soil and fertilizer N to the plant. Water treatments included rainfed and irrigation at 45–50 mm deficit beginning in the spring. Recovery of¹⁵N in the plant increased to a maximum of about 20% within 91 days after sowing (DAS 91) and then remained constant. Approximately 16% (0.8 g N m⁻²) of the fertilizer was in the stem and leaves at DAS 91 and this N was subsequently redistributed to the head. At maturity, approximately 75% of the¹⁵N assimilated by the tops was recovered in the grain. Soil N contributed 3.6 g N m⁻² to the head; 2.2 g N m⁻² was remobilized from the stem and leaves, and the balance, approximately 1.4 g N m⁻², was taken up from the soil between DAS 69 to 91. Effects of irrigation treatments on N accumulation were not significant. Residual¹⁵N fertilizer in the soil decreased with time from sowing, and at maturity 40% of the applied N was recovered in the surface 0.15 m.¹⁵N movement to depth was limited and less than 5% of the fertilizer was recovered below 0.15 m. Irrigation had no effect on the¹⁵N recovery at depth. Total recovery of the¹⁵N varied between 60 and 67% and implies that 33–40% was lost from the soil-plant system. The total recovery in the soil and plant was not affected by time or irrigation in the interval DAS 39 to 134. Losses occurred before DAS 39 when crop uptake of N was small and soil mineral N content was high. There was an apparent loss of 1.9 g fertilizer N m⁻² (i.e. 38% of that applied) between DAS 1 and 15. This loss occurred before crop emergence when rainfall provided conditions suitable for denitrification.     

Carbon and nitrogen in the root-zone of barley (Hordeum vulgare L.) supplied with nitrogen fertilizer at two rates

Below-ground carbon (C) production and nitrogen (N) flows in the root-zone of barley supplied with high or low amounts of N-fertilizer were investigated. Interest was focused on the effect of the level of N-fertilizer on the production of root-derived C and on gross immobilization (i) and gross mineralization (m) rates. The plants were grown for 46 days in a sandy loam soil. Principles of pool dilution and changes in ¹⁵N pool abundances were used in conjunction with mathematical modelling to calculate the flows of N. N was applied at a high or a low rate, as (¹⁵NH₄)₂SO₄ solution (17.11 atom% ¹⁵N excess), before sowing. Nitrification was inhibited by using nitrapyrin (N-Serve). Pots were sampled four or five times during the experimental period, i.e. 0, 22, 30, 38 and 46 days after germination. On the three last sampling occasions, samples were also collected from pots in a growth chamber with ¹⁴C-labelled atmosphere.The release of ¹⁴C, measured as the proportion of the total ¹⁴C translocated below ground, was higher in the high-N treatment, but the differences between treatments were small. Our results were not conclusive in demonstrating that high-N levels stimulate the decomposition and microbial utilization of root-released materials. However, the internal circulation of soil-N, calculated N fluxes (m), which were in accordance with C mineralization rates and amounts of unlabelled N found in the plants (PU), suggested that the decomposition of native soil organic matter was hampered in the high-N treatment. Apparently, towards the end of the experimental period, microorganisms in the low-N treatment used C from soil organic matter to a greater extent than C they used from root released material, presumably because lower amounts of mineral N were available to microorganisms in the low-N treatment. Immobilization of N appeared to be soil driven (organisms decomposing soil organic matter account for the N demand) at low-N and root-driven (organisms decomposing roots and root-derived C account for the N demand) at high-N.Abbreviations AU Ammonium N-unlabelled - AL Ammonium N-labelled - AT Ammonium N-labelled and unlabelled (total) - NU Nitrate N-unlabelled - OU Organic N-unlabelled - OL Organic N-labelled - OT Organic N-total - PU Plant N-unlabelled (shoots and roots) - PL Plant N-labelled (shoots and roots) - PT Plant N-total (shoots and roots) - SL Sink or source of N-labelled - S Source or sink of N, mainly to and from the outer part of the cylinder - SU Sink or source of N-unlabelled - m Mineralization rate - i Immobilization rate - ua Uptake of ammonium - un Uptake of nitrate - la Loss of ammonium.     

A comparison of the effects of subsoiling on plant uptake and leaching losses of sulphur and nitrogen from a simulated urine patch

Synthetic cow urine labelled with ³⁵S and ¹⁵N was applied to large, undisturbed, monolith lysimeters sampled from subsoiled and non-subsoiled areas of a grass/clover pasture. For one year following the urine application, the lysimeters were subjected to a combination of natural rainfall, simulated rainfall and simulated flood irrigations. Drainage from the lysimeters was sampled regularly and monthly (approx.) pasture cuts taken. At the end of the year, the lysimeters were destructively sampled in 50 mm depth increments for soil analysis. Leachates, plant samples and soil samples were analysed for ³⁵S and ¹⁵N.There were no significant differences in plant uptake of ³⁵S and ¹⁵N between the subsoiled and nonsubsoiled lysimeters. Initially grass showed a higher degree of labelling than clover. Total amounts of ³⁵S and ¹⁵N leached from the subsoiled lysimeters were approximately twice that leached from the nonsubsoiled ones. Leaching patterns differed substantially between the two nutrients.Total recoveries of ³⁵S (in plants, leachates and soil extracts) accounted for 82% of the applied ³⁵S for the subsoiled lysimeters and 72% for non-subsoiled ones. The unrecovered ³⁵S is considered to have been incorporated into soil organic matter. Total recoveries of ¹⁵N (in plants, soil and leachates) were similar to those for ³⁵S, but unrecovered ¹⁵N is attributed to loss by denitrification.