15N Fertilizer Research Articles

Nitrogen dynamics in co-composted drilling wastes: Effects of compost quality and 15N fertilization

A better understanding of N availability in co-composted drilling wastes is required to evaluate the potential use of the composts as growth media. We investigated N dynamics in co-composted drilling wastes by examining the changes in the concentrations and partition of applied 15N in various soil N pools ( NH 4 + , NO 3 − , dissolved organic N, microbial biomass N, and non-extractable N) in a 4-month greenhouse incubation experiment using 1-, 2-, 3-, and 4-year-old (referred to below as 1Y, 2Y, 3Y, and 4Y, respectively) composts, representing substrates with different quality. Regardless of compost age, after 4 months of incubation extractable N concentrations decreased ( P<0.05), in contrast with the increasing pattern of the non-extractable N, indicating stabilization of the extractable N into the recalcitrant soil organic fraction. Fertilizer N application increased ( P<0.05) extractable N concentrations. In the younger composts, a major part of the applied 15N was recovered in the non-extractable N fraction (44.0% for 1Y and 38.5% for 2Y) with little recovered as mineral N. On the other hand, a considerable percentage of the applied 15N (21.8% for 3Y and 18.8% for 4Y) was found in the NO 3 − pool in the older composts with relatively high mineral N but low organic C contents. This study shows that the dynamics of biologically available N and fate of applied N in the composts depend on compost quality such as mineral N and organic C contents, and compost C:N ratio. To use the co-composted drilling waste as growth media, different N management strategies need to be established for those composts with differed substrate quality.

Determination of nitrogen fixation effectiveness in selected Medicago truncatula isolates by measuring nitrogen isotope incorporation into pheophytin

Effectiveness is a term used to describe the input that a bacterial nitrogen-fixing symbiosis makes to plant nitrogen metabolism. In legumes, effectiveness is considered a polymorphic trait where specific interactions between the plant and symbiotic rhizobia contribute to the success of the interaction. Evaluation of effectiveness using model legumes like Medicago truncatula may open new avenues for genetic studies. In previous work, an isotope dilution mass spectrometry method, which uses the effect of nitrogen fixation on the nitrogen isotope composition of chlorophyll in plants grown on 15N fertilizer as a measure of effectiveness, was developed for estimating the contribution of symbiotic nitrogen fixation to plant nitrogen content. This 15N-dilution assay was used to evaluate the level of nitrogen fixation effectiveness in three Medicago truncatula lines that have been used as parents in generating recombinant inbred lines. Three Sinorhizobium meliloti strains, USDA 1600, 102F51 and MK506, differ in this measure of effectiveness on three lines of M. truncatula: Jemalong A17, DZA315.16 and F83005.5. Plant–rhizobia combinations grown in two different conditions showed comparable differences in effectiveness.

Black spruce seedlings in a Kalmia–Vaccinium association: microsite manipulation to explore interactions in the field

We established a field trial on an ericaceous-dominated clearcut in Quebec to determine the effect of Kalmia angustifolia L., Vaccinium angustifolium (Ait.), and V. myrtilloides (Michx.) on the growth and physiology of black spruce (Picea mariana (Mill.) BSP) seedlings and on soil characteristics over the first two growing seasons. Plots undergoing one of three treatments (shrub removal, humus removal, or undisturbed control) were planted with black spruce seedlings that were either unfertilized or spot fertilized at time of planting. In some of the undisturbed control plots, we also used 15NH415NO3 to compare uptake of broadcast N fertilizer by vegetation. The ericaceous shrubs had a significant negative impact on seedling growth. Growth reductions were not related to water stress, soil temperature, or soil moisture. Extractable NH4-N and P concentrations in mineral soil tended to decrease in the presence of ericaceous shrubs, but effects were not significant. Seedling foliar N concentration was also reduced in the presence of ericaceous shrubs. Of the total amount of 15N fertilizer found in vegetation, 64% was immobilized in Vaccinium spp., 31% in Kalmia, and 5% in black spruce, but spruce took up more 15N per unit of root biomass than the ericaceous shrubs. Kalmia had consistently higher predawn xylem water potentials than black spruce.

Nitrogen-15 Uptake by Pinus contorta Seedlings in Relation to Phenological Stage and Season

This study measured the amount of uptake of labeled nitrogen (15N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after 15N application, the amount of 15N recovered in seedlings as a percentage of the total 15N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total 15N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total 15N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low 15N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.

Fate of Nitrogen‐15 Applied to Irrigated Acala and Pima Cotton

Better knowledge on the fate of fertilizer N will aid in developing effective N management strategies balancing plant requirements for optimal lint yield with environmental concerns. Field studies were initiated to determine the fate of 15N fertilizer applied to Acala (Gossypium hirsutum L.) and American Pima (G. barbadense L.) cotton. Four N regimes (56, 112, 168, and 224 kg N ha−1 corrected for residual soil nitrate N levels) were established for Acala cotton grown on a Panoche clay loam [fine‐loamy, mixed (calcareous) thermic Typic Torriorthents] and on a Wasco sandy loam (coarse‐loamy, mixed, nonacid, thermic Typic Torriorthents) in 1998, 1999, and 2000. Pima cotton was evaluated on the Panoche clay loam for the same N regimes in 1999 and 2000. To trace the fate of fertilizer N, 15N‐labeled urea was applied to microplots in selected treatments and years. Acala fertilizer use efficiency by 15N dilution (FUE–15N) averaged 49% on Panoche clay loam and 43% on Wasco sandy loam. Pima FUE–15N on the Panoche clay loam averaged 48% and was not affected by N treatment. Recovery of fertilizer N in the soil was not different between the Panoche clay loam and the Wasco sandy loam in the Acala experiments and combined across both Acala and Pima trials averaged 42%. Averaged across all experiments, more than 75% of the 15N recovered in the soil was found in the top 0.9‐m layer. The total recovery of 15N fertilizer in plant and soil averaged 89% (76–98%) across all treatments, suggesting that production practices employed in this study resulted in only small losses of fertilizer N during the season of application.

Soil 13C–15N dynamics in an N2‐fixing clover system under long‐term exposure to elevated atmospheric CO2

AbstractReduced soil N availability under elevated CO2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N2‐fixing system. We studied the effects of Trifolium repens (an N2‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO2 under FACE conditions. 15N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha−1 yr−1 and the CO2 concentration was increased to 60 Pa pCO2 using 13C‐depleted CO2. The total soil C content was unaffected by elevated CO2, species and rate of 15N fertilization. However, under elevated CO2, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (fN) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO2, had a significant effect on fN values of the total soil N pool. The fractions of newly sequestered C (fC) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO2, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more 15N fertilizer than T. repens: 179 vs. 101 kg N ha−1 for the low rate of N fertilization and 393 vs. 319 kg N ha−1 for the high N‐fertilization rate. As the loss of fertilizer‐15N contributed to the 15N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N2‐fixing L. perenne system. This suggests that N2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.

Recovery of Nitrogen from Maize Residue and Inorganic Fertilizer in a Maize‐Groundnut Rotation System in Humid Tropics of Malaysia

Incorporation of crop residues after harvest may improve nitrogen (N) retention of tropical soils. A maize‐groundnut rotation experiment was established in an Ultisol to study the recovery by the crop‐soil system of N from mineral fertilizer and from the first maize residue. Out of the recommended N fertilizer for maize (150 kg N ha− 1), 60 kg N ha− 1 was applied in the form of ammonium sulphate (9.82 atom percent 15N excess) to the first maize crop and traced (for four crops) in the subsequent cropping system, with or without incorporation of crop residues. Similarly, N in the labelled residues of the first crop (maize) was also traced in the soil‐crop system with incorporation of unlabelled crop residues. Recovery of fertilizer 15N by the first crop ranged from 20 to 22%. In the subsequent four crops, this value averaged 2% (but ranged from 0.4 to 6%). The proportion of fertilizer N retained in the soil (0–50 cm) after harvest of the first crop was 35 to 44% whereas after harvest of the subsequent crops, this proportion was 30 (but ranged from 24 to 34%) in crop residue treated plots and 22% (but ranged from 18 to 27%) in plots where crop residues were removed. Maize residue‐N recovery in the subsequent three crops averaged only 5% (but ranged from 1 to 11%), being highest in the crop after crop residue incorporation. The corresponding recovery in the soil (0–50 cm) averaged across crops was 46% (but ranged from 40 to 52%). In tropical regions with high rainfall, conserving and decreasing soil N losses from mineral fertilizers could be achieved (in the long term) by the combined application of crop residues with mineral fertilizer.

Biomass Distribution and Nitrogen‐15 Partitioning in Citrus Trees on a Sandy Entisol

The area under citrus production in Florida is 332 000 ha, with a production of 10 to 12 million metric tonnes of fruit annually. Nutrient management recommendations are needed to increase N uptake efficiency and to minimize nitrate leaching below the root zone. The objectives of this study were (i) to evaluate biomass distribution of 6‐yr‐old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osbeck] on ‘Swingle citrumelo’ [Poncirus trifoliata (L.) Raf. × C. × paradisi Macfad.] rootstock grown in a sandy soil under low volume irrigation, and (ii) to estimate partitioning of 15N fertilizer applied to the soil during early spring into different tree components. We evaluated biomass of tree components (leaves, twigs, trunk, taproot, roots, and fruit), and N recovery and distribution of 15NH415NO3 (AN) and 15N‐urea (UR) (10 atom % 15N) applied to the soil surface. About 70% of dry matter biomass of trees was aboveground (AG). Length density of feeder roots was concentrated at a depth of 0 to 15 cm below the soil surface and varied from 1.87 to 0.88 cm cm−3 at 0.5‐ and 1.5‐m distance from the trunk, respectively. Total recoveries of 15N by trees were 25.5% for UR and 39.5% for AN at fruit harvest, 280 d after fertilization. Mean accumulation of applied 15N in recent leaf flush was 4.2% and that of older leaves was 2.5%. Accumulation of 15N was low in woody tissue. Since fruit represented a large sink for N (10.2 and 18.4% recovery of 15N applied as UR and AN, respectively), we confirmed the importance of N fertilization before fruit development.

Biomass Distribution and Nitrogen-15 Partitioning in Citrus Trees on a Sandy Entisol

The area under citrus production in Florida is 332 000 ha, with a production of 10 to 12 million metric tonnes of fruit annually. Nutrient management recommendations are needed to increase N uptake efficiency and to minimize nitrate leaching below the root zone. The objectives of this study were (i) to evaluate biomass distribution of 6-yr-old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osbeck] on ‘Swingle citrumelo’ [Poncirus trifoliata (L.) Raf. × C. × paradisi Macfad.] rootstock grown in a sandy soil under low volume irrigation, and (ii) to estimate partitioning of 15N fertilizer applied to the soil during early spring into different tree components. We evaluated biomass of tree components (leaves, twigs, trunk, taproot, roots, and fruit), and N recovery and distribution of 15NH415NO3 (AN) and 15N-urea (UR) (10 atom % 15N) applied to the soil surface. About 70% of dry matter biomass of trees was aboveground (AG). Length density of feeder roots was concentrated at a depth of 0 to 15 cm below the soil surface and varied from 1.87 to 0.88 cm cm−3 at 0.5- and 1.5-m distance from the trunk, respectively. Total recoveries of 15N by trees were 25.5% for UR and 39.5% for AN at fruit harvest, 280 d after fertilization. Mean accumulation of applied 15N in recent leaf flush was 4.2% and that of older leaves was 2.5%. Accumulation of 15N was low in woody tissue. Since fruit represented a large sink for N (10.2 and 18.4% recovery of 15N applied as UR and AN, respectively), we confirmed the importance of N fertilization before fruit development.

Nitrous oxide emissions following application of residues and fertiliser under zero and conventional tillage

Emissions of N2O were measured following combined applications of inorganic N fertiliser and crop residues to a silt loam soil in S.E. England, UK. Effects of cultivation technique and residue application on N2O emissions were examined over 2 years. N2O emissions were increased in the presence of residues and were further increased where NH4NO3 fertiliser (200 kg N ha−1) was applied. Large fluxes of N2O were measured from the zero till treatments after residue and fertiliser application, with 2.5 kg N2O-N ha−1 measured over the first 23 days after application of fertiliser in combination with rye (Secale cereale) residues under zero tillage. CO2 emissions were larger in the zero till than in the conventional till treatments. A significant tillage/residue interaction was found. Highest emissions were measured from the conventionally tilled bean (Vicia faba) (1.0 kg N2O-N ha−1 emitted over 65 days) and zero tilled rye (3.5 kg N2O-N ha−1 over 65 days) treatments. This was attributed to rapid release of N following incorporation of bean residues in the conventionally tilled treatments, and availability of readily degradable C from the rye in the presence of anaerobic conditions under the mulch in the zero tilled treatments. Measurement of 15N-N2O emission following application of 15N-labelled fertiliser to microplots indicated that surface mulching of residues in zero till treatments resulted in a greater proportion of fertiliser N being lost as N2O than with incorporation of residues. Combined applications of 15N fertiliser and bean residues resulted in higher or lower emissions, depending on cultivation technique, when compared with the sum of N2O from single applications. Such interactions have important implications for mitigation of N2O from agricultural soils.

Interspecific competition for nitrogen between early successional species and planted white spruce and jack pine seedlings

Relatively little is known about belowground competition for nitrogen (N) between boreal forest species. The objective of this study was to compare the relative competitiveness of three early successional boreal forest species, trembling aspen (Populus tremuloides Michx.), fireweed (Epilobium angustifolium L.), and calamagrostis (Calamagrostis canadensis (Michx.) Beauv.), on 15N uptake and the growth response by containerized white spruce (Picea glauca (Moench) Voss) and jack pine (Pinus banksiana Lamb.) seedlings. Three densities (0, 2, and 6 plants/pot) of each competitor species were interplanted with individual seedlings in pots with 15N-labelled fertilizer and grown for 3 months. The conifer seedlings had the greatest fertilizer 15N uptake (129.5 and 82.3 µg/m2 root for white spruce and jack pine, respectively) when grown in monoculture and the lowest uptake when interplanted with six competitor plants (9.2 and 8.6 µg/m2 root for white spruce and jack pine, respectively). Whether grown in monoculture or with conifer seedlings, calamagrostis took up the greatest amount of fertilizer 15N. Vegetation management practices that reduce the establishment of this grass species in the field should benefit N uptake and growth of outplanted white spruce and jack pine seedlings.

Leaching of nitrogen, potassium, calcium and magnesium in a sandy soil cultivated with sugarcane

A lysimeter experiment was carried out with sugarcane aiming to evaluate the leaching of nitrogen derived from either urea (15N) or the soil/sugarcane crop residues. The leaching of K+, Ca2+, and Mg2+ was also evaluated. The experiment was a factorial 2x4. The influencing factors were: firstly, the differential addition of two kinds of sugarcane remains to the soil, simulating conditions of cane- plantation renewal after the cane crop harvest, with and without previous straw removal by burning; secondly, four doses of N: 0, 30, 60, and 90 kg ha-1. During the experimental period the total volume of water received by the sugarcane-soil system was 2,015 mm, with 1,255 mm as precipitation and 760 mm as irrigation. The loss of N by leaching from the fertilizer (15N) was not detected. In the first three weeks the largest losses of N by leaching occurred, originating from the soil/sugarcane remains-N. The mean of leached N during the experimental period of 11 months was of 4.5 kg ha-1. The mean losses of K+, Ca2+, and Mg2+ were of 13, 320 and 80 kg ha-1, respectively.

Nitrogen Fertilizer Use Efficiency of Furrow‐Irrigated Onion and Corn

Furrow‐irrigated onion (Allium cepa L.) production, with high N fertilization rates, may be contributing NO3–N to ground water in southeastern Colorado. This study determined the growth and N uptake patterns of onion grown on a silty clay soil, N fertilizer use efficiency (NFUE) of onion, and recovery of residual N fertilizer by corn (Zea mays L.) following onion in rotation. Onion was sampled biweekly from 18 May to 15 Sept. 1998 from plots receiving 0 and 224 kg N ha−1. Nonlabeled N and labeled 15N fertilizer were band‐applied near the onion row in split applications of 112 kg N ha−1 each on 18 May and 25 June. Onion dry matter accumulation was slow from planting to about late May, followed by a rapid increase in biomass production and N uptake. Because residual soil NO3–N was high, N fertilization resulted in only a small increase in bulb yield. Greatest demand for N by onion occurred during bulb development. Fertilizer N recovery by onion was 11 and 19% for May and June N applications (average 15%), respectively. Much of the fertilizer N remained in the upper 60‐cm soil profile at harvest and had moved toward the onion bed center. Fertilizer 15N detected at 180‐cm soil depth indicated leaching losses from the root zone. The unfertilized 1999 corn crop recovered 24% of fertilizer N applied to onion for a total fertilizer N uptake by the two crops of 39%. Delaying N fertilizer application until onion bulbing begins may improve NFUE. Planting corn directly on the previous onion bed may result in greater N fertilizer recovery by corn.

Estimation of symbiotic N 2 fixation by a low-level, large-scale 15N application technique

A modified approach is presented for the 15N isotope dilution method to estimate the proportion of plant nitrogen derived from the atmosphere (Ndfa). This approach utilizes a low-level, large-scale application of 15N enriched fertilizer. In a field study, pea ( Pisum sativum L.) was cultivated in two long-term (since 1970) tillage systems (conventional: CT, minimum tillage: MT). Nitrogen (26 g NO 3–N ha −1 with 10 at.% 15N) was distributed with a conventional field sprayer to increase the plant available soil 15N level up to the desired range of δ 15N>5‰. A formula for determining the desired amounts of 15N application is presented and a model is introduced comparing the apparent Ndfa calculated by field data to the theoretically calculated true values of Ndfa. The model makes it possible to consider three basic factors: (i) the amount of plant available soil nitrogen, (ii) the distribution of 15N enrichment in different soil layers and (iii) the soil N uptake proportions of the legume and the reference plant when the amount of applied 15N fertilizer is the dependent variable. Errors in estimating Ndfa by the 15N dilution method can be quantitatively determined with different soil N uptake patterns of legumes and reference plants at varying 15N enrichment levels. The model shows that results obtained by using the low-level, large-scale 15N enrichment technique produced a slightly smaller margin of error than did the addition of larger amounts of 15N labelled fertilizer when N uptake patterns of the legume and the reference plant differ. The new technique offers distinct advantages, in that it is very practical and makes the enlargement of plots possible at low cost and with marginal effort. The results of the low-level enrichment field study were evaluated using the model as well as the results of an investigation on pea with moderate 15N enrichment by liquid manure (isotope dilution method).

Sunflower nitrogen requirement and 15N fertilizer recovery in Western Pampas, Argentina

In order to avoid nitrogen overfertilization, fertilizer rates must be adjusted to meet crop requirements. Two field experiments with sunflower ( Helianthus annuus L.) were performed in the western part of the Pampas, Argentina, to: (i) assess nitrogen fertilization effects on seed yield, grain oil content, and plant lodging, (ii) determine N requirement per unit of yield, crop recovery of fertilizer N, and whether these two parameters were affected by N and other nutrient additions. Nitrogen fertilization increased the seed yield only by 17% at one site. Crop nitrogen requirement per unit yield ( b-value) increased from 37 to 42 kg Mg −1 due to nitrogen fertilization only at the site where there was not a yield response. Therefore, if a yield response is expected, it is not necessary to use different b-values for non fertilized or fertilized crop. Reduction of seed oil content due to N addition was relatively small (2–5%), and was overcompensated by the seed yield increase at the responsive site. Recovery of fertilizer 15N was of 51%. This efficiency of absorption should be considered for making fertilizer recommendations. Application of further nutrients including P and K had no influence on seed yield.

Nitrogen Fertilizer Use Efficiency of Furrow-Irrigated Onion and Corn

Furrow-irrigated onion (Allium cepa L.) production, with high N fertilization rates, may be contributing NO3–N to ground water in southeastern Colorado. This study determined the growth and N uptake patterns of onion grown on a silty clay soil, N fertilizer use efficiency (NFUE) of onion, and recovery of residual N fertilizer by corn (Zea mays L.) following onion in rotation. Onion was sampled biweekly from 18 May to 15 Sept. 1998 from plots receiving 0 and 224 kg N ha−1. Nonlabeled N and labeled 15N fertilizer were band-applied near the onion row in split applications of 112 kg N ha−1 each on 18 May and 25 June. Onion dry matter accumulation was slow from planting to about late May, followed by a rapid increase in biomass production and N uptake. Because residual soil NO3–N was high, N fertilization resulted in only a small increase in bulb yield. Greatest demand for N by onion occurred during bulb development. Fertilizer N recovery by onion was 11 and 19% for May and June N applications (average 15%), respectively. Much of the fertilizer N remained in the upper 60-cm soil profile at harvest and had moved toward the onion bed center. Fertilizer 15N detected at 180-cm soil depth indicated leaching losses from the root zone. The unfertilized 1999 corn crop recovered 24% of fertilizer N applied to onion for a total fertilizer N uptake by the two crops of 39%. Delaying N fertilizer application until onion bulbing begins may improve NFUE. Planting corn directly on the previous onion bed may result in greater N fertilizer recovery by corn.

Losses of fertilizer-derived N from transplanted rice after heading

Microplot field experiments with 15N-labeled (NH4)2SO4 were conducted to investigate the changes in the percentage of fertilizer 15N recovery (%FNR) of transplanted rice (Oryza sativa L.) with basal (4 g m−2) and topdress (3 g m−2) applications for two years. In 1999, %FNR from the basal 15N application decreased by 6% (0.23 g N m−2)from heading + 2 d to maturity, and %FNR from the topdress 15N application decreased by 11% (0.32 g N m−2)from heading − 4 d to maturity. In 2000, %FNR from the topdress 15N application decreased by 9% (0.26 g N m−2) from heading − 1 d to maturity, whereas there was no significant decline in %FNR from the basal 15N application after heading. A decrease in %FNR occurred even when total N of rice plants showed no decrease after heading. Nitrogen harvest index of fertilizer 15N with the topdress 15N application was not significantly different from that with the basal 15N application in 1999, whereas N harvest index of fertilizer 15N with the topdress 15N application was higher than that with the basal 15N application in 2000. N harvest index of fertilizer 15N, that indicated the proportion of the 15N remobilized to panicles after heading, was substantially responsible for changes in %FNR from the basal and topdress 15N applications after heading.

Nitrogen Dynamics and Fertilizer Use Efficiency in Rice following Straw Incorporation and Winter Flooding

Incorporation of rice (Oryza sativa L.) straw, when compared with burning, affects soil N supply by increasing N and C inputs. This study determined the effects of long‐term alternative rice straw management and winter flooding on seasonal crop uptake of N, fertilizer N use efficiency (FNUE), and crop N uptake from 15N‐labeled straw. Microplots were established on two sites in California, Maxwell and Biggs, by applying 15N‐labeled fertilizer during Year 4 of a long‐term rice straw management study. At the end of the year, 15N‐labeled straw was applied to assess crop uptake of straw N in the following season. Fertilizer use efficiency by 15N dilution (FUE–15N) over the growing season at Maxwell and at final harvest at Biggs was significantly higher when straw was burned rather than incorporated. Fertilizer use efficiency by N difference was significantly greater than FUE–15N, suggesting an apparent added N interaction (ANI). Straw management did not significantly affect the uptake of residual fertilizer 15N or of straw 15N in the subsequent year. Winter flooding had no significant effect on measured parameters. Data indicate a decrease in FNUE with a concomitant and more significant increase in the plant available soil N following the change in straw management from burning to incorporation. Belowground 15N pools appear to be a larger source of N for the following crop than aboveground residue.

Evaluation of a new approach to the nitrogen-15 isotope dilution technique, to estimate crop N uptake from organic residues in the field

Field experiments were conducted to test a new approach for estimating crop N uptake from organic inputs. Soils were pre-labelled by applying 15N fertiliser to soybean [Glycine max (L.) Merr] and common bean [Phaseolus vulgaris L.] crops. Additional 14N plots which received unlabelled fertiliser were also established in the same way. The above-ground biomass from all four plots was harvested, stored and the plots left to over-winter. In the following summer 15N-labelled residues were added to the unlabelled soils and unlabelled residues were added to the 15N-labelled soils at a rate of 150 kg N ha–1. All plots were cultivated and sown with maize (Zea mays L.). Control plots that did not receive residue application or any additional fertiliser N were also set up. The plots were harvested in late autumn. Maize derived 37 kg N ha–1 and 31 kg N ha–1 from the added soybean residues, estimated using the direct and indirect approach, respectively, in plots previously sown to soybean. N derived from the added common bean residues was estimated as 26 kg N ha–1 and 24 kg N ha–1 from the direct and indirect methods, respectively. In the plots previously sown to common bean, N derived from added soybean residue was 32 kg N ha–1 using the direct method and 33 kg N ha–1 using the indirect method. N derived from common bean residues was 22 kg N ha–1 and 21 kg N ha–1 estimated using the direct and indirect approaches, respectively. It was concluded that the modified indirect technique allows a reasonable estimate of N derived from residues and that this will enable further experiments to be conducted in which N derived from more complex matrices, such as manure or sewage sludge, can be determined.

Effect of dicyandiamide and hydroquinone on the transformation of urea-nitrogen-15 in soil cropped to wheat

A pot experiment with a loam soil and spring wheat as test crop showed that an application of dicyandiamide (DCD), and especially its combination with hydroquinone (HQ), gave a much larger recovery of soil urea-15N than treatments based on the application of urea alone or urea plus HQ. Most of the urea-15N applied to soil was present as organic plus chemically fixed 15N in the DCD and DCD plus HQ treatments. These two treatments showed the smallest accumulation of urea-derived (NO3–+NO2–)-15N. Under well-drained conditions, there was a synergistic effect of the nitrification inhibitor DCD and the urease inhibitor HQ on urea-15N transformations and the recovery of fertilizer 15N in soil after the application of urea.