15N Recovery Research Articles

The fate of slurry-N fractions in herbage and soil during two growing seasons following application

Farmers are under increasing pressure to use slurry-nutrients more efficiently in order to maximise crop utilisation and minimise losses to the environment. The objective of this field experiment was to quantify the fate of three N fractions (urine-N [U], rapid faecal-N [FR] and slow faecal-N [FS]) from cattle slurry in herbage and soil. The recovery of the three slurry-N fractions was measured in the first and second year after application on a permanent grassland in Ireland. Urine and faeces were collected from cows fed with 15N-labelled herbage, or unlabelled herbage with added 15N-labelled urea and these were recombined to produce differentially labelled experimental slurries. Slurries were applied to plots, and 15N-enrichments of the herbage and three soil layers were determined. The initial recovery of 15N (6 weeks after application) in herbage was 18%, 13%, 2%, while the residual recovery (12–63 weeks) was 4%, 6% and 7% for U, FR and FS, respectively. The total slurry-N recovery in the plant-soil system was estimated to range from 45% for urine-N to 72% for faecal-N. These results increase our mechanistic understanding of slurry-N dynamics in soil-plant systems and will inform models used to predict the fate of cattle slurry applied to grassland.

Nitrogen turnover in the leaf litter and fine roots of sugar maple

In order to better understand the nitrogen (N) cycle, a pulse of 15NO3- was applied in 1998 to a sugar maple (Acer saccharum) dominated northern hardwood forest receiving long-term (1994-2008) simulated atmospheric N deposition. Sugar maple leaf litter and live fine-root 15N were quantified for four years prior to labeling and for 11 subsequent years. Continuous sampling of 15N following addition of the tracer enabled calculation of leaf litter and fine-root N pool turnover utilizing an exponential decay function. Fine-root 15N recovery peaked at 3.7% +/- 1.7% the year the tracer was applied, while leaf litter 15N recovery peaked in the two years following tracer application at approximately 8%. These results suggest shoots are primarily constructed from N taken up in previous years, while fine roots are constructed from new N. The residence time of N was 6.5 years in leaf litter and 3.1 years in fine roots. The longer residence time and higher recovery rate are evidence that leaves were a stronger sink for labeled N than fine roots, but the relatively short residence time of tracer N in both pools suggests that there is not tight intra-ecosystem cycling of N in this mature forest.

Recovery of 15N-labeled urea and soil nitrogen dynamics as affected by irrigation management and nitrogen application rate in a double rice cropping system

A sophisticated soil microcosm system and 15N-labeled urea were used to investigate nitrogen (N) use efficiency and soil N dynamics in a rice monoculture system in two successive seasons. Topsoil (0 cm–20 cm) and subsoil (20 cm–50 cm) samples were collected from a traditional double rice cropping field in the Jiangxi Province, China, and these soil samples were derived from Quaternary red clay. Treatments were randomly assigned with two irrigation regimes and three N application rates (no application control, 80% traditional rate and 100% traditional rate noted as N0, N1 and N2, respectively). The levels of 15N recovery of plants, 15N and N remaining in soil were determined. Moreover, the N dynamic of soil solution from different layers of the soil profile was surveyed. The results showed that the effects of irrigation management and N application rate varied in different rice growing seasons. Irrigation regimes had remarkable effects on grain yield and chemical 15N fertilizers (CF-15N) uptake. When compared to flood irrigation (FI), the shallow water depth with wetting and drying (WD) increased grain yield up to 5.7%–20%. Although the highest grain yield was obtained with reduced N application level, both N apparent recovery (NAR) and 15N use efficiency (the percentage of plant N uptake derived from applied N, %Ndfan) significantly decreased with increasing N inputs. However, the interaction between irrigation management and N application rate on grain yield and N use efficiency (NUE) of CF-15N were not significant. A survey of soil solutions every 5 days indicated that NH4+-N was the main residual form of N, and high NH4+-N leaching was observed. When compared to FI, WD decreased vertical NH4+-N and TN leaching, especially at 10–50 cm depths of soil profile in the second season. NH4+-N was the main N residual form in the soil profile. Therefore, in this study, the WD irrigation regime and reduced rate (N1) was the optimal irrigation and fertilizer management strategy to increase the NUE of CF-N, increase the after effects of CF-15N, decrease leaching loss of CF-15N and minimize the shallow groundwater pollution risk, which were all beneficial for the ecological environment.

Nitrogen recoveries from organic amendments in crop and soil assessed by isotope techniques under tropical field conditions

The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia (Canavalia brasiliensis) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect 15N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows’ manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g N m−2, within which 1.0 g N m−2 was from canavalia residues and 2.6 g N m−2 was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total 15N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter.

Degradation of grass-derived pyrogenic organic material, transport of the residues within a soil column and distribution in soil organic matter fractions during a 28month microcosm experiment

The microbial recalcitrance of char accumulated after vegetation fires and its transport within a soil column were studied in microcosms using 13C- and 15N-enriched pyrogenic organic material (PyOM). The PyOM from rye grass (Lolium perenne L.) was produced by charring at 350 °C under oxic conditions for 1 and 4 min to examine the impact of the charring degree. After 28 months, 13C recovery decreased to values between 62% and 65%, confirming that this material can be attacked by microorganisms and that the degradation occurs rapidly after accumulation of PyOM at the soil surface. The respective 15N recovery followed the same trend but tended to be higher (between 67% and 80%). Most of the added PyOM isotopic labels were recovered in the particulate organic matter (POM) fraction, containing between 84% and 65% of the added 13C and 15N after the first 2 months, being reduced by half at the end of the experiment. After 1 month, up to 13.8% of the 13C label and 12.4% of the 15N label were detected in the POM-free mineral fractions. This fast association of PyOM with the mineral phase indicates that physical soil properties have to be considered for the elucidation of PyOM stability. Addition of fresh unlabelled grass material as co-substrate resulted in comparable trends as for the pure PyOM but the total recovery of the isotopic labels clearly increased with respect to the amount of mineral-associated PyOM. Between 73% and 82% of the mineral-associated PyOM occurred in the clay separates (<2 μm) for which the highest values were obtained for the experiment with the more intensively charred PyOM and co-substrate addition. In summary, the study demonstrates the degradability of grass-derived PyOM. The addition of fresh plant material as an easily degradable co-substrate promoted the formation of partially decomposed PyOM and subsequently its association with the mineral phase, but did not increase the respective mineralisation rates. Detection of 13C and 15N content at different depths of the microcosm column demonstrated an additional loss of PyOM from top soil by way of mobilisation and transport to deeper horizons. All these processes have to be taken into account in order to obtain a more realistic view about the behaviour of PyOM in environmental systems and for estimation of the C and N sequestration potential.

Fate of airborne nitrogen in heathland ecosystems: a 15N tracer study

In the present study, we analyze the fate of airborne nitrogen in heathland ecosystems (NW Germany) by means of a 15N tracer experiment. Our objective was to quantify N sequestration and N allocation patterns in an ecosystem that is naturally limited by N, but that has been exposed to airborne N inputs exceeding critical loads for more than 3 decades. We hypothesized that the system has a tendency towards N saturation, which should be indicated by low N sequestration and high N leaching. We analyzed 15N partitioning (aboveground biomass and soil horizons) and investigated 15N leaching over 2 years following a 15N tracer pulse addition. 15N tracer recovery was 90% and 76% in the first and second year, respectively. Contrary to our expectations, more than 99% of the tracer recovered was sequestered in the biomass and soil, while leaching losses were <0.05% after 2 years. Mosses were the most important short-term sink for 15N (64% recovery in the first year), followed by the organic layer. In the second year, the moss layer developed from a sink to a source (23% losses), and soil compartments were the most important sink (gains of 11.2% in the second year). Low 15N recovery in the current year's shoots of Calluna vulgaris (<2%) indicated minor availability of 15N tracer sequestered in the organic layer. N partitioning patterns showed that the investigated heaths still have conservative N cycling, even after several decades of high N loads. This finding is mainly attributable to the high immobilization capacities for N of podzols in soil compartments. In the long term, the podzol A- and B-horizons in particular may immobilize considerable amounts of incoming N. Since N compounds of these horizons are not readily bio-available, podzols have a high potential to withdraw airborne N from the system's N cycle.

Feedback of grazing on gross rates of N mineralization and inorganic N partitioning in steppe soils of Inner Mongolia

Plant-microbe interactions are crucial regulators of belowground nitrogen cycling in terrestrial ecosystems. However, such interactions have mostly been excluded from experimental setups for the investigation of gross inorganic N fluxes and N partitioning to plants and microorganisms. Ungulate grazing is likely to feed back on soil N fluxes, and hence it is of special importance to simultaneously investigate grazing effects on both plant and microbial N fluxes in intact plant-soil systems, where plant-microbe interactions persist during the experimental incubation. Based on the homogenous 15NH 4 + labelling of intact plant-soil monoliths we investigated how various stocking rates (0, 2.35, 4.8 and 7.85 sheep ha−1 grazing season−1) in steppe of Inner Mongolia feedback on gross rates of N mineralization and short-term inorganic N partitioning between plant, microbial and soil N pools. Our results showed that the effect of grazing on gross N mineralization was non-uniform. At low stocking rate gross N mineralization tended to decrease but increased with higher grazing pressure. Hence, there was no significant correlation between stocking rate and gross N mineralization across the investigated grazing intensities. Grazing decreased 15N recovery both in plant and microbial N pools but strongly promoted NO 3 − accumulation in the soil and thus negatively affected potential ecosystem N retention. This appeared to be closely related to the grazing-induced decline in easily degradable soil C availability at increasing stocking rate.

Application of carbon additives to reduce nitrogen leaching from cattle urine patches on pasture

A lysimeter experiment evaluated the use of sawdust or sucrose to decrease nitrogen (N) leaching from newly deposited urine patches to a perennial ryegrass/white clover pasture on sandy loam soil. Cow urine (equivalent to 500 kg N ha−1), labelled with 15N-urea, was applied to the surface of lysimeters and sawdust (9 t ha−1) or sucrose (3, 12 or 24 t ha−1) was applied immediately afterwards. Sawdust did not decrease mineral N leaching compared with urine-only (82 kg N ha−1). Sucrose applied at 12 or 24 t ha−1 decreased mineral N leaching by 27 or 66%, compared to urine-only. Pasture dry matter yields were decreased in all sucrose treatments (range 16–29% over 247 days, but 15–73% over the first 70 days) compared with urine-only. Total recovery of 15N in pasture and soil was similar across treatments (average 66%) but the 15N partitioning between soil and pasture differed; soil recovery was 19% in the urine-only and urine + sawdust treatments, and 27–51% in the sucrose treatments, increasing with increasing rate of sucrose. In the farm situation, the risk of yield loss and the large carbon applications required may limit the practical application of this N mitigation method.

Effects of clover density on N2O emissions and plant-soil N transfers in a fertilised upland pasture

Legumes have the potential to alter nitrous oxide (N2O) emissions in grass-legume mixtures via changes in soil N availability, but the influence of legume abundance on N2O fluxes in grazed multi-species grasslands has faced little attention to date. In this paper, a combination of 15N-labelled fertilizer application and automatic chamber measurements was used to investigate N2O fluxes and soil-plant N transfers for high- and low-density clover patches in an intensively-managed, upland pasture (Auvergne, France) over the course of one growing season. During the six-month study period, N2O fluxes were highly variable. Maximum daily N2O emission was 52 g N2O-N ha−1, and was associated with fertilizer application early in the growing season. Smaller peaks of N2O emission occured in response to cutting events and fertilizer application later in the growing season. Nitrous oxide fluxes derived from 15N-labelled fertilizer peaked at 40% shortly after fertilizer application, but the dominant source of N2O fluxes was the soil N pool. Contrary to expectations, clover density had no significant effects on N content or patterns of 15N recovery in plant or soil mineral N pools. Nevertheless, we found a tendency for increased N2O-N losses from the low clover treatment. Furthermore, 15N recovery in N2O was higher in the low- compared to the high-density clover treatment during favorable growing conditions, suggesting transient shifts in plant/soil competition for N depending on legume abundance. Multiple regression analysis revealed that water-filled pore space (WFPS) and clover dry mass were the main factors driving cumulative N2O emissions in the high clover treatment, whereas variation in cumulated N2O emissions in the low clover treatment was best explained by WFPS and grass mass. We hypothesize that clover density had indirect effects on the sensitivity of N2O emissions to abiotic and biotic factors possibly via changes in soil pH. Overall, our results suggest that spatial heterogeneity in clover abundance may have relatively little impact on field-scale N2O emissions in fertilized grasslands.

Carbon and oxygen controls on N2O and N2 production during nitrate reduction

Abstract Here we provide evidence that the form of carbon compound and O2 concentration exert an inter-related regulation on the production and reduction of N2O in soil. 6.7 mM d -glucose, 6.7 mM D-mannitol, 8 mM L-glutamic acid or 10 mM butyrate (all equivalent to 0.48 g C l−1) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O2 concentrations were established at ∼2%, 10% and 21% O2 v/v for each C treatment, and 2 mM K15NO3 (25 atom % excess 15N) was applied, enabling quantification of 15N–N2 production, 15N–(N2O-to-N2) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O2 v/v) soils, where addition of butyrate and glutamic acid resulted in greater N2O production (0.61 and 0.3 μg N2O–N g−1 soil for butyrate and glutamic acid, respectively) than the addition of carbohydrates (glucose and mannitol). Although, there was no significant effect of C compound at low initial O2 concentrations (∼2% O2 v/v), production of 15N–N2 was greatest where headspace O2 concentrations were initially, or fallen to, ∼2% O2 v/v, with greatest reduction of N2O and lowering 15N–(N2O-to-N2) ratios (∼0–0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O2 concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of 15N in NH4+ from applied 15N–NO3−, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O2 v/v concentrations. Our 15N approach has provided the first direct evidence for enhancement of N2O reduction in the presence of carbohydrates and the dual regulation of C compound and O2 concentration on N2O production and reduction, which has implications for management of N2O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.

Long‐Term Manure Application and Forages Reduce Nitrogen Fertilizer Requirements of Silage Corn–Cereal Cropping Systems

Assessment of the soil N supply capacity is essential to optimize fertilizer N use. We investigated soil N supply capacity and fertilizer N recovery for three cropping systems established in 1977: silage corn (Zea mays L.)–cereal without (CC) and with 20 Mg ha−1 yr−1 manure (CCM), and silage corn–forage (3‐yr) with manure (CFM). During the present study (2005–2008), manure applications were suspended and a silage corn–silage corn–barley (Hordeum vulgaris L.)–wheat (Triticum aestivum L.) sequence was imposed to all systems. Fertilizer (15NH415NO3, 3.1 atom % 15N) was applied in 2005 to silage corn (160 kg N ha−1) and in 2007 to barley (80 kg N ha−1). The 15N recovery in silage corn and barley ranged from 40 to 59%, with the lowest values measured in CFM. Compared to the CC systems (47 kg N ha−1) in 2005, soil‐derived N in silage corn was two times higher under CCM (98 kg N ha−1), and four times higher under CFM (208 kg N ha−1). These differences decreased over years, but were still noticeable at the end of the experiment. Twenty‐two to 58% of applied 15N was recovered in the soil at harvest. More than 50% of this residual N was present in macroaggregates (&gt;0.25 mm), whereas &lt;20% was present in particulate organic matter (POM). Up to 75% of residual 15N was lost during the winter period, and little residual N was transferred to the following crop. These findings indicate that soil macroaggregates may be a preferential sink for residual fertilizer N, but this N is not present in stable forms and is vulnerable to environmental loss.

Fate of Nitrogen from Organic and Inorganic Sources in Rice-Wheat Rotation Cropping System

The lower availability of N is one of the most important limiting factors impeding crop yield enhancement among the various factors that affect crop yield under the multiple-cropping agroecosystem in China. In this study, the recovery of a single application of 15N-labeled fertilizer or residues in rice-wheat cropping system was determined, in order to provide theoretical foundation for the nitrogen management in sustainable agricultural production. A continuous trace experiment was conducted for 15N microplots by using randomized block design with four treatments and four replications (T1= 15N-labeled fertilizer with crop residue incorporation, T2 = 15N-labeled residues, T3= 14N fertilizer to generate unlabeled crop residue, and T4 = 15N-labeled fertilizer without crop residue incorporation). Our results showed that, on average, 17.17 and 12.01% of crop N was derived from N fertilizer and 15N-labeled residues, respectively during the first growing season, suggesting that approximately 82.83 or 87.99% of crop N was derived directly from soil N pool. There was a larger difference in the 15N recovery pattern in crop when N was applied as fertilizer or residues, i.e., most of crop N derived from N fertilizer was absorbed in the first growing season (92.04%), and the relevant value was 38.03% when 15N-labeled residues were applied. This implied that most of N fertilizer was recovered in the present cropping season, while a longer residue effect will be found for 15N-labeled residues. Thus, the average recovery of N fertilizer and N residue in the soil after the first growing season was 33.46 and 85.64%, respectively. The recovery of applied N in soil when N was applied as residues was significantly higher than that when N was applied as fertilizer. There was a larger difference in the total 15N recovery in plant and soil when N was applied as fertilizer or residues. By the end of the fifth or sixth cropping season, the total 15N recovery in plant and soil when N was applied as fertilizer or residues were estimated at 64.38 and 79.11%, respectively. On the contrary, there was little difference between the practices of residue incorporation and residue removal following the N fertilizer application. N fertilizer appeared to be more readily available to crops than residue-N, and residue-N replenished soil N pool, especially N in soil organic matter, much more than N fertilizer after six growing seasons. Therefore, residue-N is a better source for sustaining N content of soil organic matter. Thus, one possible management practice is to use both organic and inorganic N sources simultaneously to improve the use efficiency of N while protecting the sustainability of soil.

Herbicide application increases nitrogen (15N) exudation and root detachment of Brachiaria decumbens Stapf

Current research shows that glyphosate herbicides can increase nitrogen exudation by plant roots and, as a consequence, favor pathogenic fungal colonization in soil. In Brazilian agroecosystems, the main herbicides used in conservationist systems for glyphosate substitution are glufosinate and paraquat. However, no studies have been developed that to evaluate the effects of these herbicides on nitrogen exudation by roots. This work was carried out with the objective of evaluating the exudation of nitrogen (15N) compounds and root detachment after the application of glyphosate, glufosinate-ammonium or paraquat on Brachiaria decumbens Stapf. The ammonium concentration in the plant’s tissues and the nitrogen (15N) recovery in the plant-solution system were also evaluated. The nitrogen left by the soil-solution system and the ammonium concentration in the plant’s tissues were increased after glyphosate or glufosinate application, but they were not modified by paraquat. Nitrogen compound exudation and root detachment were increased by the desiccation of Brachiaria decumbens Stapf by the use of the following herbicides: glyphosate, glufosinate-ammonium and paraquat. This increase demonstrated that the nitrogen exudation was not affected by a particular herbicide’s mode of action.

Estimating the contribution of nitrogen from legume cover crops to the nitrogen nutrition of grapevines using a 15N dilution technique

Cover cropping is increasingly being used as a form of sustainable soil management in grapevine (Vitis vinifera L.) production. The objective of the current study was to determine the effect of a legume cover crop on a set of growth and production variables of a vineyard, and to quantify the relative importance of legume nitrogen (N) as a source of N for the vine plants and to compare this to the current recommended practice of annually applying 40 kg fertilizer-N ha-1. The study was carried out in a 5-yr-old vineyard cv. Cabernet Sauvignon at the Cauquenes-INIA Experimental Center in one of the wine production areas of Chile. The treatments were: control without cover crop (C); a legume mixture of early maturing cultivars of subterranean clover (Trifolium subterraneum L.) and burr medic (Medicago polymorpha L.) (EMC); or a legume mixture of late maturing cultivar of subterranean clover and balansa clover (T. michelianum Savi) (LMC). Average inputs of legume N generated by the EMC and LMC treatments represented 112 and 161 kg above-ground legume N ha-1yr-1, respectively. Grape dry matter production was increased significantly (P ≤ 0.05) by 48–61% and the amount of N accumulated in grape bunches was enhanced by 74–105% after 2 years of legume cover crop. However, no significant (P > 0.05) effects of cover cropping were observed on vine leaf, canes, roots or trunk biomass. To estimate the relative contribution of legume and fertilizer N to the N nutrition of grapevines 15N-enriched fertilizer (10 atom% 15N excess) was applied to the soil in micro-plots within all treatments as two split applications of 20 kg N ha-1. The 15N composition of vines grown with or without cover crops were subsequently compared. Any measured reduction in the 15N enrichment of the tissues of cover cropped vines relative to the nil cover crop control was assumed to have resulted from the uptake of legume N. Recovery of 15N in vine plant parts was used to calculate the efficiency of use of fertilizer N. Surprisingly the 15N data indicated that the fruit had the lowest reliance upon legume N for growth (only 7–13% cf 14–56% of the N in other organs), even though grape bunches were the only plant part to positively respond to the cover cropping treatments. This suggested that either the temporal patterns of N mineralization in soil and N uptake by vines over the growing season was such that much of the mineral N utilized during fruit development was not directly derived from legume residues, or that N remobilized from other plant parts within the vines was the major source of N for grape production rather than N newly assimilated from soil. The amount of legume N estimated to be recovered by cover cropped vine plants (12–15 kg N ha-1), was similar to the calculated contribution from 40 kg of fertilizer-N applied to vines (11-12 kg ha-1). This amount of N represented <10% of the amounts of N annually returned to the soil in above-ground legume material in the case of the cover cropping treatments, and was equivalent to a recovery of 27–30% of the N applied in fertilizer.

Dynamics and recovery of fertilizer 15N in soil and winter wheat crop under minimum versus conventional tillage

The development of reduced tillage in cropping systems raises the question of adaptation of N fertilization of crops in response to changes in the chemical, physical and biological soil caused by the cessation of ploughing. The purpose of the work was to investigate the fate of fertilizer 15N applied in spring to a winter wheat crop ( Triticum aestivum L.) under minimum tillage (MT) and conventional tillage (CT) and to examine the hypothesis that in MT systems plants face stronger microbial competition for N. The experiment was conducted in Northern France on a silt loam in two different rotations [sugar beet ( Beta vulgaris L.)–winter wheat–maize ( Zea mays L.)–winter wheat and seed flax ( Linum usitatissimum L.)–winter wheat–pea ( Pisum sativum L.)–winter wheat] from which we selected two crop sequences, pea–wheat and maize–wheat. At the beginning of stem elongation, 15N labelled fertilizer was applied at the rates of 100 kg N ha −1 for wheat after pea and 120 kg N ha −1 for wheat after maize. The dynamics of fertilizer 15N in soil and plant was assessed at six dates between N application and wheat harvest. In addition, soil respiration was monitored continuously on the maize–wheat CT and MT plots as a way of assessing heterotrophic microbial activity. The type of tillage (MT vs CT) had little effect on fertilizer N dynamics in the soil. The amount of labelled N immobilized was low with values ranging from 3.5 to 11.9% of applied 15N. Plant 15N recovery (above ground biomass + roots) amounted to 59–63% at flowering and harvest. The 15N balance at harvest time (i.e. the sum of 15N recovered from all plant and soil compartments) indicates that approximately 29% of the fertilizer N applied was lost whichever the tillage considered. The type of rotation (i.e. the nature of the preceding crop) had more influence on wheat N uptake than the type of tillage. The lack of observed effect of tillage on the dynamics of nitrogen fertilizer and plant recovery is in agreement with most published results obtained in other soil and climate conditions.

Contributions of Different N Sources to Crop N Nutrition in a Chinese Rice Field

N availability is one of the most important factors limiting crop yield enhancement. The recovery of applications of 15N-labeled fertilizer and crop residues in a rice-wheat cropping system was determined for up to 6 consecutive growing seasons. The crop residues from the previous season were either incorporated or removed as two different treatments. Our results showed that 16.55%–17.79% (17.17% on average) of the fertilizer N was recovered in the crop during the first growing season, suggesting that more than 80% of crop N was not directly from the N fertilizer. When 15N-labeled residues were applied, 12.01% was recovered in the crop in the first growing season. The average recoveries of fertilizer N and crop residue N in the soil after the first growing season were 33.46% and 85.64%, respectively. N from soil organic matter contributed approximately 83% of the N in the crop when 15N fertilizer was applied or 88% when crop residues were applied. There was a larger difference in the total 15N recovery in plant and soil between N applications in the forms of fertilizer and crop residues. Incorporation of crop residues following the 15N fertilizer application did not significantly promote 15N recovery in the crop or soil. On average, only additional 1.94% of N for the fertilizer-applied field or 5.97% of N for the crop residue-applied field was recovered by the crops during the 2nd and 3rd growing seasons. The total recoveries of 15N in crop and soil were approximately 64.38% for the fertilizer-applied field after 6 growing seasons and 79.11% for the crop residue-applied field after 5 growing seasons. Although fertilizer N appeared to be more readily available to crops than crop residue N, crop residue N replenished soil N pool, especially N from soil organic matter, much more than fertilizer N. Therefore, crop residue N was a better source for sustaining soil organic matter. Our results suggested that the long-term effect of fertilizer or crop residues on N recovery were different in the crop and soil. However, there was little difference between the practices of crop residue incorporation and residue removal following the N fertilizer application.

Influence of different forage grasses on nitrate capture and leaching loss from a pumice soil

AbstractNitrogen (N) uptake and loss in leachate from a pulse of 15N‐labelled synthetic cow urine applied to a pumice soil were compared in a glasshouse lysimeter experiment among the grass species, Agrostis capillaris, Dactylis glomerata, Phalaris aquatica, Lolium multiflorum, L. perenne and a L. multiflorum/A. capillaris mixture. In addition, four L. perenne treatments investigated the effect of infection by Neotyphodium lolii strains. Leachate volumes and leachate nitrate concentrations were measured. At final harvest 24 weeks after sowing, dry matter (DM) mass of shoots and roots, plant N and 15N contents, soil residual mineral N, and root diameters and lengths were measured. Endophyte had no effect on plant or nitrate variables. Lysimeters planted with L. perenne (pooled endophyte treatments) leached 48 mg of NO3‐N compared with &lt;3 mg N for the other grasses. Recovery of 15N was highest in A. capillaris (0·99), followed by D. glomerata and P. aquatica (0·89), the L. multiflorum/A. capillaris mixture (0·87), L. multiflorum (0·60) and L. perenne (0·44). Low 15N recoveries and high leaching losses from L. perenne were associated with low plant and root masses of DM and low rooting depth. High aerial mass of DM, root systems extending below 20 cm and high root masses of DM in lysimeters with A. capillaris, P. aquatica and D. glomerata contributed to a high rate of nitrate interception by these species and low leachate losses. The L. multiflorum/A. capillaris mixture was intermediate between the two species for most of the variables measured.

Improved plant nitrogen nutrition contributes to higher water use efficiency in tomatoes under alternate partial root-zone irrigation

Comparative effects of partial root-zone irrigation (PRI) and deficit irrigation (DI) on stomatal conductance (gs), nitrogen accumulation and distribution in tomato (Lycopersicon esculentum L.) plants were investigated in a split-root pot experiment. Results showed that both PRI and DI saved 25% water and led to 10.0% and 17.5% decreases in dry biomass, respectively, compared with the fully irrigated (FI) controls. Consequently, water use efficiency (WUE) was increased by 18.6% and 10.8% in the PRI and DI plants, respectively. The highest WUE in the PRI plants was associated with the highest carbon isotope composition (δ13C), indicating that the improvement of WUE might have been a result of long-term optimisation of stomatal control over gas exchange. The constantly higher xylem sap ABA concentration in PRI compared with DI plants was seemingly responsible for the greater control over stomatal conductance during the treatment. At the end of the experiment, N accumulation and 15N recovery was highest in FI, intermediate in PRI and lowest in DI. In addition, PRI plants consistently allocated more N into the upper and middle leaf layers than in the FI and DI treatments. The improved N nutrition and distribution in the canopy may indicate that PRI plants have a greater photosynthetic capacity than DI plants; this is confirmed by the observed positive linear relationship between specific leaf N content and δ13C. It is concluded that PRI improves N nutrition and optimises N distribution in the canopy, which might have been partly contributed to the high WUE in PRI tomato plants.

Multiseason Recoveries of Organic and Inorganic Nitrogen‐15 in Tropical Cropping Systems

In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A 15N‐recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single 15N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58% (average of 21%) of crop N uptake during the first growing season was derived from fertilizer. On average, 79% of crop N was derived from the soil. When 15N‐labeled residues were applied, in the first growing season 4% of crop N was derived from the residues. Average recoveries of 15N‐labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. Corresponding recoveries in the soil were 38 and 71%. An additional 6% of the fertilizer and 9.1% of the residue was recovered by crops during subsequent growing seasons. There were no significant differences in total 15N recovery (average 54%) between N from fertilizer and N from residue. After five growing seasons, more residue N (40%) than fertilizer N (18%) was recovered in the soil, better sustaining the soil organic matter N content. Long‐term total recoveries of 15N‐labeled fertilizer or residue in the crop and soil were similar. Soil N remained the primary source of N for crops. As higher rainfall and temperature tend to cause higher 15N losses, management practices to improve N use efficiency and reduce losses in wet tropical regions will remain a challenge.

The fate of nitrogen in grain cropping systems: a meta‐analysis of 15N field experiments

Intensively managed grain farms are saturated with large inputs of nitrogen (N) fertilizer, leading to N losses and environmental degradation. Despite decades of research directed toward reducing N losses from agroecosystems, progress has been minimal, and the currently promoted best management practices are not necessarily the most effective. We investigated the fate of N additions to temperate grain agroecosystems using a meta-analysis of 217 field-scale studies that followed the stable isotope 15N in crops and soil. We compared management practices that alter inorganic fertilizer additions, such as application timing or reduced N fertilizer rates, to practices that re-couple the biogeochemical cycles of carbon (C) and N, such as organic N sources and diversified crop rotations, and analyzed the following response variables: 15N recovery in crops, total recovery of 15N in crops and soil, and crop yield. More of the literature (94%) emphasized crop recovery of 15N than total 15N recovery in crops and soil (58%), though total recovery is a more ecologically appropriate indicator for assessing N losses. Findings show wide differences in the ability of management practices to improve N use efficiency. Practices that aimed to increase crop uptake of commercial fertilizer had a lower impact on total 15N recovery (3-21% increase) than practices that re-coupled C and N cycling (30-42% increase). A majority of studies (66%) were only one growing season long, which poses a particular problem when organic N sources are used because crops recover N from these sources over several years. These short-term studies neglect significant ecological processes that occur over longer time scales. Field-scale mass balance calculations using the 15N data set show that, on average, 43 kg N x ha(-1) x yr(-1) was unaccounted for at the end of one growing season out of 114 kg N x ha(-1) x yr(-1), representing approximately 38% of the total 15N applied. This comprehensive assessment of stable-isotope research on agroecosystem N management can inform the development of policies to mitigate nonpoint source pollution. Nitrogen management practices that most effectively increase N retention are not currently being promoted and are rare on the landscape in the United States.