Fate of residual 15N-labeled fertilizer in dryland farming systems on soils of contrasting fertility

Abstract

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (¹⁵N)-labeled urea (165 kg N ha^?1) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116 kg ha^?1), and next was NPK soil (60 kg ha^?1), then the MNPK soil (43 kg ha^?1) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual ¹⁵N), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual ¹⁵N in No-F soil over summer fallow was as high as 48%, and significantly (P < 0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual ¹⁵N use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied ¹⁵N. The total ¹⁵N recovery (¹⁵N uptake by crops and residual in 0–100 cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The ¹⁵N uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual ¹⁵N was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.     

控释氮肥在淹水稻田土壤上的去向及利用率

通过土壤渗漏装置、微区和田间小区试验,研究了15N标记控释氮肥在淹水稻田土壤上氮素的去向和利用率。结果表明,施用控释氮肥能明显地降低氨挥发、淋失和硝化—反硝化的损失。控释氮肥处理的氨挥发量比尿素降低54.0%,氮淋失量降低32.5%。尿素的硝化—反硝化损失量占施入氮量的34.5%,而控释氮肥的只占2.0%;控释肥料与尿素氮在0—80cm土层中的残留率相近。控释氮肥一次性全量作基肥施入土壤,水稻的氮肥利用率平均为65.6%,比尿素(基肥+追肥)高出32.2个百分点。控释氮肥的农学效率显著地高于尿素。     

Kinetics of 15N -labelled nitrogen from co-compost made from cattle manure and chemical fertilizer in a paddy field

In order to produce an effective organic fertilizer, cattle manure was cocomposted with chemical fertilizer. And the kinetics of nitrogen uptake by rice plants from the co-compost was investigated using the ¹⁵N labelled co-composts on either cattle manure or chemical fertilizer. As a control, nitrogen kinetics from the mixture of cattle manure and chemical fertilizer without co-composting was investigated. At the early stage, rice growth may have been promoted by co-composting, while, it may have been promoted by the larger N-content of cattle manure at the harvesting stage. The ratios of nitrogen uptake by rice plants and residual nitrogen in soil from the cattle manure and chemical fertilizer were determined by measuring ¹⁵N -atom%. The N -uptake ratios by rice plants from the cattle manure in the co-composted plot were about 2–4 times higher than those from the cattle manure without co-composting. However, the N -uptake ratios from the chemical fertilizer in the co-composted plot were lower than those from the chemical fertilizer without co-composting. The N -content of the rice plants derived from chemical fertilizer without co-composting decreased consistently after 28 d. The nitrogen from chemical fertilizer in the co-compost was absorbed again in the latter period of rice growth. The total nitrogen uptake by rice plants from cattle manure and chemical fertilizer was similar regardless of co-composting. However, co-composting would be advantageous at least· in terms of the following aspects: increase of the N -uptake by rice plants from cattle manure, slow-release ability of nitrogen from chemical fertilizer, decrease of nitrogen loss by denitrification.     

Fate of Fertilizer-15N to a Maize Crop Grown in Northern Zambia

Abstract

A field study with maize (Zea mays L.) was conducted in the 1988/89 cropping season to investigate the fate of ¹⁵NO₃-N-labelled NH₄ ¹⁵NO₃ applied at 40, 80 and 120 kg N ha^?1 (unlabelled N applied at 0, 80, 160 and 240 N ha^?1) with and without lime. The investigations were conducted in northern Zambia at Misamfu Regional Research Centre, Kasama on a Misamfu red sandy loam soil. The experimental design was a split plot arrangement with four replications with main plots receiving 0 and 2 Mg ha^?1 dolomitic limestone, while subplots received fertilizer N at various rates. Significant (p < 0.001) grain and DM yield responses to applied N up to 160 kg ha^?1 were observed. At higher rates little or no crop responses were observed and fertilizer use efficiency declined. Partitioning of amounts of total N and ¹⁵N in plants was in the order of seed = tassel > leaf> cob = earleaf> stem. Fertilizer N rates showed a highly significant (p < 0.001) effect on plant uptake of labelled N. Lime and its interaction with N rates had no effect on all measured parameters. Leaching of NO₃-N fertilizer to lower soil depths was in proportion to the rate of N applied, with highly significant (p < 0.001) differences among soil depths. Although higher concentrations of fertilizer-¹⁵N were recovered in the 0–20 cm depth the recovered portion at lower soil depths was still significant. Total recovery of labelled N by plant and by soil after crop harvest averaged 75, 55 and 54% of originally applied fertilizer-¹⁵N at 40, 80 and 120 kg N ha^?1, respectively. Corresponding unaccounted for ¹⁵N was 25, 45 and 46%. The most probable loss mechanism could have been by leaching to depths greater than 60 cm, gaseous losses to the atmosphere and root assimilation.     

Incorporation of nitrogen from urea fertilizer into soil organic matter in rice paddy and cassava upland fields in Indonesia

Incorporation of newly-immobilized N into major soil organic matter fractions during a cropping period under paddy and upland cropping systems in the tropics was investigated in Jawa paddy fields with and without fish cultivation and a Sumatra cassava field in Indonesia. ¹⁵N-labelled urea (¹⁵N urea) was applied as basal fertilizer, and the soil samples were collected after harvest. The percentage of distribution of the residual N in soil from ¹⁵N urea into the humic acids, fulvic acid fraction, and humin were 13.1–13.9, 19.0–20.5, and 53.4–54.3%, respectively, for the Jawa paddy soils, and 14.9, 27.4, and 52.4%, respectively, for the Sumatra cassava soil. These values were comparable to the reported ones for other climatic zones. The percentage of distribution of ¹⁵N urea-derived N into humic acids was larger than that of total N into the same fraction in all the soils. The distribution into the fulvic acid fraction was also larger for ¹⁵N urea-derived N than for total N in the Jawa soils. Humic and non-humic substances in the fulvic acid fraction were separated using insoluble polyvinylpyrrolidone (PVP) into the adsorbed and non-adsorbed fractions, respectively. Less than 5% of the ¹⁵N urea-derived N in fulvic acid fraction was detected in the PVP-adsorbed fraction (generic fulvic acids). The proportion of non-hydrolyzable N remained after boiling with 6 M HCl in the ¹⁵N urea-derived N was 9.4–13.5%, 17.3–26.7%, and 8.4–16.6% for the humic acids, generic fulvic acids, and humin, respectively. The significantly low resistance to acid hydrolysis suggested that the ¹⁵N urea-derived N was less stable than the total N in soil regardless of the fractions of humus.     

Real-time imaging of nitrogen fixation in an intact soybean plant with nodules using 13N-labeled nitrogen gas

Abstract

Real-time images of nitrogen fixation in an intact nodule of hydroponically cultured soybean (Glycine max [L] Merr.) were obtained. In the present study, we developed a rapid method to produce and purify ¹³N-labeled radioactive nitrogen gas (half life: 9.97?min). ¹³N was produced from a ¹⁶O (p, α) ¹³N nuclear reaction. The target chamber was filled with CO₂ and irradiated for 10?min with protons at an energy of 18.3?MeV and an electric current of 5?μA, which was delivered from a cyclotron. All CO₂ in the collected gas was absorbed and removed with powdered soda-lime in a syringe and replaced with helium gas. The resulting gas was injected into gas chromatography and separated and a 35?mL fraction, including the peak of [¹³N]-nitrogen gas, was collected by monitoring the chromatogram. The obtained gas was mixed with 10?mL of O₂ and 5?mL of N₂ and used in the tracer experiment. The tracer gas was fed into the underground part of intact nodulated soybean plants and serial images of the distribution of ¹³N were obtained non-invasively using a positron-emitting tracer imaging system (PETIS). The rates of nitrogen fixation of the six test plants were estimated to be 0.17?±?0.10?μmol N₂?h^?1 from the PETIS image data. The decreasing rates of assimilated nitrogen were also estimated to be 0.012?±?0.011?μmol?N₂?h^?1. In conclusion, we successfully observed nitrogen fixation in soybean plants with nodules non-invasively and quantitatively using [¹³N]N₂ and PETIS.     

The release and plant uptake of nitrogen from some plant and animal manures

Summary Field and laboratory experiments were used to examine the efficiency of N uptake from various manure forms, and at different rates of application. In a field experiment, wheat was grown on soils with different amounts of ¹⁵N-labelled legume residues. The amount of N taken up by the crop was directly proportional to the amount applied, with a recovery of between 15% and 23% of the legume N. In a second field experiment, inorganic N was applied at rates varying from 0 to 120 kg N ha^-1 in the presence and absence of poultry manure. The uptake of N by barley was 11 kg ha^-1 greater in the manured plots when no inorganic N was applied, and 23 kg ha^-1 greater when N was applied at the top rate. N uptake in a pot experiment was again shown to be directly proportional to the rate of manure application, but the amount of N taken up was strongly related to the N content of the manure. An incubation experiment demonstrated that net N mineralisation reached a maximum where residue concentrations were 1,5%. The significance of added nitrogen interactions in the context of manure-N additions is discussed.     

Changes in natural 15N abundance in paddy soils under different,long-term soil management regimes in the Tohoku region of Japan

Abstract

Long-term temporal changes in natural ¹⁵N abundance (δ¹⁵N value) in paddy soils from long-term field experiments with livestock manure and rice straw composts, and in the composts used for the experiments, were investigated. These field experiments using livestock manure and rice straw composts had been conducted since 1973 and 1968, respectively. In both experiments, control plots to which no compost had been applied were also maintained. The δ¹⁵N values of livestock manure compost reflected the composting method. Composting period had no significant effect on the δ¹⁵N value of rice straw compost. The δ¹⁵N values increased in soils to which livestock manure compost was successively applied, and tended to decrease in soils without compost. In soils to which rice straw compost was successively applied, the δ¹⁵N values of the soils remained constant. Conversely, δ¹⁵N values in soils without rice straw compost decreased. The downward trend in δ¹⁵N values observed in soils to which compost and chemical N fertilizer were not applied could be attributed to the natural input of N, which had a lower δ¹⁵N value than the soils. Thus, the transition of the δ¹⁵N values in soils observed in long-term paddy field experiments indicated that the δ¹⁵N values of paddy soils could be affected by natural N input in addition to extraneous N that was applied in the form of chemical N fertilizers and organic materials.     

Effect of long-term fertilization on 15N uptake and retention in soil

We did a pot experiment with three different fertilized soils (no fertilizer (No-F), inorganic fertilizer nitrogen, phosphorus and potassium (NPK), manure plus inorganic fertilizer (MNPK)) from a 19-year fertilizer trial. Three N treatments, (1) no N, (2) 100 mg/kg urea-¹⁵N (N), (3) 50 mg/kg urea-¹⁵N + 50 mg/kg corn straw-N (1/2N + 1/2S), were applied to each soil. The residual soil from the same treatments was used to grow second wheat crop. The MNPK soil had significantly higher nitrogen use efficiency (NUE) in the first growing season, and lower N loss than the NPK, and No-F soils. The 1/2N + 1/2S treatment decreased NUE on each soil, even though the MNPK soil still had highest NUE and lowest N loss. The residual ¹⁵N use efficiency (RNUE) in 1/2N + 1/2S treatment of MNPK soil was higher than NPK and No-F soils. We concluded that long-term application of manure plus inorganic fertilizer increased NUE and decreased N loss.     

Areal variation in the relationship between natural 15N abundances of rice (Oryza sativa L.) and soil

This study was performed to clarify whether areal variation exists in the relationship between natural ¹⁵N abundances (δ¹⁵N values) of rice (Oryza sativa L.) and soil without an applied nitrogen (N) source, and to explore possible reasons for any areal variation. We investigated the relationships between δ¹⁵N values of rice and those of unamended soil with no applied N source in two locations; Daisen and Ogata, in Akita Prefecture, Japan. The δ¹⁵N values of rice in Daisen were higher than those in Ogata from 2007 to 2009, irrespective of the cropping year. Results demonstrated areal variation in the relationship between δ¹⁵N values of rice and those of unamended soil. The variation might be attributed to variation in the δ¹⁵N of natural N input and to ammonia nitrification and subsequent denitrification. When the relationship between δ¹⁵N values of rice and those of unamended soil is used to discriminate between organic and conventional rice, the areal variation of the relationship in the target area should be taken into account, from the point of the δ¹⁵N value of natural N input and N transformation in the soil.     

Carbon and nitrogen dynamics in a forest soil amended with purified tannins from different plant species

Tannins are purported to be an important factor controlling nitrogen cycling in forest ecosystems, and the ability of tannins to bind proteins in protein-tannin complexes is thought to be the primary mechanism responsible for these effects. In this study, we examined the influence of well-characterized tannins purified from five different plant species on C and N dynamics of a forest soil A horizon. Tannic acid, a commonly used and commercially available hydrolyzable tannin (HT), and cellulose were also included for comparison. With the exception of tannins from huckleberry (Vaccinium ovatum), the amendments increased respiration 1.4-4.0 fold, indicating that they were acting as a microbial C source. Tannic acid was significantly more labile than the five purified tannins examined in this study. All treatments decreased net N mineralization substantially, through greater N immobilization and decreased mineralization. The six tannins inhibited gross ammonification rates significantly more than cellulose. This suggests that added tannins had effects in addition to serving as an alternative C source. Tannins purified from Bishop pine (Pinus muricata) were the only tannins that significantly inhibited potential gross nitrification rates, however, rates were low even in the control soil making it difficult to detect any inhibition. Differences in tannin structure such as condensed versus HTs and the hydroxylation pattern of the condensed tannin B-ring likely explain differences observed among the tannin treatments. Contrary to other studies, we did not find that condensed tannins were more labile and less inhibitory than HTs, nor that shorter chained tannins were more labile than longer chained tannins. In addition to supporting the hypothesis that reduced N availability in the presence of tannins is caused by complexation reactions, our data suggests tannins act as a labile C source leading to increased N immobilization.     

Green manure application improves rice growth and urea nitrogen use efficiency assessed using 15N labeling

ABSTRACT

Green manure is an efficient nitrogen (N) source when used as an alternative to chemical fertilizer. However, the N taken up by rice derived from green manure, chemical fertilizers or soil native N in complex nutrient systems is unclear. A pot experiment with partial substitution of urea with Chinese milk vetch (a green manure) implemented with ¹⁵N-labeled urea and Chinese milk vetch was set up to study the sources of N in rice and the fate of the fertilizers. The dry weights, N contents, N uptake, and urea N use efficiency were notably higher (by 15–16%, 4–13%, 22–30% and 182%-203%, respectively) in the Chinese milk vetch applied with urea treatment than in the urea alone treatment. The uptake of N from Chinese milk vetch and the use efficiency of Chinese milk vetch N were increased with reductions in the urea input amount. The application of Chinese milk vetch substantially changed the fate of urea: higher amounts of urea N were taken up by rice (approximately 29%) and remained as residue in the soil (approximately 15%) in the related treatments than in the treatment with urea alone (10% and 9%). More urea N than Chinese milk vetch N was taken up by rice (29% vs 20%, respectively) and lost (56% vs 14%, respectively), but less urea N than Chinese milk vetch N remained as residue in the soil (15% vs 66%, respectively). The partial substitution of chemical fertilizer with green manure is an effective method of promoting rice growth by supplying N for rice uptake and promoting more efficient N use.     

Excessive nitrogen application decreases grain yield and increases nitrogen loss in a wheat–soil system

Abstract

Excessive use of nitrogen (N) fertilizers in wheat fields has led to elevated NO₃-N concentrations in groundwater and reduced N use efficiency. Three-year field and ¹⁵N tracing experiments were conducted to investigate the effects of N application rates on N uptake from basal and topdressing ¹⁵N, N use efficiency, and grain yield in winter wheat plants; and determine the dynamics of N derived from both basal and topdressing ¹⁵N in soil in high-yielding fields. The results showed that 69.5–84.5% of N accumulated in wheat plants derived from soil, while 6.0–12.5%and 9.2–18.1% derived from basal ¹⁵N and top ¹⁵N fertilizer, respectively. The basal N fertilizer recovery averaged 33.9% in plants, residual averaged 59.2% in 0–200 cm depth soil; the topdressing N fertilizer recovery averaged 50.5% in plants, residual averaged 48.2% in 0–200 cm soil. More top ¹⁵N was accumulated in plants and more remained in 0–100 cm soil rather than in 100–200 cm soil at maturity, compared with the basal ¹⁵N. However, during the period from pre-sowing to pre-wintering, the soil nitrate moved down to deeper layers, and most accumulated in the layers below 140 cm. With an increase of N fertilizer rate, the proportion of the N derived from soil in plants decreased, but that derived from basal and topdressing fertilizer increased; the proportion of basal and top ¹⁵N recovery in plants decreased, and that of residual in soil increased. A moderate application rate of 96–168 kg N ha^?1 led to increases in nitrate content in 0–60 cm soil layer, N uptake amount, grain yield and apparent recovery fraction of applied fertilizer N in wheat. Applying above 240 kg N ha^?1 promoted the downward movement of basal and top ¹⁵N and soil nitrate, but had no significant effect on N uptake amount; the excessive N application also obviously decreased the grain yield, N uptake efficiency, apparent recovery fraction of applied fertilizer N, physiological efficiency and internal N use efficiency. It is suggested that the appropriate application rate of nitrogen on a high-yielding wheat field was 96–168 kg N ha^?1.     

Uptake of carbon and nitrogen through roots of rice and corn plants,grown in soils treated with 13C and 15N dual-labeled cattle manure compost

Nitrogen and carbon dynamics in paddy and upland soils for rice cultivation and in upland soil for corn cultivation was investigated by using ¹³C and ¹⁵N dual-labeled cattle manure compost (CMC). In a soil with low fertility, paddy and upland rice took up carbon and nitrogen from the CMC at rates ranging from 0.685 to 1.051% of C and 17.6–34.6% of N applied. The ¹³C concentration was much higher in the roots than in the plant top, whereas the ¹⁵N concentration differed slightly between them, indicating that organic carbon taken up preferentially accumulated in roots. The ¹³C recovery in the plant top tended to be higher in upland soil than in paddy soil, whereas ¹⁵N applied was recovered at the same level in both paddy and upland soils. In the experiment with organic farming soil, paddy rice took up C and N from the CMC along with plant growth and the final recovery rates of ¹³C and ¹⁵N were 2.16 and 17.2% of C and N applied. In the corn experiment, a very large amount of carbon from the CMC was absorbed, accounting for at least 7 times value for rice. The final uptake rates of ¹³C and ¹⁵N reached about 13 and 10% of C and N applied, respectively. Carbon emission from the CMC sharply increased by 2 weeks after transplanting and the nitrogen emission was very low. It is concluded that rice and corn can take up an appreciable level of carbon and nitrogen from the CMC through roots.     

Effect of chemical composition on the release of nitrogen from agricultural plant materials decomposing in soil under field conditions

Summary In two field experiments, plant materials labelled with ¹⁵N were buried separately within mesh bags in soil, which was subsequently sown with barley. In the first experiment, different parts of white clover (Trifolium repens), red clover (T. pratense), subterranean clover (T. subterraneum), field bean (Vicia faba), and timothy (Phleum pratense) were used, and in the second, parts of subterranean clover of different maturity. The plant materials were analysed for their initial concentrations of total N, ¹⁵N, C, ethanol-soluble compounds, starch, hemicellulose, cellulose, lignin, and ash. After the barley had been harvested, the bags were collected and analysed for their total N and ¹⁵N. In the first experiment the release of N was highest from white clover stems + petioles (86%) and lowest from field bean roots (20%). In stepwise regression analysis, the release of N was explained best by the initial concentrations of lignin, cellulose, hemicellulose, and N (listed according to decreasing partial correlations). Although the C/N ratio of the plant materials varied widely (11–46), statistically the release of N was not significantly correlated with this variable. The results of the second experiment using subterranean clover of different maturity confirmed those of the first experiment.     

Dynamics of microbial biomass nitrogen as influenced by organic matter application in paddy fields

The effects of annual application of rice straw or cow manure compost for 17–20 y on the dynamics of fertilizer N and soil organic N in Gley paddy fields were investigated by using the ¹⁵N tracer technique during the rice cropping season. The chloroform fumigation-extraction method was evaluated to determine the properties of soil microbial biomass under submerged field conditions at the tillering stage before mid-summer drainage, with special reference to the fate of applied NH₄ ⁺-¹⁵N.

The transfer ratios from applied NH₄ ⁺-¹⁵N to immobilized N in soil and to uptake N by rice during given periods varied with the rice growth stages and were affected by organic matter application. The accumulated amounts of netmineralized soil organic N (net-M_j ), immobilized N (I_j ), and denitrified N (D_j ) during the cropping season were estimated to be 14.0–22.5, 6.3–11.2, and 3.4–5.3 g N m^-2, respectively. Values of net-M_j and I_j were larger in the following order: cow manure compost plot > rice straw plot > plot without organic matter application, and their larger increase by the application of cow manure compost contributed to a decrease of the D_j values, as compared with rice straw application.

Values of E _N extra extractable soil total N after fumigation, increased following organic matter application, ranging from 2.1 to 5.4 g N m^-2. Small residual ratios of applied ¹⁵N in the fraction E _N at the end of the given period indicated that re-mineralization of newly-assimilated ¹⁵N through the easily decomposable fraction of microbial biomass had almost ended. Thus, the applicability to paddy field soils of the chloroform fumigation-extraction method was confirmed.     

Extractability of 15N-labeled corn-shoot tissue in a sandy and a clay soil by 0.01 MCaCl2 method in laboratory incubation experiments

Management of N fertilization depends not only on the mineral N measured at the beginning of the growing season but also on the status of the low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the ¹⁵N applied in labeled cornshoot tissue would be recovered in 0.01 M CaCl₂-extractable ¹⁵N fractions and wheter a decrease in the CaCl₂-extractable ¹⁵N fraction quantitatively followed the trend in net mineralization of the ¹⁵N applied in corn-shoot tissue during an incubation period. The effects of adding ¹⁵N-labeled young corn-shoot tissue to a sandy soil and a clay soil were investigated for 46 days in an aerobic incubation experiment at 25°C. The application of 80 mg N kg^-1 soil in the form of labeled corn-shoot tissue (24.62 mg ¹⁵N kg^-1 soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in comparison with the untreated soils during the incubation. The labeled organic-N fraction was significantly higher in the sandy soil than in the clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in ¹⁵NO _inf3 ^sup- during the incubation. Ammonification of applied plant N resulted in a significant increase in the 1 M HCl-extractable non-exchangeable ¹⁵NH _inf4 ^sup+ fraction in the clay soik, owing to the vermiculite content. The ¹⁵N recovery was analyzed by the 0.01 M CaCl₂ extraction method; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incubation, recovery increased to 47.2 and 43.8% in the sandy and clay soils, respectively. Net mineralization of the ¹⁵N applied in corn-shoot tissue determined after the 46-day incubation was 6.60 mg ¹⁵N kg^-1 soil (=34.9% of the applied organic ¹⁵N) and 4.37 mg ¹⁵N kg^-1 soil (=23.1% of the applied organic ¹⁵N) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl₂ over the whole incubation period was 3.14 and 2.33 mg ¹⁵N kg^-1 soil in the sandy and clay soil, respectively. These results indicate that net mineralization of ¹⁵N was not consistent with the decrease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl₂ to extract or desorb all of the applied organic ¹⁵N that was mineralized during the incubation period.