DEVELOPMENT OF A SOIL N TEST FOR FERTILIZER REQUIREMENTS FOR WHEAT

Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO^? ₃)- N, soil ammonium (NH ⁺ ₄)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO^? ₃-N and NH ⁺ ₄-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH ⁺ ₄-N and NO^? ₃ -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO^? ₃-N.     

Development of a Soil N Test for Fertilizer Requirements for Corn Production in Quebec

Corn requires high nitrogen (N) fertilizer use, but no soil N test for fertilizer N requirement is yet available in Quebec. Objectives of this research were (1) to determine the effects of soil nitrate (NO₃ ^?)-N, soil ammonium (NH₄ ⁺)-N, and N fertilizer rates on corn yields and (2) to determine soil sampling times and depths most highly correlated with yields and fertilizer N response under Quebec conditions. Soil samples were taken from 0- to 30-cm and 30- to 60-cm depths at seeding and postseeding (when corn height reached 20 cm) to determine soil NH₄ ⁺ and NO₃ ^? in 44 continuous corn sites fertilized with four rates of N in two replications using a quick test (N-Trak) and a laboratory method. The N-Trak method overestimated soil NO₃ ^?-N in comparison with the laboratory method. Greater coefficients of determination were observed for soil NO₃ ^?-N analyses at postseeding compared with seeding.     

Evaluating Inorganic Nitrogen and Rye-Crimson Clover Mixture Fertilization of Spring Broccoli and Lettuce by 15Nitrogen Tracing and Mass Balance

ABSTRACT

Broccoli (Brassica oleraceaL. var. italica) and lettuce (Latuca sativaL.) were grown under greenhouse conditions with nitrogen (N) from a cover crop mixture of rye (Secale cerealeL.) and crimson clover (Trifolium incarnatumL.) and ammonium nitrate (NH₄NO₃). Individual cover crop species were produced with non-enriched or enriched (5 atom % NH₄ ¹⁵NO₃) Hoagland Nutrient Solutions resulting in enriched rye [0.799% atom % ¹⁵N, 24:1 carbon (C):N ratio] and enriched clover (0.686% atom % ¹⁵N, 19:1 C:N ratio). Cover crops were applied as an equal mixture of rye and clover at 1884, 3768, and 5652 kg·ha^{? 1} dry weight to supply 26, 52, and 78 kg·ha^{? 1} N. Enriched materials were only applied at the 3768 kg·ha^{? 1} rate, either as enriched rye plus non-enriched clover or non-enriched rye plus enriched clover. Additional treatments consisted of an unfertilized control and three NH₄NO₃ fertilizer rates; 112, 224, and 336 kg·ha^{? 1} N for broccoli and 70, 140, and 210 kg·ha^{? 1} N for lettuce. Combination treatments were the standard cover crop rate (3768 kg·ha^{? 1}) plus the lowest N fertilizer rate for each vegetable. Cover crops did not increase yield of either broccoli or lettuce, and contributed only 17% of the N in broccoli and 15% of the N in lettuce. The majority of cover crop ¹⁵N remained in the soil: 54.8% and 81.3% of rye and clover N, respectively, after broccoli harvest; and 68.1% and 79.2% of rye and clover N, respectively, after lettuce harvest. Broccoli plant tissue recoveries were 8.0% of the rye and 11.0 % of the clover ¹⁵N; while lettuce plant tissue recoveries were 6.3% (rye) and 4.1% (clover). Broccoli yield could not be assessed due to lack of floret development, but dry matter accumulation was maximized at 224 kg·ha^{? 1}N. Lettuce yield and fertilizer N recovery efficiency (by mass balance) was maximized at 140 g·ha^{? 1} N.     

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.     

Soil inorganic nitrogen composition and plant functional type determine forage crops nitrogen uptake preference in the temperate cultivated grassland,Inner Mongolia

ABSTRACT

Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ ¹⁵N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO₃^?-N than NH₄⁺-N regardless of growth stages, and the NH₄⁺/NO₃^? uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH₄⁺-N rather than NO₃^?-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH₄⁺ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH₄⁺/NO₃^? uptake ratio was positively correlated with soil NH₄⁺/NO₃^? ratio. Our results suggest that the major forage crops prefer to absorb soil NO₃^?-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO₃^?-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.     

Nitrogen Balance and Fertigation Use Efficiency in a Field Coffee Crop

Brazil is a major world coffee producer, using increasing quantities of nitrogen (N) fertilizer as the monoculture expands across the savannas. The fate and efficiency of this fertilizer N were evaluated for one complete cropping cycle using ¹⁵N tracer, permitting an N balance at harvest. Annual rates of 200, 400, 600, and 800 kg N ha^?1 year^?1 of ¹⁵N-labeled urea and an unfertilized control were applied every 14 days via fertigation. The N concentration, percentage of N derived from fertilizer, quantity of N derived from fertilizer, and percentage of nitrogen derived from fertilizer per N rate was assessed for 8-year-old coffee trees. The most efficient N use was with 200 kg ha^?1 year^?1 because it presented the lowest losses and highest N recoveries in the crop. Conversely, the least sustainable rate was 800 kg ha^?1 year^?1, which presented the greatest losses and the lowest N recovery in the whole plant.     

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.     

脲酶抑制剂/硝化抑制剂对土壤中尿素氮转化及形态分布的影响

运用^15N尿素示踪技术,以壤质草甸棕壤和春小分别作供试土壤和作物进行盆栽试验,结果表明,与单^15N尿素或^15N尿素＋氢素＋氢醌（HQ）相比,配施双氰胺（DCD）尤其与HQ组合可使土壤保持较高的^15N回收率,其中有^15N占相当大的比例,在小麦孕穗期前,上述两处理可有效地保持尿素解后土壤中NH^ 4 -^15N含量,并显著降低（NO^ 3 NO^-2)-^15N的富集;促进肥料^15N固持－矿化的周转,HQ与DCD配合施用对尿素施用后土壤中残留^15N量及有机^15N的再矿化和NH^ 4-^15N含量具有一定的协同作用。     

Residual soil nitrogen credits for corn production along the upper Texas Gulf Coast region

Studies were conducted to evaluate response of dryland corn (Zea mays L.) along the upper Texas Gulf Coast to residual soil nitrate-nitrogen (NO₃-N) measured to depths of 15, 30, and 61 cm. Residual soil NO₃-N levels ranged from 3.4 to 31.6, 7.8 to 49.3, and 9.0 to 71.7 kg ha^?1, respectively, in 0 to 15, 15 to 30, and 30 to 61 cm depth increments, with cumulative NO₃-N ranging from 23.5 to 114.5 kg ha^?1 across sites-years. Where N fertilizer was reduced due to N crediting, yields and bushel weights at all 13 site-years showed no difference from those receiving full recommended N rates. A yield response to any level of added fertilizer N above the control was observed for only 6 of 13 site-years. These results indicate a high potential for success in crediting carryover soil NO₃-N to 61 cm as a means of reducing applied nitrogen fertilizer rates.     

Timing and Method of 15Nitrogen-Labeled Fertilizer Application on Grain Protein and Nitrogen Use Efficiency of Spring Wheat

ABSTRACT

Grain protein content is one of the most important quality constraints for bread wheat (Triticum aestivum L.) production in eastern Canada. A field experiment was conducted for two years (1999 and 2000) on the Central Experimental Farm, Ottawa, Canada, to study whether split application of nitrogen (N) fertilizer improved grain protein content and nitrogen-use efficiency (NUE). Two cultivars (‘Celtic,’ as N-responsive and ‘Grandin’, as N-non-responsive) were grown using three different N doses and application methods: (1) 100 kg N ha^?1 as NH₄NO₃, soil-applied at seeding with ¹⁵N₂-labeled NH₄NO₃ to microplots, (2) 60 kg N ha^?1 soil-applied at seeding plus 40 kg N ha^?1 foliar-applied at the boot stage with ¹⁵N₂-labeled urea to microplots, and (3) 90 kg N ha^?1 as soil-applied at seeding plus 10 kg N ha^?1 foliar-applied at the boot stage with ¹⁵N₂-labeled urea to microplots. Plants were sampled at heading and maturity. While dry-matter production and grain yields were not affected by the treatments in either year, N application methods influenced tissue N concentration and NUE. In 1999, extended drought stress led to significant yield reduction; in 2000, foliar application of 10 kg N ha^?1 at the boot stage significantly increased grain N concentration when grain protein was under the limit for bread quality, suggesting that later-applied N can contribute to grain protein content. At maturity, the average NUE was 22.3% in 1999 and 34.5% in 2000, but was always greater when all N was applied at seeding (42.5%) than when N was foliar-applied at the boot stage (18.5% to 24.5%). We conclude that application of a small amount of fertilizer N at the boot stage can improve the bread-making quality of spring wheat by increasing grain protein concentration.     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.     

Evaluation of the 15Nitrogen Isotope Dilution Technique for Estimating Crop Nitrogen Uptake from Organic Residues

Abstract

A field experiment was conducted to test the new approach for estimating crop nitrogen (N) uptake from organic inputs. The soil was prelabeled with ¹⁵N by applying ¹⁵N fertilizer to sunflower crop (Helianthus annuus L. var. Viki). The ¹⁴N plots, which received unlabelled fertilizer, were also set up. At harvest, ¹⁵N labeled residues were added to the unlabeled soils at a rate of 73 kg N ha^?1 (direct technique) and unlabeled residues were added to the ¹⁵N‐labeled soils at the same rate (indirect technique). Control plots without residues were also established. All plots were sown with the wheat (Triticum aestivum L. var merchouch)–fababean (Vicia faba L.)–wheat (Triticum aestivum L. var merchouch) cropping sequence.

In the cropping sequence, the first, second and third crop derived respectively 12.01, 2.4, and 1.93 kg N ha^?1 from crop residues estimated by the direct method and 14.77, 3.3, and 1.85 kg N ha^?1 estimated by the indirect method. The results showed no significant difference between the two techniques, which suggests that the new soil prelabeling technique compares well with the direct technique.     

Re-Evaluation of Soil Nitrogen Sampling Strategy Effects on Statistical Power

Soil sampling may be used as a decision-making tool for late-vegetative stage nitrogen (N) fertilizer applications in corn (Zea mays L.). Recommended sampling strategies following banded fertilizer applications commonly suggest taking cores from both on the fertilizer band (B) and off the band (O-B), however we hypothesized that soil nitrate concentrations (NO₃^?ppm) in the O-B were not influenced by N application rate. Analyzing samples from six experiments, we found there was a strong relationship between NO₃^?ppm and applied N rate in the B, but not the O-B position. Power analysis revealed that finding significant differences in applied N rates was only likely when sampling on the B and the difference in N rate was greater than 110 kg N ha^?1. This demonstrates that soil N sampling is not sensitive to small differences in applied N, and that O-B soil cores may only dilute the ability to detect these differences.

Abbreviations: B, on N fertilizer band; O-B, halfway between the corn row and the N fertilizer band; NO₃^?ppm, log-transformed nitrate-N concentration (ppm); NH₄⁺ppm; ammonium-N concentration (ppm); D1, 0–30 cm depth; D2, 30–60 cm depth; C-220, contrast of N rates differing by 220 kg N ha^?1; C-110, contrast of N rates differing by 110 kg N ha^?1; C-55H, contrast of N rates differing by 55 kg N ha^?1 at high N rates; C-55L, contrast of N rates differing by 55 kg N ha^?1 at low N rates; A:N, ratio of non-transformed ammonium-N to nitrate-N concentrations; 0N, unfertilized treatment; CV, coefficient of variation; SE, standard error.     

A 15N tracing technique-based analysis of the fate of fertilizer N: a 4-year case study in eastern China

The fate of fertilizer N applied with different irrigation amounts in tobacco fields was quantitatively studied by applying ¹⁵N double-labelled NH₄NO₃ in lysimeters. The ¹⁵N (fertilizer N originating from the fertilizer applied in 2011) in tobacco plants, ¹⁵N in soils and ¹⁵N loss were observed continuously from 2011 to 2014. The results showed that 21.6% of ¹⁵N was utilized by tobacco plants, 72.1% remained in the 0–60 cm soil layer and 6.3% was lost from the soil–plant system after the first season’s harvest (2011) of flue-cured tobacco. During the four seasons from 2011 to 2014, cumulative utilization of ¹⁵N by tobacco plants was 34.3%, while 54.2% remained in the 0–60 cm soil layer, and 11.5% was lost via mechanisms such as leaching and volatilization. The fate of ¹⁵N in terms of accumulation in plants and soils or losses from the soil–plant system from 2012 to 2014 was greatly affected by the fertilizer and irrigation management strategies in 2011. The results of this investigation suggest that the major amount of fertilizer N applied during the first season remains available in the soil for utilization by tobacco plants after 4 years.     

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

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

Denitrification in the rooting zone of cropped soils with regard to methodology and climate: A review

Summary Denitrification N losses can be determined by three methods. The first is by estimating the non-recovery of ¹⁵ N-labelled compounds (¹⁵N-balance method). Using this method, denitrification losses are deduced from the balance of an N budged (¹⁵N-labeled fertilizer), having accounted for transformations in soil, plant uptake, and leaching losses. The evolution of gaseous N from native soil N is not taken into account by this procedure. Studies on arable land with annual crops in the temperate zone have shown that of the fertilizer N applied, about 20–500% (10–70 kg N^* ha^–1) is not recovered at the end of the growth period. The second method of determining denitrification N losses is by in situ field measurement of ¹⁵ N ₂ and ¹⁵ N ₂ O production. Under this procedure, ¹⁵N-enriched N is applied to a plot and the denitrification N losses are determined by covering the soil. The method allows a quantitative estimate of the relative contributions to the emitted gas by both the original enriched source and the native soil N. N-evolution rates measured on arable land under a temperate climate are approximately the same order of magnitude as the N losses estimated by the non-recovery of ¹⁵ N method. The third measuring procedure is based on the acetylene inhibition phenomenon. This principle uses the inhibition of bacterial N₂O reduction to N₂ in the presence of acetylene (C₂H₂). The methoddetermines the denitrification of all NO₃ ^–-N irrespective of its source. Measurements on classical crop production systems show maximum N losses in the temperate climate of about 20–30 kg N^* ha^–1 during the growth period of annual crops. A similar level of denitrification is estimated for grassland sites under the same climate. In the subtropics (mediterranean climate with hot summers and mild winters), from both intensively cultivated arable land and grassland sites, N losses may exceed 200 kg^* ha^–1 year^–1. Without the use of irrigation the denitrification flux is negligible in spite of the high temperatures in this climate.     

Effects of repeated fertilizer and cattle slurry applications over 38 years on N dynamics in a temperate grassland soil

The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m³ ha⁻¹ yr⁻¹ over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m³ ha⁻¹ yr⁻¹, 170 kg N ha⁻¹), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a ¹⁵N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH₄⁺ and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH₄⁺ production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH₄⁺ production in the control treatment, toward a predominant NO₃⁻ production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH₄⁺ oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO₃⁻ production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO₃⁻ rather than NH₄⁺ production. The ¹⁵N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.     

Effect of nitrogen application on the A N value of soil

Pot experiments were conducted with two soils, from Rottenhaus and Seibersdorf in Austria, to ascertain whether the rate of fertilizer N application and the test crop would influence the amount of N available in the soil as assessed by the A-value method. ¹⁵N-labelled fertilizer was applied at rates of 10, 25, 40, 60, and 100 mg N kg^-1 soil, corresponding approximately to 20, 50, 80, 120 and 200 kg N ha^-1 respectively, and two crop species, barley (Hordeum vulgareL.) and non-nodulating soybean (Glycine max L.) were used to determine the soil A _N value under the various fertilizer regimes. The results showed that the Rottenhaus soil had a higher A _N value than the Seibersdorf soil, suggesting that the former was more fertile than the latter. The A _N values of both soils were significantly affected by the level of N application. When grown in the same soil, the two test crops showed significantly different fertilizer use efficiency and per cent N derived from fertilizer when the rate of N application exceeded 20 kg ha^-1. Thus, the A _N value as determined by the two test crops differed significantly for the same soil when the rate of N application was greater than 20 kg/ha. The difference was greater when the soil fertility level was high. The dependence of the A _N value on the level of N application and the species of crop seriously compromises the suitability of this method for determining plant-associated N₂ fixation. Hence, considerable caution is required when using this method to estimate plant-associated N₂ fixation.     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

Comparison of NH4 pool dilution techniques to measure gross N fluxes in a coarse textured soil

We compared gross N fluxes by ¹⁵N pool dilution in a coarse-textured agricultural soil when ¹⁵N was applied to the soil NH₄⁺ pool by either: (i) mixing a ¹⁵NH₄NO₃ solution into disturbed soil or (ii) injection of ¹⁵NH₃ gas into intact soil cores. The two techniques produced similar results for gross N mineralization rates indicating that NH₄⁺ production in soil was not altered by soil disturbance, method of application (gas vs. solution), or amount of N applied. This was not the case for immobilization rates, which were twofold higher when ¹⁵N label was applied to the soil NH₄⁺ pool with the mixing technique compared to the injection technique. This was attributed to the fact that more NH₄⁺ was applied with the mixing technique. Estimates of gross nitrification were accompanied by large error terms meaning differences between ¹⁵N labeling methods could not be accurately assessed for this process rate.