Comparison of Factors Affecting Soil Nitrate Nitrogen and Ammonium Nitrogen Extraction

Extraction of soil nitrate nitrogen (NO₃ ^?-N) and ammonium nitrogen (NH₄ ⁺-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO₃ ^?-N and NH₄ ⁺-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L^?1 calcium chloride (CaCl₂), 1 mol L^?1 potassium chloride (KCl), and 0.5 mol L^?1 potassium sulfate (K₂SO₄)] and concentration (0.5, 1, and 2 mol L^?1 KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (?18 ^oC, 4 ^oC, and 25 ^oC) of extracted solution. The recovery of soil NO₃ ^?-N and NH₄ ⁺-N was also measured to compare the differences of three extracting reagents (CaCl₂, KCl, and K₂SO₄) for NO₃ ^?-N and NH₄ ⁺-N extraction. Air drying decreased NO₃ ^?-N but increased NH₄ ⁺-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO₃ ^?-N and NH₄ ⁺-N concentrations compared to other treatments. The concentrations of extracted NO₃ ^?-N and NH₄ ⁺-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO₃ ^?-N and NH₄ ⁺-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO₃ ^?-N and 94.9% for NH₄ ⁺-N. K₂SO₄ was not found suitable for NO₃ ^?-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl₂ was only 57.3% for NH₄ ⁺-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L^?1 KCl could fully extract NO₃ ^?-N. A ratio of 10:1 of solution to soil was adequate for NO₃ ^?-N extraction, whereas the NH₄ ⁺-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at ?18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO₃ ^?-N concentration, whereas the NH₄ ⁺-N concentration varied greatly with storage temperature. Storing the extracted solution at ?18 ^oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO₃ ^?-N and NH₄ ⁺-N concentrations. Compared with the immediate extraction, the averaged NO₃ ^?-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH₄ ⁺-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L^?1 KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO₃ ^?-N, whereas the concentration of extracted NH₄ ⁺-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at ?18 ^oC could improve the reliability of NO₃ ^?-N and NH₄ ⁺-N results.     

Leaching Methods Can Underestimate Mineralization Potential of Soils

Aerobic incubations to estimate net nitrogen (N) mineralization typically involve periodic leaching of soil with 0.01 M calcium chloride (CaCl₂), so as to remove mineral N that would otherwise be subject to immobilization. A study was conducted to evaluate the accuracy of leaching for analysis of exchangeable ammonium (NH₄⁺)-N and nitrate + nitrite (NO₃^?+ NO₂^–)-N, relative to conventional extractions using 2 M potassium chloride (KCl). Ten air-dried soils were used, five each from Illinois and Brazil, that had been amended with NH₄⁺-N (1 g kg^?1) and NO₃^–-N (0.6 g kg^?1). Both methods were in good agreement for inorganic N analysis of the Brazilian Oxisols, whereas leaching was significantly lower by 12–48% in recovering exchangeable NH₄⁺-N from Illinois Alfisols, Mollisols, and Histosols. The potential for underestimating net N mineralization was confirmed by a 12-wk incubation experiment showing 9–86% of mineral N recoveries from three temperate soils as exchangeable NH₄⁺.     

Colorimetric interference and recovery of adsorbed ions from ion exchange resins

Abstract

Analytical interference in the colorimetric determinations of ammonium and nitrate was examined in various KCl extracts of several ion exchange resins. No analytical interference was found in the colorimetric NO₃ ^‐‐N determination in any extract of any resin. However, a mixed‐bed (cation + anion) exchange resin extract substantially affected the colorimetric determination of NH₄ ^‐ ‐N. Recovery of adsorbed ammonium and nitrate from ion exchange resins was also studied as a function of KCl extractant strength and number of extractions. The recovery of adsorbed NO₃ ^‐‐N in the first extraction increased with increasing KCl concentration, with a 2 M solution recovering about 80%. However, a 1 M KCl solution gave the greatest recovery of ammonium‐N, recovering about 75–80% of the adsorbed ammonium. The second extraction with the same concentration of KCl solution was greater with the 0.5 and 2.0 M than with the 1 M solution so that total NH₄ ⁺‐N recovery after two extractions was about the same for all three KCl concentrations. The recovery of resin‐adsorbed NH₄ ⁺‐N and NO₃ ^‐‐N appeared independent of their concentrations on the resins.     

Regression equations to monitor inorganic nitrogen changes in fallow and wheat soils

Soil NH⁺₄-N and NO^?₃-N at five soil depths (0–10, 10–20, 20–40, 40–60, 60–80 cm) and some environmental variables were measured in a field trial under fallow and wheat for 9 months.Significant linear and quadratic relationships were obtained relating soil NH⁺₄-N, NO^?₃-N, NH⁺₄-N + NO^?₃-N, and NH⁺₄-N + NO^?₃ + total-N uptake by wheat to soil heat accumulation (temperature), moisture, and rainfall. R² values generally decreased with soil depth and the maximum value (37%) was obtained for NO^?₃-N changes in the topsoil (0–10 cm).Although a considerable amount of variation in the inorganic values recorded is not included in the equations, our results suggest that the development of the above relationships particularly of the quadratic type are useful to predict crop requirements for N by measurement of environmental variables in the field.     

Effect of ammonium-, nitrite- and nitrate nitrogen on anaerobic nitrogenase activity in soil

Sandy loam soil, with added glucose, was incubated anaerobically under N₂ and subjected to repeated 1-h C₂H₂ reduction assays. In the presence of 1% glucose the addition of 50 μg NH₄⁺ ?N/g or of 20 μg NO^?₃ N/g (untreated soil contained 1.2 μg NH⁺₄?N and 7.10 μg NO^?₃-N/g) caused at least some suppression of nitrogenase activity. Activity developed when the KCl-extractable soil inorganic nitrogen concentration dropped below 35 μg/g. In the presence of 0.1 or 0.05% glucose the addition of 5 μg NH⁺₄?N/g caused some suppression of nitrogenase activity. However, activity developed when the soil NH₄⁺-N concentration dropped below about 4 μg/g. With 0.1% glucose and 5 μg added NO^?₂ N/g, activity did not develop until the soil NO^?₂ -N concentration dropped to zero. Added NO^?₃ N was rapidly reduced and denitrified to NO^?₂- N, N₂O-N and NH⁺₄ N and furthermore caused some inhibition of CO₂ evolution. The data from NH₄^?-addition experiments are consistent with a nitrogenase repression/ derepression threshold of 4 and 35μg NH⁺₄-N/g at 0.05 and 1% glucose concentrations, respectively. The data from NO^?₂- and NO^?₃-addition experiments suggest a combination of repression and toxicity effects in the presence of added NO^?₃ N.     

Improved diffusion methods for determination of inorganic nitrogen in soil extracts and water

Diffusion methods previously developed for inorganic-N analysis of soil extracts were modified to improve reliability, increase the dynamic range, extend the scope of applications, and simplify the processing of samples for N-isotope analysis. In these methods, the soil extract is treated with MgO, or MgO plus Devarda‘s alloy, in a 473-ml (1-pint) wide-mouth Mason jar to convert NH₄ ⁺-N, NO₃ ^–-N, and/or NO₂ ^–-N to NH₃-N. The NH₃ thereby liberated is collected in H₃BO₃-indicator solution in a Petri dish suspended from the Mason-jar lid and determined quantitatively by acidimetric titration. With the modifications described, analyses can be performed on 10- to 100-ml samples of water, 0.5MK₂SO₄, 1MKCl, 2MKCl, or 4MKCl, at temperatures between 20 and 30°C. Recovery from 10 or 20ml was quantitative in 18–80h with up to 4mgN; recovery from 50 or 100ml was quantitative in 3–13 days with up to 2mgN. Removal of H₃BO₃ for N-isotope analysis by the Rittenberg process was effected using methanol. Mason-jar diffusion methods are much simpler and more convenient than conventional steam distillations. Comparative studies showed that quantitative determinations are more accurate and precise by diffusion than by distillation. Received: 15 May 1996     

铵硝比和磷素营养对菠菜生长、氮素吸收和相关酶活性的影响

通过水培试验研究了不同铵硝比的氮素营养和磷素营养对菠菜生长、氮素吸收及硝酸还原酶活性（NRA）和谷氨酰胺合成酶活性（GSA）的影响。结果表明：在供磷水平相同时，菠菜的生物量随着铵硝比的降低而降低，但铵硝比为25：75与0：100两个处理之间没有显著差异；在铵硝比相同时，随着营养液中磷含量的增加，菠菜的生物量随之增加。菠菜茎叶中硝酸盐的含量随着铵硝比和磷水平的降低而升高。不同铵硝比处理，菠菜含氮量没有明显差异，随着磷水平的提高，菠菜植株含氮量有升高的趋势，但各处理之间差异不显著；受到生物量显著差异的影响，菠菜植株中氮素累积量随着铵硝比的降低和磷素水平的增加而增加。在铵硝混合营养条件下，缺磷会显著抑制菠菜对铵态氮和硝态氮的吸收，且磷索缺乏对菠菜吸收硝态氮的抑制作用要大于对铵态氮吸收的抑制作用。铵硝比相同时，随着营养液中磷索供应量的增加，菠菜茎叶中NRA显著增加；但是营养液中铵硝比较高时，会显著抑制菠菜茎叶中NRA，而铵硝比较低时，则有利于提高菠菜的NRA。缺磷会严重抑制GSA；在磷素水平相同时，随着营养液中铵比例的增加，菠菜茎叶中GSA显著增加。为此，在一些硝酸盐含量较高的土壤上栽培蔬菜时，可以采取增施适量磷肥的方法，以降低叶菜的硝酸盐含量。     

Effect of Inorganic N Composition of Fertilizers on Nitrous Oxide Emission Associated with Nitrification and Denitrification

We studied the effect of repeated application (once every 2 d) of a fertilizer solution with different ratios of NH₄ ⁺ - and NO₃ ^?-N on N₂O emission from soil. After the excess fertilizer solution was drained from soil, the water content of soil was adjusted to 50% of the maximum water-holding capacity by suction at 6 × 10³ Pa. Repeated application of NH₄ ⁺- rich fertilizer solution stimulated nitrification in soil more than NO₃ ^?-rich fertilizer. Although the evolution of N₂O through nitrifier denitrification tended to increase with the repeated addition of a fertilizer solution rich in NH₄ ⁺ rather than in NO₃ ^?, the contribution of nitrifier denitrification remained at levels of 20 to 36% of the total emission regardless of the inorganic N composition. The total emission of N₂O also tended to increase with the application of NH₄ ⁺- rather than NO₃ ^?-rich fertilizer. It was suggested that the coupled process of nitrification and denitrification at micro-aerobic sites became important when fertilizer rich in NH₄ ⁺ was applied to soil under relatively aerobic conditions.     

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.     

An evaluation of some manual colorimetric methods for the determination of inorganic nitrogen in soil extracts

Abstract

A number of manual colorimetric methods for the determination of inorganic nitrogen in 1 M KCl soil extracts were investigated to find techniques that were inexpensive, rapid, versatile and suitable for laboratories with limited analytical equipment. Three colorimetric methods for No^? ₃‐N determination were evaluated and only the copperised/cadmium reduction technique suffered no significant interference from the Cl^? present in the extracting solution. A phenol‐hypo‐chlorite (Berthelot) procedure for NH⁺ ₄‐N determination and the Griess‐Ilosvay method for NO^? ₂‐N determination were both found suitable for N determination in 1M KC1 soil extracts. The reliability and accuracy obtainable with the manual colorimetric methods described was shown to be comparable with that obtained from colorimetric analyses performed using an AutoAnalyser.     

Acetylene and N-serve effects upon N2O emissions from NH4+ and NO3− treated soils under aerobic and anaerobic conditions

N₂O emissions from soils treated with NH₄⁺-N under aerobic conditions in the laboratory were 3- to 4-fold higher than those from controls (no extra N added) or when NO₃^?-N was added. Although the emission of N₂O-N in these field and laboratory experiments represented only 0.1–0.8% of the applied fertilizer NH₄⁺-N and are therefore not significant from an agronomic standpoint, these studies have conclusively demonstrated that the oxidation of applied ammoniacal fertilizers (nitrification) could contribute significantly to the stratospheric N₂O pool.Like N-serve, acetylene was shown to be a potent inhibitor of nitrification as it stopped the oxidation of NH₄⁺-N to (NO₃⁺-N + NO₂^?)-N and hence reduced the evolution of N₂O from nitrification within 60 min after its addition.Although high amounts of NO₃^?-N were present, the rate of denitrification was very low from soils with moisture up to 60% saturation. The further increase in the degree of saturation resulted in several-fold increase of denitrification which eventually became the predominant mechanism of gaseous N losses under anaerobic conditions.     

不同铵硝比对菠菜生长、安全和营养品质的影响

通过水培试验,研究了等氮条件下5种不同铵硝比对菠菜生长和品质的影响。结果表明:(1)从铵硝比100∶0到0∶100,菠菜地上部鲜重不断增加,铵硝比为0∶100时,菠菜的鲜重达最大值;但铵硝比25∶75和0∶100两个处理菠菜的干物重没有显著差异(p<0.05)。(2)随着铵硝比的降低,菠菜茎叶中硝酸盐、亚硝酸盐的含量均表现为线性增加;菠菜茎叶中可溶性草酸的含量和营养液中铵硝比之间呈现出二次曲线相关,在铵硝比为25∶75时,菠菜茎叶中草酸含量最低。适当增施铵态氮有利于降低菠菜硝酸盐、亚硝酸盐及草酸的含量。(3)增铵可以提高菠菜Vc含量,铵硝比为50∶50的处理菠菜Vc含量最高;随着铵硝比的下降,菠菜茎叶中可溶性糖的含量逐渐降低,而粗蛋白的含量则以铵硝比25∶75处理最高。     

Uptake Kinetics of Different Nitrogen Forms by Chinese Kale

In this paper, the uptake kinetics of various nitrogens (nitrate (NO₃^?), ammonium (NH₄⁺), urea, amino acid) by Chinese kale (Brassica oleracea L. var. Bailey) were studied under hydroponic condition. The results indicated that the uptake kinetics of organic and inorganic nitrogen (N) by Chinese kale conform to the Michaelis–Menten equation, and the maximum uptake rate (Vmax) and affinity index (1/Km) showed nitrate (NO₃^—N) > ammonium (NH₄⁺-N) > urea-N > Gly-N, with significant differences between treatments (p < 0.05). Adding different types of N to NO₃^? nutrient solution had little impact on its affinity, but significantly decreased the NO₃^? Vmax, which showed NO₃^–N > NO₃^? + NH₄⁺ > NO₃^? + urea > NO₃^? + Gly. Chinese kale preferred inorganic N to organic N, with NO₃^? preceding NH₄⁺. Adding organic and NH₄⁺ N to nutrient solution reduced the NO₃^? uptake capacity by the plant.     

Biodegradation of an oily waste as influenced by nitrogen forms and sources

Biodegradation rates of oily waste in soil can be limited by mineral nutrients, particularly N and P. A laboratory incubation experiment was carried out to investigate the influence of N forms, nitrate (NO^? ₃-N) vs ammonium nitrogen (NH⁺ ₄-N), and sources, i.e., the conjugate cations/anions, on C mineralization rate (CMR) was determined daily by measuring the CO₂ evolved using gas chromatography. The CMR and the cumulative C mineralized (CCM) varied with the form and/or the source of N applied. The greatest enhancement in CMR occurred in the NO^? ₃-treatments in which the source conjugate cation was Ca⁺². The addition of P fertilizer further enhanced C mineralization rates irrespective of the form and/or the source of N added. The results show that up to 45% of the added oily waste mineralized as CO₂-C in 28 d. The residual P and N (NO^? ₃-N plus NH⁺ ₄-N) data showed that approximately 90% of the added P and N were utilized for oil decomposition. The amount of residual NO^? ₃-N appeared to have an inverse relationship with CCM. The NO^? ₃-N utilization occurred at the expense of NH⁺ ₄-N and this was particularly high in the treatments which received P.     

Nitrification and denitrification in an acidic soil of tea (Camellia sinensis L.) field estimated by δ15N values of leached nitrogen from the soil columns treated with ammonium nitrate in the presence or absence of a nitrification inhibitor and with slow-release fertilizers

Forty-two-day-old wheat (Triticum aestivum L. var. Asakazekomugi) plants were treated with complete, K-free (—K), Ca-limited (—Ca), and Mg-free (—Mg) nutrient solutions for 10 days using 2 mM NH₄NO₃ as the nitrogen source, which was replaced with 4 mM ¹⁵ NH₄C1 or Na¹⁵NO₃ for the subsequent 2 days to investigate the absorption, translocation, and assimilation of inorganic nitrogen in relation to the mineral supply. In another experiment plants were grown on NO₃ ^?, NH₄ ⁺, NH₄N0₃, and K-free and Ca-limited NH₄N0₃ nutrient solutions for 10 days, and then in the latter three treatments the nitrogen source was replaced with NO₃ ^? and half of the —K plants received K for 6 days to examine the changes in the nitrate reductase activity (NRA).

Wheat plants absorbed NH₄ ^?N and NO₃-N at a similar rate. Influence of K on the absorption of N0₃-N was stronger than that on the absorption of NH₄-N in wheat plants. The supply of K to the —K plants increased the absorption of NO₃-N, while the absorption of NH₄-N still remained at a lower rate in spite of the addition of K. A limited supply of Ca and lack of Mg in nutrient media slightly affected the absorption of NH₄-N. The influence of K was stronger on the translocation of nitrogen from roots to shoots, while Ca and Mg had little effect. When K was supplied again to the —K plants the translocation of NO₃,-N was more accelerated than that of NH₄-N. Incorporation of NH₄-N into protein was higher than that of NO₃-N in all the tissues; root, stem, and leaf. Assimilation of NH₄-N and NO₃-N decreased by the —K and —Mg treatments.

Leaf NRA of wheat plants decreased in the —K and —Ca plants. Higher leaf NRA was found when K was given again to the —K plants than when the plants were continuously grown in K-free media. Replacement of NO₃ ^? with NH₄ ⁺ as the nitrogen source caused a decline of leaf NRA, while the supply of both NH₄ ^?N and NO₃-N slightly affected the leaf NRA.     

Nitrification and ammonification in acid forest litter and humus as affected by peptone and ammonium-n amendment

Samples of strongly acid forest litter and humus from beneath Sitka spruce, heather, Scots pine and larch from two sites in north-east Scotland were incubated aerobically at 20°C in the laboratory. At the Glen Tanar site, spruce litter and larch humus showed significant nitrification and ammonification whereas spruce humus and Scots pine humus produced only NH₄⁺-N. Heather humus showed no net mineralization. At the Fetteresso site, application of fertilizer N, P and K to Sitka spruce up to 3 yr previously, significantly stimulated the production of NO₃⁻-N in both litter and humus.Amendment of the samples with organic N as peptone caused significant increases in NO₃⁻-N production in those samples that already showed nitrification. The increases in NO₃⁻-N generally represented a low proportion of the added peptone-N. Amendment with NH₄⁺-N as (NH₄)₂SO₄ either had no effect or significantly reduced NO₃⁻-N production (in larch humus). The results suggest the occurrence of heterotrophic nitrification in some of these forest samples.Net immobilization of NH₄⁺-N was typically greater in NH₄⁺-N amended than in peptone amended samples, except for heather humus which showed complete immobilization of both N sources.Total mineral N produced at the end of the aerobic incubation was correlated (P < 0.01) with NH₄⁺-N produced during a 30-day anaerobic incubation at 30°C. Net NO₃⁻-N production was greater in litter than in the corresponding humus samples and was correlated (P < 0.001) with initial organic N soluble in 1 m KCl.     

Interactions between Inorganic Nitrogen Nutrition and Root Development

Root development responds not only to the quantity of inorganic nitrogen in the rhizosphere, but to its form, NH₄⁺ or NO₃^?. Root growth of tomato showed a hyperbolic response to soil levels of inorganic nitrogen: very few roots were found in soil blocks depleted in inorganic nitrogen, roots proliferated as soils increased to 2 μg NH₄⁺-N g^?1 soil or 6 μg NO₃^?-N g^?1 soil, and root growth declined in soils with the higher levels of inorganic nitrogen. High NH₄⁺ concentrations inhibited root growth, but low concentrations promoted the development of an extensive, fine root system. Supply with NO₃^? as the sole nitrogen source led to a more compact root system. These differences in root morphology under NH₄⁺ and NO₃^? nutrition may be mediated through pH. Rice and maize roots absorbed NH₄⁺ most rapidly right at the apex and appeared to assimilate this NH₄⁺ in the zone of elongation. During NH₄⁺ assimilation, root cells must release protons, and the resulting acidification around the walls of cells in this region should stimulate root extension. By contrast, NO₃^? absorption reached a maximum in the maturation zone of rice and maize roots, and this NO₃^? was probably assimilated in more basal regions. Absorption of NO₃^? requires proton efflux, whereas NO₃^? assimilation requires proton influx. The net result under NO₃^? nutrition was only subtle shifts in rhizosphere pH that probably would not influence root elongation. The signal through which roots detect changes in rhizosphere NH₄⁺ and NO₃^? levels is still obscure. It is proposed that a product of nitrogen metabolism such as nitric oxide serves as a signal.     

Improving the Accuracy of Diffusion for Inorganic 15N Analyses of Soil Extracts and Water

Sequential diffusion techniques used to speciate inorganic nitrogen-15 (¹⁵N) during soil or water analysis are complicated by incomplete recovery of ammonium (NH₄⁺)-N, introducing error in the subsequent determination of nitrate (NO₃^–)-N. Based on studies to evaluate different strategies for minimizing cross-contamination error in Mason-jar diffusions, a simple cleaning technique was developed that involves an additional 6-h diffusion using 0.6 M boric acid (H₃BO₃) at room temperature following the recovery of NH₄⁺-N. This technique was 60–87% effective for reducing cross-contamination of unlabeled NO₃^–-N by labeled NH₄⁺-N and became more effective for controlling analytical error with decreasing sample volumes, lower NH₄⁺-N enrichment, and larger quantities of NO₃^–-N. When used with the cleaning technique described, sequential diffusions were far superior for ¹⁵N analysis of NO₃^–-N, as compared to the nonsequential approach that involves an isotope dilution calculation after separate diffusions to determine NH₄⁺-N and total mineral N.     

Sorption and Desorption Characteristics of Ion Exchange Resins to Estimate Leaching Losses in the Field

Laboratory experiments were conducted to determine sorption and desorption properties of ion exchange resins based on recovery rates. Dissolved nitrate (41.6 mM NO₃ ^? and 166.5 mM NO₃ ^?), ammonium (63.9 mM NH₄ ⁺ and 256.1 mM NH₄ ⁺), phosphate (3.5 mM PO₄ ^3– and 13.9 mM PO₄ ^3–), and potassium (16.11 mM K⁺ and 63.94 mM K⁺) were factorially combined and added at flow rates of 5 L at 3 and 9 h to an exchange resin?/?sand mixture. Six extractions with 1 M sodium chloride (NaCl) desorbed >90% of the added ions. Apparent recovery rates for nitrate (NO₃-N) averaged 99% (±6), for ammonium (NH₄-N) averaged 100% (±8), and for phosphate (PO₄-P) averaged 109% (±6). Apparent recovery rates of potassium (K) were erroneously high (151% ± 12) and largely reflected analytical interference problems with the inductively coupled plasma (ICP) method. Concentrations of dissolved ions did not affect recovery rates of any studied ion.     

不同形态氮素比例对不同小白菜品种生物量和硝酸盐含量的影响

利用NO-3 N∶NH 4 N为 10 0∶0、5 0∶5 0和 0∶10 0三个硝铵配比的营养液对 12个不结球小白菜品种进行水培试验。结果表明 :不同的硝铵配比对不同品种小白菜的生物量、叶绿素SPAD值、硝酸盐积累量等有着显著的影响 ,同一氮源培养下不同的小白菜品种间也表现出显著的差异 ;12个小白菜品种叶绿素SPAD值随营养液中的NH 4 N比例的增加而升高 ,两者间存在着显著的正相关 ;单株生物量除亮白叶 1号和五月慢在全硝培养中生物量较其他两种配比大外 ,其他 10个品种均在 5 0∶5 0硝铵营养液中表现最好 ;供试的 12个小白菜品种中有 9个品种的硝酸盐积累量随着NH 4 N比例的增加而下降 ,表明适当地配施铵态氮较纯硝营养液能获得更好的产量、更高的叶绿素SPAD值和较低的硝酸盐积累量。