氮肥品种和用量对水稻产量和镉吸收的影响研究

采用盆栽试验,研究了Cd污染土壤上,不同氮肥品种和用量对水稻产量和Cd吸收的影响。结果表明,与磷钾配施的4个氮肥处理中,施用尿素处理水稻产量最高,其次为施(NH4)2SO4和NH4Cl处理,施NH4NO3处理水稻产量最低。与施(NH4)2SO4、NH4NO3和尿素处理相比,施NH4Cl处理可显著增加水稻对Cd的吸收,并促进Cd由秸秆向籽粒的转移;而其他3种氮肥对水稻秸秆和籽粒中Cd含量的影响效应相当。适量尿素[0.2g(N)·kg-1]处理能显著降低水稻籽粒Cd含量,而不施尿素和高量尿素处理都显著提高了水稻籽粒中的Cd含量。研究表明,在Cd污染的水稻土上,采用抗Cd污染的水稻品种和优化肥、水管理措施,可使稻米中Cd含量低于国家无公害大米的限量指标。     

Nitrogen fertilization inhibits soil microbial respiration regardless of the form of nitrogen applied

We tested how amendments of different forms of nitrogen (N) affect microbial respiration rates by adding six different forms of N (NH₄NO₃, (NH₂)₂CO (urea), KNO₃, NH₄Cl, (NH₄)₂SO₄, Ca(NO₃)₂) to three distinct soils. All inorganic N forms led to a net reduction in microbial respiration, and the magnitude of the observed response (up to 60 % reduction) was consistent across all soils and negatively correlated with N concentration. Urea also reduced respiration rates in nearly all cases, but the effect was attenuated by the associated input of labile organic carbon. We observed decreases in respiration regardless of soil type, the specific N counter ion, N added as NH₄⁺ or NO₃⁻, or the effects of N form on soil pH, suggesting that decreases in respiration rates were mainly a direct result of the increase in soil N availability, rather than indirect effects caused by the form of N added.     

Estimation of critical limit for zinc in rice soils

Abstract

Twenty surface soil samples were collected from Nainital Tarai (foothills of Himalya) where “Khaira”; disease (Zn deficiency of rice) is prevalent. Rice (Oryza sativa L. variety IR‐8) was grown in pots for 8 weeks after transplanting. Experiments were conducted to determine the suitability of five soil Zn extractants: dilute acid (HCl + H₂SO₄) mixture; DTPA‐(NH₄) ₂CO₃, pH 7.3; dithizone; NH₄OAc, pH 4.6; and 2N MgCl₂ to predict Zn deficiency. Critical values for soil available Zn were established for rice by the old and new Cate and Nelson procedures¹.

Zinc extracted from the soils with dithizone; NH₄OAc, pH 4.6; 0.2N MgCl₂. and DTPA‐(NH₄) ₂CO₃ pH 7.3 was significantly correlated with the uptake of Zn by the rice plants. The correlation between Zn extracted with the dilute acid (HCl + H₂SO₄) mixture and plant Zn was not statistically significant. The ex‐tractants which extracted greater quantities of Zn gave higher critical values and vice versa. It is concluded that all extracting solutions except the dilute acid (HCl + H₂SO₄) mixture were found to he suitable for predicting available Zn in rice soils of Tarai.     

氮肥对镉在土壤-芥菜系统中迁移转化的影响

以芥菜为研究对象, 采用盆栽试验, 探讨了不同用量的5种氮肥对污染农田土壤中镉(Cd)在土壤–根系–地上部迁移累积的影响。结果表明: 5种氮肥均促进了芥菜根系对Cd的吸收, 且根系Cd含量随施氮量的增加而增加; 但根系吸收转运Cd的能力随氮肥施用量的增加呈先降后增的变化趋势。在≤200 mg(N)·kg^-1(土)的施氮水平下, CO(NH₂)₂和Ca(NO₃)₂处理能显著降低芥菜地上部Cd含量, 降低幅度分别为13%~29%和24%~30%。在施氮量相同的条件下, NH₄Cl和(NH₄)₂SO₄显著降低了土壤pH, 增加了土壤DTPA-Cd含量, 促进了芥菜对Cd的吸收。本试验条件下, 200 mg(N)·kg^-1(土)的CO(NH₂)₂在增加芥菜产量和降低芥菜地上部Cd含量等方面优于其他氮肥处理。     

Redistribution of cadmium,copper, lead,nickel, and zinc among soil fractions in a contaminated calcareous soil after application of nitrogen fertilizers

This study was conducted to evaluate the redistribution of the heavy metals Cd, Cu, Pb, Ni, and Zn among different soil fractions by N fertilizers. In a lab experiment, soil columns were leached with distilled water, KNO₃, NaNO₃, NH₄NO₃, or Ca(NO₃)₂ · 4H₂O. After leaching, soil samples were sequentially extracted for exchangeable (EXCH), carbonate (CARB), organic‐matter (OM), Mn oxide (MNO), Fe oxide (FEO), and residual (RES) fractions. Distilled water significantly increased the concentrations of Cd and Ni in EXCH fraction, while concentration of Cu and Zn did not change significantly. Application of KNO₃, NaNO₃, NH₄NO₃, or Ca(NO₃)₂ · 4H₂O significantly increased the concentrations of Cd and Zn in EXCH fraction, while concentration of Pb and Ni was decreased. Application of all fertilizers caused an increase of Cu in the OM fraction. Moreover, leaching with these solutions significantly increased Cd [except in Ca(NO₃)₂ · 4H₂O], Cu, and Zn concentrations in the CARB fraction, while Pb and Ni concentrations were decreased. With application of all leaching solutions, Zn in the EXCH, CARB, FEO, and MNO fractions was significantly increased, while Zn in the OM fraction did not change. The mobility index indicated that Ca(NO₃)₂ · 4H₂O increased the mobility of Cd, Cu, and Zn in the soil, whereas NaNO₃ decreased the mobility of Pb and Ni in the soil. The mobility index of Pb decreased by all leaching solutions. Thus, these results suggest that applying N fertilizers may change heavy‐metal fractions in contaminated calcareous soil and possibly enhance metal mobility and that N‐fertilization management therefore may need modification.     

Estimation of critical limit for zinc in rice soils

Abstract

Twenty surface soil samples wore collectod from Nalnltal Tarai (foot‐ hills of Himalya) where ‘Khaira’ disease (Zn deficiency disease of rice) was prevalent. Rice (Oryza sativa L. variety IR ‐ 8) was grown in pots for 8 weeks after transplanting. Experiments were carried out to find the suitability of five soil Zn extractants viz. dilute acid (HC1 ‐ H₂SO₄) mixture, DTPA‐ (NH₄) ₂CO₃, pH 7.3, dithizono, NH₄ ‐ Ac, pH 4.6 and MgCl₂. Critical limits of available Zn in soils were established for rice crop by old and new Cate and Nelson procedures.

Zn extracted from the soil with NH₄ ‐ Ac, pH 4.6, dithlzono, MgCl₂, and DTPA‐(NH₄) ₂CO₃, pH 7.3 was significantly correlated with the uptake of Zn by the rice plants. The correlation of Zn uptake with dilute acid mixture extractable Zn was not significant. The extractant which extracted more Zn gave higher values of critical limit and vice versa. It is concluded that all extracting solutions except dilute acid mixture were found to be suitable for predicting available Zn in rice soils of Taral.     

Stimulative effects of elemental sulfur in the presence of ammonium on chile and broccoli growth in calcareous soils

The chile pepper plant seldom responds to N and P fertilizers on fertile soils. Surplus industrial H₂SO₄ and elemental S have created interest in “mining”; calcareous soils for additional supplies of P, Ca, Mg and micronutrients. The effect of variable S, on the growth of chile and broccoli was evaluated holding other nutrients constant. Growth of chile and broccoli plants was significantly increased in the greenhouse and chile yield increased in the field. Incremental S additions increased the water extractable and desorbable Ca + Mg and P contents of soil. The total N and K content of chile plant grown in the greenhouse increased, and then decreased, P decreased, as S rates increased. Yield of dry red chile with constant N peaked at 16.5 g S m^‐2 and then decreased with increasing S in the field. Rroccoli responded more to S application than to directly applied foliar micronutrient solutions (Fe and 7n), and responded much better to (NH₄)₂SO₄ + S than to Ca(NO₃)₂ at equivalent N rates. Increased soluble Ca + Mg content of the soil in the presence of S was thought to influence plant absorption of NH₄ and/or K.     

WHEAT RESPONSE TO NITROGEN SOURCE UNDER SALINE CONDITIONS

ABSTRACT

The influence of nitrogen (N) sources on biomass yield and nutrient uptake of wheat (Triticum aestivum L.) under saline conditions was studied in a greenhouse experiment. Six different forms of N {nitrate-N as Ca(NO₃)₂, urea-N [CO(NH₂)₂], ammonium-N as (NH₄)₂SO₄, nitrate-N+urea-N, nitrate-N+ammonium-N and a control (no N fertilizer)} were factorially combined with three levels of salinity to give a total of 18 treatments that were replicated three times. Each of the five levels of applied N was at the rate of 100?kg?ha^?1. The salinity levels (ECe) were 6.2 and 12.1?dSm^?1, denoted as S ₁ and S ₂ and untreated soil (S ₀), respectively. A basal dose of phosphorus (P) and potassium (K) was also applied. Five wheat plants were grown in each pot for six weeks. Data were collected for shoot and root biomass and shoot samples were analyzed for N, P, K, calcium (Ca), magnesium (Mg), sodium (Na), chloride (Cl), and micronutrients contents. Plant growth and nutrient uptake were influenced by both salinity and source of N. As expected, increasing salinity decreased dry matter production of shoot and root, whereas N application increased plant growth across all levels of salinity. The total dry biomass (shoot and root) of wheat was significantly higher in combined N treatments than in single sources. Irrespective of N forms most of the nutrient concentrations in the shoot was increased with increasing level of salinity. Among the fertilizers the concentration of cation was higher in nitrate-treated plants than in other forms of N. Ammonium-N and urea-N tended to inhibit the uptake of cations compared to nitrate-N under saline conditions. The trend for P and Cl concentration was almost opposite to that of cations concentration in the shoot. The uptake of nutrients seemed to be influenced by cation–anion balance in soil-plant system. Nitrogen concentration of shoot was greatly enhanced by all forms of N in the following order: Ni>NiAm>Am>NiUr>Ur>control. The interactive effect of salinity and fertilizer on iron (Fe), manganese (Mn), and zinc (Zn) contents was not consistent. Among the fertilizers the concentration of trace elements in the shoot was also not significantly different. It was concluded that the plant growth and nutrient concentration of shoot could depend upon N source and level of salinity. The mixed application of both ammoniacal and nitrate forms of N could possibly be conducive to plant growth in salt affected soils.     

The effect of Zinc application on the methods which can be used to determine available Iron content of soils

This research was carried out in order to find out the effect of zinc application on the methods which can be used to determine available iron content of paddy soils. For this purpose, an experiment was carried out in greenhouse conditions by growing maize. The effect of increasing rates of zinc application on biological indexes of maize plants was determined. Eight chemical extraction methods for available iron content of soils were used and these methods are: 0.005 M DTPA+0.01 M CaCl₂+0.1 M TEA; 0.005 N HCl+0.025 N H₂SO₄; 1 N NH₄Oac; 0.01 N Na₂EDTA+1 N NH₄Oac; 2 N MgCl₂; 0.01 M Na₂EDTA +1 M (NH₄)₂CO₃; 0.01 N Na₂EDTA and 0.005 M DTPA+NH₄HCO₃. The relationships between biological indexes and chemical extraction methods were determined at the end of the experiment. The highest correlation coefficient was determined with the method 0.005 M DTPA+0.01 M CaCl₂+0.1 M TEA. The iron content of maize plants decreased with zinc application and dry matter content increased with zinc application.     

The behaviour of nitrogen fertilizers in neutral and acid loess soils II. Distribution and balances of 15 N-tagged nitrogen

A study, to investigate the remaining fertilizer-N in undisturbed soil columns from agricultural and forest soils after 60 days of percolation (see part I), was carried out. The columns were fertilized with two 15-N tagged nitrogen fertilizers (Ca(NO₃)₂ and (NH₄)₂SO₄) at a rate of 80kg N/ha. The investigation involved the distribution and fractionation of nitrogen in the soils. The soil columns were therefore cut into 8 segments and nitrogen forms analyzed were: total N, fixed-NH₄-N, exchangeable NH₄-N, NO₃-N and organic N. Consequently, the N-balances were established. It was ascertained that immobilization and gaseous losses from the fertilizers were higher after NH₄- than after NO₃-application in both soils. Immobilization in all treatments was temperature dependent and concentrated at the upper segments of the columns. The forest soil had higher incorporation than the agricultural soil. Nitrification was low in the forest soil while in the agricultural soil there was a fairly high nitrification even at 4°C. The balances showed losses of nitrogen in the range of 10 to 35 %. While this agrees with the findings of other workers in case of the agricultural soil, it leaves however, some questions unanswered in case of the forest soil.     

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO₃⁻ and NH₄⁺ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha⁻¹ yr⁻¹. Soil samples from surface (0–10 cm) and subsurface layers (10–20 cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.     

Effect of some cations,anions, and organic residues on potassium leaching and fractionation in calcareous sandy loam soil

Potassium (K) is one of the essential elements for plants. There has been enough research to determine pollution of nitrogen (N), phosphorus (P), and heavy metals in soil. However, by comparison research on the storage and transport of K has been neglected. Chemical fertilizer usage leads to serious environmental problems in Iran. Leaching of K can be affected by type of anions and cations present in the chemical fertilizers. Potassium leaching experiments were performed using 10 mM NH₄Cl, (NH₄)₂HPO₄, NH₄H₂PO₄, NH₄NO₃, NH₄OAC, CaCl₂, Ca(NO₃)₂, NaNO₃, and CO(NH₃)₂. The leaching experiment was lasted for 20 days (15 pore volumes). In addition, a set of experiments were conducted, where potato and wheat residues and poultry manure and sheep manure were added to soil at the rate of 5% and distilled water was used as the leaching solution to investigate impacts of organic residues on K leaching. In general, maximum K release was observed using NH₄Cl (566 kg ha^?1). Potato and wheat residues had maximum and minimum impacts on K leaching, respectively. Potassium fractionation was carried out after the end of the leaching experiment. The results indicated that leaching of soil in the presence of soluble salts and organic residues altered K distribution in different parts of soil.     

Nitrogen Release from Coated Urea Fertilizers in Different Soils

The nitrogen (N) release from coated urea fertilizers (Arborite and ESN), traditional N fertilizers [urea, (NH₂)₂CO], and urea ammonium nitrate (UAN) [(NH₂)₂CO, NH₄NO₃] in three North Carolina (USA) soils was measured throughout a 12-week laboratory incubation. Treatments were N source and moisture level (60% and 80% of field capacity). In both the Candor and Cecil soils, 40% of the Arborite N had released by day 2 after addition to the soil. Maximum Arborite N release was achieved by week 6 for both soils. The ESN N release began between weeks 1 and 2, and maximum release was attained by week 6 for the Candor and by week 8 for the Cecil soil. The ESN reached 70% release on Portsmouth by week 8 and Arborite had an immediate release. Field studies of these coated ureas would be needed to determine if they are economically viable over more traditional N fertilizers in North Carolina.     

A comparison of Mehlich I and Mehlich III extractants as predictors of manganese,copper and zinc availability in four Delaware soils

Abstract

The relative effectiveness of Mehlich I (.025N H₂SO₄ + .05N HCl) and Mehlich III (0.2N CH₃COOH + 0.25N NH₄NO₃ +.015N NH₄F + .013N HNO₃ + .001M EDTA) extractants as predictors of Mn, Cu and Zn uptake was assessed in a greenhouse experiment with four Delaware soils. The soils were adjusted to eight pH levels by addition of Ca(OH)₂ or elemental S, and received comparable amounts of Mn, Cu and Zn as either (1) MnSO₄ + CuSO₄ + ZnSO₄ or (2) Poultry Manure. Mehlich 1 and III extractable Mn and Zn, but not Cu, were well correlated in most instances. Excellent correlations were obtained between Mn uptake and Mehlich I and Mehlich III extractable Mn, for all soils and sources. In general, however, neither Zn nor Cu was found to correlate well with plant uptake. Based on this study, conversion to Mehlich III, as a routine soil test extractant for micronutrients, would not result in a significant improvement over the currently used Mehlich I extractant.     

Exchangeable ammonium and nitrate from different nitrogen fertilizer preparations in polyacrylamide-treated and untreated agricultural soils

 High molecular weight, anionic polyacrylamide (PAM) is currently being used as an irrigation water additive to significantly reduce soil erosion associated with furrow irrigation. PAM contains amide-N, and PAM application to soils has been correlated with increased activity of soil enzymes, such as urease and amidase, involved in N cycling. Therefore we investigated potential impacts of PAM treatment on the rate at which fertilizer N is transformed into NH₄ ⁺ and NO₃ ^– in soil. PAM-treated and untreated soil microcosms were amended with a variety of fertilizers, ranging from common rapid-release forms, such as ammonium sulfate [(NH₄)₂SO₄] and urea, to a variety of slow-release formulations, including polymerized urea and polymer-encapsulated urea. Ammonium sulfate was also tested together with the nitrification inhibitor dicyandiamide (DCD). The fertilizers were applied at a concentration of 1.0 mg g^–1, which is comparable to 100 lb acre^–l, or 112 kg ha^–1. Potassium chloride-extractable NH₄ ⁺-N and NO₃ ^–-N were quantified periodically during 2–4 week incubations. PAM treatment had no significant effect on NH₄ ⁺ release rates for any of the fertilizers tested and did not alter the efficacy of DCD as a nitrification inhibitor. However, the nitrification rate of urea and encapsulated urea-derived NH₄ ⁺-N was slightly accelerated in the PAM-treated soil. Received: 16 January 1998     

Effect of soil pH on cation and anion solubility

Abstract

The effect of soil pH on the exchangeability and solubility of soil cations (Ca, Mg, Na, K, and NH₄‐N) and anions (NO₃‐N, Cl, and P) was investigated for 80 soils, spanning a wide range in physical and chemical properties and taxonomic groups. This information is needed from environmental and agronomic standpoints to estimate the effect of changes in soil pH on leachability and plant availability of soil nutrients. Soils were incubated with varying amounts of acid (H₂SO₄) and base (CaCO₃) for up to 30 days. Although acid and base amendments had no consistent effect on cation exchangeability (as determined by neutral NH₄OAc), amounts of water‐soluble Ca, Mg, Na, K, NH₄‐N, and P decreased, while NO₃‐N and Cl increased with an increase in soil pH. The increase in cation solubility was attributed to an increase in the negative charge of the soil surface associated with the base addition. The change in surface electrostatic potential had the opposite effect on amounts of NO₃‐N and Cl in solution, with increases in N mineralization with increasing soil pH also contributing to the greater amount of NO₃‐N in solution. The decrease in P solubility was attributed to changes in the solubility of Fe‐, A1‐, and Ca‐P complexes. The logarithm of the amount of water‐soluble cation or anion was a linear function of soil pH. The slope of this relationship was closely related (R² = = 0.90 ‐ 0.96) to clay content, initial soil pH, and size of the cation or anion pool maintaining solution concentration. Although the degree in soil pH buffering increased with length of incubation, no effect of time on the relationship between cation or anion solubility and pH was observed except for NO₃‐N, due to N mineralization. A change in soil pH brought about by acid rain, fertilizer, and lime inputs, thus, affects cation and anion solubility. The impact of these changes on cation and anion leachability and plant availability may be assessed using the regression equations developed.     

Study on ammonium tolerance of cucumber plants

The influence of N form on xylem exudate and the guttation fluid concentration in cucumber plants was studied under greenhouse conditions. Plants were hydroponically grown with three NO₃:NH₄ ratios (100:0, 80:20, and 60:40) at a constant pH of 6.0 in the nutrient solutions. Plants supplied with 60:40 NO₃:NH₄ ratio displayed a significant decrease of NO₃‐N, total‐N, organic‐P, and Mn concentrations in the xylem sap and an increase of H₂PO₄‐P, SO₄‐S, Cl, B, and Zn concentrations. Potassium and Ca uptake in these plants was slightly reduced, indicating that pH control was an important factor for cationic nutrition in cucumber plants fed with NH₄. The major ions present in the nutrient solutions are concentrated in the xylem sap, particularly for NO₃, K, Ca, and Na. The NO₃:NH₄ ratio had a small effect on the ionic levels of the guttation fluid. The concentrations of all nutrients in the guttation fluid were substantially reduced, except for Cl, showing that the leaf tissues of cucumber plants remove the excess of Cl ion. Finally, in this study, secondary effects of N source on ion uptake and release were minimized by controlling nutrient solution pH.     

Different responses of ash and beech on nitrate versus ammonium leaf labeling

The effects of tree species on the N cycle in forest systems are still under debate. However, contradicting results of different ¹⁵N labeling techniques of trees and N tracers in the individual studies hamper a generalized mechanistic view. Therefore, we compared Ca(¹⁵NO₃)₂ and ¹⁵NH₄Cl leaf‐labeling method to investigate: (1) N allocation patterns from aboveground to belowground, (2) the cycles of N in soil‐plant systems, and (3) to allow the production of highly ¹⁵N enriched litter for subsequent decomposition studies. 20 beeches (Fagus sylvatica ) and 20 ashes (Fraxinus excelsior ) were ¹⁵N pulse labeled from aboveground with Ca(¹⁵NO₃)₂ and 40 beeches and 40 ashes were ¹⁵N pulse labeled from aboveground with ¹⁵NH₄Cl. ¹⁵N was quantified in tree compartments (leaves, stem, roots) and in soil after 8 d. Beech and ash incorporated generally more ¹⁵N from the applied ¹⁵NH₄Cl compared to Ca(¹⁵NO₃)₂ in all measured compartments, except for ash leaves. Ash had highest ¹⁵N incorporation [45% of the applied with Ca(¹⁵NO₃)₂] in its leaves. Both tree species kept over 90% of all fixed ¹⁵N from Ca(¹⁵NO₃) in their leaves, whereas only 50% of the ¹⁵N from the ¹⁵NH₄Cl tracer remained in the leaves and 50% were allocated to stem, roots, and soil. There was no damage of the leaves by both salts, and thus both ¹⁵N tracers enable long‐term labeling in situ field studies on N rhizodeposition and allocation in soils. Nonetheless, the ¹⁵N incorporation by both salts was species specific: the leaf labeling with ¹⁵NH₄Cl results in a more homogenous distribution between the tree compartments in both tree species and, therefore, ¹⁵NH₄Cl is more appropriate for allocation studies. The leaf labeling with Ca(¹⁵NO₃)₂ is a suitable tool to produce highly enriched ¹⁵N leaf litter for further long term in situ decomposition and turnover studies.     

Nitrogen monoxide production and consumption in an organic soil

 Factors controlling NO production, consumption, and emission rates were examined in an organic soil. Emission rates were measured in the enclosed headspaces of intact soil cores under three fertilisation treatments (unfertilised or 100 kg N ha^–1 as NH₄Cl or as NaNO₃), with and without the nitrification inhibitor C₂H₂ (20–70 μl l^–1). Nitrification was always the main source of NO emitted across the soil surface, even when the soil was nearly saturated. Fertilisation of soil with NH₄Cl increased NO emission both by stimulating NO production from nitrification, and by decreasing the NO consumption rate constant. Addition of NaNO₃ also stimulated the production of NO and N₂O during nitrification in aerobic soil slurry experiments. This effect was eliminated by adding C₂H₂ and was therefore not related to denitrification. In loose soil samples, the increase in NO-N production after NH₄Cl addition represented as much as 26% of the added N. However, in intact cores, 95% of the NO produced through nitrification was oxidised within the soil column rather than emitted to the atmosphere. We concluded that nitrification is the primary NO source from this organic soil, that surface NO emissions are much lower than gross NO production rates, and that gaseous N oxide (NO and N₂O) losses during nitrification can be affected by both soil NH₄ ⁺ and NO₃ ^–. Received: 15 December 1998     

Comparing Nutrient Availability in Low-Fertility Soils using Ion Exchange Resin Capsules

Assessing the nutrient status of low-input, low-fertility desert soils poses some unique challenges. Commonly used soil analysis procedures and resin capsules generally assess nutrient status of fertile agricultural soils. Ion-exchange resin capsules (Unibest Company, Bozeman, Mont.) provide a viable alternative. A study was conducted to determine effectiveness of resin capsules to extract low levels of nutrients applied to native soils. Loamy sand and sandy clay loam desert soils from Utah were treated with combinations of four rates of nitrogen (N) as ammonium nitrate (34–0–0), three rates of phosphorus (P) as phosphoric acid (0–72–0), and two rates of iron sulfate (FeSO₄·7H₂O) and zinc sulfate (ZnSO₄·7H₂O) (include an untreated control). Each soil treatment was implanted with a resin capsule placed into either 250 or 1000 cm³ of soil after addition of water equivalent to 50% field capacity and incubated for either 60 or 120 days at 25 °C. After the appropriate incubation time, capsules were washed and extracted using 2 M hydrochloric acid (HCl), and the extract was used to measure iron (Fe), ammonium (NH₄)-N, nitrate (NO₃)-N, sulfur (S), and zinc (Zn). Conventional soil tests were completed on incubated soils (60 or 120 days). Resin capsules reflected NH₄-N and P fertilizer applied at low rates in the loamy sand but not in the sandy clay loam. Neither Fe nor Zn application was reflected in resin capsules, but the accompanying S was clearly quantified. In comparison to conventional soil test procedures, resin capsule NH₄-N was clearly a better indicator than KCl-extractable NH₄-N; resin capsule NO₃-N was effective, but not as good an indicator as water extraction; and resin capsule P was reflective of soil applied P in loamy sand but not in sandy clay loam, whereas sodium bicarbonate was effective in both soils. Resin capsules show promise for use in low-input conditions, but additional understanding of interactions in variable soils is needed.