Charge characteristics and exchangeable cation status of Korean Ultisols and Alfisols and Thai Ultisols and Oxisols

The charge characteristics of A₁ or Ap and B₂ horizon samples of total 23 Ultisols, Alfisols and Oxisols in Korea and Thailand were studied by measuring the retention of NH₄⁺ and NO₃^? at different pH values (4–8) and NH₄NO₃ concentrations (0.1–0.005 m ). The magnitude of their negative charge (σ^?; meq/100g) was dependent on pH and NH₄NO₃ concentration (C; m ) as represented by a regression equation: log σ^?=apH +blogC +c. The values of the coefficient a (0.04–0.226), b (0.03–0.264) and c (–0.676–1.262) were correlated with the kinds of the soil and horizon and with the region where the soil exists. The retention of NO₃^? was less than 1 and 2–3 meq/100 g for the A₁ or Ap and B₂ horizon samples, respectively. The sum of exchangeable base and Al (‘effective’ CEC) was close to and higher than the magnitude of permanent charge (=σ^? measured at pH = 4.3 and at C = 0.005 m ) for one-third and two-thirds of samples, respectively. A σ^? value of 16 meq/100 g clay at pH = 7 and C = 0.01 m was found appropriate to separate the B₂ horizons of Thai Ultisols and Oxisols from those of Korean Ultisols and Alfisols. Korean Alfisols and Ultisols and Thai Ultisols were distinguished from each other on the status of exchangeable base and Al     

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.     

Presidedress soil nitrogen test for corn in Virginia

Abstract

The presidedress soil nitrate test (PSNT) and the presidedress tissue nitrogen test (PTNT) have been developed to assess residual soil nitrogen (N) sufficiency for corn (Zea mays L.) in the humid eastern U.S. We conducted field studies at 47 sites during 1990 and 1991 to evaluate the use of the PSNT and PTNT for corn in Coastal Plain, Piedmont, and Appalachian Ridge and Valley regions of Virginia. Seven rates of fertilizer N (0, 45, 90, 135, 180, 225, and 270 kg/ha) were applied at corn height of 0.40 to 0.50 m and replicated four times in a randomized complete block design. Whole corn plants and soil to a depth of 0.30 m were sampled when corn height was 0.15 to 0.30 m to estimate available soil N prior to the application of fertilizer N treatments. Corn grain yield response to fertilizer N was used to assess residual soil N availability. Nitrogen concentration of whole corn plants at 0.15 to 0.30 m height was not an accurate indicator of plant‐available soil N. Corn yields were maximized without sidedress N at the 19 sites where soil NO₃‐N was at least 18 mg‐kg^‐1 and at the 17 sites where soil (NO₃+NH₄)‐N was at least 22 mg‐kg^‐1. The PSNT predicted corn N sufficiency regardless of soil physiographic region or surface texture; however, the critical values for NO₃‐N and (NO₃+NH₄)‐N were 3 to 5 mg‐kg^‐1 lower than those established in Pennsylvania and Maryland, where cooler soil temperatures may permit greater residence time of inorganic N.     

Effect of nitrate addition on efficient use of ammonium sulfate fertilizer on corn under saline conditions. I. Pot experiment

Abstract

Plant growth in saline soils is regulated by the availability of nitrogen (N). High soil nitrate (NO₃)‐N can lead to poor water quality. Many workers think that NO₃‐N as a source for N can contribute to better plant growth in saline soils. The purpose of this work was to determine the necessity of NO₃‐N and the ratio of NO₃/ammonium (NH₄) in the N fertilizer which gives higher productivity of the biomass yield of corn. Corn (Zea mays L.) plants (Var. LG11) were grown under saline soil conditions (8.5 dS m^‐1), soils taken from the Euphrates valley (ACSAO Research Station) at Deir‐Ez‐Zor, east of Syria, from the surface layer of soil (0–25 cm). Five levels of N were applied in two forms, ammonium sulfate [¹⁵(NH₄)₂SO₄] with enrichment (1.5% a) as the NH₄‐N form and calcium nitrate [Ca(NO₃)₂] as the NO₃‐N form, besides fixed amounts of phosphorus (P) and potassium (K) for all N treatments. The corn plants were harvested at the flowering stage (56 days old), oven dried, weighed, and analyzed for total N and ¹⁵N recovery. The results indicated that the dry matter weight for treatments which received a combination of NH₄‐N and NO₃‐N gave higher dry matter yield than a single treatment of one source of N. But, NO₃‐N was more effective in improving yield than NH₄‐N. Nitrogen recoveries on the basis of added and absorbed N derived from fertilizer were significantly more affected by NO₃‐N than NH₄‐N.     

Regulation of extracellular protease activity in soil in response to different sources and concentrations of nitrogen and carbon

A large proportion of the nitrogen (N) in soil is in the form of proteinaceous material. Its breakdown requires the activity of extracellular proteases and other decomposing enzymes. The goal of our study was to better understand how carbon (C) and N availability affect soil protease activity. Several aerobic incubations were carried out with ammonium (NH₄⁺) and proteins as N sources and cellulose as the main C source. A strong increase in protease activity was observed when proteins were added, the increase depending on the amount of protein added and its solubility. Protease synthesis was clearly substrate induced, as NH₄⁺ had no effect. During this substrate induced phase, the addition of glucose but not NH₄⁺ resulted in protease repression, indicating that the level of protease synthesis was determined by the need for C rather than N. After 1 month of incubation, protease activity remained relatively constant over time and was closely related to microbial biomass N. Different concentrations of mineral N in soil solution had no direct effect on protease activity. However, during this stationary phase, protease activity could be repressed by glucose and NH₄⁺ in a treatment with low mineral N content while in treatments with a higher N availability no repression was observed. We hypothesize that the need for N determined protease activity in the treatment with limited N availability. The addition of NH₄⁺ allowed for reallocation of C and N away from protease synthesis, leading to the observed decrease in protease activity. The repression by glucose may be attributed to shifts in the pathway of microbial NH₄⁺ assimilation. The results emphasize the close links between the microbially mediated cycles of organic C and N.     

Corn (Zea mays L.) Grain and Stover Yield as Affected by Soil Residual Mineral Nitrogen

Evaluation of nitrogen (N) dynamic in soil using regression equations is important for proper determination of N fertilization. A 3-year field experiment was conducted to (1) develop the best-fitted regression model relating corn grain and stover yield to soil residual ammonium (NH₄)-N and nitrate (NO₃)-N for corn yield prediction and (2) evaluate how such a model can be beneficial to the health of ecosystem by predicting the appropriate rates of N fertilization for corn production. Soil NH₄-N and NO₃-N were determined at corn harvest at the depths of 0–30 and 30–60 cm. Nitrogen fertilizer rates and soil mineral N accounted for a maximum of 93% variation in corn grain yield. Soil mineral N enhanced corn yield more than N fertilizer. Totals of 63.1 and 14.1 kg/ha of soil residual NO₃-N and NH₄-N were found in the 0- to 60-cm depth, indicating the importance of performing soil N tests.     

改善根系生长捕获深层土中NO3-N最优化免耕玉米氮肥利用率

Subsoil acidity restricts root growth and reduces crop yields in many parts of the world. More than half of the fertilizer nitrogen(N) applied in crop production is currently lost to the environment. This study aimed to investigate the effect of gypsum application on the efficiency of N fertilizer in no-till corn(Zea mays L.) production in southern Brazil. A field experiment examined the effects of surface-applied gypsum(0, 5, 10, and 15 Mg ha~(-1)) and top-dressed ammonium nitrate(NH_4NO_3)(60, 120, and 180 kg N ha~(-1)) on corn root length, N uptake, and grain yield. A greenhouse experiment was conducted using undisturbed soil columns collected from the field experiment site to evaluate NO_3-N leaching, N uptake, and root length with surface-applied gypsum(0 and 10 Mg ha~(-1)) and top-dressed NH_4NO_3(0 and 180 kg N ha~(-1)). Amelioration of subsoil acidity due to gypsum application increased corn root growth,N uptake, grain yield, and N use efficiency. Applying gypsum to the soil surface increased corn grain yield by 19%–38% and partial factor productivity of N(PFPN) by 27%–38%, depending on the N application rate. Results of the undisturbed soil column greenhouse experiment showed that improvement of N use efficiency by gypsum application was due to the higher N uptake from NO_3-N in the subsoil as a result of increased corn root length. Our results suggest that ameliorating subsoil acidity with gypsum in a no-till corn system could increase N use efficiency, improve grain yield, and reduce environmental risks due to NO_3-N leaching.     

Effect of nitrogen source on aluminum toxicity in nonmycorrhizal and ectomycorrhizal pitch pine seedling

The effect of elevated nitrate [(NO₃‐nitrogen (N)] or ammonium (NH₄)‐N on the response of nonmycorrhizal (NM) and ectomycorrhizal (ECM) pitch pine (Pintis rigida Mill.) seedlings to aluminum (Al) was determined in experiments in which N was increased three times above ambient levels. Seedlings with and without the mycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown in sand irrigated with nutrient solution (pH 3.8) containing 0, 10, or 20 mg Al L^‐1 (0, 370, or 740 μM Al). The nutrient solution simulated that for the sandy, nutrient‐poor soil of the New Jersey Pine Barrens. Elevated NO₃‐N had no significant effect on Al toxicity in NM seedlings, but Al toxicity at ambient NH₄‐N was ameliorated by elevated NH₄‐N. Symptoms of Al toxicity in roots (thick and stunted) of ECM seedlings at ambient N levels were reduced by elevated NH₄‐N and absent at elevated NO₃‐N. When N was elevated by an increase in NO₃‐N or NH₄‐N, uptake of N and relative increases in total biomass were greater in ECM than in NM seedlings.     

Crop Utilization of Nitrogen in Swine Lagoon Sludge

Swine lagoon sludge is commonly applied to soil as a source of nitrogen (N) for crop production but the fate of applied N not recovered from the soil by the receiver crop has received little attention. The objectives of this study were to (1) assess the yield and N accumulation responses of corn (Zea mays L.) and wheat (Triticum aestivum) to different levels of N applied as swine lagoon sludge, (2) quantify recovery of residual N accumulation by the second and third crops after sludge application, and (3) evaluate the effect of different sludge N rates on nitrate (NO₃-N) concentrations in the soil. Sludge N trials were conducted with wheat on two swine farms and with corn on one swine farm in the coastal plain of North Carolina. Agronomic optimum N rates for wheat grown at two locations was 360 kg total sludge N ha^?1 and the optimum N rate for corn at one location was 327 kg total sludge N ha^?1. Residual N recovered by subsequent wheat and corn crops following the corn crop that received lagoon sludge was 3 and 12 kg N ha^?1, respectively, on a whole-plant basis and 2 and 10 kg N ha^?1, respectively, on a grain basis at the agronomic optimum N rate for corn (327 kg sludge N ha^?1). From the 327 kg ha^?1 of sludge N applied to corn, 249 kg N ha^?1 were not recovered after harvest of three crops for grain. Accumulation in recalcitrant soil organic N pools, ammonia (NH₃) volatilization during sludge application, return of N in stover/straw to the soil, and leaching of NO₃ from the root zone probably account for much of the nonutilized N. At the agronomic sludge N rate for corn (327 kg N ha^?1), downward movement of NO₃-N through the soil was similar to that for the 168 kg N ha^?1 urea ammonium nitrate (UAN) treatment. Thus, potential N pollution of groundwater by land application of lagoon sludge would not exceed that caused by UAN application.     

Effects of ammonium nitrate encapsulated with coal combustion byproducts on nutrient accumulation by corn and rye

Ammonium nitrate is a fertilizer and an explosive. Encapsulation of ammonium nitrate (NH₄NO₃) with coal combustion byproducts (fly ash or flue gas desulfurization gypsum) reduces the explosiveness of NH₄NO₃. A field study was conducted to determine the effects of encapsulated NH₄NO₃ on corn (Zea mays L.) and rye (Secale cereal L.) yield and accumulation of nitrogen (N), arsenic (As), cadmium (Cd), iron (Pb), copper (Cu), manganese (Mn), and zinc (Zn). Nitrogen rates were 56 and 112 kg ha^?1. Yields and concentrations of N and metals in corn grain, ear-leaf, and stover and in rye shoots were not affected by N source. Increased N rate resulted in increased corn ear-leaf, grain, and stover N, ear-leaf Cu, Mn, and Zn, and rye shoot yield, Cu, and Zn. For both species, metal levels did not exceed normal ranges. Coal byproduct-encapsulated NH₄NO₃ is as effective as non-encapsulated NH₄NO₃ for corn or rye production, without increasing plant metal concentrations above normal levels.     

Short-Term Response of Soil Microbial Biomass to Different Chabazite Zeolite Amendments

Zeolitites (ZTs) are rocks containing more than 50% of zeolite minerals and are known to be a suitable material for agricultural purposes by improving soil physicochemical properties and nitrogen (N) use efficiency. However, little is known about their effects on soil microbial biomass. This study aimed to evaluate short-term effects of different chabazite-rich ZT (CHAZT) amendments on soil microbial biomass and activity. A silty-clay agricultural soil was amended in three different ways, including the addition of natural (5% and 15%) and NH_4~+-enriched (10%) CHAZT. Soil dissolved organic carbon (C), total dissolved N, NH_4~+, NO_3~-, NO_2~-, microbial biomass C and N, and ergosterol were measured periodically over 16 d in a laboratory incubation. To verify the microbial immobilization of the N derived from NH_4~+-enriched CHAZT, a high15N source was used for enriching the mineral to measure the microbial biomass δ15N signature. An increase in the ergosterol content was observed in the soil amended with 5% natural CHAZT. However, no similar result was observed in the soil amended with 15% natural CHAZT, suggesting that the fungal biomass was favored at a lower CHAZT application rate. In the soil amended with NH+ 4-enriched CHAZT, microbial biomass N was related to NO_3~-production over time and inversely related to NH_4~+, suggesting high nitrification process. Isotopic measurements on microbial biomass confirmed immediate assimilation of N derived from NH_4~+-enriched CHAZT. These results suggested that the NH_4~+-enriched CHAZT used in this study supplied an immediately available N pool to the microbial biomass.     

Plant lignin and nitrogen contents control carbon dioxide production and nitrogen mineralization in soils incubated with Bt and non-Bt corn residues

Bt (Bacillus thuringiensis) corn is reported to produce lignin-rich residues, compared to non-Bt (NBt) corn, suggesting it is more resistant to decomposition. As the Bt gene is expressed selectively in stem and leaf tissue, it could affect lignin distribution in corn, which naturally has greater lignin content in roots than in stems and leaves. Our objective was to evaluate the effects of corn plant components, the Bt gene and elevated-lignin inputs on decomposition. Roots, stems and leaves from Bt corn and NBt corn isolines enriched with ¹³C and ¹⁵N were finely ground and mixed separately with soil, then incubated at 20 °C for 36 weeks. The effect of elevated lignin on decomposition was tested by adding a commercial lignin source (indulin lignin) to half of the samples. In addition to weekly CO₂ analysis and regular measurement of N mineralization, the degree of lignin degradation was evaluated at 1 and 36 weeks from the acid to aldehyde ratio (Ad/Al) of vanillyl and syringyl lignin-derived phenols. The CO₂ production and N mineralization was lower in root-amended soils than stem- and leaf-amended soils. The Bt genetic modification increased CO₂ production from stem-amended soils (P < 0.05) and decreased N mineralization in root-amended soils. The ¹³C and ¹⁵N results also showed more residue-C and -N retained in soils mixed with NBt stem residues. After 36 weeks leaf- and stem-amended soils with indulin lignin had a lower Ad/Al ratio and were less degraded than soils without exogenous lignin. In conclusion, plant lignin and nitrogen contents were good predictors of CO₂ production and N mineralization potential. Corn roots decomposed more slowly than aboveground components emphasizing the importance of recalcitrant root residues in sustaining the organic matter content of soil.     

微生物代谢产物I36N对土壤脲酶活性抑制作用研究

利用一株赭曲霉代谢产物I36N作为脲酶抑制剂研究对土壤脲酶活性的抑制作用,结果表明,I36N对我省主要5种水稻田土壤脲酶活性具有明显的抑制作用。土壤脲酶活性高,I36N抑制作用强,但抑制时间较短。抑制活性随I36N浓度而增强。与化学抑制剂NBPT相比,虽然I36N对土壤脲酶活性的抑制所需用量较大,但抑制时间延长。盆栽试验结果表明,施加I36N后,水层尿素水解速率降低,(NH4++NH3)N高峰期比对照推迟2～4天,高峰值也较低。水层pH值变化规律与(NH4++NH3)N变化一致。     

Nutrient source and tillage influences on nitrogen availability in a Southern Piedmont corn cropping system

Poultry (Gallus gallus domesticus L.) litter (PL) is a readily available nutrient source for crop production in the Southeast USA. Long-term PL application may alter availability of N and the effect may be dependent on tillage practice. Tillage [no till (NT) vs. conventional (CT)] and N source (PL vs. commercial fertilizer CF) effects on N availability and plant uptake were evaluated in years 9, 10, and 11 of a long-term cropping systems study at the United States Department of Agriculture, Agricultural Research Service, J. Phil Campbell Sr. Natural Resource Conservation Center, Watkinsville, GA, USA. Mineral N in the top 10 cm, measured in situ, varied each year and was influenced by time, tillage, and N source. In 2003 (year 9), soil mineral N content was greater in CT–CF (100 kg ha⁻¹) than in NT–PL (95 kg ha⁻¹) but in 2004 (year 10) and 2005 (year 11) it was lower in CT–CF (93 and 60 kg ha⁻¹) compared to NT–PL (140 and 71 kg ha⁻¹). Nitrogen mineralization rates were generally greater for PL than for CF treatments with the difference being almost 1 kg ha⁻¹ day⁻¹ in 2003. Mineralization rates were greater for NT–PL compared to CT–CF in 2004 and 2005. Across the three growing seasons, corn (Zea mays L.) aboveground biomass was consistently greater in the NT–PL treatment than in the NT–CF and CT–CF treatments. Correlation between aboveground biomass and N mineralization was greater for PL than for CF (0.75 vs. 0.48). Patterns of N mineralization and total soil mineral N indicated that the distribution of N through the growing season more closely matched corn N demand in PL treatments. Results indicate that improved N availability through the growing season, by combining NT and PL, can result in more profitable corn production in the southeast.     

Changes in soil solution composition and pH in urine-affected areas of pasture

Changes in soil solution composition and concentrations of exchangeable cations and mineral N in undisturbed cores of pasture soil were investigated in two experiments following applications of sheep urine to the cores. The major cations applied in the urine were K⁺ and Na⁺, and the major anions were HCO₃^? and Cl^?. Addition of urine increased concentrations of exchangeable K⁺, Na⁺ and NH₄⁺ and measured ionic strength of the soil solution throughout the surface 15 cm of soil, demonstrating that the urine moved through the core by macropore flow immediately following addition. Immediately following urine application the ionic strength in soil solution in the surface 2.5 cm of soil increased from 4–6 MM to 24–41 mM. Hydrolysis of urine-urea was extremely rapid, and in less than 1 d high concentrations of NH₄⁺-N (i.e. 270–370 mg N kg^?1) had accumulated in the surface 0–2.5 cm of the urine patch, and soil pH had risen by over one unit. Nitrification then proceeded and, after approximately 15 d, NO₃^? became the dominant form of mineral N present. During nitrification, soil pH declined and the ionic strength of the soil solution increased substantially with NO₃^? becoming the dominant anion present in solution. There were concomitant increases in the concentrations of Ca²⁺ and, to a lesser extent, Mg²⁺ in the soil solution as NO₃^? concentrations increased. After approximately 30 d, concentrations of exchangeable NO₃^? had risen to 250–330 mg N kg^?1, soil solution NO₃^? concentrations had increased to about 80 mmol, dm^?3, and ionic strength in the soil solution had increased to 130–140 mM. These results demonstrate the dominating effect of N transformations in causing large fluctuations in the pH, ionic composition and ionic strength of the soil solution in the urine patch. It was concluded that nutrient availability in the patch was affected directly by nutrient addition in urine, and also probably indirectly through the fluctuations in soil solution pH and ionic strength that occur.     

Simultaneous determination of soil aluminum,ammonium‐ and nitrate‐nitrogen using 1 M potassium chloride extraction

Abstract

Determination of soil aluminum (Al), ammonium‐nitrogen (NH₄‐N), and nitrate‐nitrogen (NO₃‐N) is often needed from the same soil samples for lime and fertilizer recommendations, but Al has to be extracted and quantified separately from NH₄‐N and NO₃‐N according to present methods. The objective of this study was to develop a reliable method for simultaneous analyses of soil Al, NH₄‐N and NO₃‐N using a Flow Injection Autoanalyzer. Thirty‐five soil samples from different locations with wide ranges of extractable Al, NH₄‐N and NO₃‐N were selected for this study. Aluminum, NH₄‐N and NO₃‐N were extracted by both 1 M and 2 M potassium chloride (KCl), and quantified using a LACHAT Flow Injection Autoanalyzer simultaneously and separately. One molar KCl was found to be a suitable extractant for all three compounds when compared to 2 M KCl. The 1 M KCl extract proposed could aid in decreasing the costs associated with simultaneous NH₄‐N, NO₃‐N, and Al analyses. Results of those three compounds analyzed simultaneously were not statistically different from those analyzed separately in 1 M KCl solution. This new procedure of simultaneous determination of NH₄‐N, NO₃‐N, and Al increases efficiency and reduces cost for soil test laboratories and laboratory users.     

Ammonia volatilization from nitrogen fertilizers surface-applied to corn (Zea mays) and grass pasture (Dactylis glomerata)

Summary The major agronomic concern with NH₃ loss from urea-containing fertilizers is the effect of these losses on crop yields and N fertilizer efficiency. In this 2-year study, NH₃ volatilization from surface-applied N fertilizers was measured in the field, and the effects of the NH₃ losses detected on corn (Zea mays L.) and orchardgrass (Dactylis glomerata L.) yield and N uptake were determined. For corn, NH₄NO₃ (AN), a urea-AN solution (UAN), or urea, were surface-broadcast at rates of 0, 56 and 112 kg N ha^–1 on a Plano silt loam (Typic Argiudoll) and on a Fayette silt loam (Typic Hapludalf). Urea and AN (0 and 67 kg N ha^–1) were surface-applied to grass pasture on the Fayette silt loam. Significant NH₃ losses from urea-containing N sources were detected in one of four corn experiments (12%–16% of applied N) and in both experiments with grass pasture (9%–19% of applied N). When these losses occurred, corn grain yields with UAN and urea were 1.0 and 1.5 Mg ha^–1, respectively, lower than yields with AN, and orchardgrass dry matter yields with urea were 0.27 to 0.74 Mg ha^–1 lower than with AN. Significant differences in crop N uptake between N sources were detected, but apparent NH₃ loss based on N uptake differences was not equal to field measurements of NH₃ loss. Rainfall following N application markedly influenced NH₃ volatilization. In corn experiments, NH₃ loss was low and yields with all N sources were similar when at least 2.5 mm of rainfall occurred within 4 days after N application. Rainfall within 3 days after N application did not prevent significant yield reductions due to NH₃ loss from urea in grass pasture experiments.     

N2O, NO, and NH3 Emissions from Soil after the Application of Organic Fertilizers, Urea and Water

Agricultural soil is a major source of nitrous oxide (N₂O), nitric oxide (NO) and ammonia (NH₃). Little information is available on emissions of these gases from soils amended with organic fertilizers at different soil water contents. N₂O, NO and NH₃ emissions were measured in large-scale incubations of a fresh sandy loam soil and amended with four organic fertilizers, [poultry litter (PL), composted plant residues (CP), sewage sludge pellets (SP) and cattle farm yard manure (CM)], urea fertilizer (UA) or a zero-N control (ZR) for 38 days. Fertilizers were added to soil at 40, 60 or 80% water-filled pore space (WFPS). The results showed that urea and organic fertilizer were important sources of N₂O and NO. Total N₂O and NO emissions from UA ranged from 0.04 to 0.62%, and 0.23 to 1.55% of applied N, respectively. Total N₂O and NO emissions from organic fertilizer treatments ranged from 0.01 to 1.65%, and <0.01 to=" 0.55%=" of=" applied=" n,=" respectively.=" the=" lower=">₂O and NO emissions from CP and CM suggested that applying N is these forms could be a useful mitigation option. Comparison of the NO-N/N₂O-N ratio suggested that nitrification was more dominant in UA whereas denitrification was more dominant in the organic fertilizer treatments. Most N was lost from PL and UA as NH₃, and this was not influenced significantly by WFPS. Emissions of NH₃ from UA and PL ranged from 62.4 to 69.6%, and 3.17 to 6.11% of applied N, respectively.     

Nitrogen source and aluminum toxicity of two sorghum genotypes differing in aluminum susceptibility

After a 35 days growth on nutrient solutions with NO^‐ ₃ NH₄NO₃ and NH⁺ ₄ as nitrogen source (pH 4.2) dry matter yield of the sorghum genotype SC0283 was much less affected by Al (1.5 mg‐¹) than that of the genotype NB9040. With NO^‐ ₃ as the sole nitrogen source only growth of the NB9040 plants was significantly reduced. Since OH^‐ efflux, shoot Al content and concentrations of all major nutrients of both genotypes were almost equal, a higher sensitivity to Al may underlie the lower Al tolerance of the NB9040 genotype. In the presence of NH.‐N Al again lowered d.m. yield of the NB plants. With SCO283 significant Al effects on d.m. yield were observed only with NH₄NO₃. Aluminum drastically increased the amount of protons released per unit of root surface area, especially with the NB9040 line. This shift in proton flux density was partly the result of a decrease of the specific root surface area and partly due to enhanced excess of catlonic nutrients taken up. With NH₄NO₃‐fed plants the latter could almost completely be attributed to a changed N preference brought about by inhibited uptake of NO^‐ ₃ and a simultaneous enhanced NH, absorption. Although both proton efflux and NH⁺ ₄ preference of the NB plants were severely increased by Al, relative yields of this genotype were not lowered by NH⁺ _4. This can probably be explained by (1) the high NH, sensitivity of this cultivar through which Al effects can be masked and (2) the continuous adjustment of the solution pH through which rhizosphere conditions were prevented.     

Rhizosphere effects of maize hybrids and N forms on Cd bioavailability in a limed soil

Replacing new corn genotypes in agricultural practices requires adequate information on the reaction of the selected hybrids to Cd uptake in Cd-polluted soil and an understanding of interactions with N fertilizers. A 2 × 2 × 3 factorial pot experiment with limed soil (pH 8), two maize (Zea mays) hybrids (Pioneer cultivar yellow and Pioneer cultivar white), two N fertilization forms (NH₄ ⁺ and NO₃ ^?) and three Cd exposures (0, 2 and 5 mg kg^?1 soil) was conducted under greenhouse conditions. Shoot dry mass increased significantly with NH₄ ⁺ nutrition compared with NO₃ ^? nutrition in both maize hybrids, with greater negative influence of Cd application combined with NH₄ ⁺ nutrition. The yellow cultivar had significantly greater shoot dry mass and lower Cd uptake than the white cultivar. Both hybrids exhibited similar N uptake in shoots and roots, with the exception of yellow cultivar with NH₄ ⁺ nutrition without Cd application. NO₃ ^? nutrition always decreased Cd uptake in both cultivars compared with NH₄ ⁺ nutrition. The N balance (mean across cultivars and Cd supply) after harvest showed most N uptake with NH₄ ⁺ nutrition (63.4%) and N_min remains in the soil with NO₃ ^? nutrition (48.7%). Soil pH decreased more with NH₄ ⁺ (?0.95 pH units) than NO₃ ^? nutrition (?0.21).