Retarded leaching of nitrate in acid soils from the tropics: measurement of the effective anion exchange capacity

Delay in the leaching of nitrate was measured in laboratory columns of repacked soils (Oxisols, Alfisols, Ultisols and Inceptisols) from South America, Africa and South-East Asia. Between one and five pore volumes were required for the frontal displacement of chloride by nitrate (2 mM adjusted to the natural pH of the soil). The delay was in good agreement with that calculated from charge (0 to 1.7 cmol positive charge kg^?1). Measurement of charge by batch equilibration with 0.5 M and then 2 mM NH₄CI at the pH of the soil overestimated the effective charge, probably due to removal of sulphate in the washings. A leaching method using only 2 mM NH₄CI gave a better estimate.     

Classifying soils at the ultimate stage of weathering in the tropics

Oxisols cover ≈ 23% of the land surface in the tropics and are utilized extensively for agricultural purposes in the tropical countries. Under the variable input types of agricultural systems practiced locally, some of these soils still appear to have problems in terms of proper soil classification and subsequently hinder attempts to implement sustainable agro‐management protocols. The definition for Oxisols in Soil Survey Staff (1999) indicates that additional input is still required to refine the definition in order to resolve some of the outstanding classification problems. Therefore, the objective of this study is to examine the properties of some Oxisols and closely related soils in order to evaluate the classification of these soils. Soils from Brazil, several countries in Africa, and Malaysia were used in this study. Field observations provided the first indication that some of the presently classified kandi‐Alfisols and kandi‐Ultisols were closer to Oxisols in terms of their properties. Water‐retention differences and apparent CEC of the subsurface horizons also supported this idea. The types of extractable Fe oxides and external specific surface areas of the clay fractions showed that many kandic horizons have surface properties that are similar to the oxic horizons. Micromorphology indicated that the genetic transition from the argillic to the oxic involves a diminishing expression of the argillic. Properties, such as CEC, become dominant. The kandic horizon is therefore inferred as a transition to the oxic horizon. It is proposed that the Oxisols be keyed out based only on the presence of an oxic horizon and an iso–soil temperature regime. The presence of a kandic horizon will be reflected at lower levels in Oxisols. The Oxisols will now be exclusive to the intertropical belt with an iso–soil temperature regime. The geographic extend of the Oxisols would increase and that of kandi‐Alfisols and Ultisols would decrease. A few kandi‐Alfisols and Ultisols in the intertropical area will have low CEC which would fail the weatherable mineral contents. The kandic subgroups of some Alfisols and Ultisols will be transitional between the low (< 16 cmol_c [kg clay]^–1)‐ and high (> 24 cmol_c [kg clay]^–1)‐activity clay soils. The proposed changes to classification will contribute to a better differentiation of the landscape units in the field. Testing of the proposed classification on some Malaysian soils showed that the new definition for Oxisols provides a better basis for the classification of the local soils and the development of meaningful soil‐management groups for plantations.     

Nitrogen flux patterns through Oxisols and Ultisols in tropical forests of Cameroon,Central Africa

We lack an understanding of nitrogen (N) cycles in tropical forests of Africa, although the environmental conditions in this region, such as soil type, vegetation, and climate, are distinct when compared with other tropical forests. Herein, we simultaneously quantified N fluxes through precipitation, throughfall, and 0-, 15-, and 30-cm soil solutions, as well as litterfall, in two forests with different soil acidity (Ultisols at the MV village (exchangeable Al³⁺ in 0–30 cm, 126 kmol_c ha^–1) and Oxisols at the AD village (exchangeable Al³⁺ in 0–30 cm, 59.8 kmol_c ha^–1)) over 2 years in Cameroon. The N fluxes to the O horizon via litterfall plus throughfall were similar for both sites (MV and AD, 243 and 273 kg N ha^–1 yr^–1, respectively). Those values were remarkably large relative to other tropical forests, reflecting the dominance of legumes in this region. The total dissolved N flux from the O horizon at the MV was 28 kg N ha^–1 yr^–1, while it was 127 kg N ha^–1 yr^–1 mainly as NO₃^–-N (~80%) at the AD. The distinctly different pattern of N cycles could be caused by stronger soil acidity at the MV, which was considered to promote a superficial root mat formation in the O horizon despite the marked dry season (fine root biomass in the O horizon and its proportion to the 1-m-soil profile: 1.5 Mg ha^–1 and 31% at the MV; 0.3 Mg ha^–1 and 9% at the AD). Combined with the published data for N fluxes in tropical forests, we have shown that Oxisols, in combination with N-fixing species, have large N fluxes from the O horizon; meanwhile, Ultisols do not have large fluxes because of plant uptake through the root mat in the O horizon. Consequently, our results suggest that soil type can be a major factor influencing the pattern of N fluxes from the O horizon via the effects of soil acidity, thereby determining the contrasting plant–soil N cycles in the tropical forests of Africa.     

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⁺.     

ELECTRIC CHARGE CHARACTERISTICS OF ANDO A, AND BURIED A, HORIZON SOILS

The electric charge characteristics of four Ando soils (A₁ and μA₁) and a Chernozemic soil (Ap) were studied by measuring retention of NH₄⁺ and Cl^– at different pH values and NH₄Cl concentrations. No positive charge appeared in the Ando soils at pH values 5 to 8.5 except for one containing allophane and imogolite. The magnitude of their negative charge (CEC; meq/l00g soil) was dependent on pH and NH₄Cl concentration (C; N) as represented by a regression equation: log CEC =a pH +b log C +c, where the values of a and b were 0.113–0.342 and 0.101–0.315, respectively. Unlike the Chernozemic soil, Ando soils containing allophane, imogolite, and/or 2:1–2:1:1 layer silicate intergrades and humus showed a marked reduction of cation retention as pH decreased from 7 to 5. This was attributed to the charge characteristics of the clay minerals and to the carboxyl groups in humus being blocked by Al and Fe.     

Electric charge characteristics of horizons of Ando (B) and Red-Yellow B soils and weathered pumices

The electric charge characteristics of five Ando (B) and two Red-Yellow B horizon soils and two weathered pumices were studied by measuring the retention of NH₄+ and Cl^? at different pH values and NH₄Cl concentrations. The magnitude of their negative charge (CEC; meq/100g) was dependent on pH and NH₄Cl concentration (C; _m) as represented by a regression equation: log CEC =apH +blog C +c. The values of the coefficient a (0.017 to 0.342), b (0.031 to 0.274) and c (1.41 to ? 1.26) were correlated and depended on the kind of clay minerals present. A similar equation: log AEC =a’ pH +b’ log C+c’ was also found for the positive charge in the Ando soils, but there was little difference in the values of a’(?0.204 to ?0.251), b’(0.181 to 0.253) and c’(2.06 to 2.46) between the soils. It was shown that the equations generally hold for soils with constant and/or variable charges and describe adsorption equilibria in which NH₄+ and H⁺, and Cl^? and OH^? compete for cation- and anionexchange sites, respectively. The significance and utility of the coefficients is discussed.     

Acidification and nutrient imbalance in forest soils subjected to nitrogen deposition

Emission of nitrogen oxides (NO_x) and ammonia (NH₃) from a fertilizer factory and the resulting input of nitrates (NO₃ ^?) and ammonium (NH₄ ⁺) into the soil were the main reasons of nitrogen (N) cycle disturbance in forest ecosystems near Novgorod, North-Western Russia (50°31′ North, 31°17′ East). Total N atmospheric input was about 100 kg/ha annually. NH₃ was a dominant pollutant, causing the increase of atmospheric precipitation pH within the polluted region compared to background territories (6.0–6.5 and 4.5–5.0, respectively). Soil acidification through NH₄ ⁺ nitrification was observed. N-NO₃ ^? concentrations in soil solution reached 20–30 mg/l, and proton (H) production was equal to 4.1 keq/ha per warm season (from April to October). Compared with soil status in 1983, pH decrease by 0.2 pH units was found in A horizon. The content of exchangeable calcium (Ca) and magnesium (Mg) decreased by the factor of 2–3 and 1.5–2 in A and B horizons, respectively. Triple increase of exchangeable aluminium (Al) content was detected in A horizon. Through recent decrease of pollutant emission, the polluted territory is now a suitable subject for recovery studies.     

Effect of a fly ash and gypsum mixture on rice cultivation

Abstract

The salt titration (ST) method was evaluated as a method to determine ZPC in comparison with the potentiometric titration (PT) method for 26 soils with variable charge clays, i.e., Oxisols and Ultisols from Thailand and Andisols from Japan. In addition to the determination of ST-pH₀ as the zero point of charge, a calculation procedure (STPT method) was adopted here in order to acquire more information from the titration curve. Furthermore, for the purpose of cross-checking of ZPC determined by the PT method, the ST procedure was successively applied to the samples analyzed by the PT method (PTST method).

The soil to solution ratios of 1: 10 to 1: 5 gave almost an identical ST-pH₀ value for every soil. The values of both ST-pH₀ and PT-ZPC ranged from 4.7 to 6.3 for the Andisols, while for the Oxisols and Ultisols, they were always below 4.2. The difference between the values of ST-pH₀ and PT-ZPC was only slight for the Andisols, whereas it was sometimes large (0.4 pH unit) for the Oxisols and Ultisols. Nevertheless, it was concluded that the ST method with its modification (STPT) was comparable to or even better than the PT method for the soil characterization work due to its convenience and simplicity.     

Ionic-strength and pH effects on the sorption of cadmium and the surface charge of soils

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp^?3 Ca(NO₃)₂, and various concentrations of NaNO₃ (0.01–1.5 mol dm^?3) as background solutions at a range of pH values (3–8). In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaNO₃ increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.     

Fruit Quality and Nitrogen,Potassium, and Calcium Content of Apple as Influenced by Nitrate: Ammonium Ratios in Tree Nutrition

Evaluations of fruit quality and nitrogen, potassium, and calcium concentration of apple (Malus domestica Borkh. cvs. Gala and Golab) grown with five treatments of NO₃^?:NH₄⁺ ratios were made in pot culture. The concentrations of NO₃^?:NH₄⁺ ratios were 2.5:0.1, 6:1, 6:0.7, 6:0.5, 6:0.3 meq L^?1. Fruit size, percent dry matter, total dissolved solids, total acidity, or juice pH was not affected by increased ammonium in the ratio. Firmness decreased as the proportion of NH₄⁺ increased. Gala and Golab differed in some of these parameters. Concentrations of N and K increased as NH₄⁺ increased, whereas Ca had no trend or decreased. Generally, the treatment of 2.5:0.1 produced fruits with lower N but higher K and Ca concentrations than the other treatments. This research showed that some parameters such as fruit weight, length, and diameter, juice pH, and dry matter were not affected significantly by NH₄⁺ concentration whereas composition was affected.     

Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake

Reports of sugarcane yield responses to silicon (Si), coupled with mounting evidence that elevated crop Si levels reduce both biotic and abiotic stresses, account for the interest in the Si nutrition of this crop. In terms of managing Si supplies to sugarcane in South Africa, uncertainties exist regarding, first, the reserves of plant-available Si in soils, and second, the reliability of soil-test methods for predicting Si availability. In this study, extractable Si was measured in 112 soils collected from sugarcane-producing fields in South Africa. Soils were selected on the basis of dominant soil types and included Inceptisols, Alfisols, Mollisols, Vertisols, Oxisols, Entisols, and Ultisols, varying widely in chemical properties, texture, and extent of weathering. Extractants employed were 0.01 M calcium chloride (CaCl₂) and 0.02 N sulfuric acid (H₂SO₄). Silicon extracted with 0.02 N H₂SO₄ ranged from 2 to 293 mg kg^?1, whereas with 0.01 M CaCl₂ the range was 5 to 123 mg kg^?1. With both extractants, extractable Si decreased significantly with decreasing pH, exchangeable calcium (Ca), and total cations. In soils with potassium chloride (KCl)–extractable Al+H levels of greater than 0.5 cmol_cL^?1, extractable Si levels were consistently low, suggesting that soluble Al is implicated in reducing plant-available Si levels. Extractable Si levels were not related to the Bache and Williams P-sorption indices of soils. In the second part of the investigation, sugarcane leaf Si concentrations from 28 sites were related to soil extractable Si levels. The CaCl₂ soil test proved markedly superior to H₂SO₄ as a predictive test for leaf Si levels.     

INFLUENCE OF CHEMICAL FORM AND CONCENTRATION OF NITROGEN ON APOPLASTIC pH OF LEAVES

The objective of this study was to investigate the effects of various forms of nitrogen (NO^? ₃, NH⁺ ₄) supplied to the roots via a nutrient solution on the apoplastic pH in intact leaves determined by fluorescence ratio imaging. In contrast to NH⁺ ₄, higher apoplastic pH values in leaves of Phaseolus vulgaris and Helianthus annuus were measured with NO^? ₃ nutrition. In this context no significant differences were found in leaves of Vicia faba and Zea mays supplied with the various forms of N. Comparative studies on apoplastic pH in leaves of Vicia faba, Zea mays and Helianthus annuus demonstrated that NO^? ₃ reductase activity in roots was responsible for the differences in NO^? ₃ concentration and pH in the leaf apoplast. Light-induced pH changes in the leaf apoplast also occur and may overlap the effects of various forms of N. Increasing concentrations of NO^? ₃ supply to the roots did not significantly affect apoplastic pH in leaves of Helianthus annuus. Depletion of NO^? ₃ in the nutrient solution led to lower apoplastic pH in leaves of Zea mays. Leaf fertilization with NH⁺ ₄ led to a decline in apoplastic pH of leaves whereas NH₃ gas exposure caused a biphasic response in apoplastic pH.     

Der Einfluß des NO3 : NH4 -Verhältnisses in der Nährlösung auf Ertrag und Gehalte an organischen und anorganischen Ionen von Tomatenpflanzen

The influence of the relation between NO₃ and NH₄ in the nutrient solution on yield and organic and inorganic ion contents of tomato plants. . Tomato plants were grown in aerated media of oppositly varying supply of (NH₄)₂SO₄-and NaNO₃ with a constant N-quantity of 15 meq/1 or a quantity of NH₄-, NO₃ or (NO₃ + NH₄) increasing from 3 to 30 meq.N/1. Yield and ion content were determined. A maximum yield was achieved by a mixed N-supply i.e. 4 to 5 parts NO₃, 1 part NH₄-N and 7, 5 meq N/l. The “(C-A) value” was calculated by the content of the cationions “C” (=K⁺ + Na⁺ + Ca⁺⁺ + Mg⁺⁺ + NH₄⁺) and the inorganic anions “A” (= NO₃^? + Cl^? + H₂PO₄^? + SO₄^–). The “(C-A)” value is equivalent to the content of the organic anions. Furthermore we determined the citrate, malate, oxalate, and pektinate content. These make up 60–80% of the (C-A) value. The NO₃- and K content increase considerably, the Ca-, Mg-, Na-, Citrate, Malate and Oxalate content increase less pronounced, the Cl-, H₂PO₄- and SO₄- content decrease as the NO₃ content increases from 0–80%. If the medium contains 80–100% NO₃ - N, the NO₃ and K content remain almost constant, while the Ca-, Mg-, citrate, malate and oxalate content especially increase in this case. The K content decreases in the presence of a high (NH₄)₂SO₄ supply. Parallel to this the value (C-A) decreases greatly, so that it is less than the sum of the determined organic anions. In this case the content of organic anions obviously does not correlate with the (C-A) value. The yield correlates with the carboxylate contents. (C-A) values of 150–170 mval/100 gm. d. m. cause a lowering of the yield. The plants contain an almost equal amount of citrate, malate and oxalate. Depending on the N-concentration of the medium, the malate content will be a bit more than the amount of citrate in case of maximum yield. If there is a lack or toxicity in the medium the citrate content will be higher than the malate content. The yield per dry weight correlates positivly with the quotients of citrate and malate.     

Characteristics of ammonium and nitrate fluxes along the roots of Picea asperata

Nitrogen (N), ammonium (NH₄⁺) and nitrate (NO₃^?), is one of the key determinants for plant growth. The interaction of both ions displays a significant effect on their uptake in some species. In the current study, net fluxes of NH₄⁺ and NO₃^? along the roots of Picea asperata were determined using a Non-invasive Micro-test Technology (NMT). Besides, we examined the interaction of NH₄⁺ and NO₃^? on the fluxes of both ions, and the plasma membrane (PM) H⁺-ATPases and nitrate reductase (NR) were taken into account as well. The results demonstrated that the maximal net NH₄⁺ and NO₃^? influxes were detected at 13–15?mm and 8–10.5?mm from the root apex, respectively. Net NH₄⁺ influx was significantly stimulated with the presence of NO₃^?, whereas NH₄⁺ exhibited a markedly negative effect on NO₃^? uptake in the roots of P. asperata. Also, our results indicated that PM H⁺-ATPases and NR play a key role in the control of N uptake.     

Hydroxyl release by maize (Zea mays L.) roots under acidic conditions due to nitrate absorption and its potential to ameliorate an acidic Ultisol

Purpose

Hydroxyl ion release by maize (Zea mays L.) roots under acidic conditions was investigated with a view to develop a bioremediation method for ameliorating acid soils in tropical and subtropical regions.

Materials and methods

Two hydroponic culture experiments and one pot experiment were conducted: pH, nitrogen state, and rhizobox condition, which investigated the effects of different nitrogen forms on hydroxyl release by maize roots under acidic conditions.

Results and discussion

The pH of the culture solution increased as culture time rose. The gradient of change increased with rising NO₃ ^?/NH₄ ⁺ molar ratios. Maize roots released more hydroxyl ions at pH 4.0 than at pH 5.0. The amount of hydroxyl ions released by maize roots at a constant pH was greater than those at a nonconstant pH. Application of calcium nitrate reduced exchangeable acidity and increased the pH in an Ultisol rhizosphere, compared with bulk soil. The increasing magnitude of soil pH was greater at higher doses of N. The absorption of NO₃ ^?–N increased as the NO₃ ^?/NH₄ ⁺ molar ratios rose, which was responsible for hydroxyl ion release and pH increases in culture solutions and rhizosphere.

Conclusions

Root-induced alkalization in the rhizosphere resulting from nitrate absorption by maize plants can be used to ameliorate acidic Ultisols.     

Influence of ambient acidity on the absorption of NO3 − and NH4 + by tomato punts

Abstract

The effects of ambient acidity on NO₃ ^? and Nh₄ ⁺ absorption by 26‐day‐old tomato plants (Lycopersicon esculentum Mill.) were examined in solution culture. The absorption rate per unit root mass was measured for 6 hr. The NO₃ ^? absorption rate from 0.4 mM NaNO, was 36% greater at pH 4.5 than at pH 6.5. In contrast, the NhY absorption rate was approximately 42% greater at pH 5.5 or 6.5 than at pH 4.5. The presence of equimolar NHr from 0.4 mM NH^NO, decreased the NO, absorption rate at pH 5.5 or 6.5 but did not reduce the rate at pH 4.5. The NO, absorption rate was inhibited less at pH 5.5 when equimolar NHr was supplied from 0.2 mM (NH₄)₂S0₄ as opposed to NH₄NO₃. At pH 5.5, the N0₃ ^? absorption rate increased with increased #OPNH₄#CP₂SO₄ concentration. The presence of equimolar NO₃ ^? supplied as either NaNO₃ or NH₄NO₃ had no effect on the NH₄ ⁺ absorption rate at pH 5.5 or 6.5. However, at pH 4.5, the NH₄ ⁺ absorption rate was slightly reduced from NH₄NO₃ solutions relative to that from a (NH₄)₂S0₄ solution.     

A field trial to evaluate the pollution potential to ground and surface waters from woodchip corrals for overwintering livestock outdoors

Outwintering beef cattle on woodchip corrals offers stock management, economic and welfare benefits when compared with overwintering in open fields or indoors. A trial was set up on a loamy sand over sand soil to evaluate the pollution risks from corrals and the effect of design features (size and depth of woodchips, stocking density, and feeding on or off the corral). Plastic‐lined drainage trenches at 9–10 m spacing under the woodchips allowed sampling of the leachate. Sampling of the soil to 3.6 m below the corral allowed evaluation of pollutant mitigation during vadose zone transport. Mean corral leachate pollutant concentrations were 443–1056 mg NH₄‐N L^?1, 372–1078 mg dissolved organic carbon (DOC) L^?1, 3–13 mg NO₃‐N L^?1, 8 × 10⁴–1.0 × 10⁶Escherichia coli 100 mL^?1 and 2.8 × 10²–1.4 × 10³ faecal enterococci 100 mL^?1. Little influence of design features could be observed. DOC, NH₄ and (in most cases) E. coli and faecal enterococci concentrations decreased 10²–10³ fold when compared with corral leachate during transport to 3.6 m but there were some cores where faecal enterococci concentrations remained high throughout the profile. Travel times of pollutants (39–113 days) were estimated assuming vertical percolation, piston displacement at field moisture content and no adsorption. This allowed decay/die‐off kinetics in the soil to be estimated (0.009–0.044 day^?1 for DOC, 0.014–0.045 day^?1 for E. coli and 0–0.022 day^?1 for faecal enterococci). The mean [NO₃‐N] in pore water from the soil cores (n = 3 per corral) ranged from 114 ± 52 to 404 ± 54 mg NO₃‐N L^?1, when compared with 59 ± 15 mg NO₃‐N L^?1 from a field overwintering area and 47 ± 40 mg NO₃‐N L^?1 under a permanent feeding area. However, modelling suggested that denitrification losses in the soil profile increased with stocking density so nitrate leaching losses per animal may be smaller under corrals than for other overwintering methods. Nitrous oxide, carbon dioxide and methane fluxes (measured on one occasion from one corral) were 5–110 g N ha^?1 day^?1, 3–23 kg C ha^?1 day^?1, and 5–340 g C ha^?1 day^?1 respectively. Ammonia content of air extracted from above the woodchips was 0.7–3.5 mg NH₄‐N m^?3.     

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).     

Nitrate leaching loss under annual and perennial pastures with and without lime on a duplex (texture contrast) soil in humid southeastern Australia

Mineral N accumulates in autumn under pastures in southeastern Australia and is at risk of leaching as nitrate during winter. Nitrate leaching loss and soil mineral N concentrations were measured under pastures grazed by sheep on a duplex (texture contrast) soil in southern New South Wales from 1994 to 1996. Legume (Trifolium subterraneum)‐based pastures contained either annual grass (Lolium rigidum) or perennial grasses (Phalaris aquatica and Dactylis glomerata), and had a control (soil pH 4.1 in 0.01 m CaCl₂) or lime treatment (pH 5.5). One of the four replicates was monitored for surface runoff and subsurface flow (the top of the B horizon), and solution NO₃^– concentrations. The soil contained more mineral N in autumn (64–133 kg N ha^?1 to 120 cm) than in spring (51–96 kg N ha^?1), with NO₃^– comprising 70–77%. No NO₃^– leached in 1994 (475 mm rainfall). In 1995 (697 mm rainfall) and 1996 (666 mm rainfall), the solution at 20 cm depth and subsurface flow contained 20–50 mg N l^?1 as NO₃^– initially but < 1 mg N l^?1 by spring. Nitrate‐N concentrations at 120 cm ranged between 2 and 22 mg N l^?1 during winter. Losses of NO₃^– were small in surface runoff (0–2 kg N ha^?1 year^?1). In 1995, 9–19 kg N ha^?1 was lost in subsurface flow. Deep drainage losses were 3–12 kg N ha^?1 in 1995 and 4–10 kg N ha^?1 in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg N ha^?1 year^?1 under control and limed annual pastures, respectively, and 6 and 8 kg N ha^?1 year^?1 under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg N ha^?1 under the four respective pastures. The increased loss of N caused by liming was of a similar amount to the decreased N loss by maintaining perennial pasture as distinct from an annual pasture.     

The inherent 'safety-net' of an Acrisol: measuring and modelling retarded leaching of mineral nitrogen

The inherent features of Acrisols with their increasing clay content with depth are conducive to reducing nutrient losses by nutrient adsorption on the matrix soil surfaces. Ammonium (NH₄⁺) and nitrate (NO₃^?) adsorption by a Plinthic Acrisol from Lampung, Indonesia was studied in column experiments. The peak of the H₂¹⁸O breakthrough occurred at 1 pore volume, whereas the median pore volumes for NH₄⁺ and NO₃^? ranged from 6.4 to 6.9 and 1.1 to 1.6, respectively. The adsorption coefficients (K_a in cm³ g^–1) measured were 1.81, 1.51, 1.64 and 1.47 for NH₄⁺ and 0.03, 0.09, 0.10 and 0.17 for NO₃^?, respectively, in the 0–0.2, 0.2–0.4, 0.4–0.6 and 0.6–0.8 m soil depth layers. The NH₄⁺ and NO₃^? adsorption coefficients derived from this study were put in to the Water, Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) model to evaluate their effect on leaching in the context of several cropping systems in the humid tropics. The resulting simulations indicate that the inherent ‘safety‐net’ (retardation mechanism) of a shallow (0.8–1 m) Plinthic Acrisol can reduce the leaching of mineral N by between 5 and 33% (or up to 2.1 g m^?2), mainly due to the NH₄⁺ retardation factor, and that the effectiveness in reducing N leaching increases with increasing depth. However, the inherent ‘safety‐net’ is useful only if deep‐rooted plants can recover the N subsequently.