The effect of inorganic nitrogen on the added nitrogen interaction of soils in incubation experiments

A laboratory incubation experiment was conducted to study the effect of indigenous inorganic N on the immobilization of applied N and on the occurrence of an added N interaction (ANI). Samples of six Mollisols from Illinois were incubated with ¹⁵N-labelled (NH₄)₂SO₄ (100 or 200 mg N kg^-1 soil), with or without the use of 0.01 M CaCl₂ to extract inorganic N (mainly NO _inf3 ^sup- ) before incubation. From 6 to 49% of the N applied was immobilized, higher percentages being obtained with unextracted soils than with the extracted soils and with the higher rate of N addition. Net mineralization of native N occurred in both the unextracted and extracted soils, but was more extensive in the unextracted soil and increased with the addition of N. The increases were accompanied by a positive ANI, which usually exceeded the amount of applied N immobilized and increased with the rate of addition. The ANI values observed with extracted soils were attributed to increased mineralization of native organic N.     

Added nitrogen interaction as affected by soil nitrogen pool size and fertilization – significance of displacement of fixed ammonium

Displacement of NH₄⁺ fixed in clay minerals by fertilizer ¹⁵NH₄⁺ is seen as one mechanism of apparent added nitrogen interactions (ANI), which may cause errors in ¹⁵N tracer studies. Pot and incubation experiments were carried out for a study of displacement of fixed NH₄⁺ by ¹⁵N‐labeled fertilizer (ammonium sulfate and urea). A typical ANI was observed when ¹⁵N‐labeled urea was applied to wheat grown on soils with different N reserves that resulted from their long‐term fertilization history: Plants took up more soil N when receiving fertilizer. Furthermore, an increased uptake of ¹⁵N‐labeled fertilizer, induced by increasing unlabeled soil nitrogen supply, was found. This ANI‐like effect was in the same order of magnitude as the observed ANI. All causes of apparent or real ANI can be excluded as explanation for this effect. Plant N uptake‐related processes beyond current concepts of ANI may be responsible. NH₄⁺ fixation of fertilizer ¹⁵NH₄⁺ in sterilized or non‐sterile, moist soil was immediate and strongly dependent on the rate of fertilizer added. But for the tested range of 20 to 160 mg ¹⁵NH₄⁺‐N kg^–1, the NH₄⁺ fixation rate was low, accounting for only up to 1.3 % of fertilizer N added. For sterilized soil, no re‐mobilization of fixed ¹⁵NH₄⁺ was observed, while in non‐sterile, biologically active soil, 50 % of the initially fixed ¹⁵NH₄⁺ was released up to day 35. Re‐mobilization of ¹⁵NH₄⁺ from the pool of fixed NH₄⁺ started after complete nitrification of all extractable NH₄⁺. Our results indicate that in most cases, experimental error from apparent ANI caused by displacement of fixed NH₄⁺ in clay is unlikely. In addition to the low percentage of only 1.3 % of applied ¹⁵N, present in the pool of fixed NH₄⁺ after 35 days, there were no indications for a real exchange (displacement) of fixed NH₄⁺ by ¹⁵N.     

Immobilization of ammonium and nitrate and their interaction with native N in three Illinois Mollisols

Summary Three Illinois Mollisols were incubated for 2 weeks at 25°C after treatment with different amounts of glucose and/or ¹⁵N-labelled (NH₄)₂SO₄ or ¹⁵N-labelled KNO₃. The objectives were: (1) to compare the immobilization and interaction of NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N with the native soil N, and (2) to study the relationship between immobilization of applied N and the added N interaction. As determined, immobilized N refers to forms not extractable with 2 MKCl (immobilized ¹⁵N+clay-fixed ¹⁵NH _inf4 ^sup+ ). In all cases, both NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH _inf4 ^sup+ than NO _inf3 ^sup- was recovered in organic forms. Although the three soils differed considerably in the amounts of applied N immobilized, similar trends in N immobilization were observed. A positive added N interaction occurred with all soils, the magnitude increasing with the rate of N addition. In the absence of glucose, higher added N interactions were obtained for NH _inf4 ^sup+ than NO _inf3 ^sup- , whereas there was very little difference between NH _inf4 ^sup+ and NO _inf3 ^sup- in the presence of glucose. The results indicate that under conditions of rapid immobilization (e.g., in the presence of glucose), NH _inf4 ^sup+ and NO _inf3 ^sup- will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- . Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.     

灌溉施用氮肥后氮素在土壤中的转化及淋失状况: 土柱模拟研究

A soil column method was used to compare the effect of drip fertigation (the application of fertilizer through drip irrigation systems, DFI) on the leaching loss and transformation of urea-N in soil with that of surface fertilization combined with flood irrigation (SFI), and to study the leaching loss and transformation of three kinds of nitrogen fertilizers (nitrate fertilizer, ammonium fertilizer, and urea fertilizer) in two contrasting soils after the fertigation. In comparison to SFI, DFI decreased leaching loss of urea-N from the soil and increased the mineral N (NH₄⁺-N + NO₃^--N) in the soil. The N leached from a clay loam soil ranged from 5.7% to 9.6% of the total N added as fertilizer, whereas for a sandy loam soil they ranged between 16.2% and 30.4%. Leaching losses of mineral N were higher when nitrate fertilizer was used compared to urea or ammonium fertilizer. Compared to the control (without urea addition), on the first day when soils were fertigated with urea, there were increases in NH₄⁺-N in the soils. This confirmed the rapid hydrolysis of urea in soil during fertigation. NH₄⁺-N in soils reached a peak about 5 days after fertigation, and due to nitrification it began to decrease at day 10. After applying NH₄⁺-N fertilizer and urea and during the incubation period, the mineral nitrogen in the soil decreased. This may be related to the occurrence of NH₄⁺-N fixation or volatilization in the soil during the fertigation process.     

Does the Ammonium Fixation Capacity of Soils Have a Significant Effect on the Nitrogen Nutrition of Wheat?

Pot experiments were conducted on three soils differing in their ammonium (NH₄ ⁺) fixation capacity [high = 161 mg NH₄-nitrogen (N) kg^?1 soil; medium = 31.5 mg NH₄-N kg^?1 soil; and no = no NH₄-N was additionally fixed], and the effect of N fertilizer forms and doses on wheat (Triticum aestivum L.) was investigated. Grain yields responded to almost all forms of N fertilizer with 80, 160, and 240 kg N ha^?1 in the high, medium, and no NH₄ ⁺ fixing soil process, respectively. Agronomic efficiency of applied N fertilizers was significantly greater in the no NH₄ ⁺ fixing soil. Thousand grain weights (TGW) of wheat grown on the high and medium NH₄ ⁺ fixing soil decreased with increasing N. Grain protein increased with increasing NH₄ ⁺ fixation capacity. Nitrogen doses and the forms of N fertilizers affected grain protein at a significance level. The combination of urea + ammonium nitrate (NH₄NO₃) was most effective in increasing grain protein content.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.     

Changes in 15N abundance and amounts of biologically active soil nitrogen

 Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added ¹⁵NH₄ ⁺ compounds could identify active N components. Dilution of ¹⁵N in KCl extracts of mineral and total N, non-exchangeable NH4₄ ⁺, and N in K₂SO₄ extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg^–1 were used. A constant level of ¹⁵N enrichment within KCl and K₂SO₄ extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH₄ ⁺ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable NH₄ ⁺ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to increased N reserve for crop production but a slower turnover. Received: 7 July 1998     

Growth and nitrogen nutrition of maize (Zea mays L.) in soil treated with the nitrification-inhibiting insecticide Baythroid

A pot experiment was conducted to compare the uptake and dry matter production potential of NH _inf4 ^sup+ and NO _inf3 ^sup- and to study the effect of Baythroid, a contact poison for several insect pests of agricultural crops, on growth and N uptake of maize (Zea mays L.). Nitrogen was applied as (¹⁵NH₄)₂SO₄, K¹⁵NO₃, or ¹⁵NH₄NO₃ and in one treatment Baythroid was combined with ¹⁵NH₄NO₃. Source of N had, in general, a nonsignificant effect on dry matter and N yield, but uptake of NO _inf3 ^sup- was significantly higher than that of NH _inf4 ^sup+ when both N sources were applied together. Substantial loss of N occurred from both the sources, with NH _inf4 ^sup+ showing greater losses. Baythroid was found to have a significant positive effect on dry matter yield of both root and shoot; N yield also increased significantly. Uptake of N from both the applied and native sources increased significantly in the presence of Baythroid and a substantial added nitrogen interaction (ANI) was determined. The positive effect of Baythroid was attributed to: (1) a prolonged availability of NH _inf4 ^sup+ due to inhibition of nitrification, (2) an increased availability of native soil N through enhanced mineralization, and (3) an enhanced root proliferation.     

CH4 oxidation in two temperate arable soils as affected by nitrate and ammonium application

The short-term effect of NaNO₃ or (NH₄)₂SO₄ application on CH₄ oxidation was measured under laboratory conditions with sieved soils collected from the top layer (0–12 cm) of a loamy and a sandy soil. The soils were incubated in sealed flasks and the CH₄ and CO₂ concentrations in headspace were measured periodically. On each gas sampling date the soils were analysed for inorganic N, electro-ultrafiltration organic N, and pH. NH ₄ ⁺ application to the loamy soil inhibited CH₄ oxidation entirely whereas in the untreated control soils CH₄ concentration decreased linearly with a rate of-41 nl CH₄ l^-1 h^-1; NO ₃ ^sup- application to this soil caused a small but significant reduction in CH₄ uptake. The CH₄-oxidizing ability of the sandy soil was low, even in the control. This was mainly a result of the disturbed soil structure after sieving. Both NH ₄ ⁺ and NO ₃ ^sup- treatments completely inhibited CH₄ uptake in this ligh-textured soil. The adverse impact of NH ₄ ⁺ persisted during the entire incubation, although in the loamy soil only 17% of the NH ₄ ⁺ added was recovered after 168 h. The negative effect of NO ₃ ^sup- was probably caused by an increase in osmotic potential. Immediate inhibition of CH₄ oxidation after inorganic N addition was demonstrated in two arable soils, although the effect was directly related only in part to soil N transformations.     

Influence of Organic Matter Vis-à-Vis Humic Acid on the Transformation of Inorganic and Organic Forms of Nitrogen in a Typic Haplustept Soil

A laboratory experiment was conducted to study the changes in inorganic and organic forms of nitrogen (N) in a Typic Haplustept soil treated with mustard cake vis-à-vis humic acid in the presence and absence of inorganic N. Results revealed that irrespective of treatments, significantly higher amount of soluble nitrate (NO₃^-), hydrolysable ammonium (NH₄⁺), non-hydrolysable and total N were accumulated in the soil treated with mustard cake in the presence of inorganic N. However, on the other hand, a humic acid-treated system showed significantly higher content of exchangeable NH₄⁺ and hexosamine N. Application of humic acid alone leads to the accumulation of a significantly higher amount of total hydrolysable and unidentified N in the soil. Among the different treatments, NH₄⁺ fixation was more in mustard cake followed by humic acid-treated soil. Humic acid is more susceptible to mineralization than mustard cake, particularly with respect to total N accumulation in soils.     

Microbial biomass and mineralization-immobilization of nitrogen in some agricultural soils

Summary The chloroform fumigation-incubation method (CFIM) was used to measure the microbial biomass of 17 agricultural soils from Punjab Pakistan which represented different agricultural soil series. The biomass C was used to calculate biomass N and the changes occurring in NH₄ ⁺-N and NO₃ ^–-N content of soils were studied during the turnover of microbial biomass or added C source. Mineral N released in fumigated-incubated soils and biomass N calculated from biomass C was correlated with some N availability indexes.The soils contained 427–1240 kg C as biomass which represented 1.2%–6.9% of the total organic C in the soils studied. Calculations based on biomass C showed that the soils contained 64–186 kg N ha^–1 as microbial biomass. Immobilization of NCO₃ ^–-N was observed in different soils during the turnover of microbial biomass and any net increase in mineral N content of fumigated incubated soils was attributed entirely to NH₄ ⁺-N.Biomass N calculated from biomass C showed non-significant correlation with different N availability indexes whereas mineral N accumulated in fumigated-incubated soils showed highly significant correlations with other indexes including N uptake by plants.     

Transformation of 15N-labelled leguminous plant material in three contrasting soils

Summary Two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) were incubated with ¹⁵N-labelled soybean tops for up to 20 weeks at 30°C. Mineralization of soybean ¹⁵N was slightly more rapid in the Pakistani soils, and after 20 weeks of incubation, 50%, 53%, and 56% of the applied ¹⁵N was accounted for as (NH₄ ⁺+NO₃ ^–)-N in Drummer, Hafizabad, and Khurrarianwala soils, respectively. Potentially mineralizable N (determined by anaerobic incubation) varied between 1.5% and 10% of the applied ¹⁵N in the three soils at different stages of incubation; somewhat higher percentages were mineralizable in the Pakistani soils than in the Drummer soil. From 3.7% to 9% of the applied ¹⁵N was accounted for in the microbial biomass. From 10% to 32% of the applied N was recovered in the humic acid and fulvic acid fractions of the organic matter by sequential extraction with Na₄P₂O₇ and NaOH; from 12% to 49% was recovered in the humin fraction. Of the three soils, Drummer soil contained more ¹⁵N as humic and fulvic acids. In all cases, the ¹⁵N was approximately equally distributed between the humic and fulvic acid fractions. A significant percentage of the humin ¹⁵N (52%–78%, equivalent to 8%–34% of the applied ¹⁵N) occurred in non-hydrolyzable (6 N HCl) forms. Of the hydrolyzable ¹⁵N, 42%–51% was accounted for as amino acid-N followed in order by NH₃ (17%–30%), hydrolyzable unknown forms (20%–22%), and amino sugars (6%–2%). The recovery of applied ¹⁵N for the different incubation stages was 87±22%. Recovery was lowest with the Khurrarianwala soil, presumably because of NH₃ volatilization losses caused by the high pH of this soil.     

Does the potential ammonium fixation of soils have an impact on the optimum nitrogen fertilizer rate?

Field experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH₄) fixation capacity [high = 161 mg fixed NH₄-N kg^?1 soil, medium = 31.5 mg fixed NH₄-N kg^?1 soil and no = nearly no fixed NH₄-N kg^?1 soil]. On high NH₄⁺ fixing soil, 80 kg N ha^?1 Urea+ ammonium nitrate [NH₄NO₃] or 240 kg N ha^?1 ammonium sulfate [(NH₄)₂SO₄]+(NH₄)₂SO₄, was required to obtain the maximum yield. Urea + NH₄NO₃ generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH₄⁺ fixing soil, 80 kg N ha^?1 urea+NH₄NO₃ was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH₄⁺ fixing soil than in the others. Grain protein was highly affected by NH₄⁺ fixation capacities and N doses. Harvest index was affected by the NH₄⁺ fixation capacity at the 1% significance level.     

Effect of pH on nitrogen mineralization in crop-residue-treated soils

Summary This study compares N mineralization in soils treated with crop residues [corn (Zea mays L.), soybean (Glycine max (L.) Merr.), sorghum (Sorghum vulgare Pers.)] or alfalfa (Medicago sativa L.) at three adjusted soil pH values (4, 6, and 8); pH was adjusted with dilute H₂SO₄ or KOH. A sample of soil (20 g) was treated with 0.448 g plant material (equivalent to 50t ha^–1), mixed with 20 g silica sand adjusted to the pH of the soil, and packed in a leaching tube. The soil-sand mixture was leached with 100 ml 5 mM CaCl₂ adjusted to the same pH as that of the treated soil to remove the initial mineral N, and incubated at 30°C. The leaching procedure was repeated every 2 weeks for 20 weeks. Results from three soils showed that N mineralization increased as the soil pH increased. In one soil (Lester soil), significant amounts of NH ₄ ⁺ -N accumulated at pH 4 during the first 12 weeks. Treatment with corn and soybean residues resulted in a marked reduction in N mineralization, especially at pH 4. The percentage of organic N mineralized from sorghum residue and alfalfa added to soils increased as the soil pH increased; the values ranged from 7.7% to 37.0% for sorghum and from 17.2% to 30.1% for alfalfa.     

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⁺.     

Comparison of Chemical Methods for Assessing Nitrogen Mineralization in Two Calcareous Soils Treated with Organic Materials

Reliable and quick methods for measuring nitrogen (N)–supplying capacities of soils (NSC) are a prerequisite for using N fertilizers. This study was conducted to develop a routine method for estimation of mineralizable N in two calcareous soils (sandy loam and clay soils) treated with municipal waste compost or sheep manure. The methods used were anaerobic biological N mineralization, mineral N released by 2 M potassium chloride (KCl), ammonium (NH₄ ⁺) N extracted by 1 N sulfuric acid (H₂SO₄), NH₄ ⁺-N extracted by acid potassium permanganate (KMnO₄), and NH₄ ⁺-N released by oxidation of soil organic matter using acidified potassium permanganate. The results showed that oxidizable N extracted by acid permanganate, a simple and rapid measure of soil N availability, was correlated with results of the anaerobic method. Oxidative 0.05 N KMnO₄ was the best method, accounting for 78.4% of variation in NSC. Also, the amount of mineralized N increased with increasing level of organic materials and was greater in clay soil than sandy loam soil.     

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

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

Estimating microbial biomass N in soils with and without living roots: Limitations of a pre-extraction step

Special net-closed soil containers were used in a pot experiment with low and high plant densities to give soil samples with and without roots. Soils from the containers were analysed either by the fumigation-extraction method or by a modified procedure starting with a pre-extraction and sieving step to remove plant roots from the samples. In the extracts NO ₃ ^- -N, NH ₄ ⁺ -N, organic N, and total N were measured. Microbial biomass N was calculated from the differences in total N in fumigated and unfumigated soils. Different plant densities had almost no influence on the values of the N compounds using either method. In soils with roots, significantly more organic N (and total N) was found by the fumigation-extraction method compared to soils without roots while no differences were obtained using pre-extractions and sieving. Though the organic N content in pre-extracts from soils with roots was significantly higher than from soils without roots, the NO ₃ ^- -N and NH ₄ ⁺ -N content was lower. Significant differences in biomass N in soils with and without roots were found only with the fumigation-extraction method. Biomass N levels calculated using the results after pre-extraction and sieving were about 50% lower than levels detected using fumigation-extraction alone. With the use of special net-closed soil containers, not only were soil samples produced with and without roots, but it was also possible to induce a rhizophere in the soils. A comparison of the two methods using these soils clearly demonstrated that the method used has profound influence on the final biomass N results. While higher biomass levels were found by fumigation-extraction in soils with roots, because root N becomes extractable after fumigation, the use of a pre-extraction and a sieving step may underestimate the total biomass N content due to the pre-extraction of microbial N (especially from rhizosphere microorganisms) from the sample. Nevertheless, pre-extraction and sieving followed by fumigation-extraction does seem to be the preferable method for biomass N measurement in comparative studies, because in most cases only minor errors will occur.     

Calcium carbonate solubilization through H‐proton release from some legumes grown in calcareous saline‐sodic soils

Increasing levels of CO₂ and H⁺ proton in the rhizosphere from some legumes may play an important role in calcite dissolution of calcareous salt affected soils. Soils planted with white and brown varieties of cowpea (Vigna unguiculata L.) and hyacinth bean (Dolichos lablab L.) relying on either fertilizer N (KNO₃) or N‐fixation were compared against soils to which gypsum was applied and a control without plants and gypsum application to study the possibility of Ca²⁺ release from calcite and Na⁺ leaching. As compared to plants relying on inorganic N, leachates from all pore volumes (0·5, 1·0, 1·5, 2·0 pore volume) in lysimeters planted with N‐fixing hyacinth bean contained significantly higher concentrations of HCO with lower concentrations from lysimeters planted with white cowpea relying on N‐fixation. However, the lowest concentrations of HCO were recorded in the gypsum and control treatments. In initial leaching, lysimeters planted with N‐fixing plants maintained similar leachate Ca²⁺ and Na⁺ concentrations compared to gypsum amended soils. However, gypsum amended soils were found to have a prolonged positive effect on Na⁺ removal. It might be concluded that some legumes that are known to fix N in calcareous salt affected soils may be an alternative ameliorant to the extremely expensive gypsum through calcite solubilization and a consequent release of Ca²⁺. Copyright © 2009 John Wiley & Sons, Ltd.