A comparison of different indices for nitrogen mineralization

Indices of N mineralization in soils of contrasting texture, pH, and organic matter contents were compared at different dates during the growing season. The indices were derived from a 12-week aerobic incubation, determination of the amount of microbial biomass at the start of the incubation, determination of the increase in NH ₄ ⁺ after boiling with 2 M KCl for 2h, and extraction of total soluble N with 0.01 M CaCl₂. Cumulative mineral N increased linearly with time in the course of the incubations. Rates of mineralization in soil samples taken in March 1989 and 1990 were significantly correlated with soluble organic N, while correlations between the mineralization rate and the increase in NH ₄ ⁺ after boiling with 2 M KCl for 2 h were poor for sandy soils and absent for loamy soils. Correlations between NH ₄ ⁺ after boiling with 2 M KCl for 2h and the soil N concentration were highly significant, but no general relationship was found between the mineralization rate and the soil N concentration. Neither biomass N nor biomass C was significantly correlated with the mineralization rate or with one of the chemical indices. Among the methods tested, soluble organic N extracted with 0.01 M CaCl₂ was the only method with any promise for routine measurement of the mineralization capacity of the individual sites.     

Nitrogen compounds extracted by electroultrafiltration (EUF) or CaCl2 solution and their relationships to nitrogen mineralization in soils

The objective of the investigation was to identify the most important organic N-containing fractions extracted from soils by electroultrafiltration (EUF) or a CaCl₂ solution, respectively, and their importance for nitrogen mineralization. The investigation comprised 19 agricultural and one forest top soil. Net N mineralization was tested in Mitscherlich pot experiments with three treatments: (1) fallow soil without N fertilizer, (2) soil cultivated with rye grass without N fertilizer, (3) soil cultivated with rye grass with N fertilizer. The highest proportion of N in the extracts was the amino N fraction (amino acids + peptides) amounting to approximately 60% of the total N extracted by CaCl₂ and to about 40% of the total N extracted by EUF. The proportion of amino sugars from total N extracted was in average 10% for the CaCl₂ and 5.2% for the EUF extracts. The proportion of heterocyclic N bases derived from nucleic acids amounted in average to 4.8% and 3.6% for the CaCl₂ and EUF extract, respectively. Amino N (amino acids + peptides) were correlated best with net N mineralization (EUF, r = 0.81***, CaCl₂, r = 0.86***). The correlation between amino sugars and net N mineralization was r = 0.55* for the EUF extract and r = 0.49* for the CaCl₂ extract. The heterocyclic N bases did not correlate with net N mineralization. Correlations between Norg extracted by CaCl₂ versus net N mineralization were higher than those obtained by the EUF extract. Net N mineralization was about four times higher in the fallow soils than in the treatment with grass and no N fertilizer. In the treatment with grass + N fertilizer on average no net N mineralization occurred, moreover there was a tendency of N immobilization. It is assumend that in the treatments with grass cultivation, organic C released by roots stimulated the assimilation of mineral N and amino acids by soil microorganisms resulting in a low net N mineralization. Net N mineralization led to a highly significant depletion in the Norg pools and particularly in the amino N and amino sugar pools in the treatment with grass and without N fertilizer. This depletion was particularly evident in the CaCl₂ extracts. The results justify the conclusion that the Norg obtained with both extraction methods originates from a dynamic N pool into which N flows in and out. The amino N extractable with EUF or CaCl₂ is a reliable indicator for the net N mineralization potential of soils.     

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.     

Extractability of 15N-labeled corn-shoot tissue in a sandy and a clay soil by 0.01 MCaCl2 method in laboratory incubation experiments

Management of N fertilization depends not only on the mineral N measured at the beginning of the growing season but also on the status of the low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the ¹⁵N applied in labeled cornshoot tissue would be recovered in 0.01 M CaCl₂-extractable ¹⁵N fractions and wheter a decrease in the CaCl₂-extractable ¹⁵N fraction quantitatively followed the trend in net mineralization of the ¹⁵N applied in corn-shoot tissue during an incubation period. The effects of adding ¹⁵N-labeled young corn-shoot tissue to a sandy soil and a clay soil were investigated for 46 days in an aerobic incubation experiment at 25°C. The application of 80 mg N kg^-1 soil in the form of labeled corn-shoot tissue (24.62 mg ¹⁵N kg^-1 soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in comparison with the untreated soils during the incubation. The labeled organic-N fraction was significantly higher in the sandy soil than in the clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in ¹⁵NO _inf3 ^sup- during the incubation. Ammonification of applied plant N resulted in a significant increase in the 1 M HCl-extractable non-exchangeable ¹⁵NH _inf4 ^sup+ fraction in the clay soik, owing to the vermiculite content. The ¹⁵N recovery was analyzed by the 0.01 M CaCl₂ extraction method; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incubation, recovery increased to 47.2 and 43.8% in the sandy and clay soils, respectively. Net mineralization of the ¹⁵N applied in corn-shoot tissue determined after the 46-day incubation was 6.60 mg ¹⁵N kg^-1 soil (=34.9% of the applied organic ¹⁵N) and 4.37 mg ¹⁵N kg^-1 soil (=23.1% of the applied organic ¹⁵N) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl₂ over the whole incubation period was 3.14 and 2.33 mg ¹⁵N kg^-1 soil in the sandy and clay soil, respectively. These results indicate that net mineralization of ¹⁵N was not consistent with the decrease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl₂ to extract or desorb all of the applied organic ¹⁵N that was mineralized during the incubation period.     

Prediction of mineralizable nitrogen in soils on the basis of an analysis of extractable organic N

Most of the nitrogen (N) in agricultural soils is organically bound, while the N uptake by plants and also the N losses from the soil-plant system into the environment are as inorganic N. The electro-ultrafiltration (EUF) method and the extraction by a CaCl₂ solution extract an organic N fraction (Norg) that is thought to provide information about the amount of rapidly mineralizable N in soils. This paper aims to illustrate various aspects regarding the biological meaning of the Norgfractions extracted by these two extraction methods and also the opportunities and limitations for predicting the mineralizable N based on an Norg analysis. From an evaluation of numerous data on EUF and CaCl₂ extract-able Norg fractions we concluded that these methods extract N compounds which can be used as indices for easily mineralizable soil N. However, both methods extract only some of the rapidly mineralizable N in soils, and some of the Norg ecxtracted appears to be from the more recalcitrant soil organic N. This was particularly true for the EUF-method. It may therefore be desirable to improve both the extractability and the selectivity of the extraction methods. This may be achieved by measuring extractable amino-N compounds instead of the total extractable Norg. Evaluating the numerous field experiments done during the last decade shows that the calibration factors obtained for extractable Norg were not the same for different growing seasons, geographical regions and management practices. Theoretically, for each combination of these factors separate calibrations would be necessary. It is this inflexibility which appears to be the most serious drawback for the use of extractable Norg fractions in practice. A possible solution may be to combine the flexibility of a simulation model with additional information obtained by the analysis of extract-able soil organic N. Further work in this direction may be desirable.     

Extraction of soil nitrogen by chloroform fumigation – A new index for the evaluation of soil nitrogen supply

The N extracted after chloroform (CHCl₃) fumigation was determined as a possible index of soil N supply to plants. The relationships between extractable N following fumigation and reference indices such as total N, alkali-hydrolyzable N, N released by the Stanford short-term incubation method, and the N extracted by KCl and by CaCl₂, were measured in nine soils of differing soil N supply capacity. A highly significant correlation was achieved between the extractable N released by fumigation and the N released by the Stanford method, i.e. a short-term aerobic incubation (r = 0.87). Similarly, the correlation between extractable N by fumigation and the N uptake by ryegrass was highly statistically significant (r = 0.93). Using the N extracted following fumigation has the advantage that laboratory results are available in two days and are both reproducible and of high precision. Therefore, the N extracted following fumigation is a valid, timesaving and precise index of soil N supply capacity.     

Effect of tillage and residue management on enzyme activities in soils

Recent interest in soil tillage and residue management has focused on low-input sustainable agriculture. In this study we investigated the effect of three tillage systems (no-till, chisel plow, and moldboard plow) and four residue placements (bare, normal, mulch, and double mulch) on the activities of four amidohydrolases (amidase, L-asparaginase, L-glutaminase, and urease) in soils from four replicated field-plots. Correlation coefficients (r) for linear regressions between the activities of each of the enzymes and organic C or pH and between all possible paired amidohydrolases were also calculated. The results showed that the effects of tillage and residue management on pH in the 28 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different tillage and residue-management practices studied, being the greatest in soils with notill/double mulch treatment, and the least with no-till/bare and moldboard/normal treatments. Within the same tillage system, mulch treatment resulted in greater organic C content compared with normal or bare treatment. The activities of the amidohydrolases studied were generally greater in mulch-treated plots than in non-treated plots, and were significantly correlated with organic C contents of soils, with r values ranging from 0.70^*** to 0.90^***. Linear regression analyses of enzyme activities on pH values (in 0.01 M CaCl₂) of the 28 surface soils showed significant correlations for L-asparaginase, L-glutaminase, and urease, with r values of 0.74^***, 0.77^***, and 0.72^***, respectively, but not for amidase (r=0.24). The activities of the four amidohydrolases studied in the 40 soil samples tested were significantly intercorrelated, with r values ranging from 0.72^*** to 0.92^***. The activities of the four amidohydrolases decreased with increasing soil depth of the plow layer, and were accompanied by a decrease in organic C content.     

Extractability of labelled microbial biomass N by electro-ultrafiltration and CaCl2 extraction

A laboratory soil incubation and a pot experiment with ryegrass were carried out in order to examine the extractability of microbial biomass N by using either 10-mM CaCl₂ extraction or the electro-ultrafiltration (EUF) method. The aim of the experiment was to test the hypothesis whether the organic N (Norg) extracted by EUF or CaCl₂ from dried soil samples represents a part of the microbial biomass. For the laboratory incubation a ¹⁵N-labelled Escherichia coli suspension was mixed with the soil. For the pot experiment a suspension of ¹⁵N-labelled bacteria was applied which had previously been isolated from the soil used. Soil samples of both treatments, with and without applied bacterial suspension, were extracted by EUF and CaCl₂. The extractability of applied microbial biomass was estimated from the difference in extractable Norg between the two treatments. In addition, the N isotopic composition in the upper plant matter, in the soil, and in organic and inorganic N fractions of EUF and CaCl₂ extracts was analysed. Both experiments showed that the applied microbial biomass was highly accessible to mineralization and thus represented potentially mineralizable N. However, this mineralizable N was not extractable by CaCl₂ or by the EUF method. It was, therefore, concluded that the organic N released on soil drying and which was thus extractable was derived from the non-biomass soil organic matter. The result suggests that both extraction methods may provide a suitable index for mineralizable N only in cases where the decomposable organic substrates are derived mainly from sources other than the living soil biota.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Evaluation of Chemical Extraction Methods for Determining Plant-Available Potassium in Some Soils of West Bengal,India

The effectiveness of eight chemical extraction methods was evaluated on 15 Indian soils for the prediction of plant-available potassium (K⁺) to Sudan grass (Sorghum vulgare var. sudanensis) grown in modified Neubauer technique. Average amounts of soil K⁺ extracted were in descending order: Morgan’s reagent > 0.5 M sodium bicarbonate (NaHCO₃) > neutral 1N ammonium acetate (NH₄OAc) > 1N nitric acid (HNO₃) > 0.02 M calcium chloride (CaCl₂) > 0.1N HNO₃ > Bray and Kurtz No.1> distilled water. The highest simple correlation with plant K⁺ uptake was obtained with NH₄OAc-K⁺ (r = 0.866**) and the lowest with CaCl₂-K⁺ (r = 0.45*). To develop the predictive models using stepwise regression, plant K⁺ uptake was used as the dependent variable and the extractable soil K⁺, pH, sand, silt and organic carbon (C) contents as the independent variables. Based on the final R², the NH₄OAc model was found to be the best predictor of plant-available K⁺ in the soils when used along with sand and organic C.     

Sulphur mineralization and release of soluble organic sulphur from camp and non-camp soils of grazed pastures receiving long-term superphosphate applications

Summary Topsoils (0–75 mm) from four soil types with different sulphate retention capacities were collected from stock camp and non-camp (main grazing area) sites of grazed pastures in New Zealand which had been annually fertilized with superphosphate for more than 15 years. These soils were analysed for different S fractions and incubated at 30°C for 10 weeks using an open incubation technique in order to assess the extent of S mineralization and the release of soluble soil organic S from camp and non-camp soils during incubation. The soils were preleached with 0.01 M KCl, followed by 0.04 M Ca(H₂PO₄)₂ before being incubated. Pre-incubation leachates and weekly 0.01 M KCl leachates were analysed for mineralized S (i.e., hydriodic acid-reducible S) and total S. Soluble organic S was estimated as the difference between these two S fractions. Results obtained show higher cumulative amounts of all three S fractions in leachates over a 10-week incubation period in camp than in non-camp soils, suggesting that higher mineralization occurred in camp soils. Cumulative amounts of mineralized S from camp and non-camp soils showed a linear relationship with duration of incubation (R ²0.985^***), while the cumulative release of soluble organic S followed a quadratic relationship (R ²0.975^***). A significant proportion (14.6%–40.8%) of total S release in KCl leachates was soluble organic S, indcating that organic S should be taken into account when assessing S mineralization. Mineralized S and soluble organic S were best correlated with 0.01 M CaCl₂-extractable soil inorganic S (R ²=0.767^***) and 0.04 M Ca(H₂PO₄)₂-extractable soil inorganic S(R ²=0.823^***), respectively. Soil sulphate retention capacity was found to influence amounts of mineralized S and soluble organic S, and thus periodic leaching with KCl to remove mineralized S from soils may not adequately reflect the extent of soil S mineralization in high sulphate-retentive soils. In low (<10%) sulphateretentive soils, increasing the superphosphate applications from 188 to 376 kg ha^–1 year^–1 increased S mineralization but not amounts of C-bonded and hydriodic acid-reducible soil S fractions.     

Effect of freeze-thaw events on mineralization of soil nitrogen

Summary In humid regions of the United States there is considerable interest in the use of late spring (April–June) soil NO ₃ ^– concentrations to estimate fertilizer N requirements. However, little information is available on the environmental factors that influence soil NO ₃ ^– concentrations in late winter/early spring. The influence of freeze-thaw treatments on N mineralization was studied on several central Iowa soils. The soils were subjected to temperatures of-20°C or 5°C for 1 week followed by 0–20 days of incubation at various temperatures. The release of soluble ninhydrin-reactive N, the N mineralization rate, and net N mineralization (mineral N flush) were observed. The freeze-thaw treatment resulted in a significant increase in the N mineralization rate and mineral N flush. The N mineralization rate in the freeze-thaw treated soils remained higher than in non-frozen soils for 3–6 days when thawed soils were incubated at 25°C and for up to 20 days in thawed soils incubated at 5°C. The freeze-thaw treatments resulted in a significant release of ninhydrin-reactive N. These values were closely correlated with the mineral N flush (r ²=0.84). The release of ninhydrin-reactive N was more closely correlated with biomass N (r ²=0.80) than total N (r ²=0.65). Our results suggest that freeze-thaw events in soil disrupt microbial tissues in a similar way to drying and re-wetting or chloroform fumigation. Thus the level of mineral N released was directly related to the soil microbial biomass. We conclude that net N mineralization following a spring thaw may provide a significant portion of the total NO ₃ ^– present in the soil profile.     

Winter cover cropping influence on nitrogen mineralization,presidedress soil nitrate test,and corn yields

The mineralization and availability of cover crop N to the succeeding crop are critical components in the management of soil N to reduce N leaching. The effects of several leguminous and non-leguminous cover crops on soil N availability, N mineralization potential, and corn (Zea mays L.) yield were examined. The cover crops had variable effects on soil N availability and corn yield and N uptake. Because of the rapid mineralization of the cover crops following incorporation, the inorganic N levels in the soil sampled in mid-May 1992 (4 weeks after incorporation of cover crops), rather than the potentially mineralizable N, rate constants, initial potential mineralization rate, or cumulative N mineralized over 14 weeks, correlated well with N concentrations, C:N ratios, or the N added in the cover crops. However, the inclusion of potentially mineralizable N with inorganic N in a multiple regression improved the variability in the corn yield and the N uptake accounted for. Since extensive mineralization had occurred before the 21 May sampling, the potentially mineralizable N was affected more by the soil organic N and C than by the N concentrations of the cover crops. The presidedress NO₃ ^--N test levels were well predicted by the inorganic and potentially mineralizable N (R ²=0.89, P<0.01), although the test levels were better in predicting corn yield and N uptake. If the available soil N test needs to be made earlier than recommended by the presidedress NO₃ ^--N test, both inorganic and potentially mineralizable N are needed to better predict the corn yield and N uptake in the soils.     

Extraction of soil nitrogen by chloroform fumigation – A new index for the evaluation of soil nitrogen supply

The N extracted after chloroform (CHCl₃) fumigation was determined as a possible index of soil N supply to plants. The relationships between extractable N following fumigation and reference indices such as total N, alkali-hydrolyzable N, N released by the Stanford short-term incubation method, and the N extracted by KCl and by CaCl₂, were measured in nine soils of differing soil N supply capacity. A highly significant correlation was achieved between the extractable N released by fumigation and the N released by the Stanford method, i.e. a short-term aerobic incubation (r = 0.87). Similarly, the correlation between extractable N by fumigation and the N uptake by ryegrass was highly statistically significant (r = 0.93). Using the N extracted following fumigation has the advantage that laboratory results are available in two days and are both reproducible and of high precision. Therefore, the N extracted following fumigation is a valid, timesaving and precise index of soil N supply capacity.     

Phosphate buffer–extractable organic nitrogen as an index of soil‐N availability for sorghum and pearl millet

The availability of soil nitrogen (N) is usually quantified by the amount of mineralized N as determined after several weeks of soil incubation. Various alternative methods using chemical solvents have been developed to extract the available organic N, which is easily mineralized. We compared one such solution, neutral phosphate buffer (NPB), with conventional incubation and 0.01 M–CaCl₂ extraction, as measures of soil N available to two major cereal crops of the semiarid tropics, based on the total N uptake by plants in a pot experiment. Mineralized N had the highest correlation with N uptake by pearl millet (Pennisetum glaucum L., r = 0.979***) and sorghum (Sorghum bicolor [L.] Moench, r = 0.978***). NPB‐extractable N was also highly correlated with N uptake (pearl millet, r = 0.876***; sorghum, r = 0.872***). Only one major peak was detected when NPB extracts were analyzed using size‐exclusion high‐performance liquid chromatography, regardless of soil properties. In addition, the organic N extracted with NPB was characterized by determining the content of peptidoglycan, the main component of bacterial cell walls. Although the characteristics of NPB‐extractable organic N are still unclear, it offers a promising quick assay of available N.     

Status and distribution of soil sulphur fractions,total nitrogen and organic carbon in camp and non-camp soils of grazed pastures supplied with long-term superphosphate

Summary Topsoils (0–75 mm) from four different soil types were collected from stock camp and non-camp (main grazing area) areas of grazed pastures in New Zealand, which had been fertilised annually with superphosphate for more than 15 years, in order to assess the effects of grazing animals on the status and distribution of soil S fractions and organic matter. These soils were analysed for organic C, total N, total S, C-bonded S, hydriodic acid-reducible S, 0.01 M CaCl₂, and 0.04 M Ca(H₂PO₄)₂-extractable S fractions, and soil pH. Soil inorganic and organic S fractions extracted by NaHCO₃ and NaOH extractants were also determined. The results obtained showed that camp soils contain higher soil pH, organic C, total N, total S, organic (C-bonded S and hydriodic acid-reducible S) and inorganic S fractions, NaHCO₃-and NaOH-extractable soil S fractions but a lower anion retention capacity than non-camp soils, attributed to a higher return of plant litter and animal excreta to camp soils. In both soils, total S, organic S, C-bonded S, and hydriodic acid-reducible S were significantly correlated with organic C (r0.90***, ***P0.001) and total N (r0.95***), suggesting that C, N, and S are integral components of soil organic matter. However, C: N : S ratios tended to be lower in camp (60: 5.6: 1–103: 7.2: 1) than in non-camp soils (60:6.1:1–117:8.3:1). Most (>95%) of the total soil S in camp and non-camp soils is present as organic S, while the remainder is readily soluble and adsorbed S (i.e. Ca(H₂PO₄)₂-extractable S). C-bonded S and hydriodic acid-reducible S constituted 55%–74% and 26%–45% of total S, respectively, reflecting a regular return of plant litter and animal excreta to the grazed pastures. NaHCO₃, and especially NaOH, extracted significantly higher amounts of total soil S (13%–22% and 49%–75%, respectively) than Ca(H₂PO₄)₂ or CaCl₂ (<5%). In addition, NaHCO₃ and NaOH-extractable soil S fractions were significantly rorrelated with soil organic S (r0.94***), C-bonded S (r0.90***) and hydriodic acid-reducible soil S (r0.93***). Differences between soils in either camp or non-camp areas were related to their sulphate retention capacities, as soils with high sulphate retention capacities (>45%) contain higher levels of C-bonded and hydriodic acid-reducible S fractions than those of low sulphate retention soils (<10%). Long-term annual superphosphate applications significantly increased the accumulation of soil organic and inorganic S fractions, and organic C and total N in the topsoil, although this accumulation did not occur when the superphosphate application rates were increased from 188 to 376 kg ha^-1 year^-1.     

Sulfur mineralization rates and potentials of soils

Summary Field-moist soil and glass beads mixtures were packed in glass tubes and leached with 100 ml of 5 mM CaCl₂ and incubated at 20 or 30°C. The leaching procedure was repeated every 2 weeks for 14 weeks. The leachates were analysed for SO _inf4 ^sup2– and NO₃ ^–. The S uptake by three successive croppings of corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] at 40- or 60-day intervals, respectively, or three cuttings of ryegrass (Lolium multiflorum L.) at 30-day intervals were studied under greenhouse conditions. Results showed that significantly greater amounts of S were mineralized at 30°C than at 20°C in each of 13 Iowa and 7 Chilean surface soils. Expressed as percentages of organic S in soils, the amounts of S mineralized in the Iowa surface soils in 14 weeks at 20 and 30°C ranged from 1.2% to 9.8% and from 2.4% to 17.5%, respectively. The corresponding values for the Chilean soils ranged from 0.9% to 7.2%6 and from 1.4% to 12.1%. The Q₁₀ values of S mineralization ranged from 1.7 to 4.4 (average 2.5) for the Iowa soils and from 1.7 to 3.1 (average 2.1) for the Chilean soils. The cumulative S mineralized at 20°C in 14 weeks was significantly correlated with the cumulative N mineralized (linear model, r=0.72^**; quadratic model, r=0.84^***). Similarly, the cumulative S mineralized at 30°C was significantly correlated with the cumulative N mineralized at this temperature (linear model, r=0.81^***; quadratic model, r = 0.82^***). The potentially mineralizable S pool (S₀), calculated by using an exponential equation for the S mineralized at 20°C, ranged from 5 to 44 mg kg^–1 for the Iowa soils and from 10 to 25 mg kg^–1 for the Chilean soils. The corresponding values obtained by using a reciprocal-plot technique ranged from 6 to 48 mg kg^–1 and from 12 to 26 mg kg^–1, respectively. The S₀ values calculated for S mineralized at 30°C, in general, were higher than those obtained at 20°C. The S mineralization rate constant (k) and the time required to mineralize 50% of S₀ (K _t), calculated by using the cumulative SO _inf4 ^sup2– released during 14 weeks of incubation, varied considerably among the soils. Up take of S by corn and soybean (tops+roots) were, in general, lower than the total SO _inf4 ^sup2– mineralized in 14 weeks at 20°C.     

Available Nitrogen for Corn and Winter Cereal in Spanish Soils Measured by Electro‐ultrafiltration,Calcium Chloride,and Incubation Methods

Abstract

Comparison of methods is necessary to develop a quick and reliable test that can be used to determine soil‐available nitrogen (N) in an attempt to increase the efficiency of N fertilizers and reduce losses. The objectives of this research were to compare the fractions extracted by the calcium chloride (CaCl₂) and the electro‐ultrafiltration (EUF) methods and to correlate them to the mineralization rate (k) obtained from a 112‐d incubation of 61 soil samples. Thirty‐five soil samples were collected from cornfields and 26 from winter cereal fields. Subsamples were either aerobically incubated to calculate k or extracted by the EUF and CaCl₂ methods to identify three fractions: nitrate (NO₃ ^?)‐N, ammonium (NH₄ ⁺)‐N, and Norg‐N. The Norg‐N extracted by both methods was larger in soils from cornfields than in soils from winter cereal fields. In samples from cornfields, the Norg‐N fraction obtained by the EUF method was correlated to the Norg‐N measured by the CaCl₂ method (r=0.46). Soil N content was related to k in samples from cornfields (r=0.40) but not in samples from winter cereal fields. Also, k was correlated to inorganic N content extracted by both chemical methods. The CaCl₂ method was a reliable alternative for laboratories to determine soil‐available N for corn but not for winter cereal.     

Sugar yield,nitrogen uptake by sugar beet and optimal nitrogen fertilization in relation to nitrogen soil analyses and several additional factors

To test the relative usefulness of different methods of chemical analysis for soil nitrogen fractions in the assessment of the fertilizer nitrogen needs of sugar beet, different doses of nitrogen were applied in field experiments laid out during the years 1985–1991. The chemical methods used were N mineral (NO _– ³ +NH ₊ ⁴ ) analyses on soil samples taken in late winter, and extraction with 0.01 M CaCl₂ from soil samples taken the preceding autumn and in late winter. The results of the chemical methods were evaluated in models using estimated optimum nitrogen fertilization, nitrogen present in beets or beets+leaves at leaf maximum and sugar yield as variables. In addition, parameters such as estimates of possible rooting depth and mineralization capacity of the soil were also included in the model. All models for estimating nitrogen fertilization need showed low R ² values. The two methods of soil chemical analysis yielded similar R ² values for nitrogen uptake in plots both with and without nitrogen fertilization. The N mineral method was least useful in predicting sugar yield. Addition of the covariables rooting depth and mineralization capacity appreciably improved the explanatory value of the models with 0.01 M CaCl₂, especially when the analytical results of soil samples taken in autumn were used. For the N mineral method the addition of covariates was found to have far less influence.     

Effects of filter type and extraction efficiency on nitrogen mineralization measurements using the aerobic leaching soil incubation method

Aerobic incubation of soils with sequential leachings to extract mineralized N is often used to determine N mineralization potential and N availability in the laboratory. This study used tropical forest soils with differing mineralogy and texture to address: (1) the effects of filter type and equilibration time on soil moisture and N mineralization and (2) the N extraction efficiency of 0.01 M CaCl₂, minus-N nutrient solution (containing 0.004 M CaCl₂) and 2 M KCl. Use of glass microfiber filters compared to cellulose acetate or polyethersulfone membrane filters resulted in a lower moisture content for both low-and high-clay soils. However, filter type did not affect N mineralization. Under 47 kPa suction, soil moisture equilibration occurred between 240 and 360 min regardless of filter type. Extraction efficiency for mineralized N using 0.01 M CaCl₂ or minus-N nutrient solution was lower in forest soils of smectitic mineralogy and soils with a higher proportion of macroaggregates. However, with the exception of allophanic soils, the cumulative amount of N mineralized measured in a long-term incubation for approximately 1 year was not different when either a leaching or an unleached incubation method was used. These results indicate that researchers may wish to conduct preliminary evaluations to determine whether their incubation method will achieve a desired uniform moisture level and N extraction efficiency.     

An evaluation of plant-available soil nitrogen in selected sandy soils by electro-ultrafiltration, KCl, and CaCl2 extraction methods

 Improving the precision in estimating the nitrogen (N) requirement for citrus trees on sandy soils is important for increasing N efficiency by the trees and minimizing potential losses of N in commercial citrus production areas. In this study, representative Florida soils were sampled from major citrus production areas and the electro-ultrafiltration (EUF) technique was used to measure the concentrations of total EUF-extractable nitrogen (EUF-N_t), ammonium-N (EUF-NH₄ ⁺–N) and nitrate-N (EUF-NO₃ ^––N). Available organic N (N_org) was calculated as: EUF-N_t–(NH₄ ⁺–N+NO₃ ^––N). The N concentrations in the EUF extraction were greater than those by the KCl or CaCl₂ method. The N_org fraction, estimated by the EUF method, varied from 4.4 to 40.8 mg kg^–1 soil, equivalent to 10 to 91 kg N ha^–1 (for the top 15 cm depth soil) and was positively correlated with the total soil N determined by the Kjeldahl method. The presence of appreciable amounts of N_org in these soils indicates that these soils contain high proportions of the total soil N in easily mineralizable N_org forms. This study demonstrates that the EUF-extractable organic bound N must be considered in developing N fertilizer recommendations for citrus. Received: 13 January 1999