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Awad Elkarim, A. H. And Usta, S. Department of Soil Science, Faculty of Agriculture Ankara University ABSTRACT Laboratory experiments were conducted to evaluate two biological aerobic and anaerobic short-term incubation methods for assessing potentially available organic soil nitrogen and estimating the ability of soils under investigation to supply nitrogen. Status of some major known fractions of soil nitrogen such as amino acid-N and amino sugar-N which may considered to be the most predominant sources of mineralization were extracted under reflux by using 6N HCl. Soil organic matter, soil total-N and soil inorganic nitrogen which initially present in the soil were considered and correlated with the biological procedures. Surface soil samples (0-20 cm ) were collected from Polatli State Farm soils to represent criteria of arid and semi-arid region, which is usually cultivated with cereals, and normally received high doses of nitrogen fertilizers. Results obtained from the study showed that highly significant correlations were found when the total hydrolyzable ammonia-nitrogen, total hydrolyzable nitrogen and soil total nitrogen were compared with the results of the two incubation methods. Whereas, soil organic matter showed less significant correlation. Amounts of amino acid-N and amino sugar-N in the soil hydrolysate showed that no significant correlation were found when compared with those two incubation methods. INTRODUCTION During the last decades numerous biological and chemical methods attempting to provide a reliable and simple index of soil nitrogen availability were proposed. Most of biological procedures recently proposed involve determination of ammonium-N produced during short-term incubation periods (7-25 days) under controlled aerobic and anaerobic conditions (Bremner , 1965 ; Geist , 1977 ; Gianello and Bremner, 1986; Stanford,1982; Jalil et. al. 1996). These methods have a rational basis in that, the release of mineral N during the incubation periods involved microbially based processes as does the release of N during the growing season in the field (Smith and Shengxui, 1993) . Therefore, many of the literature cited elsewhere adopted the incubation procedures to be the best laboratory method currently available for the assessment of potentially available soil-N in soils ( Gianello and Bremner, 1986 ) . Although these biological methods encountered with many limitations as they are not simple and rapid enough for use in soil testing laboratories when compared with chemical methods, the incubation techniques provide a fairly satisfactory index of organic soil nitrogen availability .Nevertheless,variability of soils and climate makes accurate prediction of soil nitrogen availability by using such methods difficult. In soils of arid and semi-arid regions which usually cultivated with cereal crops and generally received appreciable amount of N fertilizers, predictions of soil ability to supply available N with reasonable accuracy during the growing season is very important. With such predictions of soil nitrogen availability it could be able to provide some recommendations to optimize the quantities of N fertilizers to be applied in such soils. Evaluation of such techniques in assessing potentially available soil N and as a result of that to state a reliable index of soil N availability in arid and semi-arid soils have studied to a very limited extent This is may be due partially to the generally low levels of native soil nitrogen in these soils as well as to the apparent high losses of the applied nitrogen fertilizers by NH3 volatilization. Status of some major known fractions of soil N such as total hydrolysable N, total hydrolyzable NH4, amino acid-N , polymer at amino sugar-N and acid insoluble-N and their relation to the potentially available soil N measured by biological incubation methods may help in obtaining a better understanding of such soils to provide available nitrogen to plant.The objective of this study was to evaluate two short-term incubation procedures in assessing of potentially available soil N in arid and semi-arid soils collected from Central Anatolia A further goal was established to show the relationship between the potentially available soil-N measured by incubation methods and status of some major known fractions of soil N extracted by 6 N HCl. MATERIALS & METHODS The soils used in the study were collected from 31 different sites in Polatli State Farm (latitude: 32º 00' to 32º 30' N; longitude: 39º 30' to 39º 00' E ) mainly used for production of cereal crops. The soil samples (0-20 cm) were taken from uncultivated parcels and were air-dried and crashed to pass through 2-mm screen and stored at -5 °C . Analysis reported in Table 1 shows that some physical and chemical properties of the soils used in the study .  Anaerobic incubation method: The anaerobic incubation procedure (waterlogged) used in the study was previously described by Waring and Bremner, (1964) and modified and developed by Keneey (1982) and strongly recommended for the assessment of the mineralizable-N during incubation period. The method involves incubation the soil samples under waterlogged conditions in enclosed test tubes with as little head space as possible in the tubes. The amounts of ammonium-N produced when 5 g of soil sample incubated under waterlogged conditions (12 ml of distilled water) at 40±2°C for 7 days were determined by using steam distillation apparatus. Also the amount of ammonium-N in the soil sample before incubation was determined by the same procedure, and mineralisable-N was calculated from the difference in the results of the two analysis. Analytical procedures The hydrolysis procedure used herein is based on the observation that maximal release of some major known fractions of soil organic-N(6N HCl acid-extractable forms) such as amino acid-N, amino sugar-N and nearly maximal release of total-N, was obtained by hydrolysis under reflux for about 6-12 hours using 3 ml of 6N HCl g-1 of soil (Stevenson 1982). Five g of finely ground (< 100 mesh) soil were placed into round-bottom flask fitted with a standard-taper (24/40) groundglass joint, 2 drops of octyl alcohol and 20 ml of 6N HCl were added. The flask contents were mixed thoroughly by swirling and placed in electric heating mantle then connected to a Liebig condenser fitted with a (24/40) ground-glass joint and then heated under reflux for 12 hours. After completion of hydrolysis the flask allowed to cool and its content filtered. The filtrate was neutralized to pH 6.5 by cautious addition of NaOH using a pH meter to follow the course of the neutralization. Determinations of different forms of soil nitrogen were carried out in the neutralized hydrolysate as described by Bremner (1965). All forms of N were converted to NH4 which was distilled under alkaline conditions in H3BO3-mixed indicator solution and determined by titration with standard 0.005N H2SO4. RESULTS AND DISCUSSION Results given in Table 2 shows the potentially available soil-N in mg NH4-N kg-1 soil as measured by aerobic incubation method described by Keeney and Bremner (1967) and anaerobic incubation method as described by Waring & Bremner (1964) then modified by Keeney (1982) in the 31 soils of Central Anatolia. As shown in Table 2 the quantities of mineralized N in aerobic incubation method ranged between 6.34 and 74.33 and averaged 21.78 mg NH4-N kg-1 soil .As compared to aerobic incubation method greater amounts of mineralized N ranging between 26.01 and 111.32 and giving a mean value of 63.87 mg NH4-N kg-1 soil was obtained with anaerobic incubation method. These findings were in agreement with that of Gianello and Bremner (1986) for Brazilian soils. The reasons of obtaining greater amounts of mineralized N in anaerobic methods that only NH4-N was measured and the losses of ammonia gases were avoided in enclosed system during the anaerobic conditions. Significant correlation was found between the aerobic and anaerobic incubation methods used for the assessment of potentially available soil-N when applied in Central Anatolian soil. (r = 0.72 p < 0.001). Soil-N parameters such as total N content and soil organic matter were evaluated to be used as indexes of soil nitrogen availability in test soils, because several workers have found that the results of many biological procedure assessing soil-N availability are closely related to total soil-N which in turn is closely related to organic matter content (Keeney 1982). According to the results given in Table 3. total soil-N, as an index of N availability, was found to be significantly correlated with the potentially available soil-N measured by the two incubation methods (r = 0.70 and 0.66 respectively). Whereas, the correlation coefficient between soil organic matter, as an index of N availability, and the two biological methods was found to be comparatively low . (r = 0.44 and 0.47 respectively).   As presented in Table 4 highly significant correlation was established when we compared the amount of total hydrolysable NH4-N extracted by reflux with 6N HCl and the aerobic and anaerobic incubation methods ( r = 0.79 and 0.73 respectively ). However, the correlation between the two incubation methods and total hydrolysable N determined in the same soil hydrolysate ( r = 0.66 and 0.57 respectively ) was not as good as for the total hydrolysable NH4. Amounts of some major known fractions of soil N such as amino acid-N and amino sugar-N in the soil hydrolysate showed no significant correlation when compared with these two incubation methods. Inconclusıon, the total hydrolysable ammonium-N extracted under reflux with 6N HCl showed the highest correlation with the potentially available soil-N measured with the two incubation methods. As a result of that, the total hydrolysable ammonium-N could be suggested for the prediction of N availability in such soils.  REFERENCES Bremner, J. M. (1965). Nitrogen availability indexes. In C.A.Black et al.(ed.) Methods of soil analysis, Part 2. Agronomy 9:1324-1345. Am. Soc. of Agron Madison, Wis. Geist, J. M. (1977) . Nitrogen response relationships of some volcanic ash soils. Soil Sci. Soc. Am. J. 41 :996-1000. Gianello, C., Bremner, J. M. (1986) . Comparison of chemical methods of assessing potentially available organic nitrogen in soil. Comm. Soil Sci. Plant Anal. 17:215-236. Jalil, A., Campbell, C.A., Shoenau, J ,Henry,J.L. , Jame, Y. M. and Lafon,G. P. (1996) Assessment of two chemical extraction methods as indices of available nitrogen.Soil Sci. Soc. Am. J. 60:1954-1960. Keeney, D.R. (1982). Nitrogen availability indexes .In Methods of Soil Analysis. Part 2nd edn Eds A.L. Page, R.H. Miller and D.R. Keeney, pp 711-733 Am Soc. Agron.Madison,Wisconsin. Smith,K.A. and Li. Shengxiu (1993). Estimation of potentially mineralizable N in soil by KCl extraction . 1.Comparison with pot experiments. Plant and Soil 157:167-174 Stanford, G. (1982). Assessment of soil nitrogen availability . In Nitrogen in Agricultural soils. Ed. F.J. Stevenson. pp,651-688. Am. Soc. Agron. Madison Wisconsin. Stevenson, F. J. (1982). Nitrogen-Organic forms. In methods of Soil Analysis. Part 2nd edn Eds, pp 625-642 Am. Soc. . Agron.Madison,Wisconsin. Waring, S.A. and J. M. Bremner (1964). Ammonium production in soil under water logged condition as an index of nit |