Proc. 5th Sci. Conf. / SRC Iraq, Baghdad 7 11 Oct. 1989 Vol. I Part 2 NITROGEN MINERALIZATION IN SOME CALCAREOUS SOILS OF IRAQ Faiz G. Aziz, Hamad M. Salih, Barzan I. Khayatt, M. A. Umran Department of Soil and Land Reclamation, Scientific Research Council, Baghdad, P.O. Box 2416, IRAQ Abstract: A study was conducted to evaluate N mineralization in 12 calcareous soils of varying characteristics by employing long and short terms of incubation. Mineralizable and inorganic-n were correlated with N uptake by wheat grown in these soils in a pot experiment. The results showed a significant release of mineralizable-n even at the later periods of incubation. The mean of mineralization during the 16 weeks of incubation ranged from 4-15 ppm per week. Correlation coefficient between the two incubation methods at the corresponding interval of incubation (0-2 weeks) was highly significant. Total soil N showed to be an important factor in predicting the soil potentiality for N mineralization. In both incubation methods adding the value of N mineralized at a particular time of incubation to the initial inorganic-n or NO3-N improved the correlation coefficients of these two forms of N with N-uptake by wheat. It appears that employing short term incubation gave a reliable N availability index in the calcareous soils. INTRODUCTION The need for a soil test for N availability has been recognized since beginning of present century. The past several years have witnessed interest in assessing N availability in soils as a basis for more accurately predicting the N fertilizer recommendation. Generally, the soil N taken up by plants is derived from two sources; the mineral N present in the soil at the time of planting and N mineralized during growth of the current crop (1). With the rising use of N fertilizer, the complexity of assessing soil N availability has also increased because both mineral N initially present in the root zone and N mineralized during the cropping season have become significant variables (2). 97
Proc. 5th Sci. Conf. / SRC Iraq, Baghdad 7 11 Oct. 1989 Vol. I Part 2 In the soils of humid regions, when the variability in N-supplying capability of soil is not taken into account, most N fertilizer recommendations would be inaccurate (3,4). The arid and semi-arid soils are known to contain low organic matter, accordingly high response to the N fertilizer is expected by crops. However, application of N fertilizer more than demanded by plant may have a negative effect on crop yield. On the other hand, seasonal fluctuations of the soil condition in Mediterranean type climate are generally extreme. Temperature may range from below 0 to as high as 60 o C, while, moisture content may range from field capacity to below the conventional wilting point. Such variation in the environment has a great impact on the dynamic of N transformations in soil (5). Better understanding the rate of N mineralization in dry land farming would help predicting of inorganic-n availability and allow for timely correction of N deficiencies through fertilizer application. The amounts of mineralized-n in two weeks of incubation as described by Keeney (6) considered to be the most accurate method for assessing N availability (4). On the other hand, it has been concluded that the amounts of N accumulated over extended periods of aerobic incubation was the most useful measurement for the soil potential to supply inorganic-n (7,8). In Iraq, soils which mainly characterized as calcareous soils with low organic matter content, there is clear lack on the information concern N availability index. El-Aggory et al. (9) concluded that initial NO3-N may be useful in predicting N fertilizer need in soils low in organic matter. Aziz et al. (10) found in a silt clay loam soil a significant correlation between N-uptake by wheat and inorganic-n using different N fertilizer sources. Al-Akabi (11) measured important quantities of mineralizable N in soils of southern Iraq. Accordingly the present study intended to evaluate the two methods of soil incubation (long and short) for predicting N availability for wheat grown in different calcareous soils, and to obtain more information concern the rate of mineralizable N in these soils. MATERIALS AND METHODS Pot experiment: Surface soil samples (0-0.25 m) were collected from selected locations to represent the major soils under cultivation in Iraq (Tables 1 and la). 98
Proc. 5th Sci. Conf. / SRC Iraq, Baghdad 7 11 Oct. 1989 Vol. I Part 2 The soil samples were air dried and crushed to pass through a 2 mm sieve. Sub-samples from each were taken and further crushed to pass through 100 mesh sieve and used for determination of chemical properties. The soil samples were uniformly packed at the rate of 9.0 kg soil per pot (0.29 height and 0.29 m inner diameter). A basal dose of N as urea, P as triple superphosphate and K as potassium sulfate at the rate of 36, 10, and 7 mg kg -1 soil, respectively were mixed with the upper 5 cm of soil surface in the pots. Ten seeds of Maxipak wheat (Triticum aestivum L.) were planted in each pot on Nov. 24 th, 1986 and thinned to 8 plants after germination. The irrigation was performed to maintain the soil moisture at 50% of the soil saturation percentage of each soil by adding water daily or every other day. On April 19 th, 1987 the plants were harvested above the soil surface, oven dried at 65 o C for 48 hours, ground for chemical analysis. Chemical analysis Soil extract of 1:2.5 (Soil:Water) was used for measuring the electrical conductivity and soil ph. Calcium carbonate and cation exchange capacity (CEC) were determined by acid neutralization and sodium acetate methods respectively (12). Total and available N were determined by semi-micro Kjeldahl procedure as described by Bremner (13). Organic matter was measured by Walkely and Black method. Available P was extracted by 0.5 N NaHCO3, while available K was extracted by neutral ammonium acetate and measured using spectrophotometer and flame photometer, respectively. The standard pipette method was used for soil particle analysis. Plant analysis for total N was performed by digestion of the ground plant sample with concentrated sulfuric and salicylic acids. The N-uptake was calculated by multiplying the N% in the plant material by its oven dry weight. Incubation methods The methods of aerobic incubation at 30 o C were used. The first method (A) was the long term incubation procedure described by Stanford and Smith (7) involving determination of N produced from a 20 g soil sample mixed with 20 g of 30-60 mesh quartz in a leaching tube (0.035 m in diameter and 0.15 m in height). 99
Proc. 5th Sci. Conf. / SRC Iraq, Baghdad 7 11 Oct. 1989 Vol. I Part 2 Hundred ml of 0.01 M KC1 was used for leaching achievement in 4 to 5 increments, and the excess solution was removed under Vacuum (60 cm Hg). The leaching procedure was repeated for 0, 2, 2, 2, 2, 4 and 4 weeks (cumulative: 2, 4, 6, 8, 12 and 16 weeks). The leachate obtained was analyzed for NH4-N and NO3-N (14). The second method (B) was the short term incubation procedure described by Keeney and Bremner (15). It involved determination of N produced when a 10 g soil sample was mixed with 30 g of 30-60 mesh quartz, moistened with 6 ml of distilled water and incubated for 2 and 4 weeks, hundred ml of 2 M KC1 was used as inorganic-n extractant. All incubations were carried out in duplicate. RESULTS AND DISCUSSION The amounts of inorganic-n (NH4-N+NO3-N) extracted by 0.01M KC1 (Method A) from different soils, and after different time intervals of incubation are presented in Table 2. There was a wide range of soil inorganic- N, as measured in pre-incubation of soil leachates. Soils from Ishaki and Hilla (No.7 and 10) had the lowest and highest initial inorganic-n, respectively. Although, most of the soils contained relatively low organic matter, significant amounts of inorganic-n were released even at the later periods of incubation (12 and 16 weeks). These results agreed with the finding of Al- Akabi (11). It appears that there was a tendency for a relatively constant rate of N-mineralization in each individual soil throughout the 16 weeks of soil incubation; a period in which many agricultural crops complete their N- uptake. The means of mineralized-n (method A) ranged from 3.5-9.9 mg N kg -1 soil week -1. However, the means obtained in the studied soils were lower than those of temperate regions (3,16). This may be attributed to the relatively lower organic matter content of the arid regions. Cumulative amounts of mineralized-n in four selected soils are shown in Fig.1. It is worth to mention that the cumulative amount of mineralized-n in each soil studied was linear with time of incubation. These relationships were significant at 0.01 level. Similar results were found by Al-Akabi (11), Tabatabi and Al-Khafaji (17), Flowers and Arnold (18). However, by using nutrient solution devoid of N, Stanford and Smith (7) found a linear 102
Proc. 5th Sci. Conf. / SRC Iraq, Baghdad 7 11 Oct. 1989 Vol. I Part 2 those from Akra, Altoon-Kopri and Hilla. In these soils NO3-N produced during two weeks incubation was more than that produced in the other soils (data not shown), consequently more N was accumulated. However, the mean values of mineralized N in both methods over all the different soils were very close (x=19.3, 19.4) and the correlation coefficient between them at the end of two weeks incubation was highly significant (r=0.81). There was also highly significant correlation (r=0.76) between the cumulative N mineralized in long term incubation (A) and that of short term (B). This indicated that the both methods can be used to predict the capability of calcareous soils to release available N. Total N and organic matter contents of the native soils were correlated with total mineralized-n and NO3 fraction at successive time of incubation (Table 4). In both incubation methods, at certain time of incubation, initial total N showed to be more correlated with mineralized- N than organic matter. These results indicated that total N in the native soil is more important in predicting soil potentiality for N mineralization. These results are in accordance with that found by Viassak (19), Hadas et al. (20). It can also be noticed that, in method (A), the correlation coefficient between total N and mineralized-n was not improved by increasing the time of incubation. However, in method (B), increasing time of incubation from two to four weeks improved the correlation between total N and mineralized-n or NO3. So, the results suggested that two weeks of soil incubation is accurate as the long term in predicting the capability of soils for N mineralization. On the other hand, NO3-N produced during incubation had lower correlation coefficients with total N and organic matter in both methods which probably reflect the importance of NH4-N fraction of mineralized-n during incubation. Data presented in Table 5 shows the correlations between N-uptake by wheat and mineralized-n or NO3 alone or plus native inorganic-n or NO3 in the both methods. Adding the amount of N mineralized during a certain time of incubation to the initial inorganic-n or NO3-N, improved the correlation of these two N forms with N-uptake by wheat. Such improvement was so clear with the NO3-N form. Similar results were reported in humid regions by Stanford et al. (1). However, El-Aggory et al. (9) found in some Iraqi soils that the N indices were highly correlated with initial NO3-N and no significant 106
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