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Mahmut Basri Halitligil 1 , Ali Akın 1 , Adem İlbeyi 2 1 Ankara Nuclear Research and Training Center, Turkish Atomic Energy Authority, Ankara 2 Rural Services Research Institute, General Directory of Rural Services, Ankara-Turkey ABSTRACT In order to obtain information on potato yield, N uptake, N fertilizer residue in the soil and the portion of N fertilizer leached below 200 cm of soil depth, 9 field experiments were conducted at three different locations in 1992, 1993 and 1994. The N rates used in these experiments were 0, 200, 400, 600, 800 and 1000 kg N/ha with a completely randomized block design with 3 replications. Nitrogen fertilizer was applied in two equal portions; one at planting and one just before the first irrigation. Although all yield data were used to find out the marketable tuber yield - the N rate response curve and the fate of applied fertilizer N was determined only for the 400 and 1000 kg N/ha rates. Isotope microplots were established where 15N-labelled ammonium sulphate was applied at % 5.0 and 2.5 atom excess enrichments for 400 and 1000 kg N/ha rates, respectively. At harvest, marketable and dry tuber yield was determined for all N rates. Dry tuber and leaf plus vine yields were determined for the isotope microplots and they were analyzed for the % N and % 15N atom excess. The percent N derived from fertilizer ( % Ndff ) and N use efficiency ( % NUE ) were calculated for the plant samples. The 15N-labelled residue left in 0 - 200 cm soil was also determined. The amount of N fertilizer leached beyond 200 cm soil depth was also calculated. 15N-labelled nitrate and total nitrate of the groundwaters from wells were determined at different dates. Our results show that the optimum marketable tuber yield was obtained with 600 kg N/ha. Tuber N uptake was increased slightly, while leaf plus vine N uptakes increased considerably as the N rate was increased from 400 to 1000 kg N/ha. The percent NUE values decreased nearly by half and the amount of N fertilizer in the 0 - 200 cm soil layer increased more than 3 times when the N rate was increased from 400 to 1000 kg N/ha. Nearly half of the applied fertilizer N ( 45.6 % ) at 400 kg N/ha and more than half of the applied fertilizer N (60.8 % ) at 1000 kg N/ha was still in 0 - 200 cm soil depth after harvest. Four times more N fertilizer was leached beyond 200 cm soil depth when 1000 kg N/ha nitrogen was applied in stead of 400 kg N/ha rate. Our results also indicate that the potential of contamination of the groundwaters do exist due to leaching of the applied N fertilizer. INTRODUCTION Due to their light textured soils, farmed lands of the Derinkuyu, Suvermez and Kuyulutatlar region of the Nev?ehir province of Central Anatolia, Turkey, are mainly used for potato cultivation. Nearly fourty thousand hectares are under (Anonymous, 1987 and 1989). Recently, farmers began to use new high yielding varieties and as a result of this they began to apply very high amounts of N fertilizers (sometimes more than 900 kg N/ha) and very frequent and high irrigation rates in order to get much higher yields. Some researchers conducted fertilizer N experiments with potatoes in this area. However, the aim of those investigations was only to find out the plant response to several N rates. Consequently, no direct data are available about the fate of fertilizer N, the amount taken up by the potato crop and especially the unrecovered N residue in the soil profile or the leached portion of fertilizer N to the groundwater. Therefore, the main objectives of this study were : a.) to find out the marketable potato tuber yield, the N rate relationship under sandy textured soils and b.) to find out the fate of 15N labelled ammonium sulphate fertilizer ( fertilizer N in the potato plant, in the soil, and the portion lost or leached ) applied at rate of 400 and 1000 kg N/ha. MATERIAL AND METHODS Nine field experiments were conducted at different locations in the Nevşehir province (at Merkez Suvermez and Kuyulutatlar in 1992; at Kuyulutatlar 1, Suvermez, and Kuyulutatlar 2 in 1993 and at Merkez, Kuyulutatlar 1 and Kuyulutatlar 2 in 1994 ). This province is located in the Cappadocia region of the Central Anatolia Plateau. Mean annual rainfall is about 410 mm of which most falls in winter and spring. All experiments were carried out on typically sandy textured soils belonging to the Regosol great soil group. The soils have a low organic matter content and water retention properties. They are low in P2O5 content, high in K2O content, neutral in pH and have no salinity and drainage problems (Table 1). The experimental design used in all experiments was a completely randomized block with three replications. The N rates were 0, 200, 400, 600, 800 and 1000 kg N/ha applied as (NH4)2 SO4. N fertilizer was applied in furrows in two equal portions : one at planting and one just before the first irrigation. 100 kg P2O5/ha, as triple superphosphate was applied to all plots at seed bed preparation. Each plot measured 4.2 m x 5.1 m = 21.42 m2 with 70 and 50 cm between and within row spacing, respectively. The high cost of 15N-labelled fertilizer and the differences in recommendations of the N rate to the potato crop, as well as the N rate local farmers generally apply, dictated the use of isotope microplots (0.9 m x 1.4 m = 1.26 m2) only for the rates 400 and 1000 kg N/ha, for which % 5.0 and 2.5 15N atom excess ( % 15N a.e. ) enrichments were used, respectively. Each isotope microplot contained 6 plants. 15N-labelled fertilizer, in granular form, were also applied in furrows, half at planting and the other half just before the first irrigation. About 11 to 17 irrigations were applied (Table 2) during the growing season with sprinklers being the only irrigation method used in the region. At the end of September or at the beginning of October plants from each plot ( 2.8 m x 3.6 m = 10.08 m2 ) were harvested and separated into tuber and leaf plus vine. Marketable tuber yields were determined from all N treatments in order to find the N fertilizer response curve of the potato crop. Afterwards, they were dried at 70o C, ground and screened through a 2 mm screen for the % N analysis. For the 15N balance study, tuber and leaf plus vine of two out of six plants from each 400 and 1000 kg N/ha treatment were dried and the total dry matter yield was determined. After drying they were ground and passed through a 2 mm screen to be analysed the % N and % 15 N a.e. which was done with the micro Kjeldahl method (Keltech) and the Dumma dry combustion method and emission spectrophotometer (Jasco 150), respectively, according to Axmann et al.(1990). The nitrogen yield ( kg N/ha ), % N derived from fertilizer ( % Ndff ), N fertilizer yield ( kg N/ha ) and % Nitrogen use efficiencies ( % NUE ) values were calculated. Soil samples from 0 - 200 cm depth, with 20 cm increments, were taken from each experimental site just before planting, and the total N content was determined. Also, after the potato harvest soil samples were taken again from the same depths of the 0, 400 and 1000 kg N/ha rate plots. Total N, NO-3 and 15N-labelled NO-3 determinations carried out. Fertilizer N residues in the 0-200 cm soil layer were determined. Also, by subtracting the amount of plant N uptake plus fertilizer N residue in 200 cm soil depth from the amount of 15N applied , the amount of N fertilizer leached below 200 cm soil depth ( which is the unaccounted or lost portion ) was calculated at both 400 and 1000 kg N/ha rates. Analysis of variance the marketable tuber yields for each experiment and the regression analysis including all the data obtained in three years were done according to Steel and Torrie, (1960). Also, for the 15N data the standard deviations for dry matter, N yield, fertilizer N in the 200 cm soil layer, for each treatment at each location and year were calculated. For the averages over locations and years, standard deviations were also calculated. RESULTS AND DISCUSSION a.) Marketable potato tuber yield - N rate relationship data : In all experiments, the marketable tuber yields were significantly increased ( P < 0.01 ) by N fertilization in comparison to the unfertilized treatments. The N rates which produced the highest marketable yields varied for each experiment. From the 9 experiments, the highest tuber yields was obtained with three 400 kg N/ha rate, four 600 kg N/ha rate and one 200 kg N/ha rates. In only one experiment there was no significant difference in tuber yield among the N rates. The necessary amount of N fertilizer that should be applied for the optimum potato tuber yields under our experimental conditions was found to be around 600 kg N/ha. In other words, around 2857 kg ammonium sulphate fertilizer ( 21 % ) per hectare must be applied to get maximum tuber yield. This is a very high amount of N fertilizer which can only be explained with the frequently (almost every 5 to 7 days) applied high rates (1000 mm ) of sprinkler irrigation. Other investigations on potato tuber yield - N rate relationship under sandy textured soils showed varying results. Lauer (1985) conducted N rate experiments with potato on sandy to loamy sandy soils and reported that for the optimum tuber yields and economic return 340 kg N/ha fertilizer should be applied. However, the researcher who had conducted N rate experiments in Nevşehir, ( Turkey ) found N rate - yield responses similar to ours. Karaca and Demir (1994), conducted N-rate experiments under sandy loamy soils and they reported that optimum marketable tuber yields were obtained with 600 kg N/ha, although the economical rate that should be applied was found to be 500 kg N/ha. In conclusion, all these results clearly indicate that N rates above 600 kg N/ha would be a waste of money in respect to the optimum marketable tuber yields. b.) 15N balance data: Tuber and total dry matter yields showed slight decreases, whereas leaf plus vine dry matter yields showed slight increases with the increasing N rate from 400 to 1000 kg N/ha when the averages over locations and years are considered ( Table 3 ). Also, tuber N uptake was increased slightly, leaf plus vine N uptakes increased considerably as the N rate was increased. These results do coincide with the findings of Lauer (1985). They indicate that excess N to potatoes crop will show up as increased N, especially in the vines. Percent Ndff increased from 60 to 67, while the Ndff increased ( kg N/ha ) from 174 to 211, when N rate was increased from 400 to 1000 kg N/ha, respectively, when averaged over year and location . Also N uptake from the soil N pool, increased with fertilization (110.0 and 116.4 kg N/ha for 0 and 400 kg N/ha rate ,respectively). However, with increasing N rate, less N uptake from the soil was observed ( 116.4 and 102.8 kg N/ha for 400 and 1000 kg N/ha rate, respectively ). Averaged NUE values decreased from 42 to 20.8 , and the N fertilizer residue in the 0-200 cm soil depth increased from 182.2 to 608 kg N/ha, respectively, with increasing N rate. We obtained low % NUE values, which could be expected under irrigated sandy soils ( Korte and Sotirion, 1980; Rawitz et al.,1980; Westermann and Kleinkopt, 1985; Saffigna et al., 1977; Lauer, 1985) Under such soil and management conditions ways of increasing the efficiency of N use must be investigated. Efficiency of N use can be improved by more frequent application of N fertilizer during the growing period ( Lauer,1985 ). Our data show that nearly half of the applied N fertilizer ( 45.6 % ) at 400 kg N/ha and more than half of the applied N fertilizer ( 60.8 % ) at 1000 kg N/ha was still in the 0 - 200 cm soil depth after harvest. Also, the amount of 15N-labelled NO-3 found was two fold higher at 1000 than at 400 kg N/ha at each 20 cm increment of the 0 - 200 cm soil depth ( Fig. 2 ). This trend was true at each location and year as it can be seen clearly from the figure. Due to rapid nitrification occuring in well aerated sandy soils, there is a potential for leaching of nitrate. In our study, the amount of fertilizer N leached below 200 cm was not actually measured. It was calculated and therefore standard deviation values were not calculated for them. As can be clearly seen from Table 6, under our experimental conditions, much more fertilizer N ( more than 4 fold ) was leached beyond 200 cm soil depth when 1000 kg N/ha nitrogen was applied in comparison to 400 kg N/ha (44 and 181 kg N/ha,respectively). CONCLUSIONS From our results, it is confirmed that N application by farmers of very high rates ( more than 900 kg N/ha ) to the potato crop in the Nevşehir province is unnecessary. There are two basic reasons for this : a.) the optimum marketable tuber yield can be obtained at maximum 600 kg N/ha in the region. So, there is no need for higher N fertilizer rates; and b.) in fact, with the very high N rate, farmers are potentially polluting the groundwaters. This is very important because they are using the water from wells for irrigation and drinking which can cause serious health problems in the region in the future. Nitrogen fertilization research in this potato growing area should continue extensively in order to find ways to increase the % NUE. This can be accomplished by applying fertilizer N much more frequently, especially with irrigation water. In other words, fertigation practices, which can increase the % NUE and limit the nitrate movement in the soil, while decreasing the nitrate contamination of groundwaters, would be more intensively investigated.     REFERENCES Anonymous. 1989. Agricultural structure and production. State Statistics Institute. Publication No. 1055, Ankara Axmann, H., Sebastianelli, A. and Arrillaga. J.L. (1990) Sample prepation techniques of biological material for isotope analysis. In Use of Nuclear Techniques in Studies os Soil-Plant Relationships. ( Edited by G. Hardarson) IAEA, Vienna. p. 41-54 Karaca, M. and Demir, Z. 1994. N rate and application time influences to potato tuber yields and inorganic N residue in the soil. Final report. Field Crops Research Institute. Publication No. 3434, Ankara. Lauer, D.A. 1985. Nitrogen uptake patterns of potatoes with high-frequent sprinkler - applied N fertilizer. Agronomy Journal 77: 193 - 197. Rawitz, E., Burns, S., Etkin, H., Hardiman, R. and Hillel, D. 1980. Fate of fertilizer nitrogen in irrigated fields under semi-arid conditions. In ' Soil Nitrogen as Fertilizer or Pollutant ' proceedings of a research coordination meeting held at Piracicaba, Brasil, during 3 - 7 July 1978 supported by IAEA. Published by IAEA, in Austria, page 195 - 236. Steel, R.G.D. and Torrie, J.H. 1960 Principle and procedures in statistics. McGraw Hill, New York, 481 p Westermann, D.T. and Kleinkopf,G.E. 1985. Nitrogen requirements of potatoes. Agronomy Journal 77: 616 - 621. |