اثرات نوع و مقدار نيتروژن و آبياري بر جذب نيتروژن ذرت علوفه اي و نيترات باقي مانده خاك

The Effects of Source and Rate of Nitrogen Fertilizer and Irrigation on Nitrogen Uptake of Silage Corn and Residual Soil Nitrate

بهبود مدیریت نیتروژن در رابطه با تولید ذرت رابطه تنگاتنگی با مقدار رطوبت خاك دارد. در این مطالعه اثرات نوع و مقادیر كود نیتروژنی و آب آبیاری بر تولید ذرت علوفه ای مورد بررسی قرار گرفت. این تحقیق بصورت كرتهای دوبار خرد شده با سه تكرار به مدت سه سال در ایستگاه كشاورزی اراك به اجرا در آمد. كرت اصلی شامل دور آبیاری پس از 70 ، 100 و 130 میلیمتر تبخیر تجمعی از تشتك كلاس A و كرت فرعی شامل دو منبع كود اوره و نیترات آمونیم و كرت های فرعی فرعی 5 مقدار نیتروژن (0، 100، 200، 300 و 400 كیلوگرم نیتروژن در هكتار) بود. نتایج نشان داد كه تنش آبی تولید علوفه خشك را در سطح یك درصد، به صورت منفی و معنی دار كاهش داده ولی مقدار نیتروژن در این راستا دارای تاثیر مثبت و معنی دار بود. مقدار نیتروژن در سطح یك درصد بر روی غلظت نیتروژن گیاه، تاثیر مثبت و معنی دار داشت، بطوریكه مقدار آن از 97/0 درصد در تیمار شاهد به 36/1 درصد در تیمار 300 كیلوگرم نیتروژن در هكتار رسید. جذب كل نیتروژن گیاه در شرایط رطوبتی متفاوت به صورت مثبت و معنی دار تحت تاثیر مقدار نیتروژن قرار گرفت. همچنین اثرات متقابل آبیاری، مقدار و منبع نیتروژن در سالهای مختلف، بر نیترات باقیمانده در هر دو عمق 0 تا 30 و 30 تا 60 سانتیمتر در سطح یك درصد مثبت و معنی دار بود. معادلات رگرسیون خطی نشان داد كه با افزایش مقدار كود نیتروژن، نیترات باقیمانده در هر دو عمق 30-0 و 60-30 سانتیمتری زیاد شد و با مصرف 400 كیلوگرم نیتروژن در هكتار، این مقدار در عمق های یاد شده به ترتیب به 7/24 و 1/26 میلی گرم در كیلوگرم رسید. بنابراین با در نظر گرفتن راندمان جذب كل نیتروژن، مقدار نیترات باقیمانده در خاك و عملكرد بهینه، مقدار 200 كیلوگرم نیتروژن در هكتار از منبع اوره و آبیاری پس از 100 میلی متر تبخیر برای ذرت علوفه ای در شرایط مشابه این آزمایش ، قابل توصیه می باشد.

Introduction: Growing irrigation demand for corn production, along side with draws of ground water from stressed water sources, should be limited due to scarce resources and environmental protection aspects. Nitrogen fertilizer applied at rates higher than the optimum requirement for crop production may cause an increase in nitrate accumulation below the root zone and pose a risk of nitrate leaching. Improving nitrogen management for corn production has a close relation with soil water content. In this study, we investigated the effects of source and rate of nitrogen fertilizer and irrigation on silage corn production and nitrogen concentration, nitrogen uptake and residual soil nitrate in two depths. Materials and Methods: This experiment carried out as split spli- plot in a Randomized Complete Block design (RCBD) with three replications, in Arak station (Agricultural research center of markazi province, 34.12 N, 49.7 E; 1715 m above mean sea level) during three years. The soil on the site was classified as a Calcaric Regosols (loamy skeletal over fragmental, carbonatic, thermic, calcixerollic xerochrepts). Main plots were irrigation treatments based on 70, 100 and 130 mm cumulative evaporation from A class Pan. Sub plots were two kinds of nitrogen fertilizers (Urea and Ammonium nitrate) and sub sub-plots were five levels of nitrogen rates (0, 100, 200, 300 and 400 kgN.ha-1). Nitrogen fertilizer rates were split into three applications: 1/3 was applied at planting, 1/3 at 7-9 leaf stage and 1/3 remainder was applied before tasseling as a banding method. Phosphorus was applied at a rate of 150 kg.ha-1in each season and potassium at a rate of 30kg.ha-1 (only in first growth season) based on soil testing as triple super phosphate and potassium sulfate, respectively. The corn variety of single cross 704 was planted at 20 m2 plots. The plants were sampled at dough stage from the two rows and weighted in each plot. Plant samples were dried in a forced air oven at 70ºC for at least 3 days before weighting. Total N concentration in the plant samples were determined using kjeldahl method. Nitrogen uptake by plants was calculated based on the total N concentration in plants multiplied by dry matter. Residual nitrate concentrations were determined in soil samples (0-30 and 30-60 cm depths) by diazo method. Combined analysis of variance was accomplished using the MSTAT-C software. Mean comparisons were done using Duncan multiple rang test (DMRT). Results: The results showed that the main effect of water stress on dry matter yield was negative and significant (P <0.01) but fertilizer rates were positive and significant (P <0.01). The maximum (8791 kg.ha-1) and minimum (6906 kg.ha-1) of dry matter were obtained in I1 and I3 treatments, respectively. The main effect of nitrogen levels on wet yield were significant (P <0.01) and there were significant differences between all treatments. Dry matter and wet yield were increased up to 300 kgN.ha-1 and decreased when applied nitrogen was more than this. The irrigation was more important in the application of ammonium nitrate than urea in wet yield. The effects of nitrogen fertilizer rates were positive and significant on plant nitrogen concentration. The amounts of nitrogen concentration were increased from 0.97 to 1.36 percent. Nitrogen uptake was affected positively and significantly by irrigation and nitrogen rate. The mean Comparisons of irrigation and nitrogen levels interactions on nitrogen uptake of plants showed that the maximum amounts were gained by I1C4 and I3C1 treatments. The results of the same fertilizer application levels showed that the reduction of irrigation can reduce the nitrogen uptake.Total nitrogen uptake efficiencies for irrigation treatments were 0.45, 0.37 and 0.35, respectively. The interaction effectsof irrigation, nitrogen rate, nitrogen source and year on residual nitrate concentrations were significant (P <0.01), in two depths. Liner regression equations showed that residual nitrate amounts were increased due to increasing of nitrogen rate. In the C5 (400 kgN.ha-1) treatment, residual nitrate concentrations in 0-30 and 30-60 cm soil depths were 24.7 and 26.13 mg.kg-1, respectively. Conclusion: Based on the results of this project which was conducted in Arak station, 200 kgN.ha-1 as urea and irrigation after 100 mm evaporation were recommend with considering the nitrogen uptake efficiency, residual nitrate concentration, and optimum yield. Along with increasing the levels of nitrogen, the amounts of residual nitrate were increased in both depths, regardless of the nitrogen source.