نوع مقاله : مقالات پژوهشی
نویسندگان
1 دانشگاه تبریز
2 دانشگاه تهران
3 دانشگاه گیلان
4 موسسه تحقیقات آب و خاک
چکیده
پتانسیل اکسایش و کاهش یکی از مهمترین عوامل در تعیین ویژگیهای شیمیایی خاکهای غرقاب بوده و تأثیر زیادی بر میزان فعالیت و جذب عناصر غذایی دارد. شرایط احیاء در خاکهای خیس غیراشباع، مجموعهای از تغییرات شیمیایی و الکتروشیمیایی ایجاد میکند. منطقه مورد مطالعه با وسعت حدود 40 هزار هکتار در استان گیلان واقع شده است. تغییرات مورفولوژیکی، فیزیکوشیمیایی قابل توجهی در اثر شرایط اکسایش و کاهش در خاکرخهای مورد مطالعه ایجاد شده است. برای این منظور پتانسیل اکسایش- کاهش، اشکال مختلف آهن از جمله آهن محلول، بلورین، بیشکل و کل در خاکرخهای واقع در واحدهای مختلف فیزیوگرافی مورد بررسی قرار گرفت. آهن بیشکل در افقهای سطحی همه خاکها با مقدار متوسط 3/24 گرم بر کیلوگرم نسبت به عمق بیشتر بوده که با مقدار ماده آلی همبستگی مثبت داشت. هوادیدگی بیشتر در سطح، شرایط اکسیداسیون و احیاء و ماده آلی زیاد مانع از تبلور آهن بیشکل شده است. مقادیر کمتر پتانسیل اکسایش- کاهش با میانگین 7/145 میلیولت و rH با متوسط 6/19 در خاک اراضی پست و ساحلی دلالت بر شرایط احیاء شدید در این خاکها دارد. بررسی اختلاف آهن آزاد و بیشکل و نسبت آنها نشان دهنده مقادیر کمتر آهن متبلور با میانگین 8/6 گرم بر کیلوگرم در خاکهای اراضی پست و ساحلی بوده و دلالت بر تحول کمتر خاکهای واقع در این واحدها به دلیل بالا بودن سطح آب زیرزمینی و نوسان آن نسبت به خاکهای اراضی واحدهای فلاتها و تراسهای فوقانی و آبرفتهای رودخانهای با مقدار متوسط آهن متبلور 8/15 گرم بر کیلوگرم دارد. همچنین مقایسه میانگین مقادیر شکلهای مختلف آهن با استفاده از آزمون LSD اختلاف معنیدار در واحدهای فیزیوگرافی را نشان داد.
کلیدواژه‌ها
عنوان مقاله [English]
Analysis of Eh Condition and Evolutional Trend of Paddy Soils in a Toposequence
نویسندگان [English]
- javad seyedmohammadi 1
- leila esmaeelnejad 2
- hassan ramezanpour 3
- kamran eftekhari 4
1 University of Tabriz
2 University of Tehran
3 University of Guilan
4 Soil and Water Research Institute, Karaj
چکیده [English]
Introduction: Paddy soils are important and the base of agriculture in Guilan province. It is necessary to recognize these soils for understanding of their limitations and optimum use. Unsaturated soil submerging is the cause of collection of chemical and electrochemical process that has significant effects on soil fertility. Eh, rH and pH are important indexes that are used to investigate oxidation and reduction condition in submerged soils and have abundant effects on activity and sorption rate of nutrients. Decrease of Eh and rH in poorly drainage of paddy soils affects availability and solubility of nutrient. Different Fe forms are used for analysis of soils evolution trend and submerging influences on changes of Fe forms. The aim of the present study was conducted to investigate the effect of redox potential changes on soil characteristics and analysis of soils evolutional trend in different physiographic units.
Materials and Methods: The study area with 40000ha (at the east of Rasht city) is located between 49° 31' to 49° 45' E longitude and 37° 7' to 37° 27' N latitude in North of Guilan Province, Northern Iran, in the southern coast of the Caspian sea with different water table depth. The climate of the region is very humid with the mean annual precipitation of 1293.6 mm. The mean annual temperature is 15.8°C. The soil moisture and temperature regimes are Aquic, Udicand Thermic, respectively. The parent materials are derived from river sediments. The soils formed on the plateaues and upper terraces, river alluvial plain and lowland physiographic units were classified as Inceptisols and the soils formed on coastal plain physiographic unit as Entisols. Air-dried soil samples were crushed and passed through a 2mm sieve. Particle-size distribution, organic carbon and cation exchange capacity were determined by hydrometric, wet oxidation and ammonium acetate methods, respectively. Eh by Eh electrode, total iron, free iron and amorphous iron were determined using nitric acid, dithionite-citrate-bicarbonate and ammonium oxalate methods, respectively. The means of different Fe forms values compared through LSD test.
Results and Discussion: It can be seen especial morphological and physicochemical characteristics in studied paddy soils with high groundwater table due to artificial submerging in rice growing seasonDifferent Fe mottles such as orange mottles include lepidocrocite mineral was observed in studied soils. Low redox potential with average 145/7mV and rH with average 19.6 in lowland and coastal soils implicate intense reduction condition. In lowland soils Eh was lower than other units and it was lower in top horizons than to sub horizons in all of units. Eh index had inverse relationship with organic matter, because of high organic matter amount caused high activity of anaerobic micro-organisms, increase of iron reduction and reduction soils degree decrease. rH index amounts showed that studied soils had reduction condition and presence of brown iron and black manganese minerals proved this condition. CEC was high in top soil of physiographic units due to high amount of organic matter and clay content. Clay particles in plateaues were lower than other units because of alteration and suitable aeration and showed high evolution in these soils. Clay coatings were not observed due to high ground water table and its alternative fluctuation. Results showed amorphous iron in surface horizons with average amount of 24.3g kg-1 was higher than subsurface in all soils and had positive correlation with organic matter, because of high activity of anaerobic micro-organisms that prevent from transformation of amorphous iron to crystallized iron, therefore amorphous iron amount increased in presence of organic matter. Pedogenic iron was high in A and B horizons with regard to BC and C horizons due to aeration and weathering. In lowland and coastal land Fed was lower than plateaues and upper terraces and river alluviums units because of ground water presence and its alternative fluctuation. Fed-Feo index showed crystallized iron oxides, high amount of Fed-Feo index proved soils evolution and high weathering. Feo/Fed ratio was related to amorphous pedogenic iron and high amount of this index showed few evolution of soil. Fed/Feo and Fed-Feo indexes indicated the lower rate of crystallized iron with average 6.8g kg-1 in lowland and coastal soils and implicated the lower evolution of these units' soils, due to higher surface groundwater and its more fluctuation than soils of plateaues, upper terraces and river alluviums unites with average amount of crystallized iron 15/8g kg-1.The comparison of different Fe forms using LSD method showed significant difference at the 0.01 level for different Fe forms values in different physiographic units.
Conclusion: Submerging, high groundwater table and severe fluctuation caused noticeable changes in morphological, physical, chemical and electrochemical properties of studied paddy soils. Noticeable organic matter amount added to soil and their burial by puddling operation and slow decomposition were effective factors in redox potential changes of studied wet soils. In equal anaerobic condition, more organic matter caused to decrease redox potential in surface horizons of soils with aquic condition and reverse, lower organic matter caused increasing in redox potential. lower amount of Eh and rH proved severe reduction condition in lowland. Investigation of Fed-Feo and Feo/Fed showed that their amount in lowland and coastal land were lower than plateaus and river alluviums, therefore lowland and coastal soils had lower evolution. Mean comparison of different Fe forms values using LSD method showed significant difference at the 0.01 level for different Fe forms in different physiographic units.
کلیدواژه‌ها [English]
- Chemical and Electrochemical Processes
- Different Physiographic Unit
- Fe
- soil evolution
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