كليدواژه :
نيترات , ظرفيت تبادل كاتيوني , زيست‌توده , سطح ويژه , آمونيوم
چكيده فارسي :
كانی‌های رسی تأثیر زیادی بر پویایی ماده آلی خاك دارند و رس‌ها با حفاظت فیزیكی از ماده آلی سرعت تجزیه آن را كاهش می‌دهند. هدف این پژوهش بررسی تأثیر نوع و مقدار رس و نوع كاتیون تبادلی بر معدنی شدن نیتروژن آلی و مقدار نیتروژن زیست‌توده میكروبی بود. برای این منظور مقادیر مشخصی از كانی‌های رسی كائولینیت، ایلیت و مونت‌موریلونیت اشباع شده با كاتیون¬های سدیم، كلسیم و آلومینیوم با شن خالص مخلوط شدند تا خاك‌های مصنوعی با مقدار رس، كاتیون تبادلی و نوع رس متفاوت در سه تكرار تهیه شود. آزمایش به‌صورت فاكتوریل در قالب طرح كاملاً تصادفی (3×3×3) اجرا شد. بقایای گیاهی یونجه به خاك‌های مصنوعی اضافه و خاك‌ها با فلور میكروبی یك خاك طبیعی تلقیح و به مدت 60 روز خوابانیده شدند و مقدار نیتروژن آمونیومی و نیتراتی هر 15 روز یك‌بار اندازه‌گیری شدند. نتایج نشان داد در مدت دو ماه خوابانیدن، درصد نیتروژن معدنی شده، در شن خالص بیشتر از خاك‌هایی با مقادیر 5 و 10 درصد رس بود كه این امر نشان می‌دهد مقدار رس بر ظرفیت خاك‌ها در نگهداشت نیتروژن آلی مؤثر است. نتایج همچنین نشان داد كه با افزایش مقدار رس، مقدار نیتروژن زیست‌توده میكروبی افزایش یافت. بیشترین و كمترین درصد نیتروژن معدنی شده و نیتروژن زیست‌توده میكروبی به ترتیب در خاك‌هایی با كاتیون¬های تبادلی كلسیم و آلومینیوم اندازه‌گیری شدند. درصد نیتروژن معدنی شده در مدت دو ماه خوابانیدن در خاك‌هایی با رس كائولینیت حداكثر و در خاك‌هایی با رس مونت‌موریلونیت حداقل بود. مقدار نیتروژن زیست‌توده میكروبی نیز در خاك‌هایی با رس مونت‌موریلونیت كمتر از خاك‌هایی با رس كائولینیت و ایلیت بود.
چكيده لاتين :
Introduction: Mobilization and stabilization of organic matter in soils represent a set of complex processes involving the processing and decomposition of organic matter by diverse communities of soil fauna and microorganisms, as well as chemical-physical interactions with mineral particles of soil. Clay minerals have high effects on the soil organic matter dynamics. Clay minerals with the physical protection of organic matter play an important role in reducing the rate of decomposition of organic matter. The effects of soil texture on the soil organic matter dynamics have been investigated in many studies, but the effects of exchangeable cations and clay types on mineralization of organic nitrogen and microbial biomass nitrogen have not been given much attention. For this reason, the aim of this study was to evaluate the effects of types and clay contents and exchangeable cations on the mineralization of organic nitrogen and microbial biomass nitrogen.
Material and Methods: Appropriate amounts of homoionic Na-, Ca- and Al- clays from Georgia kaolinite, Illinois illite and Wyoming montmorillonite were mixed with pure sand to prepare artificial soils with different clay contents, exchangeable cations, and clay types. The artificial soils have zero, 5 and 10% clay from Georgia kaolinite, Illinois illite and Wyoming montmorillonite that their clay minerals saturated with Ca, Na and Al. Alfalfa plant residues were incorporated into the artificial soils and the soils were inoculated with microbes from a natural soil and incubated for 60 days and concentration of NH4-N and NO3-N were measured every 15 days. In the artificial soil samples, microbial biomass nitrogen was measured by the fumigation-extraction method in the end time of incubation period.
Results and Discussion: The results of this study showed that the percentage of mineralized nitrogen in the two-month incubation period, was higher in the pure sand than in soils containing 5% and 10% clay, indicating that clay contents influence the capacity of soils to protect and store organic nitrogen. Microbial biomass nitrogen increased as the amount of clay in the soil increased. The highest and lowest amounts of microbial biomass nitrogen measured in soils with 10% clay (9.26 mg per 50 g dry soil) and pure sand (4.31 mg per 50 g dry soil), respectively. There was a significant influence of exchangeable cations on the percentage of mineralized nitrogen and microbial biomass nitrogen. The microbial biomass nitrogen and the percentage of mineralized nitrogen were highest in Ca-soils and lowest in Al-soils. The percentage of mineralized organic nitrogen in two months of incubation period was highest in soils with Georgia kaolinite clay and lowest in soil with Wyoming montmorillonite clay. The amounts of microbial biomass nitrogen in soils with Wyoming montmorillonite clay were lower than soils with Georgia kaolinite and Illinois illite clays. The percentage of mineralized organic nitrogen increased as the incubation period increased. The results of this study indicated that organic nitrogen mineralization rates and microbial biomass nitrogen were affected by types and clay contents and exchangeable cations and interaction of organic matter with clays and is an important process as it slows soil organic matter decomposition.
Conclusions: Mixing the alfalfa residues with artificial soils and incubation samples allowed to study the effects of types and clay contents and exchangeable cations on the percentage of NH4+-N, NO3--N, mineralized nitrogen, and microbial biomass nitrogen. Soils with different clay contents have different surface areas and cation exchange capacities; therefore, it is concluded that organic nitrogen storage of soils is, partly, controlled by the surface areas, cation exchange capacity and physical protection provided by the soils. Nitrogen mineralization and the amounts of microbial biomass nitrogen were different in soils with different exchangeable cations. It is concluded that exchangeable cations exert their influence on microbial biomass and hence nitrogen dynamics by controlling the size and activity of the microbial population through modifying the physicochemical characteristics of microbial habitats. Since various clay minerals have different specific surface areas and cation exchange capacity and the physicochemical changes induced in the soil environment as a result of variations of exchangeable cations is much greater in soils with higher cation exchange capacity and specific surface area. It seems the effects of clay mineralogy on the dynamics of organic materials and microbial biomass, in part, arise from the type of exchangeable cations present on the exchange sites of the clay minerals.
