اثر متقابل رهاسازی اراضی و شرایط اقلیمی بر ذخایر ماده آلی در ذرات اولیه خاک در مراتع مناطق استپی

اگرچه مطالعات زیادی در خصوص اثر رهاسازی اراضی کشاورزی انجام گرفته است ولی اطلاعات بسیار کمی در خصوص تاثیر شرایط آب و هوایی بر احیاء زمین‌های کشاورزی بعد از رهاسازی آن‌ها وجود دارد. به همین منظور در این مطالعه اثر متقابل رهاسازی اراضی کشاورزی و شرایط اقلیمی بر ذخایر ماده آلی ذرات اولیه خاک مورد بررسی قرار گرفت. نمونه‌های مرکب خاک از عمق 30-0 سانتی‌متری در سه تکرار از دو منطقه با شرایط بارندگی متفاوت شامل مناطق حفاظت‌شده شیدا و خرگوش در استان چهارمحال و بختیاری از چهار کاربری مرتع، اراضی کشاورزی و اراضی رها شده زراعی در توالی زمانی 15-10 و 40-15 سال، تهیه و میزان توزیع کربن و نیتروژن در اجزای مختلف ذرات اولیه خاک تعیین گردید. نتایج نشان داد که کشت و کار در اراضی بکر بسته به موقعیت اقلیمی می‌تواند اثرات مثبت و منفی بر اکوسیستم‌های طبیعی داشته باشد. به تبع در منطقه شیدا با وجود میزان بارندگی و وضعیت خوب پوشش طبیعی مراتع کشت و کار در اراضی بکر اثرات منفی در جهت کاهش ماده آلی داشته است. در حالی که در منطقه خرگوش با وجود میزان بارندگی اندک و کیفیت پایین مراتع، کشت و کار اثرات سوء ای نداشته است. در کلیه کاربری‌های مدیریتی مقدار کربن و نیتروژن به ترتیب در ذرات رس، سیلت و شن بیشتر بود. رهاسازی اراضی کشاورزی و مراتع در منطقه خرگوش تاثیری بر غلظت کربن و نیتروژن سه جزء شن، سیلت و رس نداشت اما در منطقه شیدا افزایش زمان رهاسازی کربن اجزای شن و سیلت را افزایش داد ولی تاثیری بر کربن جزء رس نداشت. نتایج نشان داد رهاسازی اراضی کشاورزی بسته به شرایط آب‌و‌هوایی هر منطقه جهت استقرار مجدد پوشش گیاهی و افزایش ترسیب کربن خاک می‌تواند اثرات متفاوت و بالقوه‌ای بر احیای کربن مراتع داشته باشد.

The Effect of Interaction of Land Abandonment and Climate Conditions on Restoration of Organic Matter in Primary Soil Particles in the Rangeland of Steppe Regions

Introduction: Although many studies have been done on the effects of agricultural land abandonment, there is very little information about the impact of climate conditions on the restoration of abandoned agricultural lands. Human has changed most of rangelands to agricultural lands causing a decrease in carbon sequestration, depending on land management and tillage operations. One of the methods for rebuilding the land cover is the land abandonment, which results in enhanced organic carbon and decreased CO2 emission. Understanding the storage and dynamics of soil organic matter, especially in relation to changing land use, is fundamental to evaluate the role of soil as a carbon source or sink. After land use change from rangeland to cropland, agricultural practices decrease the C stored in soils and cause a net release of C into the atmosphere, which has strongly influenced the atmospheric CO2 levels and global C balance over the last centuries. For this purpose, this study aimed to assess the effect of interaction between agricultural land abandonment and climatic conditions on organic material reserves of primary soil particles.;Materials and Methods: The study area was located in semisteppe rangelands of Sheida and Khargosh in about 60 km northwest of Shahrekord city, ChaharmahalvaBakhtiari province, central Iran. In this study, four treatments including rangeland, agricultural and cultivated land abandoned in the time series of 1015 and 1540 Year were selected. The sample plots were placed in the distance of transects, and the soil samples were collected from 030 cm depths with different rainfall conditions from two abovementioned regions in three replications. For each region, the soil samples were transferred to the laboratory and then analyzed. The selected locations had same soil shape, topography, parent material, and slope. The soil samples of three plots were then combined and 24 samples were prepared. The distribution of carbon and nitrogen concentrations was determined at different soil particle components.;Results and Discussion: The results showed that the rangeland change to cultivated land did not have a significant effect on the amount of organic carbon, total nitrogen, and total carbon to total nitrogen ratio. However, the values of these indicators decreased significantly in the Sheida region. Under all land management, the amount of carbon and nitrogen of soil particles increased with decreasing the particle size from sand to clay. Hence, the abandoned agricultural land and rangelands did not significantly affect the amount of carbon and nitrogen concentration in sand, silt and clay particles. The amount of carbon, however, increased with the abandonment time and nonagronomic activity of carbon in sand and silt particles, although the carbon content of clay particle was not influenced. Agricultural practices may negatively or positively impact natural ecosystem depending on climatic condition and soil quality in unchanged lands. However, despite suitable climatic conditions (in terms of precipitation) and land cover in the rangelands over Sheida, the cultivation adversely influenced the soil quality and organic matter of the unchanged land. Although, the precipitation and soil quality were relatively lower in Khargosh region, the agricultural activities seem not to negatively affect the land quality. Moreover, rangelands change to cultivated lands did not have a significant effect on the amount of soil nitrogen in this region. The greatest nitrogen amount was measured in clay fractions of cultivated and abandoned lands for 40 years, and the minimum nitrogen content was detected in sand particles of lands abandoned for 15 years. The highest and lowest amount of nitrogen over all three fractions was, respectively, found for unchanged and abandoned lands in Sheida region. Therefore, the cultivated land depending on climate condition and management may considerably increase or decrease the organic carbon content in sand, silt and clay particles.;Conclusion: The results indicated that the agricultural land abandonment may differently affect the rangelands restoration measures such as the vegetation reclamation and soil carbon sequestration depending on climatic condition.