فهرست مطالب صبا باقریفام

 Soil is one of the main drivers of global warming through losing carbon in the form of CO2. On the other hand, its ability to sequester carbon is a suitable option for reducing CO2 emissions. Therefore, even few changes in carbon sequestration or decomposition of soil organic carbon affect the global atmospheric CO2 content. Although the soils of arid and semi-arid regions have low organic carbon content, they can sequester substantial amounts of carbon due to the large area of these regions. So, the Rothamsted carbon model was used to predict the impact of future climate changes on the amount of CO2 emissions and low soil organic carbon stocks in the semi-arid arable lands of Razavi Khorasan province. This model is one of the most widely used models for the study of soil organic carbon turnover and has been evaluated in a variety of ecosystems including grasslands, forests and croplands and in various climate regions. The RothC model is consists of five conceptual soil carbon pools, four active fractions and a small amount of inert organic matter (IOM) that is resistant to decay. The active pools splits into: Decomposable Plant Material (DPM), Resistant Plant Material (RPM), Microbial Biomass (BIO) and Humified Organic Matter (HUM). This model is able to reveal the effect of soil texture, temperature, rainfall, evaporation, vegetation and crop management on the soil organic carbon turnover process.

 The Rothamsted carbon model was calibrated and validated using data measured in 2020 and available data from the long-term field experiments in the semi-arid agricultural lands of Jolge Rokh. Then, by analyzing the climate change of the study area, the impact of climate change until the end of the current century on the amount of CO2 cumulative emissions, total organic carbon (TOC) and active carbon pools model were modeled and compared in the current climate and also climate change conditions.

 The comparison between the measured and simulated soil organic carbon values by the model shows the potential of the model to provide predictions with acceptable accuracy. The outcome of comparisons revealed that R2, Root Mean Square Error (RMSE), Mean Difference (MD), Mean Absolute Error (MAE) and Model efficiency were 0.97, 2.78, 2.11, 2.33 and 0.70 respectively. Assessment of climate changes in the region (during 1981-2020) showed a decrease in precipitation and a significant increase in temperature over the past 40 years. Climate change simulation was carried out by temperature increasing and decreasing the precipitation until the end of the current century, indicated the decrease of all active carbon pools. It was found that DPM, RPM, BIO, HUM and TOC decreased respectively to 2.41, 2.72, 2.51, 1.04 and 1.32% compared to the current climatic conditions, while the cumulative CO2 emission increased by 1.26%. Temperature rising leads to increase the rate modifying factor (a) by 2.20%, which enhances microbial respiration and decomposition rate of organic carbon and CO2 emissions (carbon output). However, it also increases the ecosystem's net primary productivity (carbon input). Decreases in rainfall and increase in potential evapotranspiration cause a reduction of the rate modifying factor (b) to 0.23%, which on one side reduces the activity of microorganisms and carbon biodegradation; but on the other side, it decreases the vegetation cover and following that reduces CO2 trapping during the photosynthesis process and transfers it to the soil. It seems that in arid and semi-arid climates where the lack of moisture is the most important limiting factor of the plants growth; the role of precipitation in carbon decomposition and sequestration is greater than temperature.

 The Rothamsted carbon model is suitable for regional simulations because it requires only easily obtainable inputs. Therefore RothC is an appropriate tool for estimating long-term effects of climate change and agricultural management (such as application of manures, returning plant residues to the soil, crop rotations, conservation tillage etc.). The RothC model validation in the cold semi-arid agricultural lands of the region, shows the ability of model to properly simulate the pattern of organic carbon changes. Also, simulation of soil organic carbon changes under the climate changes conditions indicates an increase in cumulative CO2 emissions and decrease in soil organic carbon pools of the study area. The methodology can be applied to other regional estimations, provided that the relevant data are available. The predictions allowed to identify the land management potential to carbon sequestration. Such information demonstrate a beneficial tool for evaluation of past land management effects on soil organic carbon trends and also estimation of future climate change effects on soil organic carbon stocks and CO2 emissions.

Soil organic carbon is a key element in determining soil quality, health, and fertility. Due to the complexity of the structure and relationships of soil organic carbon pools, the use of models is beneficial in identifying the reaction of these pools to the change in ecosystem conditions. So, by using the RothC model, the effect of global warming and climate change on the amount of soil organic carbon pool of the agricultural ecosystem of southeastern of Mashhad was investigated. Therefore, the model was calibrated and validated using data measured in 2020 and available long-term data. Comparing the measured values of soil organic carbon and the simulated values by the model, the coefficient of determination (R2) was 0.89. Root means square error (RMSE): 3.45, mean difference (MD): 1.84, mean absolute error (MAE): 2.79, and model efficiency (EF) was 0.73, demonstrating the validity and suitability of the model. The modeling of the future climate changes of Mashhad showed a decrease in rainfall and an increase in temperature and evaporation, leading the amount of total soil organic carbon (TOC) would decrease by 1.13% compared to the current conditions. Considering the decomposition rate constant of the model's four active carbon pools, humus exhibited the slowest decomposition rate of 0.96%. At the same time, decomposable plant materials (DPM), resistant plant materials (RPM), and microbial biomass (BIO) were decreased by 1.18%, 2.21%, and 2.10%, respectively, compared to the current climate condition. Moreover, over time, the decomposition rate decreased due to the decay of active organic matter pools that are easily decomposed.

Soil organic carbon is one of the most important basis for assessing soil quality, health, and fertility. Therefore, accurate, rapid, and inexpensive measurement of soil organic carbon is essential to determine efficient management and conservation practices, which might increase carbon sequestration or preservation in soils. Soil carbon measurement methods are generally classified into two categories: ex-situ and new in situ methods. The first category includes traditional field-collected samples and chemical laboratory degradation of soil organic matter using wet and dry combustion methods. These methods are often expensive, time-consuming, and environmentally destructive. In many cases, common sampling and laboratory errors have resulted in these approaches to be less precise. In situ methods, including infrared reflectance spectroscopy, laser-induced breakdown spectroscopy, inelastic neutron scattering, and remote sensing, are considered more efficient, especially in large-scale national studies as utilizing such methods may increase the speed and accuracy of measuring and significantly reduce the cost of conventional field sampling and laboratory analysis. National quality monitoring of Iranian agricultural soils showed that in more than 60% of lands, soil organic carbon content is less than one percent and further studies have confirmed the declining trend of organic carbon storages in most parts of the country. On one hand, more than 80% of Iran's land area is located in an arid and semi-arid climate whose soils are low in organic carbon content, and on the other hand, the salinity and predominance of calcareous soils in vast areas of Iran have become controversial issues, which have made the attempts to find the correct method of measuring the organic carbon of these soils more complicated. Although the use of an automated carbon analyzer is considered as the standard method for comparing the ex-situ soil carbon measurements, the incomplete conversion of carbonate to CO2 under the combustion conditions in highly calcareous soils might happens which is problematic. Therefore, the use of wet combustion methods is more popular because it’s not affected by the presence of carbonates and the direct calculation of organic carbon. All soil organic carbon measurement methods have strengths and weaknesses, so its choice depends on the characteristics of the studied soils, the purpose of the project as well as the financial considerations, and the allocated budget. Considering the lack of comprehensive studies in this field, the present study aims to help to gain a better understanding of the results of the Walkley Black method, as the basic method of measuring soil organic carbon in Iran, and to assist with developing a standard and robust protocol for measuring the organic carbon of saline and calcareous soils of Iran, and finally to comparatively evaluate the soil carbon measurement methods by outlining their strengths and weaknesses.

Soil is known as a suitable reservoir for carbon sequestration. The objective of this study was to investigate the capability of haloxylon on sequestration of carbon in sand dunes of Yahya Abad, Sabzevar, in northeastern Iran. Eighty soil samples were taken from two depths of 0-5 and 5-10 cm under and between haloxylon trees. Eight soil profiles under and between haloxylon trees were described and sampled to explore subsurface variations of soil organic carbon (SOC). The results showed that SOC in 0-5 cm has increased from 0.18% in between trees to 0.79% under trees and similarly, in 5-10 cm has increased from 0.11 to 0.29%. SOC has decreased exponentially with increasing depth. A strong correlation was found between soil organic carbon and electrical conductivity in 0-10 cm depth due to accumulation of salts in haloxylon litters that has increased both SOC and electrical conductivity of surface soil. The SOC storage under and between haloxylon trees at 0-100 cm depth were estimated 27718 kg ha-1 and 14915 kg ha 1 respectively, that diferenemnce of these values represents carbon sequestration. The amount of carbon sequestration to depth of 10 and 40 cm was 5367 and 10155 kg ha-1 that is 36 and 68% of total carbon sequestration. The results of this study confirmed the importance of haloxylon in carbon sequestration in desert environments.

Large areas of Iran are located in arid and semi-arid climatic regions and their soils are naturally subjected to erosion and degradation. Be aware of how management practices and topographic features influence soil organic carbon and aggregate stability is a very important issue from the land management point of view. In this study, four different land use management sites (preserved pasture, grazing pasture, overgrazing pasture and rain fed lands) positioned in the hill slopes of Sisab experimental station (Bojnourd) were considered. In each site, soil samples from 0-15 cm of summit, backslope and toeslope in north and south-facing slopes were sampled. Soil samples were analyzed by using standard methods. The results showed that sand content in south-facing slopes was higher compare to north facing slopes, while silt content, soil organic carbon and mean weight diameter (MWD) were higher in north-facing slopes compared to south facing slopes. The maximum soil organic carbon and MWD were observed in the toeslope of north-facing slope while the minimum were monitored in backslope of south-facing slopes. The results also showed that sand content increased to 41 and 39% in overgrazing and grazing pastures respectively compared to preserved pasture and clay content decreased by 6%. Soil organic carbon decreased from 9.6 (g kg-1) in preserved pasture to 6.4, 8.2 and 9.1 (g kg-1) in cultivated land, overgrazing and grazing pastures, respectively. MWD decreased by 55, 35 and 17 percent in rain fed, overgrazing pasture and grazing pasture compared with preserved pasture, respectively. Despite the strong correlation between MWD and soil organic carbon (R2=0.81) in all sites, the aggregate stability in north-facing slopes was mainly influenced by organic carbon while the role of calcium carbonate was notable in south-facing slopes.