عنوان مقاله [English]
Carbon sequestration is one of the most important approaches to reduce CO2 concentration in the atmosphere. Increase of CO2 in the atmosphere has prompted renewed interest in increasing the stocks of carbon (C) in the world’s croplands to mitigate climate change and also improve soil quality IPCC (2000). To better characterize, predict and manage soil C dynamics, more precise and accurate estimates of C inputs to the soil is required. The C fixed in plants by photosynthesis and added to the soil as above- and below-ground litter, is the primary source of C in ecosystems (Warembourg & Paul, 1977). Predicting the changes in C stocks (notably in soils), therefore, depends on reliable estimates of net primary productivity (NPP) and the proportion of the NPP returned to the soil (Paustian et al., 1997). The annual NPP in agroecosystems, and the distribution of C in plant parts, is usually calculated from agricultural yield, the plant component most often measured.
For carbon sequestration estimation, it is necessary to evaluate the effects of management practices on soil organic carbon (SOC) dynamics in a wide range of production systems and climatic zones. Soil organic carbon is essential for maintaining fertility, water retention, and plant production in terrestrial ecosystems. The amount of SOC stored within an ecosystem, dependes on the quantity and quality of organic matter returned to the soil matrix, the soils ability to retain organic carbon (a function of texture and cation exchange capacity), and biotic influences of both temperature and precipitation. The abiotic influences on SOC dynamics, such as moisture, temperature, aeration and the composition of plant residues are reasonably well understood.
The objective of this study was to evaluate the amount of carbon sequestration by agor-ecosystems and also the amount of CO2 emitted from agro-ecosystems in Iran.
Material and methods
The amount of carbon input for seven main crops including cereal (wheat, barley, rice and maize), forage crops (alfalfa), industrial crops (cotton) and legume (chickpea) were calculated in different climate types of Iran and finally, the amount of carbon sequestration and CO2 emission for different crops were estimated. Plant C allometric functions developed for the crops together with The Introductory C Balance Model (ICBM; Andrén and Kätterer, 1997) to describe SOC dynamics for the cropping systems were employed in this study. The model has two compartments, called Young and Old soil C, and five parameters: i, re, h, kY and kO. Annual inputs of soil C to topsoil from crop and manure are summarized in i. The parameter re (decomposer activity factor, see above) is multiplied by kY and kO, respectively, to determine the actual decomposition rates of the young and old pools for a given year. Parameter h, the humification coefficient, determines the fraction of the input that goes through Young and into Old (humus, or refractory component), and is about 0.1 for most agricultural crops and about 0.3 for manure. Then we adapted the ICBM soil climate and decomposer activity parameter (re) to account for the major effects of managing and climatically parameters. The re parameter usually is calculated from sub-parameters based on climate, soil type, crop type, intensity of cultivation and so on.
Results and discussion
The average of carbon input during 20 years showed that the warm-dry climate had the highest carbon input and cold climate had the lowest amount. The highest carbon input fluctuation was obtained in cold climate by 29.13% per year and the lowest fluctuation was related to warm-dry climate by 8.82% per year. Trend of carbon input changes among different crops illustrated that the highest carbon input was gained by alfalfa and cotton and the lowest was for chickpea. Alfalfa and cotton had the highest sequestrated carbon to the soil in all years and the lowest was observed in chickpea. The highest and lowest carbon sequestration was related to warm-dry and warm-wet climate, respectively. The highest amount of CO2 emission was observed in warm-wet climate (450 kg.ha-1.year-1) as average of 20 years and the lowest was gained in cold climate (125 kg.ha-1.year-1). The results showed that the average of CO2 emission in 20 years was 580 kg carbon for alfalfa which had the highest amount and chickpea had the lowest CO2 emission (78.8 kgc.ha-1). A significant relation was observed between CO2 emission with carbon input to the soil and also with temperature.
In essence, it was shown that by increasing the temperature and decreasing the humidity of regions, the value of carbon input was reduced. Among the study crops, alfalfa and cotton had the highest sequestrated carbon to the soil. The highest and the lowest amount of CO2 emission was related to warm-wet and cold climate, respectively.