Simulate the Effect of Climate Change on Development, Irrigation Requirements and Soybean Yield in Gorgan

Atmospheric CO2 concentration has continuously been increasing during the past century and it is expected to increase from current 384 ppm to 550 ppm in 2050. This increase is expected to increase global temperature by 1.4 to 5.8 oC which can have major effects on crop plants. Since both CO2 and temperature are among the most important environmental variables that regulate physiological and phenological processes in plants, it is critical to evaluate the effects of CO2 and air temperature on the growth and yield of key crop plants.
Warming of Earth's atmosphere can increase dark respiration and photorespiration in C3 plants. Rate of photosynthesis is affected by temperature, Therefore, rate of biochemical reactions, morphological reactions, CO2 and energy exchange with the atmosphere could be affected by temperature.
Increase in CO2 concentration causes further yield improvement in C3 plants (Such as wheat, rice and soybeans) in comparison with C4 plants (Such as corn, sorghum and sugarcane). In general, increasing CO2 concentration affects plant processes in two ways:direct effect on physiological processes in plant and indirect effect by changes in temperature and rainfall.
Studying climate change effects including increase in temperature and CO2 concentration can help understanding adaptation strategies to reach higher and sustainable crop yields. Therefore, the objective of this research was to examine the effects of temperature and CO2 changes on days to maturity, irrigation water requirement, and yield in soybean under irrigation conditions of Gorganusing SSM-iLegume-Soybean model.
The model SSM-iLegume-Soybean simulates phenological development, leaf development and senescence, crop mass production and partitioning, plant nitrogen balance, yield formation and soil water and nitrogen balances. The model includes responses of crop processes to environmental factors of solar radiation, temperature and nitrogen and water availability. The soybean model was used to run different scenarios including combination of -1, -2, -3, -4, 0, 1, 2, 3, 4, 5, 6, 7, 8 oC changes in temperature and CO2 concentration of 350, 400, 450, 500, 550, 600, 650, 700 ppm. Actual weather data in Gorgan (latitude 37 degrees 45 minutes north, longitude 54 degrees 30 minutes east) of 1980 to 2009 was used as baseline climate and then changed to obtained future temperature climates. To account for direct effect of CO2 concentration, two model parameters of radiation use efficiency and transpiration efficiency coefficient were changed for higher CO2 concentration (350 ppm as current conditions). Increasing CO2 concentration from 350 to 700 ppm will increase radiation use efficiency by 23% and transpiration efficiency coefficient by 37%. By running the model for each year under each scenario, output of the model recorded and analyzed using response surface method in SAS.
Decreasing temperature increased days to maturity from 130 to 175 days. However, increase in temperature from 1 to 6 oC decreased days to maturity from 130 to 115 days due to higher development rate. No effect of CO2 on phenological development was assumed.
At each temperature, increasing CO2 concentration from 350 to 700 ppm, decreased irrigation water requirement by 30 to 40 mm which is a result of reducing stomata conductance and increase in transpiration efficiency. Temperature increase from 3 to 8oC also decreased irrigation water requirement by 90 mm due to shortening growing season and irrigation number.
Decrease in temperature more than 2oC decreases crop yield by 10 to 20 g m-2, but increase in CO2 concentration will compensate this decrease. Increasing temperature by 2 to 3 oC will increase crop yield by 20 g m-2. Increase in temperature from 3 to 8 oC decreases crop yield from 400 g m-2 to 500 g m-2. Yield reduction due to this temperature rise will occur later as a result of increase in CO2 concentration.
The effect of temperature and CO2 concentration were studied in soybean by SSM-iLegume-Soybeanmodel. The results indicated that yield reduction increase in CO2 concentration postpones the negative effect of higher temperature on soybean yield. On the other hand, super-optimal temperatures will decrease positive impact of increase in CO2 concentration. Therefore, with regard to the effect the following strategies proposed: improve in irrigation method, development of drought and high-temperature tolerant cultivars, increase in water use efficiency, early sowing and development of longer-duration cultivars.