Continuous cropping under elevated CO2: Differential effects on C4 and C3 crops, soil properties and carbon dynamics in semi-arid alfisols

The atmospheric CO2 concentration is increasing by 1.8 μmol mol−1 year−1 and expected to reach 700 μmol mol−1 by the end of 21st century which has a direct and indirect effect on rainfed agriculture. We conducted a study at Central Research Institute for Dryland Agriculture (CRIDA), Hyderabad, India during 2005 to 2010 in order to understand the differential effects of elevated CO2 on continuous cropping in C3 and C4 crops with special reference to soil properties and carbon dynamics. The study was conducted with two elevated levels of CO2 of 550 and 700 μmol mol−1 along with Ambient Control (AC). C3 crops-castor, groundnut, sunflower, black gram and pigeonpea and C4-crops pearl millet and sorghum were grown during 2005-2010 under rainfed conditions. Irrespective of crops, there was a significant increase in root, shoot and total biomass with elevated CO2 levels, except in sunflower wherein these parameters decreased with elevated CO2 at 550 μmol mol−1. Soil pH in surface soils under different CO2 levels showed an increasing trend ranging from 7.6 in AC to 8.0 in 700 μmol mol−1. Soil organic carbon (SOC) concentration (g kg−1) increased from 4.9 at AMB to 6.0 at 550 μmol mol−1. Regression models indicated a significant predictability of enzyme activity through different carbon variables. It was also observed that carbon management index and carbon pool index decreased at 700 μmol mol−1 levels of CO2. Our results indicate that, under rainfed conditions and low fertility soil conditions, the plant biomass significantly increased under elevated CO2 levels but the root: shoot ratio decreased in certain C3 plants like castor and black gram which had a greater impact on carbon sequestration and improvement in water retention. Higher root biomass of C4 plants significantly contributed to higher C input and subsequently increased the carbon stock in the soil.