Carbon Sequestration and Greenhouse Gas Emissions: Lessons Learnt from Long-Term Fertiliser Experiment on Rice-Wheat Cropping System.

Climate change and global warming is a serious concern of present century. Depending upon management practices, soil can act as both source and sink of carbon. Fertilizer management practices have profound influence on crop productivity hence carbon storage capacity and greenhouse gas (GHG) emissions. Long-term fertilizer experiments are vital tools to study such effects for formulating strategies to mitigate the adverse effects of climate change. Rice-wheat cropping system is a prominent cropping system of Indo Gangetic Plains of India and has dominant share in carbon feedback of agriculture in India. Present study reports the 15 years long-term effects of integrated nutrient management on soil organic carbon fractions, carbon pools, carbon management index (CMI), carbon sequestration and greenhouse gas emissions under intensive rice-wheat system in Typic Ustochrept soil of semiarid region in Northern India. Different treatments comprising of combinations chemical fertilizers and organic manures were; control, 100%NPK, 100%NPK+green manure (GM), 100%NPK+straw incorporation (SI) and 100%NPK+farmyard manure (FYM). Continuous cultivation of rice-wheat system without any fertilizer application showed build-up of 1.05 g kg^-1 SOC as compared to antecedent value in surface soil. Integrated nutrient management showed positive and variable influence on various carbon fractions i.e. water soluble carbon, particulate organic carbon, labile carbon and total organic carbon. The non-labile carbon fraction was maximum in 100%NPK+SI(3.50 g kg^-1) followed by 100%NPK+FYM(3.09 g kg^-1) and minimum in 100%NPK+GM(2.60 g kg^-1). The sensitivity index for labile carbon (51-85%), water soluble carbon (67-131%), particulate organic carbon (64-159%) and hot water soluble carbon (38-131%) were the most sensitive fractions as compared to soil organic carbon (SOC) whose sensitivity index ranged between 29-59%. The maximum value of CMI was recorded in 100%NPK+FYM followed by 100%NPK+SI, 100%NPK+GM and minimum in 100%NPK. The carbon sequestration rate after 15 years was maximum in FYM (0.90 Mg ha^-1 year^-1) followed by SI (0.62 Mg ha^-1 year^-1) and GM (0.49 Mg ha^-1 year^-1) and minimum in NPK (0.40 Mg ha^-1 year^-1). Global warming potential, yield-scaled global warming potential were worked out to study the effect of long-term integrated nutrient management treatments on GHG emissions and food security. Total GWP in rice-wheat cropping system was minimum in control (8.04 Mg CO₂-eq ha^-1) and maximum in 100%+SI (14.47 Mg CO₂-eq ha^-1). Total yield–scaled GWP of different treatment for rice-wheat cropping system was the maximum in control (2.52 Mg CO₂-eq Mg^-1) followed by 100%NPK+SI (2.50 Mg CO₂-eq Mg^-1), 100%NPK+GM (1.74 Mg CO₂-eq Mg^-1) and the minimum in 100%NPK (1.65 Mg CO₂-eq Mg^-1). This suggested that conjoint application of chemical fertilizers and FYM resulted in more grain production without substantial GHG emissions, while straw application resulted in higher GHG emissions as compared to the application of chemical fertilizer alone. Meanwhile, straw incorporation helps to improve quality and quantity of soil organic carbon by avoiding the emission of greenhouse gas during straw burning in open fields.

Keywords:rice-wheat, integrated nutrient management, carbon sequestration, greenhouse gas emissions, global warming potential