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We describe a summary model for the dynamics of carbon and nitrogen under varying weather, crop and soil conditions to investigate the role of soil organic carbon and nitrogen in yield formation in rice-based cropping systems of the Indo-Gangetic Plains (IGP). The model consists of three modules: soil organic matter (SOM), soil (SOIL), and crop growth (CROP). The SOM module consists of three pools: fresh, labile and stable. Carbon and nitrogen dynamics in the model are described in terms of carbon turnover, assimilation and dissimilation, with nitrogen linked through distinct C/N ratios. The SOIL module has two soil layers of variable depth (often 15cm each), and the labile and stable pools are replicated for these two layers. Water, crop and soil management practices and aeration characteristics of these layers affect the C and N dynamics of their respective pools. The CROP module calculates potential and actual biomass, leaf area index (LAI), evapotranspiration, total N demand, actual N uptake and N stress. Actual biomass is first partitioned between above (leaves, stem and storage organs) and below ground (roots and rhizodeposition), and a harvest index (HI) is applied to the aboveground biomass to calculate the yield. Independent data sets of 9 long-term experiments (LTEs) from the IGP of India were used for calibration and validation of the model. After calibration and validation, the model was used to explore the impact of various crop and soil management practices in rice-based cropping systems on different sites of the IGP. Overall, the model satisfactorily reproduced observed crop yields, SOC dynamics and total soil N dynamics for various cropping systems at different sites. Nitrogen mineralized from soil organic nitrogen contributed 5–75% to total N uptake in fertilized and non-fertilized treatments. With a recommended NPK application, a trend of significant decrease in simulated rice yield was found only at Ludhiana-3, and Pantnagar. Similarly, the simulated wheat yield also showed a significant trend of decline at Ludhina-3, Pantnagar and Barrackpore. Simulated SOC had a significant decline at Pantnagar, whereas at Ludhiana-3, it showed a significant increase. An increase in SOM is not always associated with an increase in yield, as the factor(s) improved by an increase in SOM may not be the limiting factor for crop growth. Depletion of many major macro (NPK) and micro (Zn, Fe, Mn, etc.) nutrients, essential for plant growth even with a balanced NPK fertilizer application and a maintained SOC, can lead to a decline in productivity. [Copyright &y& Elsevier]

Abstract: Soil organic carbon sequestration rates over 20 years based on the Intergovernmental Panel for Climate Change (IPCC) methodology were combined with local economic data to determine the potential for soil C sequestration in wheat-based production systems on the Indo-Gangetic Plain (IGP). The C sequestration potential of rice–wheat systems of India on conversion to no-tillage is estimated to be 44.1MtC over 20 years. Implementing no-tillage practices in maize–wheat and cotton–wheat production systems would yield an additional 6.6MtC. This offset is equivalent to 9.6% of India''s annual greenhouse gas emissions (519MtC) from all sectors (excluding land use change and forestry), or less than one percent per annum. The economic analysis was summarized as carbon supply curves expressing the total additional C accumulated over 20 year for a price per tonne of carbon sequestered ranging from zero to USD 200. At a carbon price of USD 25MgC−1, 3MtC (7% of the soil C sequestration potential) could be sequestered over 20 years through the implementation of no-till cropping practices in rice–wheat systems of the Indian States of the IGP, increasing to 7.3MtC (17% of the soil C sequestration potential) at USD 50MgC−1. Maximum levels of sequestration could be attained with carbon prices approaching USD 200MgC−1 for the States of Bihar and Punjab. At this carbon price, a total of 34.7MtC (79% of the estimated C sequestration potential) could be sequestered over 20 years across the rice–wheat region of India, with Uttar Pradesh contributing 13.9MtC. [Copyright &y& Elsevier]