Modeling long-term dynamics of carbon and nitrogen in intensive rice-based cropping systems in the Indo-Gangetic Plains (India)

Soil organic matter (SOM) is an essential component of any sustainable crop production system, both as a nutrient source for the crop and a physical conditioner for the soil. Land use systems based on (flooded) rice- aerobic upland crop rotations, with their annual cycles of wet and dry soil conditions, puddling and plowing, are unique in their influence on SOM dynamics. Recent reports have related yield 'stagnation' in rice-wheat systems in parts of the Indo-Gangetic Plains (IGP) to a decline in SOM quantity and quality. For exploration of the long-term effects of intensive cultivation on soil C and N dynamics, and the consequences for crop yields, a summary model has been developed, based on insights in the underlying processes, to investigate the role of soil organic matter in yield formation in rice-based cropping systems in the IGP and to identify possible reasons for declining yields in the region.Following a review of existing SOM models with emphasis on approaches and principles, to identify processes relevant for long-term dynamics of C and N in rice-based cropping systems, the different components of the systems (SOM and crop) were investigated. A simple analytical model (Yang's model) used to analyze soil carbon balances for different sites in the IGP, showed that carbon demand to maintain soil organic carbon (SOC) levels depends on their initial level, and SOC dynamics are governed by crop performance, determining the rate of carbon input into a soil, and the carbon input through organic amendments. A crop growth model (LINTUL3) was developed describing crop growth of rice for N-limited situations. Nitrogen stress in the model is quantified through the nitrogen nutrition index, a measure of relative crop (leaf and stem) nitrogen content. Subsequently, the knowledge was integrated into a summary model, comprising three modules: a soil organic matter (SOM) module, a soil (SOIL) module, and a crop growth (CROP) module. SOM in the model comprises 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. Turnover of fresh SOM depends on substrate composition. Maximum relative turnover rates of the labile and stable pools are pool-specific and actual rates are influenced by environmental (temperature, moisture, texture, pH) and management (tillage and puddling) factors. Data sets of nine long-term experiments from the IGP of India were used to calibrate and validate the model. In general, the model satisfactorily reproduced observed crop yields, SOC dynamics and total soil N dynamics for various cropping systems at different sites. With recommended fertilizer NPK applications, a significant decline in yield was found only at two sites: Ludhiana-3 and Pantnagar. Nitrogen mineralized from soil organic matter contributed 20-80% to total N uptake for different treatments. The model results show that 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. The study concludes that SOM is not always a direct measure of a soil's nitrogen supplying capacity and the importance of SOM as a nutrient source for the crop depends on the relative contribution of other N sources. This suggests that SOM dynamics are not the sole reason for observed yield stagnation.