Soil organic carbon build-up and dynamics in rice-rice cropping systems

Intensive irrigated rice system is the most important food production system in the world. Continued population growth necessitates increased rice production to meet the increased need of food production through increased rice productivity to ensure food security. In contrast, the recent slowdown in yield growth in intensive irrigated rice-based cropping systems as a result of deterioration of soil health and decline in productivity level is a serious cause for concern. The objective of this research was to investigate the soil organic carbon (SOC) build-up and its dynamics in rice-rice cropping system by including a green manure (GM) crop Sesbania rostrata Berm. either during fallow and/or intercrop at 4 : 1 ratio as additive series without changing rice geometry. The results revealed that there is a gradual build-up of SOC when S. rostrata included and in situ incorporated at flowering stage as a basic means of improving soil quality in rice-rice cropping system. Further, the soil organic matter fractions viz. humic acid and fulvic acid were also improved in all GM-incorporated systems. In general, the proportion of fulvic acid was higher than humic acid. Cropping system involving three GMs viz. S. rostrata-rice/S. rostrata-rice/S. rostrata registered significantly higher SOC and was 10.63 percent higher than traditional rice-rice cropping system (A(1)). This was followed by S. rostrata-rice/S. rostrata-rice system. Although the traditional rice-rice cropping system (A(1)) also conserved SOC (0.29 and 1.69 % higher than initial during first and second annual cropping cycle, respectively), the magnitude was very low. Repeated application of S. rostrata as GM improved SOC, which formed the basis for sustainable management of soil resources. It can be concluded that S. rostrata would affect the rate of loss of SOC in cropping systems and its ultimate level in soils.