We conducted a series of field experiments to study the effects of rice straw compost (RSC) and chemical fertilizer application on soil carbon and silicon under a double rice cropping system for 25 crops successive in the Mekong Delta, Vietnam. The results showed that the continuous application of RSC (6 Mg ha -1 as fresh weight for each crop) caused a higher yield and more available silicon than treatment without RSC. Available silicon in the surface (0 -10 cm from the ground surface) soil was 36.3 to 38.7 mg Si kg -1 in treatment with RSC, while it was 24.2 to 30.0 mg Si kg -1 for treatment without RSC. Regardless the amount of chemical fertilizer applied, the rice yields for treatment without RSC were lower than those for treatment with RSC and entailed less chemical fertilizer. Moreover, RSC application used in combination with moderate doses of chemical fertilizer increased total carbon in the soil at a rate of 356 to 401 kg C ha -1 year -1. Our results suggested that Si availability in paddy soil must be maintained or increased in order to maintain rice yield, for which the application of rice straw compost offers an option. Our results also showed that the application of RSC increased total-C in the soil, suggesting that paddy fields can be used effectively for carbon sequestration.
We conducted a 12-year field experiment to study the combined effects of rice straw compost and chemical fertilizer application on a double rice cropping system in the Mekong Delta, Vietnam and established 7 treatments using a randomized block design with 3 replications. We calculated the yields of treatments relative to those of conventionally fertilized plots (i.e. with no added rice straw compost), and analyzed their annual trends. In the plots with rice straw compost, we observed a positive trend over 12 continuous wet cropping seasons. In comparison with conventional fertilization, the application of rice straw compost with reduced chemical fertilizer can maintain rice productivity over a longer period. We analyzed the nutrient status of the rice straw at harvest over 4 cropping seasons and observed that the Si concentration of the rice straw in conventionally fertilized plots was significantly lower than that in the plots where rice straw compost was applied. The N, P and Mg concentrations of the rice straw increased with the fertilizer application rate, while the concentrations of K, Ca, Mn, Fe, Zn, and Cu did not differ significantly among treatments. Our results suggest that continuous removal of rice straw may reduce the Si availability in soil, thereby decreasing rice productivity. In addition, we verified that rice straw compost can be an effective source of silicon for paddy rice.
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