The use of Azolla caroliniana compost as organic input to irrigated and rainfed rice ecosystems: Comparison of its effects in relation to CH4 emission pattern, soil carbon storage, and grain C interactions

Abstract

The factors governing the interactive relationship between organic inputs and release of methane from rice fields are still relatively poorly understood. In this context, the significance of Azolla caroliniana-compost (AC) has been assessed as a component of integrated nutrient management in reference to a few other organic inputs (green manure (GM), rice husk dust (RHD), and cow dung (CD) in two rice ecosystems (irrigated and rainfed). Overall, emissions of methane in irrigated and rainfed conditions ranged between ~0.75–1.2 and ~0.71–1.0 g m−2, respectively. Its emission was strongly correlated with microbial biomass C (r = 0.659–0.720: P < 0.05) and respiration (r = 0.590–0.828: P < 0.05) in soil. Results of regression analysis suggest that crop yield and grain carbon stock should play a significant role in reducing methane emissions from rice soils in both types of ecosystems. Interestingly, AC application led to about 1.1–1.4 folds greater soil carbon sequestration and 1.2–1.4 folds lower global warming potential when compared to CD and GM treatments; AC in turn recorded 2.8% and 5.1% growth in grain yield relative to CD and GM, respectively. It is also observed that friable organic inputs (e.g., AC and RHD) could efficiently suppress methane approximately by 30–36% through the enhancement of porosity, C-storage, and recalcitrant C fractions in soil. Thus, AC and RHD are potential candidates for sustainable rice cultivation in the subtropics with low methane emission and high productivity.