Azolla planting reduces methane emission and nitrogen fertilizer application in double rice cropping system in southern China

Abstract

Rice paddies are a major source of methane. How to reduce the methane emission in the paddy field without decreasing the yield has become a major concern of scientists, environmental groups, and agricultural policymakers worldwide. Azolla, used as a dual crop in rice cultivation, has multiple agronomic benefits. However, the effects of the dual cropping of Azolla on methane emissions of double rice cropping paddies have not yet been reported. Here, we conducted a 3-year field experiment to evaluate the impacts of rice + Azolla on methane emission and rice yield in a double rice cropping system. The results indicated that the rice + Azolla without N fertilizer and with moderate N fertilizer (200 kg N ha −1 a −1 ) significantly reduced methane emissions over the rice cycle by 12.3 and 25.3% compared with the conventional rice cropping with common N fertilizer (400 kg N ha −1 a −1 ), respectively. The reason for the trend was because the dual cropping of Azolla has significant effect on dissolved oxygen and soil redox potential, which are key factors for methane emission in this study. The rice yield under the rice + Azolla with moderate N fertilizer annually averaged 12.7 Mg ha −1 , which was comparable with that of the conventional rice cropping with common N fertilizer. Moreover, the rice + Azolla with moderate N fertilizer had the lowest yieldscaled methane (25.2 kg Mg −1 grain yield). Here, we showed for the first time that Azolla planting allows sustainable rice production coupled with methane mitigation in double rice cropping systems.

Leguminous Cover Crop Astragalus sinicus Enhances Grain Yields and Nitrogen Use Efficiency through Increased Tillering in an Intensive Double-Cropping Rice System in Southern China

“…In this study, the positive effect of milk vetch on the subsequent rice crops is in agreement with results from Yu et al [23] and Zhu et al [17], which showed a 5-10% yield increase by including legumes such as bean or vetch in a rice-winter crop system. Similar positive effects on rice yield have been observed in multiple cropping systems such as rice-Azolla [24] and rice-fish [25]. In our study, milk vetch and subsequent early and late rice crops constituted a temporal multicropping system that resulted in enhanced rice crop yields.…”

“…Others observed enhanced crop yield and N uptake with residue incorporation of a suitable C/N ratio and when applied at the right time or in a right way [12,28,29]. In this study, the low C/N ratio of the vetch biomass residue (20)(21)(22)(23)(24)(25) and 10 days between vetch incorporation and rice transplanting allowed enough time for N release coinciding with early rice growth resulting in increased grain yield and N uptake. The increase in late rice grain yields and N uptake in vetch covering treatments could be explained by a higher N return from early rice straw biomass incorporation before late rice transplanting.…”

Azolla incorporation and dual cropping influences CH4 and N2O emissions from flooded paddy ecosystems

“…Additionally, Azolla has recently gained considerable importance for its multifaceted uses (Yadav et al 2014;Kollah, Patra, and Mohanty 2016;Shukla et al 2018;Chakraborty et al 2019). Use of Azolla as green manure can either be through its incorporation into paddy soil at the beginning of land preparation before rice transplanting or grown as a dual crop along with rice plants (Xu et al 2017). Dual cropping of Azolla with rice was reported to either increase (Chen et al 1997;Ying et al 2000) or decrease (Bharati et al 2000;Kimani et al 2018) CH 4 emission fluxes from flooded rice paddies with varying observations of N 2 O emissions reported as well (Chen et al 1995;Zou et al 2005;Kimani et al 2018).…”

Growing Azolla to produce sustainable protein feed: the effect of differing species and CO2 concentrations on biomass productivity and chemical composition

“…As a result of their high growth and N 2 fixation rates, Azolla ferns have been used as a bio‐fertilizer in rice paddies in south‐east Asia for several centuries . Using Azolla as bio‐fertilizer increases the sustainability of rice cultivation by reducing fertilizer requirement as well as reducing nitrogen and greenhouse gas (GHG) emissions from rice paddies . However, a direct use of Azolla biomass for feed production may provide more environmental benefits as well as economic value to farmers, but is not yet common on a large scale.…”