Neglecting the fallow season can significantly underestimate annual methane emissions in Mediterranean rice fields

Abstract

Paddy rice fields are one of the most important sources of anthropogenic methane. Improving the accuracy in the CH4 budget is fundamental to identify strategies to mitigate climate change. Such improvement requires a mechanistic understanding of the complex interactions between environmental and agronomic factors determining CH4 emissions, and also the characterization of the annual temporal CH4 emissions pattern in the whole crop cycle. Hence, both the growing and fallow seasons must be included. However, most of the previous research has been based on single-factor analyses that are focused on the growing season. In order to fill this gap, a study was conducted in a Mediterranean rice agrosystem (Ebre Delta, Catalonia) following a farm-to-farm approach with the purpose of 1) evaluating the cumulative and temporal pattern of CH4 emission, and 2) conducting a multi-variate analyses to assess the associative pattern, relative contribution and temporal variation of the main explanatory variables concerning the observed CH4 emissions. Measurements of CH4 emissions and agronomic and environmental parameters in 15 commercial rice fields were monitored monthly, during a whole crop field cycle. The temporal pattern of CH4 emission followed a bi-modal distribution peaking in August and October. The cumulative annual CH4 emissions from rice fields amounted 314 kg CH4 kg ha-1, of which ca. 70% were emitted during the fallow season. The main controlling factors of the CH4 emission rate in the growing season were positive related to water level and plant cover, while soil redox was negatively related. The main controlling factors in the fallow season were water level (negatively related, conversely to the growing season), as well as straw incorporation and soil temperature (positively related). The results of this study highlight the importance of the often neglected fallow season in the accurate estimation of CH4 emissions and, thus, the necessity of measurement programs that cover the whole crop field cycle. This information is the first step for setting effective mitigation strategies based on straw and water management.

Acclimation of methane emissions from rice paddy fields to straw addition

“…Our experiments and meta-analysis, the IPCC methodologies, and the CH4MOD model all focus on CH 4 emissions during the growing season. However, CH 4 emission during the fallow season can be substantial in rice systems where paddies remain flooded year-round ( 28 ). Under these conditions, annual CH 4 emission with straw addition may acclimate to a smaller extent because the mechanism responsible for acclimation is contingent on the presence of rice plants.…”

Greenhouse gas emission, water quality and straw decomposition as a function of rice postharvest field management

“…The peaks observed in both years coincide with the highest temperature and where the straw was incorporated with a knife-roller. During the off-season, the most determinant factors for CH 4 emissions are temperature, water depth and straw management (Martínez-Eixarch et al 2018). According to Das and Adhya (2012), moisture and high temperature provide a favorable environment for the methanogenesis that is maximized by the better adaptation of the population of methanogenic bacteria to the environment.…”

“…Under straw management, the water depth contributes the most to the total methane emissions (Sander et al 2018). The greater incorporation of straw into organic matter stimulates hydrolytic microbial activity under anaerobic conditions, resulting in accumulation of products from fermentation, increasing methanogenesis in the anoxic layer (Martínez-Eixarch et al 2018) and consequently increasing CH 4 emissions.…”

The main drivers of methane emissions differ in the growing and flooded fallow seasons in Mediterranean rice fields

“…Because CH 4 emissions were the priority of this study, samples were not collected during the pre-sowing season, when the fields are dry and emissions are minimal, in order to focus the sampling effort on the flooded or soilsaturated periods, when methane emission is expected. Furthermore, negligible CH 4 emission rates were found in May of 2015 (Martínez-Eixarch et al 2018), so that, for the sake of optimizing resources, in the second year of the study gas sampling was started in June.…”

“…To address such complexity a two-year field study was conducted in a Mediterranean rice growing area in the Ebre Delta (Catalonia, Northeaster Spain) based on a farm-to-farm approach covering the whole range of agronomic and environmental variability of the area. In a previous paper resulting from this study (Martínez-Eixarch et al 2018), we identified the key dynamic variables modulating CH 4 fluxes, including environmental (soil physic-chemical parameters such as temperature, pH, redox and conductivity) and agronomic (water level, plant cover) factors. In the present report, we focus on the relative effect of static factors (rather than dynamic) on cumulative (rather than fluxes) greenhouse gas emissions and the resulting global warming potential.…”