Water management reduces greenhouse gas emissions in a Mediterranean rice paddy field

Abstract

Rice paddy fields are one of the largest anthropogenic sources of methane (CH4), the second most important anthropogenic greenhouse gas (GHG) after carbon dioxide (CO2). For this reason most studies on the GHG budget in these agricultural systems focus on the evaluation of CH4 production. However, these systems also exchange other GHGs with the atmosphere, such as CO2 and nitrous oxide (N2O). To estimate the total global warming potential (GWP) of rice cultivation, a field experiment was carried out in a Mediterranean rice paddy field in the Po Valley (Italy), the largest rice producing region in Europe. For two consecutive years, ecosystem CO2 and CH4 fluxes were assessed using the eddy covariance technique and CH4 and N2O fluxes were measured with closed chambers. The net biome productivity indicated a nearly carbon (C) neutral system in 2009 while it accumulated C in 2010, due to the application of organic fertilizers and the midseason drainage of the otherwise flooded field, the latter having the additional benefit of leading to lower water consumption. The rice paddy field acted as a strong GHG source with a GWP of 1148g CO2-eqm−2yr−1 in 2009 and decreased four-fold in 2010 (289g CO2-eqm−2yr−1). In both years, the site was a large CH4 source. Differences in the GHG budget between the two years of measurements were mainly caused by the lower CH4 emissions in 2010 (21.0g CH4 m−2 compared to 37.4gCH4m−2 in 2009), probably driven by drainage of the otherwise flooded field in the middle of the growing season during 2010 and moderately larger CO2 uptake. The increased N2O fluxes (29%), had a marginal contribution to the GWP. However, midseason drainage, which needs to be evaluated in combination with the concurrent application of organic fertilizers, resulted in small decreases of yield. Our results therefore suggest that an adequate management of the water table level reduces CH4 fluxes and has the potential to decrease the GWP and water losses through evapotranspiration of rice paddy fields, confirming that full GHG budgets should be assessed in combination with yields in order to develop and evaluate effective mitigation strategies.