Microbial processes such as mineralization, nitrification and denitrification regulate nitrogen dynamics in the soil. The last two processes may produce nitrous oxide (N2O). In this work N2O fluxes were quantified at four moments of the rice cycle, sowing, tillering, panicle initiation and maturity, in two sites that differed mainly in their soil organic matter (OM) content, Salto (higher OM) and Treinta y Tres. Potential net N mineralization, ammonium oxidation and denitrification as well as the most probable numbers (MPN) of ammonia oxidizers and denitrifiers were determined. Potential N mineralization did not vary with the soil type and increased at rice maturity. Neither ammonia oxidation potential nor MPN were different among the soils. However, the soil with higher OM exhibited higher activity and MPN of denitrifiers, irrespective of the rice stage. In turn, at the latest phases of the crop, the MPN of denitrifiers increased coinciding with the highest mineralization potential and mineral N content of the soil. Significant differences in N2O flux were observed in Salto, where the highest emissions were detected at rice maturity, after the soil was drained (44.2 vs 20.8g N-N2O/ha d in Treinta y Tres). This work shows the importance of considering the soil type and end-of-season drainage of the rice field to elaborate GHGs (greenhouse gases) inventories.

Los procesos microbianos como la mineralización, la nitrificación y la desnitrificación regulan la dinámica del nitrógeno en el suelo. Estos 2 últimos son los principales responsables de la emisión de óxido nitroso (N2O). En este trabajo se determinaron los flujos de N2O en momentos clave del ciclo de cultivo del arroz en 2 sitios que diferían principalmente en el contenido de materia orgánica del suelo (MO), en las localidades de Salto (mayor MO) y de Treinta y Tres. Dichos momentos clave fueron a la siembra, en macollaje, en primordio floral y a la madurez. También se determinó el potencial de mineralización neta de N y las actividades y los números más probables (NMP) de oxidantes de NH4+ y de desnitrificantes. El potencial de mineralización de N, así como la actividad y el NMP de oxidantes de NH4+, no variaron con el tipo de suelo. Sin embargo, la actividad y el NMP de desnitrificantes fueron mayores en el suelo con mayor contenido de MO, independiente de la etapa del cultivo. A su vez, en las etapas finales del ciclo del cultivo, el NMP de desnitrificantes aumentó coincidiendo con el mayor potencial de mineralización y el mayor contenido de N mineral del suelo. Solo se observó un incremento en el flujo de N2O en el suelo de Salto a la madurez del arroz y cuando el suelo ya había sido drenado (44,2g N-N2O/ha d, frente a 20,8g N-N2O/ha d en Treinta y Tres). Esta investigación señala la importancia de estudiar las emisiones en distintos tipos de suelos y de continuar la medición luego del drenaje del cultivo de arroz para la elaboración de los inventarios de gases de efecto invernadero.

ABSTRACT Water management impacts both methane (CH4) and nitrous oxide (N2O) emissions from rice paddy fields. Although controlled irrigation is one of the most important tools for reducing CH4emission in rice production systems it can also increase N2O emissions and reduce crop yields. Over three years, CH4 and N2O emissions were measured in a rice field in Uruguay under two different irrigation management systems, using static closed chambers: conventional water management (continuous flooding after 30 days of emergence, CF30); and an alternative system (controlled deficit irrigation allowing for wetting and drying, AWDI). AWDI showed mean cumulative CH4 emission values of 98.4 kg CH4 ha−1, 55 % lower compared to CF30, while no differences in nitrous oxide emissions were observed between treatments ( p > 0.05). No yield differences between irrigation systems were observed in two of the rice seasons ( p > 0.05) while AWDI promoted yield reduction in one of the seasons ( p< 0.05). When rice yield and greenhouse gases (GHG) emissions were considered together, the AWDI irrigation system allowed for lower yield-scaled total global warming potential (GWP). Higher irrigation water productivity was achieved under AWDI in two of the three rice seasons. These findings suggest that AWDI could be an option for reducing GHG emissions and increasing irrigation water productivity. However, AWDI may compromise grain yield in certain years, reflecting the importance of the need for fine tuning of this irrigation strategy and an assessment of the overall tradeoff between relationships in order to promote its adoption by farmers.

Los arrozales son fuente de dos importantes gases de efecto invernadero (GEI), metano y óxido nitroso. Como un paso inicial hacia la obtención de información local, se midieron las emisiones de CH4 y N2O del suelo y de las plantas de arroz mediante la técnica de la cámara estática en experimentos en invernáculo y a campo en el este de Uruguay. En el experimento en invernáculo, se estudió el efecto del momento de inundación (21 y 45 días después de la emergencia) y de la fertilización nitrogenada (0 y 50 kg N ha-1) sobre las emisiones. La inundación temprana y la fertilización nitrogenada tendieron a aumentar las emisiones de N2O. En el experimento a campo, se estudió el efecto de la cobertura invernal y de la fertilización nitrogenada (0 y 82 kg N ha-1). Se detectaron mayores flujos de CH4 durante la etapa reproductiva de la planta en el tratamiento fertilizado con cobertura invernal previa de raigrás. El flujo de N2O fue máximo después de los baños. Los resultados indican que el uso del cultivo de cobertura podría incrementar las emisiones de GEI durante el ciclo del arroz. A pesar de las distintas prácticas de manejo del cultivo empleadas en Uruguay, los flujos de CH4 y N2O se encuentran dentro de los valores informados previamente para arrozales de otras partes del mundo.

Irrigated rice fields are major sources of two important greenhouse gases (GHG), methane and nitrous oxide. As an initial step towards obtaining local information, emissions of CH4and N2O from rice paddy soil were measured by the static chamber technique in greenhouse and field experiments conducted in eastern Uruguay. In the greenhouse experiment, the effect of two flooding moments (21 and 45 days after emergency) and nitrogen fertilization (0 and 50 kg N ha-1) on gas emissions was studied. Early flooding and nitrogen fertilization tended to increase N2O emissions. In the field experiment, effect of winter soil cover crop and nitrogen fertilization (0 and 82 kg N ha-1) were tested. Higher CH4fluxes were observed mainly during the reproductive stage of the plant in the N-fertilized treatment with ryegrass winter crop. N2O flux peaked at flushing. Results indicate that the use of cover crops might increase GHG emissions during the rice cycle. Despite differences in agronomic management practices employed in Uruguay, CH4and N2O fluxes are within magnitudes previously reported for rice fields worldwide.