Aquaponics: integrating fish feeding rates and ion waste production for strawberry hydroponics

Aquaponics is the science of integrating intensive fish aquaculture with plant production in recirculating water systems. Although ion waste production by fish cannot satisfy all plant requirements, less is known about the relationship between total feed provided for fish and the production of milliequivalents (mEq) of different macronutrients for plants, especially for nutrient flow hydroponics used for strawberry production in Spain. That knowledge is essential to consider the amount of macronutrients available in aquaculture systems so that farmers can estimate how much nutrient needs to be supplemented in the waste water from fish, to produce viable plant growth. In the present experiment, tilapia (Oreochromis niloticus L.) were grown in a small-scale recirculating system at two different densities while growth and feed consumption were noted every week for five weeks. At the same time points, water samples were taken to measure pH, EC25, HCO3 ��� , Cl ��� , NH4 + , NO2 ��� , NO3 ��� , H2PO4 ��� , SO4 2��� , Na + , K+ , Ca 2+ and Mg 2+ build up. The total increase in mEq of each ion per kg of feed provided to the fish was highest for NO3 - , followed, in decreasing order, by Ca 2+ , H2PO4 ��� , K+ , Mg 2+ and SO4 2��� . The total amount of feed required per mEq ranged from 1.61- 13.1 kg for the four most abundant ions (NO3 ��� , Ca 2+ , H2PO4 ��� and K+ ) at a density of 2 kg fish m���3 , suggesting that it would be rather easy to maintain small populations of fish to reduce the cost of hydroponic solution supplementation for strawberries.

Fertilization Management: Assessing New Techniques and Key Interactions to Increase Crop Yields With Less N2O Emissions

El ��xido nitroso (N2O) es un potente gas de efecto invernadero (GHG) proveniente mayoritariamente de la fertilizaci��n nitrogenada de los suelos agr��colas. Identificar estrategias de manejo de la fertilizaci��n que reduzcan estas emisiones sin suponer un descenso de los rendimientos es vital tanto a nivel econ��mico como medioambiental. Con ese prop��sito, en esta Tesis se han evaluado: (i) estrategias de manejo directo de la fertilizaci��n (inhibidores de la nitrificaci��n/ureasa); y (ii) interacciones de los fertilizantes con (1) el manejo del agua, (2) residuos de cosecha y (3) diferentes especies de plantas. Para conseguirlo se llevaron a cabo meta-an��lisis, incubaciones de laboratorio, ensayos en invernadero y experimentos de campo. Los inhibidores de la nitrificaci��n y de la actividad ureasa se proponen habitualmente como medidas para reducir las p��rdidas de nitr��geno (N), por lo que su aplicaci��n estar��a asociada al uso eficiente del N por parte de los cultivos (NUE). Sin embargo, su efecto sobre los rendimientos es variable. Con el objetivo de evaluar en una primera fase su efectividad para incrementar el NUE y la productividad de los cultivos, se llev�� a cabo un meta-an��lisis. Los inhibidores de la nitrificaci��n dicyandiamide (DCD) y 3,4-dimetilepyrazol phosphate (DMPP) y el inhibidor de la ureasa N-(n-butyl) thiophosphoric triamide (NBPT) fueron seleccionados para el an��lisis ya que generalmente son considerados las mejores opciones disponibles comercialmente. Nuestros resultados mostraron que su uso puede ser recomendado con el fin de incrementar tanto el rendimiento del cultivo como el NUE (incremento medio del 7.5% y 12.9%, respectivamente). Sin embargo, se observ�� que su efectividad depende en gran medida de los factores medioambientales y de manejo de los estudios evaluados. Una mayor respuesta fue encontrada en suelos de textura gruesa, sistemas irrigados y/o en cultivos que reciben altas tasas de fertilizante nitrogenado. En suelos alcalinos (pH ��� 8), el inhibidor de la ureasa NBPT produjo el mayor efecto. Dado que su uso representa un coste adicional para los agricultores, entender las mejores pr��cticas que permitan maximizar su efectividad es necesario para posteriormente realizar comparaciones efectivas con otras pr��cticas que incrementen la productividad de los cultivos y el NUE. En base a los resultados del meta-an��lisis, se seleccion�� el NBPT como un inhibidor con gran potencial. Inicialmente desarrollado para reducir la volatilizaci��n de amoniaco (NH3), en los ��ltimos a��os algunos investigadores han demostrado en estudios de campo un efecto mitigador de este inhibidor sobre las p��rdidas de N2O provenientes de suelos fertilizados bajo condiciones de baja humedad del suelo. Dada la alta variabilidad de los experimentos de campo, donde la humedad del suelo cambia r��pidamente, ha sido imposible entender mecan��sticamente el potencial de los inhibidores de la ureasa (UIs) para reducir emisiones de N2O y su dependencia con respecto al porcentaje de poros llenos de agua del suelo (WFPS). Por lo tanto se realiz�� una incubaci��n en laboratorio con el prop��sito de evaluar cu��l es el principal mecanismo bi��tico tras las emisiones de N2O cuando se aplican UIs bajo diferentes condiciones de humedad del suelo (40, 60 y 80% WFPS), y para analizar hasta qu�� punto el WFPS regula el efecto del inhibidor sobre las emisiones de N2O. Un segundo UI (i.e. PPDA) fue utilizado para comparar el efecto del NBPT con el de otro inhibidor de la ureasa disponible comercialmente; esto nos permiti�� comprobar si el efecto de NBPT es espec��fico de ese inhibidor o no. Las emisiones de N2O al 40% WFPS fueron despreciables, siendo significativamente m��s bajas que las de todos los tratamientos fertilizantes al 60 y 80% WFPS. Comparado con la urea sin inhibidor, NBPT+U redujo las emisiones de N2O al 60% WFPS pero no tuvo efecto al 80% WFPS. La aplicaci��n de PPDA increment�� significativamente las emisiones con respecto a la urea al 80% WFPS mientras que no se encontr�� un efecto significativo al 60% WFPS. Al 80% WFPS la desnitrificaci��n fue la principal fuente de las emisiones de N2O en todos los tratamientos mientras que al 60% tanto la nitrificaci��n como la desnitrificaci��n tuvieron un papel relevante. Estos resultados muestran que un correcto manejo del NBPT puede suponer una estrategia efectiva para mitigar las emisiones de N2O. Con el objetivo de trasladar nuestros resultados de los estudios previos a condiciones de campo reales, se desarroll�� un experimento en el que se evalu�� la efectividad del NBPT para reducir p��rdidas de N y aumentar la productividad durante un cultivo de cebada (Hordeum vulgare L.) en secano Mediterr��neo. Se determin�� el rendimiento del cultivo, las concentraciones de N mineral del suelo, el carbono org��nico disuelto (DOC), el potencial de desnitrificaci��n, y los flujos de NH3, N2O y ��xido n��trico (NO). La adici��n del inhibidor redujo las emisiones de NH3 durante los 30 d��as posteriores a la aplicaci��n de urea en un 58% y las emisiones netas de N2O y NO durante los 95 d��as posteriores a la aplicaci��n de urea en un 86 y 88%, respectivamente. El uso de NBPT tambi��n increment�� el rendimiento en grano en un 5% y el consumo de N en un 6%, aunque ninguno de estos incrementos fue estad��sticamente significativo. Bajo las condiciones experimentales dadas, estos resultados demuestran el potencial del inhibidor de la ureasa NBPT para mitigar las emisiones de NH3, N2O y NO provenientes de suelos arables fertilizados con urea, mediante la ralentizaci��n de la hidr��lisis de la urea y posterior liberaci��n de menores concentraciones de NH4 + a la capa superior del suelo. El riego por goteo combinado con la aplicaci��n dividida de fertilizante nitrogenado disuelto en el agua de riego (i.e. fertirriego por goteo) se considera normalmente una pr��ctica eficiente para el uso del agua y de los nutrientes. Algunos de los principales factores (WFPS, NH4 + y NO3 -) que regulan las emisiones de GHGs (i.e. N2O, CO2 y CH4) y NO pueden ser f��cilmente manipulados por medio del fertirriego por goteo sin que se generen disminuciones del rendimiento. Con ese prop��sito se evaluaron opciones de manejo para reducir estas emisiones en un experimento de campo durante un cultivo de mel��n (Cucumis melo L.). Los tratamientos incluyeron distintas frecuencias de riego (semanal/diario) y tipos de fertilizantes nitrogenados (urea/nitrato c��lcico) aplicados por fertirriego. Fertirrigar con urea en lugar de nitrato c��lcico aument�� las emisiones de N2O y NO por un factor de 2.4 y 2.9, respectivamente (P < 0.005). El riego diario redujo las emisiones de NO un 42% (P < 0.005) pero aument�� las emisiones de CO2 un 21% (P < 0.05) comparado con el riego semanal. Analizando el Poder de Calentamiento global en base al rendimiento as�� como los factores de emisi��n del NO, concluimos que el fertirriego semanal con un fertilizante de tipo n��trico es la mejor opci��n para combinar productividad agron��mica con sostenibilidad medioambiental en este tipo de agroecosistemas. Los suelos agr��colas en las ��reas semi��ridas Mediterr��neas se caracterizan por su bajo contenido en materia org��nica y bajos niveles de fertilidad. La aplicaci��n de residuos de cosecha y/o abonos es una alternativa sostenible y eficiente desde el punto de vista econ��mico para superar este problema. Sin embargo, estas pr��cticas podr��an inducir cambios importantes en las emisiones de N2O de estos agroecosistemas, con impactos adicionales en las emisiones de CO2. En este contexto se llev�� a cabo un experimento de campo durante un cultivo de cebada (Hordeum vulgare L.) bajo condiciones Mediterr��neas para evaluar el efecto de combinar residuos de cosecha de ma��z con distintos inputs de fertilizantes nitrogenados (pur��n de cerdo y/o urea) en estas emisiones. La incorporaci��n de rastrojo de ma��z increment�� las emisiones de N2O durante el periodo experimental un 105%. Sin embargo, las emisiones de NO se redujeron significativamente en las parcelas enmendadas con rastrojo. La sustituci��n parcial de urea por pur��n de cerdo redujo las emisiones netas de N2O un 46 y 39%, con y sin incorporaci��n de residuo de cosecha respectivamente. Las emisiones netas de NO se redujeron un 38 y un 17% para estos mismos tratamientos. El ratio molar DOC:NO3 - demostr�� predecir consistentemente las emisiones de N2O y NO. El efecto principal de la interacci��n entre el fertilizante nitrogenado y el rastrojo de ma��z se dio a los 4-6 meses de su aplicaci��n, generando un aumento del N2O y una disminuci��n del NO. La sustituci��n de urea por pur��n de cerdo puede considerarse una buena estrategia de manejo dado que el uso de este residuo org��nico redujo las emisiones de ��xidos de N. Los pastos de todo el mundo proveen numerosos servicios ecosist��micos pero tambi��n suponen una importante fuente de emisi��n de N2O, especialmente en respuesta a la deposici��n de N proveniente del ganado mientras pasta. Para explorar el papel de las plantas como mediadoras de estas emisiones, se analiz�� si las emisiones de N2O dependen de la riqueza en especies herb��ceas y/o de la composici��n espec��fica de especies, en ausencia y presencia de una deposici��n de orina. Las hip��tesis fueron: 1) las emisiones de N2O tienen una relaci��n negativa con la productividad de las plantas; 2) mezclas de cuatro especies generan menores emisiones que monocultivos (dado que su productividad ser�� mayor); 3) las emisiones son menores en combinaciones de especies con distinta morfolog��a radicular y alta biomasa de ra��z; y 4) la identidad de las especies clave para reducir el N2O depende de si hay orina o no. Se establecieron monocultivos y mezclas de dos y cuatro especies comunes en pastos con rasgos funcionales divergentes: Lolium perenne L. (Lp), Festuca arundinacea Schreb. (Fa), Phleum pratense L. (Php) y Poa trivialis L. (Pt), y se cuantificaron las emisiones de N2O durante 42 d��as. No se encontr�� relaci��n entre la riqueza en especies y las emisiones de N2O. Sin embargo, estas emisiones fueron significativamente menores en ciertas combinaciones de especies. En ausencia de orina, las comunidades de plantas Fa+Php actuaron como un sumidero de N2O, mientras que los monocultivos de estas especies constituyeron una fuente de N2O. Con aplicaci��n de orina la comunidad Lp+Pt redujo (P < 0.001) las emisiones de N2O un 44% comparado con los monocultivos de Lp. Las reducciones de N2O encontradas en ciertas combinaciones de especies pudieron explicarse por una productividad total mayor y por una complementariedad en la morfolog��a radicular. Este estudio muestra que la composici��n de especies herb��ceas es un componente clave que define las emisiones de N2O de los ecosistemas de pasto. La selecci��n de combinaciones de plantas espec��ficas en base a la deposici��n de N esperada puede, por lo tanto, ser clave para la mitigaci��n de las emisiones de N2O. ABSTRACT Nitrous oxide (N2O) is a potent greenhouse gas (GHG) directly linked to applications of nitrogen (N) fertilizers to agricultural soils. Identifying mitigation strategies for these emissions based on fertilizer management without incurring in yield penalties is of economic and environmental concern. With that aim, this Thesis evaluated: (i) the use of nitrification and urease inhibitors; and (ii) interactions of N fertilizers with (1) water management, (2) crop residues and (3) plant species richness/identity. Meta-analysis, laboratory incubations, greenhouse mesocosm and field experiments were carried out in order to understand and develop effective mitigation strategies. Nitrification and urease inhibitors are proposed as means to reduce N losses, thereby increasing crop nitrogen use efficiency (NUE). However, their effect on crop yield is variable. A meta-analysis was initially conducted to evaluate their effectiveness at increasing NUE and crop productivity. Commonly used nitrification inhibitors (dicyandiamide (DCD) and 3,4-dimethylepyrazole phosphate (DMPP)) and the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) were selected for analysis as they are generally considered the best available options. Our results show that their use can be recommended in order to increase both crop yields and NUE (grand mean increase of 7.5% and 12.9%, respectively). However, their effectiveness was dependent on the environmental and management factors of the studies evaluated. Larger responses were found in coarse-textured soils, irrigated systems and/or crops receiving high nitrogen fertilizer rates. In alkaline soils (pH ��� 8), the urease inhibitor NBPT produced the largest effect size. Given that their use represents an additional cost for farmers, understanding the best management practices to maximize their effectiveness is paramount to allow effective comparison with other practices that increase crop productivity and NUE. Based on the meta-analysis results, NBPT was identified as a mitigation option with large potential. Urease inhibitors (UIs) have shown to promote high N use efficiency by reducing ammonia (NH3) volatilization. In the last few years, however, some field researches have shown an effective mitigation of UIs over N2O losses from fertilized soils under conditions of low soil moisture. Given the inherent high variability of field experiments where soil moisture content changes rapidly, it has been impossible to mechanistically understand the potential of UIs to reduce N2O emissions and its dependency on the soil water-filled pore space (WFPS). An incubation experiment was carried out aiming to assess what is the main biotic mechanism behind N2O emission when UIs are applied under different soil moisture conditions (40, 60 and 80% WFPS), and to analyze to what extent the soil WFPS regulates the effect of the inhibitor over N2O emissions. A second UI (i.e. PPDA) was also used aiming to compare the effect of NBPT with that of another commercially available urease inhibitor; this allowed us to see if the effect of NBPT was inhibitor-specific or not. The N2O emissions at 40% WFPS were almost negligible, being significantly lower from all fertilized treatments than that produced at 60 and 80% WFPS. Compared to urea alone, NBPT+U reduced the N2O emissions at 60% WFPS but had no effect at 80% WFPS. The application of PPDA significantly increased the emissions with respect to U at 80% WFPS whereas no significant effect was found at 60% WFPS. At 80% WFPS denitrification was the main source of N2O emissions for all treatments. Both nitrification and denitrification had a determinant role on these emissions at 60% WFPS. These results suggest that adequate management of the UI NBPT can provide, under certain soil conditions, an opportunity for N2O mitigation. We translated our previous results to realistic field conditions by means of a field experiment with a barley crop (Hordeum vulgare L.) under rainfed Mediterranean conditions in which we evaluated the effectiveness of NBPT to reduce N losses and increase crop yields. Crop yield, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification potential, NH3, N2O and nitric oxide (NO) fluxes were measured during the growing season. The inclusion of the inhibitor reduced NH3 emissions in the 30 d following urea application by 58% and net N2O and NO emissions in the 95 d following urea application by 86 and 88%, respectively. NBPT addition also increased grain yield by 5% and N uptake by 6%, although neither increase was statistically significant. Under the experimental conditions presented here, these results demonstrate the potential of the urease inhibitor NBPT in abating NH3, N2O and NO emissions from arable soils fertilized with urea, slowing urea hydrolysis and releasing lower concentrations of NH4 + to the upper soil layer. Drip irrigation combined with split application of N fertilizer dissolved in the irrigation water (i.e. drip fertigation) is commonly considered best management practice for water and nutrient efficiency. Some of the main factors (WFPS, NH4 + and NO3 -) regulating the emissions of GHGs (i.e. N2O, carbon dioxide (CO2) and methane (CH4)) and NO can easily be manipulated by drip fertigation without yield penalties. In this study, we tested management options to reduce these emissions in a field experiment with a melon (Cucumis melo L.) crop. Treatments included drip irrigation frequency (weekly/daily) and type of N fertilizer (urea/calcium nitrate) applied by fertigation. Crop yield, environmental parameters, soil mineral N concentrations, N2O, NO, CH4, and CO2 fluxes were measured during the growing season. Fertigation with urea instead of calcium nitrate increased N2O and NO emissions by a factor of 2.4 and 2.9, respectively (P < 0.005). Daily irrigation reduced NO emissions by 42% (P < 0.005) but increased CO2 emissions by 21% (P < 0.05) compared with weekly irrigation. Based on yield-scaled Global Warming Potential as well as NO emission factors, we conclude that weekly fertigation with a NO3 --based fertilizer is the best option to combine agronomic productivity with environmental sustainability. Agricultural soils in semiarid Mediterranean areas are characterized by low organic matter contents and low fertility levels. Application of crop residues and/or manures as amendments is a cost-effective and sustainable alternative to overcome this problem. However, these management practices may induce important changes in the nitrogen oxide emissions from these agroecosystems, with additional impacts on CO2 emissions. In this context, a field experiment was carried out with a barley (Hordeum vulgare L.) crop under Mediterranean conditions to evaluate the effect of combining maize (Zea mays L.) residues and N fertilizer inputs (organic and/or mineral) on these emissions. Crop yield and N uptake, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification capacity, N2O, NO and CO2 fluxes were measured during the growing season. The incorporation of maize stover increased N2O emissions during the experimental period by c. 105 %. Conversely, NO emissions were significantly reduced in the plots amended with crop residues. The partial substitution of urea by pig slurry reduced net N2O emissions by 46 and 39 %, with and without the incorporation of crop residues respectively. Net emissions of NO were reduced 38 and 17 % for the same treatments. Molar DOC:NO3 - ratio was found to be a robust predictor of N2O and NO fluxes. The main effect of the interaction between crop residue and N fertilizer application occurred in the medium term (4-6 month after application), enhancing N2O emissions and decreasing NO emissions as consequence of residue incorporation. The substitution of urea by pig slurry can be considered a good management strategy since N2O and NO emissions were reduced by the use of the organic residue. Grassland ecosystems worldwide provide many important ecosystem services but they also function as a major source of N2O, especially in response to N deposition by grazing animals. In order to explore the role of plants as mediators of these emissions, we tested whether and how N2O emissions are dependent on grass species richness and/or specific grass species composition in the absence and presence of urine deposition. We hypothesized that: 1) N2O emissions relate negatively to plant productivity; 2) four-species mixtures have lower emissions than monocultures (as they are expected to be more productive); 3) emissions are lowest in combinations of species with diverging root morphology and high root biomass; and 4) the identity of the key species that reduce N2O emissions is dependent on urine deposition. We established monocultures and two- and four-species mixtures of common grass species with diverging functional traits: Lolium perenne L. (Lp), Festuca arundinacea Schreb. (Fa), Phleum pratense L. (Php) and Poa trivialis L. (Pt), and quantified N2O emissions for 42 days. We found no relation between plant species richness and N2O emissions. However, N2O emissions were significantly reduced in specific plant species combinations. In the absence of urine, plant communities of Fa+Php acted as a sink for N2O, whereas the monocultures of these species constituted a N2O source. With urine application Lp+Pt plant communities reduced (P < 0.001) N2O emissions by 44% compared to monocultures of Lp. Reductions in N2O emissions by species mixtures could be explained by total biomass productivity and by complementarity in root morphology. Our study shows that plant species composition is a key component underlying N2O emissions from grassland ecosystems. Selection of specific grass species combinations in the context of the expected nitrogen deposition regimes may therefore provide a key management practice for mitigation of N2O emissions.

Soil moisture content determines the effect of the urease inhibitor NBPT on N2O emissions

Among the mitigation strategies to prevent nitrogen (N) losses from ureic fertilizers, urease inhibitors (UIs) have been demonstrated to promote high N use efficiency by reducing ammonia (NH3) volatilization. In the last few years, some field experiments have also shown its effectiveness in reducing nitrous oxide (N2O) losses from fertilized soils under conditions of low soil moisture. An incubation experiment was carried out with the aim of assessing the main biotic mechanisms behind N2O emissions once that the UIs N-(n-butyl) thiophosphoric triamid (NBPT) and phenil phosphorodiamidate (PPDA) were applied with Urea (U) under different soil moisture conditions (40, 60 and 80 % water-filled pore space, WFPS). In the same study we tried to analyze to what extent soil WFPS regulates the effect of these inhibitors on N2O emissions. The use of PPDA in our study allowed us to compare the effect of NBPT with that of another commercially available urease inhibitor, aiming to see if the results were inhibitor-specific or not. Based on the results from this experiment, a WFPS (i.e. 60 %) was chosen for a second study (i.e. mesocosm experiment) aiming to assess the efficiency of the UIs to indirectly affect N2O emissions through influencing the pool of soil mineral N. The N2O emissions at 40 % WFPS were almost negligible, being significantly lower from all fertilized treatments than that produced at 60 and 80 % WFPS. When compared to U alone, NBPT+U reduced the N2O emissions at 60 % WFPS but had no effect at 80 % WFPS. The application of PPDA significantly increased the emissions with respect to U at 80 % WFPS whereas no significant effect was found at 60 %. At 80 % WFPS, denitrification was the main source of N2O emissions for all treatments. In the mesocosm study, the application of NBPT+U was an effective strategy to reduce N2O emissions (75 % reduction compared to U alone), due to a lower soil ammonium (NH4 +) content induced by the inhibitor. These results suggest that adequate management of the UI NBPT could provide, under certain soil conditions, an opportunity for mitigation of N2O emissions from fertilized soils.

Management of irrigation frequency and nitrogen fertilization to mitigate GHG and NO emissions from drip-fertigated crops

Drip irrigation combined with split application of fertilizer nitrogen (N) dissolved in the irrigation water (i.e. drip fertigation) is commonly considered best management practice for water and nutrient efficiency. As a consequence, its use is becoming widespread. Some of the main factors (water-filled pore space, NH4+ and NO3���) regulating the emissions of greenhouse gases (i.e. N2O, CO2 and CH4) and NO from agroecosystems can easily be manipulated by drip fertigation without yield penalties. In this study, we tested management options to reduce these emissions in a field experiment with a melon (Cucumis melo L.) crop. Treatments included drip irrigation frequency (weekly/daily) and type of N fertilizer (urea/calcium nitrate) applied by fertigation. Crop yield, environmental parameters, soil mineral N concentrations and fluxes of N2O, NO, CH4 and CO2 were measured during 85 days. Fertigation with urea instead of calcium nitrate increased N2O and NO emissions by a factor of 2.4 and 2.9, respectively (P < 0.005). Daily irrigation reduced NO emissions by 42% (P < 0.005) but increased CO2 emissions by 21% (P < 0.05) compared with weekly irrigation. We found no relation between irrigation frequency and N2O emissions. Based on yield-scaled Global Warming Potential as well as NO cumulative emissions, we conclude that weekly fertigation with a NO3���-based fertilizer is the best option to combine agronomic productivity with environmental sustainability. Our study shows that adequate management of drip fertigation, while contributing to the attainment of water and food security, may provide an opportunity for climate change mitigation.

Corrosi��n inducida por contaminantes hidrosolubles en la intercara metal/pintura

El objetivo principal de este trabajo es profundizar en el conocimiento del fen��meno de la corrosi��n subpelicular inducida por contaminantes hidrosolubles en la intercara metal/pintura. La contaminaci��n salina del substrato es una situaci��n com��n en la pr��ctica: la superficie met��lica suele estar expuesta a atm��sferas contaminadas antes de ser recubierta, limpieza previa del metal con abrasivos contaminados, etc. La eliminaci��n total de estos contaminantes resulta muy dif��cil de conseguir incluso con las t��cnicas m��s sofisticadas de limpieza. Esta investigaci��n se centra en la determinaci��n del efecto de la naturaleza del contaminante y la naturaleza y espesor del recubrimiento en el proceso de corrosi��n subpelicular del acero. En la investigaci��n se utilizaron dos barnices de naturaleza diferente: poliuretano y vin��lico; y se aplicaron a tres espesores diferentes. Los contaminantes empleados en este trabajo fueron: NaCl, NH4C1, CaCl2, Na2S04, (NH4)2S04, NaN03, NH4N03, Ca(N03)2. Los ensayos se realizaron en una c��mara de condensaci��n de humedad permanente. Los tiempos de exposici��n fueron 100, 300 y 600 horas. La velocidad de corrosi��n se evalu�� gravim��tricamente, mediante la t��cnica de p��rdida de peso. Se realizaron estudios de permeabilidad al ox��geno y al agua de pel��culas libres de substrato, evaluaci��n de la velocidad de corrosi��n de probetas sin pintar inmersas en soluciones salinas de los contaminantes seleccionados, conductividad de dichas soluciones salinas, solubilidad del ox��geno en las soluciones salinas, adherencia en seco y en h��medo a diferentes tiempos de exposici��n. Se aporta evidencia respecto al control ejercido en el proceso corrosivo por el ox��geno que permea a trav��s de la pel��cula, mientras que la permeaci��n de agua controla la p��rdida de adherencia del recubrimiento. Ambas permeabilidades dependen de la naturaleza del recubrimiento y de su espesor. Se ha investigado la influencia de la naturaleza del contaminante en la intercara metal/pintura. La naturaleza del cati��n parece quedar enmascarada por el efecto definitivo del ani��n. La concentraci��n salina ejerce asimismo un efecto importante en la corrosi��n subpelicular. ABSTRACT The main aim of this work is to study in depth the knowledge of underfilm corrosi��n induced by hydrosoluble contaminants at the metal/paint ��nterface. The saline contamination of the substrate is a common situation in practice: metallic surfaces use to be exposed to polluted atmospheres, previous cleaning of the metal with contaminated abrasives, etc. Total elimination of these contaminants is hard to obtain even with modern cleaning techniques. This research is focused in determining the effect of contaminant nature, coating nature and its thickness on the steel underfilm corrosi��n process. In this work we used two varnishes with different nature: polyurethane and vinyl; they were applied in three different thicknesses. The saline contaminants employed were: NaCl, NH4C1, CaCl2, Na2S04, (NH4)2S04, NaN03, NH4N03/ Ca(N03)2. The tests were carried out in a condensation humidity chamber. The period of exposure were 100, 300 and 600 hours. Corrosi��n rate was assessed by weight loss. Simultaneously, studies on oxygen and water permeability of free films, assessing on corrosi��n rate of uncoated samples immersed in saline solutions of the selected contaminants, conductivity of these solutions, oxygen solubility in saline solutions, wet and dry adhesi��n of the polyurethane varnish at different periods of exposure, were carried out. There is clear evidence about control on corrosi��n process of oxygen that passes through the coating, while the passing of water controls the loss of adhesi��n of the coating. Both, water and oxygen permeation, depend on the nature and thickness of the coating. It has been researched the inf luence of the nature of contaminant at the metal/paint interface. The nature of the cati��n seems to be "masked" by the definitive effect of the nature of ani��n. The saline concentration also exerts an important effect on underfilm corrosi��n.