Background and aims Grazing can affect the stock and flow of C between above and below-ground vegetation layers. Components of below-ground stratum are one of the less studied. The goals of this research were: 1) to characterize and estimate the vertical distribution of below-ground biomass in grazed and ungrazed areas during a growing season, and 2) to evaluate grazing effects on below-ground net primary production (BNPP). Methods Below-ground biomass was cored four times to 100 cm depth during a growing season on three paired grazed-ungrazed areas in South-central Uruguayan grasslands. BNPP was estimated using both field data and CENTURY model. Results On average, below-ground biomass was higher in grazed (1417 gm −2 ) than in ungrazed areas (945 gm −2 ) and showed a marked reduction in relation with soil depth. Turnover rates were 0.40 and 0.37 years −1 in grazed and ungrazed areas respectively. Field data and CENTURY simulation showed higher BNPP in grazed areas (1.86; 0.77 gm −2 days −1 respectively) than in ungrazed areas (1.07; 0.67 gm −2 days −1 respectively). Conclusions Grazed areas showed higher belowground biomass, BNPP and turnover that ungrazed areas. Grazing has an important role in regulating both stock and dynamics of C in grassland ecosystems.

Land use change affects soil organic carbon (SOC) and generates CO emissions. Moreover, SOC depletion entails degradation of soil functions that support ecosystem services. Large areas covered by dry forests have been cleared in the Semiarid Chaco Region of Argentina for cropping expansion. However, deforestation impacts on the SOC stock and its distribution in the soil profile have been scarcely reported. We assessed these impacts based on the analysis of field data along a time-since-deforestation-for-cropping chronosequence, and remote sensing indices. Soil organic C was determined up to 100cm depth and physically fractionated into mineral associated organic carbon (MAOC) and particulate organic C (POC). Models describing vertical distribution of SOC were fitted. Total SOC, POC and MAOC stocks decreased markedly with increasing cropping age. Particulate organic C was the most sensitive fraction to cultivation. After 10yr of cropping SOC loss was around 30%, with greater POC loss (near 60%) and smaller MAOC loss (near 15%), at 0-30cm depth. Similar relative SOC losses were observed in deeper soil layers (30-60 and 60-100cm). Deforestation and subsequent cropping also modified SOC vertical distribution. Soil organic C loss was negatively associated with the proportion of maize in the rotation and total crop biomass inputs, but positively associated with the proportion of soybean in the rotation. Without effective land use polices, deforestation and agricultural expansion can lead to rapid soil degradation and reductions in the provision of important ecosystem services.

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A todos los que me acompañaron y ayudaron en este camino, principalmete a José por todo lo que me enseñó y me enseña dia a dia. Por encontrar la forma de bancarme las veces que estuve sin beca, eternamente agradecido… A Josefina, mi compañera de vida, por el apoyo constante e inconmensurable. Gracias por tanto, gracias por todo lo vivido, gracias por todo lo que vendrá… A mi familia, por estar siempre… Al LART y al LabFo, por la compañía y el inmejorable lugar de trabajo… A Lola, por hacer que todo sea posible… A Cami B., Tamara P., HD, Sebu A., Cristian D., Félix M., Fede P. y Juan C. por el apoyo brindado durante el ensayo de campo y los muestreos, y por bancarse los intensos calores del Chaco salteño… A Andorrana SA y Anta Del Dorado SA, por permitirme instalar el experimento en sus campos… , por ayudarme de una u otra forma… A Gerva y Marcos, por contribuir y estar cuando lo necesité… Al Depto. de Métodos Cuantitativos y a Susana P., por abrirme las puertas… A la UBA y al CONICET, por contribuir fuertemente en mi formación… A todos los que hicieron posible esta tesis...A todos y a cada uno, ¡muchas gracias! v Declaro que el material incluido en esta tesis es, a mi mejor saber y entender, original producto de mi propio trabajo (salvo en la medida en que se identifique explícitamente las contribuciones de otros), y que este material no lo he presentado, en forma parcial o total, como una tesis en ésta u otra institución.

RESUMEN.La productividad primaria neta (PPN) se define como la tasa a la cual se acumula biomasa por unidad de área y de tiempo, y se la considera una variable integradora del funcionamiento de los ecosistemas. El desmonte y los cambios en el uso del suelo que afectan gran parte de la región del Chaco Semiárido pueden impactar sobre las ganancias totales de C y su estacionalidad. Trabajos previos en la región utilizan índices espectrales de vegetación como subrogado de la PPN. Sin embargo, arriban a resultados dispares y sólo se circunscriben a lo que sucede con la parte aérea (PPNA), sin considerar los cambios que ocurren en la porción subterránea (PPNS), que suele variar entre tipos fisonómicos de vegetación. Sobre la base del modelo de eficiencia en el uso de la radiación, de información espectral derivada de sensores remotos y de relaciones PPNS/PPN de la bibliografía, el objetivo de este trabajo fue cuantificar el efecto de la transformación en el uso y la cobertura del suelo sobre la PPN, y su variación estacional. En particular, se evaluó el cambio de la PPN en sistemas agrícolas y silvopastoriles con un componente leñoso variable, respecto del bosque original. En general, se observó que los usos del suelo que presentaban una mayor productividad aérea mostraban, al mismo tiempo, los menores niveles de productividad subterránea. El doble cultivo trigo-maíz presentó la mayor PPN total, significativamente superior a la de los sistemas silvopastoriles entre 5 y 25% de cobertura arbórea, y a los cultivos de trigo-soja y soja de primera. Sin embargo, su productividad no varió significativamente de la del bosque, las pasturas megatérmicas y el maíz de primera. Los resultados obtenidos no sólo aportan información sustancial para concluir cuantitativamente en términos de la magnitud del cambio en unidades de materia seca, sino que también permiten establecer un orden o ranking más apropiado de las coberturas y usos del suelo en cuanto a la productividad total.[Palabras clave: índices espectrales, modelo de Monteith, eficiencia en el uso de la radiación, sistemas silvopastoriles] ABSTRACT. Agricultural and silvopastoral systems in the semi-arid Chaco. Impacts on primary productivity. Net primary production (NPP) is defined as the rate at which biomass accumulates per unit area and time and is considered an integrating variable of the ecosystem function. Deforestation and land use changes observed in the semi-arid Chaco region may affect total C gains and seasonality. Previous work in the region uses spectral vegetation indexes as a subrogate of the NPP. However, they arrive at contrary results and only concluded about to what happens with the aboveground production, without considering the changes that occur in the belowground production, which usually varies between physiognomic types of vegetation. Based on the model of radiation use efficiency, spectral information obtained from remote sensing and BNPP/NPP relationships of the bibliography, the goal of this work was to quantify the effect of the land use and cove...

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A�������. The Río de la Plata Grasslands (RPG) are one of the largest areas of open ecosystems (grasslands, shrublands and savannas) in the world. Historically, these systems have experienced, and continue to experience, an enormous loss of natural habitats. Moreover, their importance has been largely invisible in comparison to forested systems. The remaining area of open ecosystems in the RPG region varies according to the source from 38 to 58% of the original area. Open Ecosystems (OE) are a special case of agroecosystems because they can combine the supply of both provisioning and regulating and supporting ecosystem services (ES). Preserving the provision of ES in these natural habitats depends, in part, on understanding the role of the two main disturbances operating in them: grazing and fire. Although these two disturbances are natural components of OE, both are manipulated by humans. In this paper we reviewed the role played by fire and grazing in the structure and functioning of the RPG starting from the late Pleistocene and Holocene, and summarizing current evidence on the effects of fire and grazing on vegetation, fauna and biogeochemical processes. The evidence indicates that among agricultural activities, direct grazing systems in OE have the lowest environmental footprint. At the same time are the key for habitat preservation and ES supply. Overall, the OE of the RPG still represent a high proportion of the area, are capable of covering 2.5% of the world's population needs of high quality protein and, at the same time, guarding the regulation of key processes.