Arid land degradation diminishes the proportion of precipitation conducted to infiltration and increases the proportion lost to run-off and evaporation. Consequently, we expect that the effects of annual precipitation on shrub growth vary with land degradation as a result of changes in soil available water. Chuquiraga avellanedae is the dominant shrub and the main indicator of land degradation in semiarid rangelands of northeastern Patagonia. We chose two communities with a different degree of land degradation: an herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Vegetative growth of C. avellanedae was determined non-destructively using a double-sampling approach. Soil water content was estimated for the two communities using a soil water balance model. Linear regressions were used to evaluate the relationships between shrub growth and (i) annual precipitation and (ii) mean available water during the period of high vegetative growth in the soil layer that each plant community concentrates their roots. In SS, with elevated clay content, there were more roots of C. avellanedae in the upper layers of soil while in HSS, with coarse-textured soil, C. avellanedae had more roots in deeper layers. Vegetative growth of C. avellanedae, both in HSS and SS communities, was positively related to annual precipitation but, for a given precipitation, C. avellanedae presented higher vegetative growth in HSS than in SS. We also found a positive relationship between vegetative growth and soil available water, and this relationship did not differ between communities. SS presented lower water availability because of lower infiltration rates. Our results showed that, irrespective of the degree of land degradation, plants respond directly to water content of the soil layers where most roots are present at a specific window of time.

Fil: Rostagno, Cesar Mario. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Centro Nacional Patagonico. Instituto Patagonico para el Estudio de los Ecosistemas Continentales; Argentina

Our objective was to examine the effects of inter-annual variation of precipitation on productivity of two dominant species (Chuquiraga avellanedae, an evergreen shrub, and Nassella tenuis, a perennial grass) in two communities of contrasting soil degradation: a herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Data were collected during two consecutive years with different annual precipitation. Aboveground productivity was determined nondestructively using a double sampling approach. The number of inflorescences per plant was recorded too. Perennial grass productivity was lower in SS than in HSS in both years, while shrub productivity was lower in SS only during the year of below average precipitation. With rising precipitation the perennial grass increased the number of inflorescences while the evergreen shrub augmented vegetative biomass. In summary, the effects of precipitation on plant productivity are community dependent; abiotic factors, such as superficial and sub-superficial soil characteristics, and biotic factors, such as leaf area index (LAI) or tussock sizes, may interact to influence the responses of species to precipitation. Our results suggest that if precipitation increased, this would favor the dominance of shrubs over grasses.

In arid and semiarid rangelands, soil erosion has been widely considered an important soil degradation process and one of the main factors responsible for declining soil fertility. In this study, we determined the sediment production and the enrichment ratios of clay, organic C, and total N by using rainfall simulations on runoff plots (0.60 x 1.67 m) in three plant communities of northeastern Patagonia: grass (GS), degraded grass with scattered shrubs (DGS), and degraded shrub steppes (DSS). Our results clearly indicate that spatial variability in soil loss rate and enrichment process exists as a result of the local differences in both plant composition and soil surface characteristics. Sediment production was significantly lower in the GS (14.2 g m(-2)) compared with the DGS and DSS (38.2 and 51.5 g m(-2), respectively). In the GS, the enrichment ratio of clay was significantly greater (3.9) and enrichment ratio of organic C was lower (3.1) than in the DGS and the DSS, though differences in enrichment ratios of total N were not significant. The high rate of soil loss and nutrients through overland-flow may limit the opportunities that promote the pathway from DGS back to GS community, favoring the dominance of shrubs. (c) 2012 Elsevier Ltd. All rights reserved.

Particulate organic-matter (MOP) is a dynamic soil component that quickly and selectively responds to management changes and to different disturbance regimes. We compared two techniques to determine the MOP content in different soils and assessed the potential of this variable as an indicator of soil quality. We studied the soils of three ecological sites located in the province of Chubut under different management schemes or affected by different disturbance regimes: Media Luna exclosure (MLC), Media Luna-grazing (MLP), Puerto Madryn bioturbed (PMB), Puerto Madryn nonbioturbed (PMNB), Punta Ninfas uneroded (PNNE) and Punta Ninfas eroded (PNE). We collected soil samples from the 0 to 5 cm (PM and PN) or 0 to 10 cm (ML) depths to determine total soil organic-matter (MOT) and MOP using wet combustion MOPh (Walkley and Black method) and weight loss-on-ignition in a high temperature oven set at 430° C (MOPi). In the Mollisols and the Aridisols from Puerto Madryn, MOPi was higher than MOPh; in the Aridisols from Punta Ninfas, however, the differences between MOPi and MOPh were not significant. In soils with high calcium carbonate (PMB and PMNB) or allophane (MLC and MLP) contents, the loss-on-ignition method overestimated MOP contents. The MOT contents were highly correlated to both MOPh and MOPi for all soils (r2= 0.89 and r2 = 0.73, respectively). Both MOT and MOP were sensitive to land management and soil erosion. The soil that displayed the greatest MOP/ MOT ratio was the PNNE. On the contrary, the PNE site showed the lowest values for this ratio, which would indicate that the low MOT content of this soil is predominantly associated to the < 0.053 mm fraction, and that the erosion process decreased the MOP reserves more than the MOT. For the Puerto Madryn soils, the MOPi / MOT ratio was greater than 1. This seems to be mainly due to the overestimation of the loss-on-ignition method in soils with abundant carbonates. Both MOT and MOP, determined by the two methods, can be used to detect differences in management or the state of soil degradation. However, MOP seemed to be more affected by soil erosion and sheep grazing compared to MOT. These results would allow us to consider MOP as a good indicator of changes in soil quality by either soil erosion or sheep grazing.

La materia orgánica particulada (MOP) es un componente dinámico del suelo, que responde selectiva y rápidamente a los cambios de manejo y distintos tipos de disturbios. El objetivo de este estudio fue comparar dos técnicas de determinación de MOP en distintos suelos y evaluar el potencial de esta variable como indicadora de calidad de los mismos. Se estudiaron tres sitios ecológicos de la provincia del Chubut con distinto manejo o afectados por distintos disturbios: Media Luna Clausura (ML C), Media Luna Pastoreo (ML P), Puerto Madryn bioperturbado (PM B), Puerto Madryn no bioperturbado (PM NB), Punta Ninfas no erosionado (PN NE) y Punta Ninfas erosionado (PN E). En muestras de suelos extraídas de 0 a 5 cm (PM y PN) y de 0 a 10 cm (ML) de profundidad se determinó la materia orgánica total (MOT) y la MOP por vía húmeda (MOPh) (Método Walkley y Black) y por ignición en mufla (MOPi) a 430 °C. En los Molisoles de ML y en los Aridisoles de PM la MOPi fue mayor que la MOPh, mientras que para los Aridisoles de PN no se registraron diferencias significativas entre los dos métodos. Para los suelos con carbonatos (PM) o con alofanos (ML), el método por ignición sobreestimó los contenidos de MOP. La MOT presentó una alta correlación con la MOPh y con la MOPi para todos los suelos en conjunto (r² = 0,89 y r² = 0,73, respectivamente). El suelo que presentó una mayor relación MOP/MOT fue PN NE. En contraste, el suelo PN E, mostró los valores más bajos para esta relación, lo que indicaría, por un lado, que la poca MO que contienen estos suelos está predominantemente asociada a la fracción< de 0,053 mm, y por el otro, que la erosión afecta en mayor medida las reservas de MOP que las de MOT. Para los suelos PM, la relación MOPi/MOT fue mayor que uno, debido a la sobreestimación del método por ignición en estos suelos con abundante carbonatos. Tanto la MOT como la MOP determinada por los dos métodos, han permitido detectar diferencias entre tipos de manejo o estado de degradación de los suelos. De todas maneras, la MOP es proporcionalmente más afectada por la erosión y el pastoreo que la MOT, lo cual nos permitiría considerarla como un buen indicador de cambios en la calidad de los suelos bajo condiciones de pastoreo o afectados por procesos erosivos.Particulate organic-matter (MOP) is a dynamic soil component that quickly and selectively responds to management changes and to different disturbance regimes. We compared two techniques to determine the MOP content in different soils and assessed the potential of this variable as an indicator of soil quality. We studied the soils of three ecological sites located in the province of Chubut under different management schemes or affected by different disturbance regimes: Media Luna exclosure (ML C), Media Luna-grazing (ML P), Puerto Madryn bioturbed (PM B), Puerto Madryn nonbioturbed (PM NB), Punta Ninfas uneroded (PN NE) and Punta Ninfas eroded (PN E). We collected soil samples from the 0 to 5 cm (PM and PN) or 0 to 10 cm (ML) depths to determine total soil organic-matter (MOT) and MOP using wet combustion MOPh (Walkley and Black method) and weight loss-on-ignition in a high temperature oven set at 430° C (MOPi). In the Mollisols and the Aridisols from Puerto Madryn, MOPi was higher than MOPh; in the Aridisols from Punta Ninfas, however, the differences between MOPi and MOPh were not significant. In soils with high calcium carbonate (PM B and PM NB) or allophane (ML C and ML P) contents, the loss-on-ignition method overestimated MOP contents. The MOT contents were highly correlated to both MOPh and MOPi for all soils (r²= 0.89 and r² = 0.73, respectively). Both MOT and MOP were sensitive to land management and soil erosion. The soil that displayed the greatest MOP/ MOT ratio was the PN NE. On the contrary, the PN E site showed the lowest values for this ratio, which would indicate that the low MOT content of this soil is predominantly associated to the < 0.053 mm fraction, and that the erosion process decreased the MOP reserves more than the MOT. For the Puerto Madryn soils, the MOPi / MOT ratio was greater than 1. This seems to be mainly due to the overestimation of the loss-on-ignition method in soils with abundant carbonates. Both MOT and MOP, determined by the two methods, can be used to detect differences in management or the state of soil degradation. However, MOP seemed to be more affected by soil erosion and sheep grazing compared to MOT. These results would allow us to consider MOP as a good indicator of changes in soil quality by either soil erosion or sheep grazing.

En los ecosistemas aridos, la precipitacion anual es considerada un factor determinante de su funcionamiento. Los cambios en su estacionalidad y/o en la frecuencia en que ocurren eventos extremos de lluvia podrian tener un importante impacto ecologico y biogeoquimico, dependiendo del estado de conservacion del suelo. El objetivo de este trabajo fue evaluar la influencia de lluvias estacionales de gran magnitud (riegos de 40 mm) sobre la disponibilidad de N (amonio y nitrato) en suelos del NE de Chubut con distintos niveles de erosion. Se eligieron dos comunidades vegetales que representan dos situaciones de erosion de suelos: una estepa herbacea con arbustos con suelos no erosionados (EHA) y una estepa arbustiva erosionada (EA). El experimento de un ano de duracion tuvo un diseno factorial de tres factores: estacion de riego (invernal y primaveral), dos niveles de riego (con y sin riego) y dos comunidades (EHA y EA). Para los tratamientos de riego, el agua se agrego en un unico momento durante el invierno y/o la primavera, simulando un evento de precipitacion grande (40 mm). Se colectaron muestras de suelo superficial (0-5 cm) y sub-superficial (5-15 cm) de las parcelas en 4 fechas: inmediatamente antes del riego, a los 3, 6 y 14 dias luego de su aplicacion. No se encontraron diferencias en el N total entre los suelos de las comunidades. El riego invernal no tuvo efecto en los contenidos de amonio y nitrato de los suelos de la EHA ni de la EA. En cambio, en primavera se encontro mayor contenido de nitrato en la capa superficial del suelo de la EHA. Nuestros resultados sugieren que la falta de respuesta en el contenido de nitrato en los suelos erosionados (EA) luego de un evento de lluvia grande seria consecuencia de la menor infiltracion y del menor aumento del contenido hidrico en la EA respecto a la EHA.