Elizabeth J. Jacobo

The role of the permanent soil seed bank in early stages of a post-agricultural succession in the inland Pampa, Argentina

We studied the soil seed bank composition in four old fields of different ages, after abandonment from agriculture. Complete seed bank composition was assessed by direct seed separation from soil samples and identification to species. Most species found in the seed bank were not important in the present seral communities. Seed of the species that dominated the early succession were generally not found. Additionally, there were very few propagules rather than on the germination of in situ propagules. We suggest that pampean grasses evolved under that the course of post-agricultural succession will depend strongly on the pattern of arrival of exogenous propagules rather than the germination of in situ propagules. We suggest that pampean grasses evolved under disturbances of low intensity and/or a disturbance regime dominated by small gaps, in which open areas could be rapidly colonized from the edges and/or by remnant vegetative propagules. The changes produced by the introduction of agriculture triggered the invasion by exotic species adapted to the new disturbance regime.

Germination behaviour of Italian ryegrass in flooding pampa rangelands

Lolium multiflorum Lam., a winter annual grass species originating from the Mediterranean region, has become widespread in native grasslands of the flooding pampa (Argentina). The dynamics of dormancy release of L. multiflorum seeds in simulated natural conditions was studied. Dormancy was overcome by exposing imbibed seeds to mild cool temperatures (15°C or below) for brief periods (pre-chilling). After-ripening progressively released dormancy, at a higher rate in seeds that remained under the canopy (30/20°C) than those that were dry-stored (20°C). Germination was promoted by red light only when the after-ripening requirement was partially fulfilled or after pre-chilling treatment. These results provide useful knowledge for designing grazing strategies aimed at obtaining well-developed ryegrass canopy of high forage quality during winter and spring.

Improvement in rangeland condition of the Flooding Pampa of Argentina through controlled grazing

Abstract The Flooding Pampa grasslands situated in temperate Argentina were ungrazed historically, but now support primarily breeding herds of cattle. These extensive, flat, infertile grasslands experience seasonal floods. Although summer droughts are usual, grasses maintain productivity during the entire year and produce almost 6 t ha‐1 a‐1. Continuous grazing has caused deterioration of these grasslands in terms of floristic composition and soil properties (salinisation). Stocking rate has been adversely affected. Controlled grazing systems have been applied with the objective of preventing deterioration. The main characteristics of this system are the concentration of animals in large herds, non‐selective grazing of dormant vegetation during autumn and winter, and selective grazing during spring and summer. Rotational grazing ensures adequate rest for grazed plants and promotes tillering and establishment of cool season grasses. A system of controlled grazing has shown an improvement in floristic compo...

Glyphosate Alters Aboveground Net Primary Production, Soil Organic Carbon, and Nutrients in Pampean Grasslands (Argentina)*

ABSTRACT We have previously demonstrated that recurrent application of glyphosate causes dramatic shift in the vegetation structure of the native grasslands of Flooding Pampa. As these structural changes might alter functional processes such as primary production, carbon, nitrogen and phosphorus cycling, this study aims to evaluate functional changes associated with the application of glyphosate in these temperate grasslands. We measured aboveground net primary production (ANPP) during two consecutive years, and the concentration of organic carbon, nitrogen and phosphorus in the soil during the following six years after primary production measurements ended in glyphosate treated and non- treated (control) paddocks of a commercial livestock far. We related the vegetation data, basal cover, species richness and diversity, obtained in a previous study conducted in the same paddocks of the livestock farm, with ANPP data obtained in this one. Late summer applications of glyphosate greatly reduced the biomass contribution of warm-season perennial grasses and legumes and increased the contribution of cool season annual grasses, altering the seasonal pattern of ANPP. As the reduction of the spring and summer productivity could not be compensated by the increase of cool-season productivity, the annual ANPP was lower in the glyphosate-treated paddocks than in control paddocks. Glyphosate applications also decreased soil organic carbon and phosphorus concentration, probably because of the reduction of ANPP, the changes of its seasonal distribution and the shift in the floristic composition of the community, which may modify the amount and quality of the litter. We found a linear positive relationship between basal cover, species richness and species diversity with ANPP, which suggest that the negative effects on ecosystem functioning would be directly related with the changes in vegetation structure caused by glyphosate application.

Tidal Suppression Negatively Affects Soil Properties and Productivity of Spartina densiflora Salt Marsh

ABSTRACT In order to intensify cattle utilization, embankments were constructed to avoid tidal ingressions in Samborombon Bay, Argentina, The objective of this study was to evaluate the effect of tidal suppression and cutting frequency of a salt marsh dominated by Spartina densiflora Brongn. Two paddocks of a commercial cow-calf operation farm, one prevented from tidal flooding and another exposed to overflow from natural tidal pattern (control), were the main plots of the nested design. The experiments were carried out during a dry (2008–2009) and a wet growing season (2012–2013). Two defoliation frequencies, simulating light and moderate grazing pressure, were performed in the subplots nested within each main plot. Soil organic matter and N content were lower and soil structural instability index was much higher in the embankment than in the control treatment. Soil salinity during the dry growing season was higher in the embankment than in the control treatment. Bare soil was higher under embankment treatment and high defoliation frequency exacerbated this response. Relative contribution of Spartina densiflora was lower under embankment than control treatment and the changes of floristic composition depended on the growing season. Aboveground net primary production (ANPP) in the wet growing season was almost 70% higher than in the dry growing season. Embankment reduced ANPP and high defoliation increased ANPP with respect to low defoliation frequency in the control paddock, to a much higher extent in the wet season. Dry matter digestibility of S. densiflora was not affected by treatments. Crude protein was higher in control paddocks under high frequency. Our results showed that tidal suppression by embankment was not effective to increase productivity and forage value of S. densiflora saltmarsh but caused soil and structural changes that may negatively alter ecosystem processes of this vulnerable grassland of high importance for biodiversity conservation.