Norberto Bartoloni

Rotational Grazing Effects on Rangeland Vegetation at a Farm Scale

Abstract We evaluated the adequacy of rotational grazing to improve rangeland condition in the Flooding Pampa region, eastern Argentina, comparing the floristic composition dynamic of the 2 main plant communities under rotational and continuous grazing over a study period of 4 years (1993–1996). The experiment was conducted in commercial farms located in 4 sites of the Flooding Pampa region. In each site, a couple of farms, one managed under rotational grazing (implemented in 1989) and an adjacent one managed under continuous grazing at a similar stocking rate (1 AU·ha−1), constituted the replications of the experiment. Basal cover of species, litter, and bare soil were monitored in midslope and lowland grassland communities on each farm. Total plant basal cover in midslope and in lowland communities remained unchanged over the whole experimental period under both grazing methods. Under rotational grazing, litter cover was higher in both communities while the amount of bare soil showed a significant reduction in lowlands and a tendency to be lower in midslope. Basal cover of legumes, C3 annual and C3 perennial grasses was higher, while cover of C4 prostrate grasses was lower under rotational grazing in the midslope community. In the lowland community, rotational grazing effects were evident only in the drier years, when higher cover of hydrophytic grasses and legumes and lower cover of forbs occurred. Plant species diversity did not change in response to grazing. In conclusion, rotational grazing promoted functional groups composed of high forage value species and reduced bare soil through the accumulation of litter. These changes indicate an improvement in rangeland condition and in carrying capacity. As the stocking rate was approximately 60% higher than the average stocking rate of the Flooding Pampa region, we believe that productivity and sustainability may be compatible by replacing continuous with rotational grazing.

A comparison of indexing methods to evaluate quality of soils: the role of soil microbiological properties

The study evaluates and compares two procedures for selecting soil quality indicators (used for the construction of soil quality indices, SQI) by using diverse chemical, physical, and biological properties, and evaluates the role of soil microbiological properties in the construction of SQI. Different soil environments were selected from an extensive agricultural production site in the rolling pampa, Buenos Aires, Argentina. The plots included an undisturbed soil, a grassland soil, and continuous tilled soils with four different surface horizon depths (25, 23, 19, and 14 cm). Various properties were measured, and a minimum dataset was chosen by principal component analysis (PCA) considering all measured soil properties together (procedure A), or the PCA was performed separately according to classification as physical, chemical, or biological soil properties (procedure B). The measured soil properties involved physical, chemical, and biochemical properties determined by standard protocols used in routine laboratory analysis (simple SQI, SSQI) or more laborious protocols to determine microbial community structure and function by phospholipid fatty acid (PLFA) and catabolic response profile (CRP), respectively (complex SQI, CSQI). The selected properties were linearly normalised and integrated by the weight additive method to calculate SSQI A, SSQI B, CSQI A, and CSQI B indices. Two microbiological SQI (MSQI) were also calculated; MSQI 1 considered only biological properties according to the procedure used for calculating SQI; MSQI 2 was calculated by considering three selected microbiological parameters representing the size (microbial biomass carbon), activity (soil basal respiration), and functional diversity (evenness, determined by CRP) of the microbial communities. All of the constructed indices show the same differences among the study sites. The inclusion of CRP and PLFA data in the indices slightly increased, or did not increase, the index sensitivity. Microbiological indices had the same sensitivity as the indices integrated by physical, chemical, and biological properties. An evaluation of the SQI constructed by both procedures found no difference in sensitivity. However, SQI constructed by procedure B allowed evaluation of the effects of management practices on physical, chemical, and biological soil properties.

Arbuscular mycorrhizae of dominant plant species in Yungas forests, Argentina

In Argentina the Yungas forests are among the ecosystems most affected by human activity, with loss of biodiversity. To assess the arbuscular mycorrhizal (AM) colonization and the arbuscular mycorrhizal fungi (AMF) spore numbers in these ecosystems, the roots of the most dominant native plants (one tree, Alnus acuminata; three herbaceous, Duchesnea indica, Oxalis conorrhiza, Trifolium aff. repens; and one shrub, Sambucus peruviana) were studied throughout the year from two sites of Yungas forests. Assessments of mycorrhizal colonization (percent root length, intraradical structures) were made by washing and staining the roots. Soil samples of each plant species were pooled and subsamples were obtained to determine AM spore numbers. The herbaceous species formed both Arum- and Paris-type morphologies, whereas the tree and the shrub species formed respectively single structural types of Arum-and Paris-type. AM colonization, intraradical fungi structures and AMF spore numbers displayed variation in species, seasons and sites. D. indica showed the highest AM colonization, whereas the highest spore numbers was observed in the rhizosphere of A. acuminata. No correlation was observed between spore numbers and root length percentage colonized by AM fungi. Results of this study showed that Alnus acuminata is facultatively AM. The AM colonization, intraradical fungi structures and AMF spore numbers varied in species depending on phenological, climatic and edaphic conditions.

Native arbuscular mycorrhizal fungi in the Yungas forests, Argentina.

The arbuscular mycorrhizal fungal (AMF) communities from the Yungas forests of Argentina were studied. The AMF species present in the rhizosphere of some dominant native plants (one tree: Alnus acuminata; three herbaceous species: Duchesnea indica, Oxalis conorrhiza, Trifolium aff. repens; and one shrub: Sambucus peruviana) from two sites (Quebrada del Portugués and Narváez Range) of the Yungas forests were isolated, identified and quantified during the four seasons of the year. Twenty-two AMF morphotaxa were found. Spore density of some AMF species at each site varied among seasons. The genera that most contributed to the biodiversity index were Acaulospora for Quebrada del Portugués and Glomus for Narváez Range. High diversity values were observed in the Yungas forests, particularly in the spring (rainy season). We concluded AMF differed in species composition and seasonal sporulation dynamics in the Yungas forests.

Arbuscular mycorrhizal fungi in saline soils: vertical distribution at different soil depth

Arbuscular mycorrhizal fungi (AMF) colonize land plants in every ecosystem, even extreme conditions such as saline soils. In the present work we report for the first time the mycorrhizal status and the vertical fungal distribution of AMF spores present in the rhizospheric soil samples of four species of Chenopodiaceae (Allenrolfea patagonica, Atriplex argentina, Heterostachys ritteriana and Suaeda divaricata) at five different depths in two saline of central Argentina. Roots showed medium, low or no colonization (0–50%). Nineteen morphologically distinctive AMF species were recovered. The number of AMF spores ranged between 3 and 1162 per 100 g dry soil, and AMF spore number decreased as depth increased at both sites. The highest spore number was recorded in the upper soil depth (0–10 cm) and in S. divaricata. Depending of the host plant, some AMF species sporulated mainly in the deep soil layers (Glomus magnicaule in Allenrolfea patagonica, Septoglomus aff. constrictum in Atriplex argentina), others mainly in the top layers (G. brohultti in Atriplex argentina and Septoglomus aff. constrictum in Allenrolfea patagonica). Although the low percentages of colonization or lack of it, our results show a moderate diversity of AMF associated to the species of Chenopodiaceae investigated in this study. The taxonomical diversity reveals that AMF are adapted to extreme environmental conditions from saline soils of central Argentina.

Use of irradiated Musca domestica pupae to optimize mass rearing and commercial shipment of the parasitoid Spalangia endius (Hymenoptera: Pteromalidae)

Abstract This paper examines the potential for using irradiated Musca domestica pupae as suitable hosts of the parasitoid Spalangia endius for its use in biological control programs. Prior to being exposed to parasitoids, M. domestica pupae were gamma irradiated at 500 Gy and maintained for up to 2 months in anoxia at 6°C. The parasitization percentage, estimated by parasitoid emergence, decreased 25% after 26.5 days, 50% after 53.2 days, and 58% after 60 days. This was compared to a control group of S. endius parasitoids reared on cold-stored non-irradiated pupae whose emergence percentage decreased by 25% after 7.7 days, 50% after 15.5 days, and 72% after 22 days. Fecundity and adult longevity of parasitoids emerging from irradiated pupae were evaluated as indicators of fitness. There were no significant differences in fitness between parasitoids raised on irradiated, cold-stored pupae and the standard, live pupae presently being used in biocontrol programs. If this procedure is implemented for the mass rearing process of S. endius, it could allow the production of surplus stocks of pupae, improved efficiency, reduced rearing costs, and allow commercial shipments of non-parasitized host pupae.

Discrete observations of the spatial distributions of the Helix aspersa snail in an outdoor system

O. Gonzalez, G. Perez Camargo, M. Membiela, D. Frezza, N. Bartoloni, and C. Vieites. 2009. Discrete observations of the Helix aspersa land snails’ spatial distribution in an outdoor system. Cien. Inv. Agr. 36(1):123-130. In this study, adult Helix apersa land snails with previously established periodicity were observed at three densities in their fi nishing stage in order to contribute to the body of knowledge about spatial distribution and certain behavior patterns in an outdoor system. From November 8 to December 18, 2006, snails that were in their fi nal fattening phase were grouped at three densities (50 snailm ², 100 snailm ², 200 snailm ²) and randomly placed in nine cubicles, with three replicates of each density. Twelve observations of the snail behavior were made between 12 am and 1 pm. The categories selected as “discrete acts” include: observing the snails in the shelters, on Swiss chard leaves (Beta vulgaris), adhered to the netting, on the balanced feed, on the water supply tray, and on the ground. A statistical analysis employing marginal and partial frequency tests was completed on the density x category x time interactions. It was concluded that around midday, the majority of the snails remained in the artifi cial shelters and on the Swiss chard monoculture leaves. In addition, differences were not detected for the rest of the categories in regard to the spatial distribution of the individuals in the three densities analyzed. If lower densities than the ones most commonly employed in studies (200 snailm ²) are used in an open outdoor system in the fattening stage, the individuals will use the available space in a similar way, thereby exhibiting similar behavior patterns.

FLORAL ATTRACTIVENESS AND REWARDS AFFECT RESILIENCE OF HOST-POLLINATOR SYSTEMS: A GENETIC MODEL

A genetic model was designed to depict what the resilience of host-pollinator interactions may be according to well known genetic rules and assuming an underlying genetic basis for (a) the attraction that flowers exert on pollinators, and (b) the fitness gain by the pollinator from visiting a flower. We explore the possible trajectories that a plant-pollinator system describes under certain bound conditions determined by a whole complex of attractiveness and reward scores. Such scores represent genetic relations between two diallelic loci assumed to control both traits. To see how the system would behave over time we created eight different scenarios, differing in the orientation they impose on the system. Half of these situations are of a reinforcing type (indicating a similar input both for attractiveness and rewards) and the remaining ones are conflicting (indicating opposite inputs). A numerical simulation was carried over seventy-five generations starting from Hardy–Weinberg equilibrium populations. We detected some general behavioral patterns in the final structure of frequencies. One of these may be viewed as a resilient type of structure (i.e., without memory of the initial population frequencies) which we hypothesize, may reflect the typical attraction-reward structure generally observed in nature. Another pattern is characterized by a pronounced lose of heterozygotes in the final structure, caused by the fixation of the most attractive phenotypes at the expense of the least attracting ones, independently of the pollinator genotype.