Semiarid Argentinean Pampas Research Articles

Socio-technical transitions towards sustainable development: A proposal for learning assessment in livestock SMEs in the semi-arid Pampas of Argentina

The urgency to achieve the Sustainable Development Goals in the face of climate change has increased interest in studies on innovation niches in food systems. Strategic Niche Management in the framework of the Multi-Level Perspective considers that deep learning must be achieved in experimenting with innovations and demands greater understanding of instrumental aspects to guide the evolution of a niche. This study proposes to assess the progress of innovative performance towards sustainability of small and medium livestock enterprises in the Southwest of Buenos Aires, a fragile region of the Pampas biome of Argentina, as a result of learning generated in rural extension activities of the National Institute of Agricultural Technology (INTA) in order to identify leverage points in the process of technological change towards more sustainable systems. An exploratory-descriptive study was carried out using qualitative-quantitative methods. Ten producers were interviewed and the MESMIS method was proposed for the construction of indicators and assessment of learning goals towards sustainability. A rapprochement of producers from modal practices to more innovative ones was observed, reflecting conditions for the progress and consolidation of the niche. However, they need for greater professionalization and institutional articulation. The study generated a measurement instrument for the self-assessment of the niche enterprises and the planning of INTA's intervention strategies. It sought to broaden empirical research in Latin America, which is still scarce in approaches to socio-technical transitions, and to advance in the evaluation of experiments in transformative innovations on the agenda for the literature.

Outdoor culture of Halamphora coffeaeformis in the semi-arid Pampa of Argentina: A comprehensive analysis of triacylglycerol production for biodiesel

The feasibility of microalgal triacylglycerol (TAG) production for biodiesel remains constrained by high production costs under outdoor conditions. The aim of this study was to evaluate the capacity of an Argentinian strain of the diatom Halamphora coffeaeformis grown in outdoor raceway ponds to produce TAG suitable for biodiesel. The strain was cultured in seawater enriched with a biofertilizer during summer and spring in the semi-arid Pampa of Argentina. The following variables were analysed: 1) growth rate; 2) TAG accumulation; 3) lipid content and quality; 4) dissolved nutrient and physicochemical properties; and 5) climatic conditions. Intracellular TAG accumulation was evaluated with fluorometry by Nile Red. The growth rate and biomass productivity of the species were similar in the two seasons, with approximate values of 1.35 div. day−1 and 61.25 kg.ha−1.day−1, respectively. Total lipid content increased about 4-fold with respect to inocula in the 2 cultures, reaching up to 44.7 % ash free dry weight (AFDW) in summer and 36.4 % AFDW in spring. However, lipid accumulation differed between seasons: neutral lipid accumulation began at 8 days of cultivation in summer and at 19 days in spring. Salinity was the main stressor that accelerated the TAG accumulation in the species, the nutrient levels did not reach limiting values. In spring, precipitation decreased salinity levels, reducing stress and delaying the onset of TAG accumulation. The biodiesel properties inferred from the fatty acid profile of the species meet international standards and their quality ensures good performance in cold climates. Furthermore, the reported lipid productivity of the species (~25.6 kg.ha−1.day−1) is higher than that of the soybean oil used commercially for biodiesel. Based on the results obtained, H. coffeaeformis has potential for exploitation as a renewable source of TAG for biodiesel production in arid and semi-arid environments with access to brackish or marine water.

Projecting Climate Change Effect on Soil Water Fluxes and Urea Fertilizer Fate in the Semiarid Pampas of Argentina

Projecting Climate Change Effect on Soil Water Fluxes and Urea Fertilizer Fate in the Semiarid Pampas of Argentina

Elemental composition of wind-blown sediments from contrasting textured soils

• Element composition of aeolian sediments of two different textured soils was studied. • Ca and Mg were transported at low heights in both soils. • Organic C and N were transported at great heights in both soils. • S was transported at low and high heights, possibly in its organic and mineral forms. • The transport along height was less segregated in sandy loam- than in sandy soil. Wind eroded sediment is sorted by height by both size and density. Less information is available about the elemental composition and enrichment ratios of these sediments. Because of that, the concentration of elements in wind-blown sediments of a sandy- and a sandy loam soil of the semiarid Argentinean Pampas were analyzed. With this purpose, a wind tunnel study was performed for obtaining the saltating sediments transported at different heights, and a sorption-based laboratory procedure was used to separate the particulate matter. Results showed that Ca and Mg were transported at low- and C and N at high heights. These tendencies were attributed to the accumulation of Ca and Mg in coarse minerals, probably apatite and Ca-feldspars, which are heavily transported by the wind, and to the accumulation of C and N in low-dense organic substances, fresh debris and polysaccharides, easily transported by the wind. Total S did not show a preference to be transported at any height, possibly due to its accumulation in organic and mineral substances. The transport of C and N was more segregated in the sandy- than in the sandy loam soil, in agreement with the better aggregation of the latter, in which some of its organic substances (carboxylic acids, amides and aromatics), form coarse and heavily transportable aggregates. From the obtained results it can be deduced that C and N would be widespread larger than Ca and Mg. Assembling of data obtained to mass fluxes, becomes relevant for modeling the biogeochemical cycles.

How aridity variations affect Prosopis caldenia growth in transitional forests in the semiarid Argentinean Pampas

The semiarid Pampas in central Argentina, occupied by caldén’s (Prosopis caldenia Burkart) woodlands (caldenales), had been affected by several wet and dry periods in the last century. Nowadays, the caldenales cover about 17 million hectares of central Argentina and their current state is critical due to massive deforestation rates. Caldén has an important dendrochronological potential since it produces sensitive tree rings linked to climate oscillations, fire events, competence, insect breakout, among others. The standard dendrochronological analysis can be improved with new advances in serial modeling and multivariate ordination techniques for handling problems related to the comparison of dendrochronological samples. We applied a statistical algorithm, BIOdry, which integrates conventional procedures for modeling patterns between annual diameter increments and drought. P. caldenia dendrochronological data were used accounting for multiple sources of variation from the sample design, and comparing patterns from contrasting climatic portions of the study site. The relation between diameter growth dynamics of caldén with temperature and precipitation fluctuations along the second half of the last century was analyzed. Populations at the Northern limit of its natural distribution area were selected. Monthly temperature and precipitation were evaluated in order to identify relative water surplus and deficit periods and an annual aridity index (AAI) was calculated. The objectives of the present work were: to analyze, describe and model response patterns between the P. caldenia diameter growth increment and the AAIs. A common growth response pattern and differences in tree and population level were also analysed. The algorithm BIOdry showed a good behavior, according to the considered statistical parameters (all the fixed effects were statistically significant). Several and common problems associated with the nature of the tree-ring data in modeling (pseudorreplication, autocorrelation, and nested random effects) were efficiently addressed. P. caldenia diameter growth, at tree level significantly responds to different AAI variation intensities along the studied period. A high frequency growth variability at population level was detected when trees establishment happened. These associations ended when trees become older. Low frequency growth variability at population level was significant throughout the analyzed period, presumably linked to age and management history. These results provide news insights in relation to the climate impact on growth dynamics of caldén in the Argentinean pampas and the value of new statistical tools in order to improve dendrochronological studies.

Spectral indices from aerial images and their relationship with properties of a corn crop

Identification of areas with similar restrictions to crop productivity could improve the efficiency to manage agricultural systems, guarantee stable yields, and reduce the effect of droughts in rainfed systems. The ability of any vegetation index to discriminate N and moisture-related changes in leaf reflectance would present an important advantage over the present diagnostic system which involves soil-testing for moisture and available N. The purpose of the study was to calibrate different vegetation indices regarding their capacity to identify water and nitrogen availability for rainfed corn crops in the semiarid Pampas of Argentina. A field experiment with corn with a control without fertilization (N0), and fertilized with 120 kg ha−1 of nitrogen (N120) was used. Two sites, Low (L) and High (H), were identified within the field, according to their altimetry, a multi-spectral aerial photography was taken from a manned airplane during flowering stage of the corn crop, and four spectral indices were calculated (NDVI, green NDVI, NGRDI, (NIR/GREEN)-1). At six georeferenced points at each site soil texture, organic matter, available phosphorus, nitrogen and moisture contents as well as corn aerial biomass and grain yield were determined. The two sites differed in most of the evaluated soil properties, crop biomass and grain yield. The spectral information obtained at crop flowering showed clear differences between sites H and L for all four indices, indicating that any of these would be able to detect the differences in soil moisture and fertility among these environments. Both (NIR/GREEN)-1 and green NDVI had the best correlation with crop yield determined in the field, and therefore could be considered most appropriate for estimating corn yields from images taken at flowering. For estimation of N requirements, green NDVI differentiated best between fertilized and non-fertilized crop in the moisture limited environment (H), while (NIR/GREEN)-1 performed better in the site where soil moisture was non-limiting (L).

First record of sexual forms of Aphis craccivora Koch (Hemiptera: Aphididae) on Medicago sativa L. (Fabales: Fabaceae)

Sexual reproduction of Aphis craccivora Koch is recorded for the first time in Semiarid Pampas of Argentina. This type of reproduction was found on Medicago sativa L. A brief description and illustrations of eggs, oviparous females and winged males are given

RWEQ – Wind erosion predictions for variable soil roughness conditions

The soil surface roughness is a main factor in all wind erosion prediction models, including the Revised Wind Erosion Equation (RWEQ). The objective of this study was to test the erodibility of two typical soils of the semiarid Argentinean Pampas under three different tillage conditions (compared to a flat surface) at three wind velocities using a wind tunnel and to evaluate the performance of the RWEQ model. Results showed that all rough surfaces were less eroded by wind than a flat surface (FS) in both soils and all wind velocities. An exception was LB (lister-bedder) in the Haplustoll that showed similar erosion than FS. Wind erosion increased rapidly above 16.5 m s−1 wind velocity in all tillage conditions. The relative wind erosion (RE) calculated with the RWEQ (K′ factor) fitted well with measured RE, except for K′ < 0.1 (rougher surface) where the measured RE were much higher than the predicted. More than 70% of RE variability was explained by the oriented roughness (Kr) in both soils. The aforementioned indicates that Kr can be used instead of K′ (a value that contains both, Kr and the random roughness – Crr factors) to predict wind erosion with RWEQ in the studied soils. Absolute wind erosion amounts predicted with RWEQ fitted well with measured data only for DT, mainly at low wind velocity. For the other tillage tools, the model did not apply well as it underestimated the erosion for the rougher soil surface condition (LB) and overestimated it for the less rough surface (DH).

Resistance in alfalfa to Aphis craccivora Koch

The cowpea aphid, Aphis craccivora Koch (Aphididae), is considered to be one of the major pests in the semiarid Pampas of Argentina and in other alfalfa (Medicago sativa L.) growing areas of the world. In the present study the antibiotic, antixenotic, and tolerance resistance of eight alfalfa cultivars to A. craccivora, were investigated under laboratory conditions at 24 ± 1 °C, 65 ± 10% relative humidity, and 14:10 h photoperiod. Antibiosis experiments showed significant differences in the developmental time and adult longevity of the cowpea aphid among the alfalfa cultivars. Intrinsic rate of natural increase (rm) for apterous aphids varied significantly with alfalfa cultivars on which aphids were reared. This value ranged from 0.04 to 0.16 females female-1 d-1, which was the lowest on 'Medina'. Additionally, the estimated net reproductive rate (Ro) and finite rate of increase (X) for apterous aphids were the lowest on 'Medina'. For the antixenosis experiment significant difference was found in aphid's preference to the alfalfa cultivars. 'Carmina' and 'Victoria' were the least preferred by the apterous aphids. For the tolerance experiment 'Carmina', 'Monarca', 'SPS6550' and 'Victoria' were more tolerant than the other cultivars to A. craccivora. Therefore, our results demonstrated that among the investigated cultivars 'Medina' displayed antibiosis to A. craccivora, 'Carmina' and 'Victoria' expressed antixenosis and tolerance, and 'Monarca' and 'SPS6550' were tolerant to this aphid pest.

Aboveground and belowground biomass allocation in native Prosopis caldenia Burkart secondaries woodlands in the semi-arid Argentinean pampas

The woodlands in the south-west of the Argentinean pampas are dominated by Prosopis Caldenia Burkart (calden). The current deforestation rate of this woodlands is 0.82% per year. Different compensation initiatives have begun that recognize the role of forests as environmental service providers. The financial incentives they offer make it necessary to quantify the amount of carbon stored in the forest biomass. A model for estimating calden biomass was developed. Thirty-eight trees were selected, felled and divided into sections. An equation system was fitted using joint generalized regression to ensure the additivity property. A weighted regression was used to avoid heteroscedasticity. In these woodlands fire is the main disturbance and it can modify tree allometry, due this all models included the area of the base of the stem and tree height as independent variables since it indirectly collects this variability. Total biomass and the stem fraction had the highest R2_Adj. values (0.75), while branches with a diameter less than 7 cm had the lowest (0.58). Tree biomass was also analyzed by partitioning into the basic fractions of stem, crown, roots, and the root/shoot ratio. Biomass allocation was greatest in the crown fraction and the mean root/shoot ratio was 0.58. The carbon stock of the caldenales considering only calden tree biomass is 20.2 Mg ha−1. While the overall carbon balance of the region is negative (deforestation and biomass burning, the remnant forested area has increased their calden density and in an indirect way his carbon sequestration capacity could also be increased.

Wind erosion risk in agricultural soils under different tillage systems in the semiarid Pampas of Argentina

The effect of plant residues, plant canopy and non-erodible soil aggregates on wind erosion has been mostly evaluated under controlled wind tunnel conditions. Little is known about their combined effect under field conditions. Wheat of different growth lengths are widely cropped in the semiarid Pampas of Argentina (SAP) under different tillage systems. Aim of this study was to measure the soil cover in wheat types of different growth lengths, cropped under three tillage systems, and their effects on wind erosion in a semiarid environment of Argentina. Measurements of the soil coverage with crops canopy, stubble and soil aggregates were done 15 days each during a wet (2005) and a dry (2006) year on a sandy loam Entic Haplustoll of the semiarid Pampas. On the basis of climatic and soil coverage data wind erosion was estimated with the Revised Wind Erosion Equation (RWEQ). Results showed that wind erosion was lower in no-till (NT) than in vertical- (VT) or conventional tillage (CT) in all wheat types due to high soil coverage with plant residues (83% of total soil cover during fallow). In contrast, during fallow in CT and VT, a 16% of soil was covered with non-erodible aggregates (64% of total cover) and plant residues (32%). As a result, the hazard of wind erosion was high in CT and VT (899 and 1002 kg ha −1, respectively). Regarding the wheat types, wind erosion amounts of CT and VT were, in average of both sampling years, lower in long cycle wheat (LCW, 635 kg ha −1) than in intermediate cycle wheat (ICW, 980 kg ha −1) and short cycle wheat (SCW, 1237 kg ha −1). The higher wind erosion of SCW was produced by the simultaneous occurrence of minimum soil coverage and high climatic erosivity just after crop seeding. In NT wind erosion was low in all cases (between 0 and 31 kg ha −1). However, high wind erosion amounts (1500 kg ha −1) can occur in NT after crop seeding. SCW cropped with CT and VT must be avoided in the studied region, in order to make an efficient wind erosion control. No-till was the best system for controlling wind erosion, though, moderate wind erosion amounts can occur in this system during short periods of time after seeding of all wheat types.

Cultivation effects on the distribution of organic carbon, total nitrogen and phosphorus in soils of the semiarid region of Argentinian Pampas

Cultivation of native land can reduce the quality of soil by decreasing topsoil contents of organic carbon, total nitrogen, and phosphorus in the semiarid Pampas of Argentina. The objective of this study was to analyze the changes produced by cultivation on organic carbon (OC), total nitrogen (TN) and phosphate (inorganic and organic fractions) in two aggregate sizes of three different semiarid soils of Argentina as a function of soil depth. The study was carried out on three soils (loamy Hapludoll, loamy Haplustoll and sandy loam Haplustoll), with two uses compared at each site. Generally, the Caldenal savanna-like ecosystem (native soil) and a cultivated counterpart with annual crops for more than 60 years (cultivated soil) were compared. Results showed that all soils had similar distribution patterns with depth of OC, TN, total inorganic phosphorus (Pi), organic phosphorus (Po) and available phosphorus (Pa) in the 100–2000 μm and < 100 μm aggregates of both the native and cultivated soils. Contents of OC, TN and Pi in the whole soil (< 2000 μm) decreased sharply with depth in native soils (OC: 50%, TN: 45% and Pi: 23%), but they had a more homogeneous distribution in cultivated soils. These tendencies were attributed to the stratification occurring in non-disturbed native soils and the mixing of the upper 20 cm in cultivated soils. Cultivation produced losses of OC (73%) and Po (64%) from the 100–2000 μm aggregates at both depths in the sandy loam Haplustoll, while it produced losses of TN (52%) and Pi (42%) from the < 100 μm aggregates in the upper 10 cm of the loamy Hapludoll. OC and Po losses of the sandy loam Haplustoll were probably produced by the mineralization of organic compounds, whereas TN and Pi losses of the loamy Hapludoll were produced by wind erosion and plant uptake. Larger relative decreases of OC and Po than TN in coarse aggregates (100–2000 μm) and Pi in fine aggregates (< 100 μm) with cultivation indicate that coarse aggregates are less stable than fine aggregates (< 100 μm), and therefore more sensitive to changes in soil quality due to changes in land use. We concluded that management practices that tend to improve the formation of large aggregates as no-till or permanent pastures should be used in coarse textured Haplustolls in order to prevent large OC and Po losses. Management practices that include soil plowing will produce a breakdown of coarse aggregates and will moderately decrease OC, Po, TN, Pa and Pi contents in loamy Haplustolls and Hapludolls. Nevertheless, the destruction of coarse aggregates will increase the risk of losses of these elements by wind erosion.

Clay mineralogy, cation exchange capacity and specific surface area of loess soils with different volcanic ash contents

The loessical soils of the semiarid Argentinean Pampas (SAP) contain variable amounts of volcanic ashes. Their influence on the mineral composition and some physicochemical properties of soils like soil specific surface area (SSA) and cation exchange capacity (CEC) have been still not elucidated. Because of that we analyzed 24 topsoil samples (0 to 20 cm) of soils with high and low volcanic ashes contents for clay mineralogy and its influence on soil chemical and physical properties and CEC and SSA. Results showed that ash enriched soils were placed to the south and ash free soils to the north-east of the SAP in agreement with the volcanic ash deposition pattern of 1932 Quizapu vocano eruption. Mineralogy of the clay fraction was dominated by amorphous minerals and less crystallized and expansible smectites in ash enriched soils, and by illites in ash free soils. No differences in crystallinity or expansibility were found between montmorillonites of ash free and ash enriched soils. Fine sized clays (< 0.2 μm) were dominated by illites and illite–montmorillonite intergrades in ash free soils and by amorphous materials in ash enriched soils. Such results indicated that montmorillonites tend to form more when volcanic ashes are present but their crystallinity and expansibility do not change with ashes content. SSA was positively related with silt contents in both ash free and ash enriched soils ( r 2 = 0.70, p < 0.05). The clay fraction (< 2 μm) explained only 9% of SSA variability and the fine clay fraction (< 0.2 um) did not affect SSA. The influence of silt on SSA was attributed to the existence of 2 : 1 minerals in the silt fraction. These results also indicated that ash contents did not influence SSA. CEC correlated positively with clay ( R 2 = 0.67, p < 0.001) in ash enriched soils, and with organic matter in ash free soils ( R 2 = 0.837, p < 0.001) but it did not correlate with fine clay contents in any of both soil types. These trends were explained on the basis of the higher CEC conferred by smectites in ash enriched soils and by OM in ash free soils. It can be deduced that soil degradation processes producing losses of fine sized particles, like wind or water erosion, will decrease SSA in both the ash enriched and the ash free soils, but only CEC in ash enriched soils. Organic matter losses due to excessive cultivation will decrease CEC in ash free soils.

Long-Term Maize, Sorghum, and Millet Monoculture Effects on an Argentina Typic Ustipsamment

Maize, sorghum, and millet monoculture effects on soil properties of a Typic Ustip samment in the semiarid pampa of Argentina were evaluated after 27 years in a plot experiment. A permanent Eragrostis curvula pasture was used as a reference. Millet was the most destructive crop to the soil because it decreased dry aggregate stability by 10%, soil organic matter (OM) by 30%, extractable K by 20%, available P by 44%, inorganic P by 11%, available Fe by 20%, available Zn more than 90%, available Cu by 30%, and available Mn by 26%, and it increased wet aggregate instability by 75%. Grain sorghum was less destructive to the soil than millet because it only decreased OM by 30% and extractable K by 24%. All studied crops decreased the cation exchange capacity of the soil between 20 and 30%, the pH values, and the concentration of soil extractable Mg2+by 38 to 63%. Soil extractable Ca2+ was decreased 30% by the Eragrostis pasture and 40% by maize. The negative effect of millet and grain sorghum on OM was attributed to a low coverage of the soil with plant residues left by these crops as well as the long exposition of the bare soil during temperate and wet periods. Decreases of soil nutrient contents were attributed to plant uptake in all cases, except the Zn concentrations, which were related to variations on soil pH and phosphate concentrations. It was concluded that maize, the most commonly cultivated crop in the region studied, did not affect physical and chemical soil properties to a large extent. Conversely, millet had the most negative effect on physical and chemical properties of the soil.

Tillage effects on soil properties and crop production in the subhumid and semiarid Argentinean Pampas

Crops have been grown in the subhumid and semiarid Argentinean Pampas for over 50 years using tillage methods (intensive plowing) introduced from more humid regions. As a result, soil degradation due to erosion by wind and water has been moderate to severe. Experiments to control erosion through use of conservation systems were initiated at six semiarid and subhumid sites in the Argentinean Pampas between 1976 and 1990 and conducted for 5 to 11 years. In this paper, we summarize the effect of conventional and conservation tillage treatments on some soil properties and crop yields. Most soil physical, chemical, and biological properties were improved by conservation systems, but the rate of change differed due to climatic and soil differences among sites, with the difference due to tillage generally being greater at the more humid sites with loamy soils than at the drier sites with sandier soils. Crops with a large N requirement yielded less with conservation than with conventional tillage systems. Use of conservation tillage can improve or stabilize soil conditions in the region, but crop nutrient requirements must be met to achieve optimum crop yields when conservation tillage is used. Research regarding the plant nutrient requirements under conservation tillage is being conducted. Also, conservation tillage is gaining acceptance in the Argentinean Pampas region.

Relationship Between Apex Developmental Stage, Tiller Density and Yield In Durum Wheat (Triticum durum Desf.)

In the semiarid Pampas of Argentina, durum wheat yields are highly dependent on rain distribution and N availability, and consequently very variable. The aim of this study was to determine if simple phenotypic parameters, evaluated at the spike differentiation stage, which in term are influenced by prevailing environmental conditions, can be used to predict grain and protein yield, with a certain precision at an early enough stage of the crop cycle to allow interventive decisions. In order to assess the influence of the developmental stage of the main stem apex (APEX), the number of tillers per square meter (TILL) and the dry matter production (DMW), upon grain yield, thirteen trials were conducted in the Semiarid Pampas over two years, with two durum wheat cultivars (Buck Cristal and Buck Candisur), two fertilization treatments: unfertilized (YUNF) and 138 kg N ha-1 (YFER) at the end of tillering. Multiple linear regression analysis was performed. Kernel weight, protein content and protein production per unit area were also regressed with APEX, TELL and DMW. For each model variables were selected with the “forward” selection STEPWISE procedure in the BMDP New System computer program package. In the best model, APEX and TILL significantly predicted grain yield of unfertilized plots at the spike differentiation period (R2 = 0.596; p ≤ 0.01). These variables also correlated significantly with kernel weight and protein production, although these results must be considered as preliminary ones.

Contents and weathering state of volcanic ashes and their effects on phosphate sorption in soils of the semiarid pampa of Argentina

Eolian parent materials of soils of the semiarid Argentinean Pampas show high volcanic ash contents. Weathering products of volcanic ashes can sorb anions in many soils. The objective of this study was to analyze the volcanic ash distribution in soils of the semiarid Pampas, its weathering state, and the influence of its weathering products on phosphate sorption. Sixteen soils, classified as Entic Hapludolls and Entic Haplus‐tolls, were studied at different landscape positions of five sites placed in a SW‐NE‐oriented climosequence within the semiarid Pampa. Results showed that volcanic ashes accumulated more in the subsoil of SW‐placed profiles and in the topsoil of NE‐placed profiles. Topsoil accumulated ashes of NE‐placed profiles were deposited by the eruption of Quizapu volcano in 1932. Subsoil accumulated ashes of SW‐placed profiles were deposited by older volcanic eruptions. Acid oxalate soluble Al (Alo), an index of the degree of volcanic ash weathering, was higher in the subsoil of profiles placed at the SW extremity of the sequence. The larger effect of ash age rather than climatic conditions on ash weathering rate was deduced. Allophane contents were low in all soils (<0.8%). Prevailing clay minerals were illite and very low crystallized intergrade illite‐smectites. Soils with the highest volcanic ash content also held the largest amounts of illite‐smectite intergrades. Alo contents explained 52% of sorbed phosphates, and only 36% of fixed P. By including the contents of exchangeable Ca2+ and CaCO3 into the model, 73% of sorbed P variations and 58% of fixed P variations were explained. No measurable effects of volcanic ashes on phosphate dynamics are expected to exist in soils placed to the north of the studied region, but in soils placed to the south, which have high contents of CaCO3 and exchangeable Ca2+, amorphous volcanic ash, weathering products can retain large amounts of phosphates.

Effects of Cattle Ingestion on Viability and Germination Rate of Calden (Prosopis Caldenia) Seeds

Calden (Prosopis caldenia Burkart) is the dominant tree of the xerophytic open forest in the semiarid pampa of Argentina. Calden has gradually increased its distribution throughout the region during the past century as a result of livestock grazing in the pampa forest. Calden has an indehiscent legume fruit that is consumed by livestock during the February-April fruit shedding period (FSP). Both free seeds and pod segments (1 seed inside the endocarp) can be found in cattle dung. Free seeds and pod segments coming from fruits (uningested) and dung (excreted) were collected during the fruit shedding period on 22 February (D1), 7 March (D2) and 27 March (D3) to compare viability and germination rate. Viabilities of uningested free seeds and pod segments averaged 95% and 65%, respectively, and were unaffected (P less than or equal to 0.01) by date of collection. Excreted free seed from D1 was 37% viable, which was lower (P less than or equal to 0.01) than the 72% viable for D2 and D3. Excreted pod segments averaged only 10% viable seeds. Ingestion increased (P less than or equal to 0.01) germination rate of free seed for D1 and D3. However, excreted seed displayed a range of delayed germination response. This diversity would increase the probability of seed germination for a variety of environmental and site conditions.

Agroecosystems' performance in the semi-arid pampas of Argentina and their interactions with the environment

The main objectives of this work were to identify systems which showed different biological and economic performance, to elucidate their interactions with the biophysical and socio-economic environments, and to study some variables associated with varying system performance. Data from 34 farms in the semi-arid pampas of Argentina were analysed by cluster and linear correlation analysis. Farms were classified into two major groups: (1) the cattle production systems in the areas of lower rainfall; and (2) the cattle—crop systems associated with higher rainfall regimes. The procedures adopte were successful in identifying systems of different performance, and in selecting variables with a high degree of association with performance. Ten biological and 14 economic variables were selected to test their degree of association with systems productivity and their sensitivity to variations in the environment. Variables were then grouped into three sets concerned with environmental, ecological and managerial factors, and their relative impacts on systems performance were estimated. Results showed that: (1) systems did not necessarily coincide in terms of biological and economic performance; (2) the level of technology employed was a common factor associated with both biological and economic performance; and (3) the administrative ability farmers appeared to have high degree of association with the economic performance of the systems. A generalised model was proposed to describe the behaviour of systems with different performance, and their interactions with the environment. No clear relationship between sensitivity of systems to the environment and the selected variables was found.