Southeastern Pampas Research Articles

Nitrogen deficiency in maize fields of the Southern Pampas does not affect kernel number but reduces weight per kernel.

Nitrogen (N) requirement of maize has been increasing over the past decades, but there has been a lack of adjustment in fertilization management practices across different regions in Argentina. The higher N requirement in modern hybrids is associated with a higher post-flowering N accumulation. Additionally, soil fertility in the Argentina Pampas has consistently decreased in recent years. Whether the effects of N deficiency are expressed in the post-flowering period is still unclear at field level. The objective of this work was to quantify the yield reduction attributed to N deficiency and to identify the physiological basis of this effect at the field level for typical high-tech farms across the south-eastern Pampas of Argentina. Nineteen experiments were conducted during the 2020/2021 growing season in the south-eastern Pampas, evaluating two N managements i) Farmers' N level; ii) High N input. Results showed that the additional N input increased grain yield by an average of 6.4% (average grain yield difference of 712 kg ha−1). Weight per kernel was the only yield component that responded to N fertilization. Farmers' N management did not affect N accumulation or growth-related variables such as leaf area index, green leaves per plant, PAR interception, nitrogen sufficiency index until flowering, but the farmers' N level reduced these variables during the grain filling period leading to differences in aerial biomass and N accumulated at physiological maturity. Farmers' N level led to decreased post-flowering N uptake during grain filling period, which reduced the photosynthetic capacity of the canopy. These findings suggest that N management strategies and models need to be reviewed for high-tech maize systems.

Occurrence of chlorpyrifos and organochlorine pesticides in a native bumblebee (Bombus pauloensis) living under different land uses in the southeastern Pampas, Argentina

Pollinators such as Apidae bees are vital for ecosystems and food security. Unfortunately, their populations have declined due to several factors including pesticide use. Among them, the organophosphate insecticide chlorpyrifos, poses a global threat, while legacy compounds like organochlorine pesticides (OCPs) easily bioaccumulate, increasing the concern. Bombus pauloensis, a widely distributed native bee in Argentina, is used for commercial pollination; however, information regarding their health status is scarce. This study assessed chlorpyrifos and OCP levels in B. pauloensis (workers and males) and related environmental matrices living from three different land uses schemes, by means of GC-ECD and GC–MS. The ornamental horticulture field (OP) showed the highest total pesticide concentrations in workers (13.1 ng/g), flowers and soils, whereas the organic agriculture field (OA) exhibited the lowest. Chlorpyrifos was the most abundant compound, accounting for at least 20 % of pesticide load across all matrices. The food production horticulture field (FH) had the highest chlorpyrifos concentration in workers, males and soils (5.0, 4.4 and 3.3 ng/g, respectively), suggesting a local greater usage, whereas OA showed the lowest. Regarding OCPs groups, Drins and DDTs were predominant in most matrices, with FH males registering the highest levels (4.0 and 2.5 ng/g, respectively), closely followed by OP. However, metabolites' contribution indicated historical use and atmospheric inputs in all sites. Multivariate analyses confirmed the significance of site and bumblebee sex to explain pesticide composition. Males from all sites exhibited higher chlorpyrifos levels than workers and this trend was similar for some OCP groups. Overall, OA differed from FH and OP, indicating a correlation between production modes and pesticide profiles. This study demonstrates the value of B. pauloensis as a pesticide biomonitor but also offers insights into its populations' health in the area. In this sense, this information could be useful towards the preservation of this crucial pollinator.

Location matters: survival of artificial nests is higher in small grassland patches and near the patch edge

Nest survival is an important part of breeding success in grassland ecosystems, and the location of nests can determine vulnerability to different predators. We conducted an experiment with artificial nests to evaluate jointly the predation rate on nests at different spatial scales (landscape, patch and tussock) and the relative abundance of potential nest predators (small mammals and birds) in a temperate grassland area. In November 2014 and 2016, we installed 288 artificial nests in Common Pampas Grass Cortaderia selloana grasslands in the southeastern Pampas region, Argentina. The nests were placed in two 10‐ha plots in a continuous grassland patch (c. 900 ha) within a reserve and in two small grassland patches (1.5 and 1.8 ha) in an agricultural matrix (landscape‐scale), at the patch edge and inside the patches (patch‐scale), and at two heights within the tussock grass (tussock‐scale). In 2016, we also conducted live trapping of small mammals and surveyed birds along strip transects at the sampling sites. Nests located in patches within an agricultural matrix and near the edge had greater relative survival than those set in the reserve and inside the patches, respectively. This might be explained by the lower relative abundance of small mammals that we found outside the reserve. Artificial nest survival values recorded at the landscape‐scale contrasted with those previously observed for natural nests. Our results could be partly explained by differences in nest density between agro‐patches and those within the reserve. Future studies could also evaluate the role of parental nest defence on nest survival.

Correspondence between thermal biology and locomotor performance in a liolaemid lizard from the southeastern coastal Pampas of Argentina

The behavioral and physiological mechanisms of thermoregulation and the morphological traits of lizards result in a particular range of body temperatures, which influence performance and ultimately fitness. We studied the thermal biology and locomotor performance of the lizard Liolaemus wiegmannii from the coastal dunes in the southeastern Pampas of Argentina. During the austral summer, we examined the link between thermoregulation and optimal locomotor performance. Liolaemus wiegmannii faced a stressful environment due to high risk of overheating; despite this, the species was able to achieve field body temperatures (Mean Tb ± SD = 35.58 ± 2.86 °C) than expected by chance (i.e., the null model) and suitable for sustaining its physiological performance. Locomotion in this species was thermally-sensitive, with lizards showing high-performance bouts at a relatively wide range of body temperatures (30–38 °C). Lizards exhibited a mean maximum running speed of 1.30 m/s at 37.3 °C (i.e., optimal temperature for locomotion) which was within the set point range of preferred temperature (Tset = 35.4–37.5 °C). Therefore, we found a correspondence between thermal optimum and preferred temperature. Our findings suggest that L. wiegmannii, like other lizard species with a broad distribution, is capable of performing well across a wide range of temperatures despite the spatiotemporal thermal fluctuations of the environment.

Forage yield, water- and solar radiation-productivities of perennial pastures and annual crops sequences in the south-eastern Pampas of Argentina

Livestock production systems of Argentina show an ongoing process of change in the composition of their forage base, with a gradual increase in the proportion of their area assigned to forage crop sequences (FCS) −in particular that involving successive winter and summer annual forage crops–, at the expense of the area assigned to perennial pastures (PP). However, there are several concerns regarding the efficient use of available environmental resources due to differences in the extent of the establishment periods and due to the occurrence of fallow periods in FCS, as well as regarding the species involved in these systems. Therefore, the main objective of this study was to evaluate the water (WP) and solar radiation productivity (RP) of PP [pure stands of alfalfa (Medicago sativa L.), fertilized and unfertilized stands of tall fescue (Festuca arundinacea Schreb.), and fertilized and unfertilized mixed stands of alfalfa/tall fescue] and FCS [including a summer alternative and oats (Avena sativa L.); the summer alternative was either maize (Zea mays L.) or intercropped maize/soybean (Glycine max L.)] through the analysis of their components, i.e. water (WC) and radiation capture (RC), and their water (WUE) and radiation use efficiency (RUE). Resource capture (WC and RC) was defined as the ratio between captured (evapotranspiration and intercepted solar radiation) and annual available resource (rainfall and incident solar radiation). Resource use efficiency (WUE and RUE) was computed as the dry matter (DM) yield per unit of captured resource. A field experiment was conducted during two consecutive years under rainfed conditions and under non-limiting nitrogen availability in the south-eastern Pampas of Argentina. Perennial pastures were evaluated during both their initial establishment year (Y1) and when already fully established (Y2). Annual-based forage DM yield ranged between 13.0 and 32.4 Mg ha−1 in Y1 and between 5.1 and 23.5 Mg ha−1 in Y2. Differences in resource capture between PP and FCS were directly associated with the crop establishment and fallow periods depending on the nature of the considered resource. The highest WP (2.75–3.64 g DM m−2 mm−1) and RP (0.93–1.42 g DM MJ−1) were reached by FCS, which did not differ (P > 0.05) from that of fully established fertilized PP. Our results demonstrate that resource productivity in FCS is not necessarily higher than for PP, once PP are fully established.

El Holoceno Medio en valles fluviales del sudeste pampeano: ¿Escasez de sitios o de unidades portadoras? Una perspectiva geoarqueológica

En este trabajo se presenta el estudio estratigráfico y cronológico de siete sitios arqueológicos localizados en las cuencas de los arroyos Cortaderas, Cristiano Muerto y Tapalqué (provincia de Buenos Aires, Argentina). El objetivo es evaluar si la ausencia y/o baja representación de unidades estratigráficas del Holoceno medio en estos cursos de bajo orden está condicionando la señal arqueológica en el sudeste pampeano. Los resultados indican que los depósitos fluviales correspondientes a este período resultan escasos y que, con frecuencia, existen discordancias erosivas entre las unidades datadas en el Holoceno tardío y aquellas correspondientes al Pleistoceno tardío u Holoceno temprano sobre las que apoyan. Este hecho también se observa en sitios localizados en otros cursos fluviales de bajo orden del sudeste pampeano. En consecuencia, la baja señal arqueológica del Holoceno medio en muchos valles puede deberse, al menos en parte, a este sesgo estratigráfico. Adicionalmente, los suelos/paleosuelos desarrollados en estas secuencias fluviales concentraron la evidencia cultural, lo cual genera una imagen de pulsos de ocupación humana y de hiatos arqueológicos que podría considerarse principalmente un resultado de la dinámica fluvial. Tal panorama estratigráfico en los cursos de bajo orden no sólo es válido para la región analizada sino que posee implicaciones para secuencias fluviales de numerosos valles en Argentina.

Environmental Challenges to Adopt Climate-Smart Agriculture for Cereals Cultivated in the South Eastern Pampas of Argentina

Environmental Challenges to Adopt Climate-Smart Agriculture for Cereals Cultivated in the South Eastern Pampas of Argentina

Vertical distribution of root biomass and soil carbon stocks in forage cropping systems

Forage cropping systems may differentially affect the vertical distribution of roots and soil organic C stock (C-OM). In annual crops sequences (ACS) and perennial pastures (PP), we assessed the association between root biomass, C-OM, C in mineral-associated organic matter (C-MAOM), and C in particulate organic matter (C-POM) and its vertical distribution. Root biomass, C-OM, C-MAOM and C-POM were measured in a Petrocalcic Argiudol at five soil depths up to 100 cm during two years in the south-eastern Pampas of Argentina. The field experiment comprised 28 plots including five perennial pastures and two annual crop sequences. Associations between variables were assessed by regression analysis and non-linear models. Overall, ACS and PP had similar cumulative root biomass. Both, root biomass and soil C stocks exponentially decreased with soil depth. Soil C stocks associated to root biomass tended to stabilize over a threshold value of root biomass for C-MAOM and C-OM. C-POM tended to stabilize over a threshold value only in treatments that included tall fescue. Our results highlights the key role of roots to improve soil C stocks through the design of forage crop rotations that include crops able to increase root inputs to the soil, such as tall fescue.

Landscape genetics in the subterranean rodent Ctenomys “chasiquensis” associated with highly disturbed habitats from the southeastern Pampas region, Argentina

Studies of genetic differentiation in fragmented environments help us to identify those landscape features that most affect gene flow and dispersal patterns. Particularly, the assessment of the relative significance of intrinsic biological and environmental factors affecting the genetic structure of populations becomes crucial. In this work, we assess the current dispersal patterns and population structure of Ctenomys "chasiquensis", a vulnerable and endemic subterranean rodent distributed on a small area in Central Argentina, using 9 polymorphic microsatellite loci. We use landscape genetics approaches to assess the relationship between genetic connectivity among populations and environmental attributes. Our analyses show that populations of C. "chasiquensis" are moderately to highly structured at a regional level. This pattern is most likely the outcome of substantial gene flow on the more homogeneous sand dune habitat of the Northwest of its distributional range, in conjunction with an important degree of isolation of eastern and southwestern populations, where the optimal habitat is surrounded by a highly fragmented landscape. Landscape genetics analysis suggests that habitat quality and longitude were the environmental factors most strongly associated with genetic differentiation/uniqueness of populations. In conclusion, our results indicate an important genetic structure in this species, even at a small spatial scale, suggesting that contemporary habitat fragmentation increases population differentiation.

Compositional Fidelity and Taphonomy of freshwater Mollusks from Three Pampean shallow Lakes of Argentina

Abstract. Three shallow lakes from the southeastern pampas, which are different in their physicochemical characteristics, were studied with the aim of comparing live/dead fidelity and preservation of mollusk shells. The studied lakes were Las Mostazas, Los Carpinchos and Nahuel Ruca, which proved different in terms of their specific richness, abundance, compositional fidelity and taphonomic alteration. The lake that presented the higher values of conductivity (3.34±0.72), hardness (286±83 mgl-1) and carbonates saturation index (SIcalcite;= 1.95±0.21) was the one for which lower specific richness in living assemblages was recorded and differences in richness and diversity between the living and the dead assemblages became apparent. It was also the lake where the number of species was higher in death assemblages (R= 5) than in living communities (R= 2). Such characteristics suggest a higher residence time in the taphonomically active zone. On the other hand, the lakes featuring lower values of the same char...

Population genetic structure and historical dispersal patterns in the subterranean rodent Ctenomys “chasiquensis” from the southeastern Pampas region, Argentina

Gene flow plays an essential role in the evolutionary history of the organisms and helps to identify those historical landscape features that most likely had affected the dispersal patterns. In this work, we assess, using mitochondrial control region DNA sequences, the historical migration patterns and population structure in Ctenomys “chasiquensis”, a highly, vulnerable and endemic subterranean rodent distributed in a very small area from the central part of Argentina. We used Bayesian and maximum likelihood approaches to evaluate the effects of historical gene flow among populations. Moreover, we used Bayesian skyline plots, tests of neutrality and mismatch distributions to assess the potential changes in population size through time. Our analyses show that populations of C. “chasiquensis” are moderate structured at regional level and this population pattern is probably the result of an asymmetric historical gene flow essentially from the South-West to the North-West, further of a recent demographic population expansion in the North-West, in conjunction with an important degree of isolation in some populations over its eastern geographical range. Evidently, historical gene flow seems to have been more frequently on the West. Finally, a close relationship appears to exist between the major climatic episodes occurred during the Late Quaternary in the central region of Argentina and the main historical demographic changes inferred for C. “chasiquensis”. The current distribution of C. “chasiquensis” appear to be a relicts of a more extended historical distribution in the Argentinean Pampas in the Late Pleistocene, with a perceptible population decline at the beginning of the Holocene. In this context, Bayesian demographic inferences showed a small but constant increment of population expansion of this species from approximately 90,000 to 11,000 years BP, after which a period of decrease in population size (that started in the early Holocene and continues nowadays) was observed.

Climate downscaling over southern South America for present-day climate (1970-1989) using the MM5 model. Mean, interannual variability and internal variability

This work focuses on evaluating the ability of the MM5 regional model to represent the basic features of present climate over South America. The spatial distribution of seasonal means and the interannual variability, as well as annual cycles for precipitation and near-surface temperature have been evaluated. The internal variability has also been investigated. The analysis has two objectives: one of them is to quantify the dynamic downscaling ability to represent the current climate and the other is to identify critical aspects of the regional climate model in South America in order to interpret the reliability of future projections for the end of the twenty-first century in the A2 scenario of the IPCC Special Report on Emissions Scenarios. In general, the MM5 model is able to reproduce adequately the main general features, seasonal cycle and year-to-year variability of near surface variables over South America. The spatial distribution of temperature is well represented, but some systematic errors were identified, such as an overestimation in central and northern Argentina and an underestimation in the mountainous regions throughout the year. The general structure of precipitation is also well captured by the regional model, although it overestimates the precipitation in the Andean region (specifically in central and southern Chile) in all seasons and underestimates the rainfall over tropical latitudes. The annual cycle of precipitation is adequately represented in the subregions analyzed, but its representation is better over La Plata basin (LPB), Cuyo (CU) and southeastern Pampas (SEP). The annual cycle of mean temperature is well represented, too. The model systematically overestimates the interannual variability of temperature and underestimates the interannual variability of precipitation. From the analyses of interannual and internal variability, as well as the biases, it can be concluded that regardless the season, the simulated precipitation is reliable at subtropical latitudes, Uruguay, southern Brazil and east-central of Argentina, but is less reliable over areas of complex topography. For temperature, the regional model is reliable over subtropical latitudes, Uruguay and the south of Brazil only during winter, but it is less reliable or it is even in the limit of reliability over central and southern Chile all along the year. Therefore, it is concluded that the MM5 model is a useful tool for the generation of regional climate change scenarios and for the evaluation of regional climate change scenarios over southern South America.

Modelling long-term effects of cropping intensification reveals increased water and radiation productivity in the South-eastern Pampas

Wheat/soybean double crop provides a reliable platform for cropping intensification in many subtropical and temperate areas, even in those with a short growing season as the South-eastern Pampas of Argentina. However, the long-term impact of double cropping as part of feasible cropping sequences on resource productivity and the whole sequence performance is unknown. We propose that cropping intensification, based on wheat/soybean double cropping would (i) improve the annual water and radiation capture and productivity, and (ii) reduce unproductive water losses estimated on an annual basis. We tested these hypotheses through long term simulations (30 years), using DSSAT models locally calibrated and tested for wheat (W) (Triticum aestivum L.), soybean (S) (Glycine max [L.] Merr.) and maize (M) (Zea mays L.). Intensification was quantified with the index ISI=number of crops in rotation/duration of rotation. Pairs of sequences with similar crop composition but different degree of intensification were compared, i.e. W–S (ISI=1yr−1) vs W/S double crop (ISI=2yr−1), W–S–M (ISI=1yr−1) vs W/S–M (ISI=1.5yr−1) and W–S–M–S (ISI=1yr−1) vs W/S–M–S (ISI=1.33yr−1). The study also included feasible or traditional rotations of our region. The increase in intensification improved annual resource capture and therefore water and radiation productivity. Proportion of maize in sequences, irrespective of ISI, additionally increased resource productivity by increasing both water use efficiency (WUE) and radiation use efficiency (RUE). Across sequences, WUE and RUE were strongly associated. This correlation was involved in the link between water and radiation productivity. The increase in water productivity was related (P<0.0001) to a reduction in water loss, mainly accounted by runoff. In the long term, sequences with high intensification (ISI≥1.5yr−1) had as high stability and productivity as traditional sequences of our region, based on wheat-summer crop with ISI=1yr−1. Overall, wheat conferred stability to the sequences whereas maize conferred productivity. Our study, accounting for the ability of whole cropping systems to capture resources on an annual basis, gives new tools to develop more efficient and sustainable cropping sequences.

Breeding Biology of Spectacled Tyrant (Hymenops perspicillatus) In the Southeastern Pampas Region, Argentina

The Spectacled Tyrant (Hymenops perspicillatus) is a flycatcher that inhabits open lands, grassy areas near water bodies, marshes, fields and pastures in southern South America. Our aim was to describe the breeding biology of Spectacled Tyrants in native tall grasslands, specifically by studying their reproductive phenology and breeding success. The study was developed in the southeastern Pampas region, Argentina. We conducted nest searching and monitoring during the 2010–2011 and 2011–2012 breeding seasons. We found 88 nests. The breeding season was from mid-October to late January. Clutch size was 2–3 eggs, mode = 2. Incubation and nestling periods were 15.8 ± 0.7 days and 14.7 ± 0.6 days, respectively. Nest predation was higher during the incubation period than during laying and nestling periods. The probability of nest success from egg-laying to fledging was 0.24. This species shows life-history traits in accordance with other South American temperate birds.

Delineation of management zones with measurements of soil apparent electrical conductivity in the southeastern pampas

Peralta, N. R., Costa, J. L., Balzarini, M. and Angelini, H. 2013. Delineation of management zones with measurements of soil apparent electrical conductivity in the southeastern pampas. Can. J. Soil Sci. 93: 205–218. Site-specific management demands the identification of subfield regions with homogeneous characteristics (management zones). However, determination of subfield areas is difficult because of complex correlations and spatial variability of soil properties responsible for variations in crop yields within the field. We evaluated whether apparent electrical conductivity (ECa) is a potential estimator of soil properties, and a tool for the delimitation of homogeneous zones. ECamapping of a total of 647 ha was performed in four sites of Argentinean pampas, with two fields per site composed of several soil series. Soil properties and ECawere analyzed using principal components (PC)–stepwise regression and ANOVA. The PC–stepwise regression showed that clay, soil organic matter (SOM), cation exchange capacity (CEC) and soil gravimetric water content (θg) are key loading factors, for explaining the ECa(R2≥0.50). In contrast, silt, sand, extract electrical conductivity (ECext), pH values and [Formula: see text]-N content were not able to explain the ECa. The ANOVA showed that ECameasurements successfully delimited three homogeneous soil zones associated with spatial distribution of clay, soil moisture, CEC, SOM content and pH. These results suggest that field-scale ECamaps have the potential to design sampling zones to implement site-specific management strategies.

Environmental factors affecting yield variability in spring and winter rapeseed genotypes cultivated in the southeastern Argentine Pampas

Rapeseed yields in Argentina are low (averaging 1400kg/ha nationwide) with a high inter-annual variability. One of the limiting factors for improving yields is the lack of information on the adaptability of the cultivars, especially in the main rapeseed-producing area, the southeastern Pampas. The objectives of this study were to (i) quantify and analyze the yield variability of winter and spring rapeseed hybrids introduced in Argentina, (ii) identify the main environmental factors that affect the yields of the spring and winter genotypes in the southeastern Pampas, and (iii) model and validate rapeseed yields from environmental variables in the pre- and post-flowering periods. Principal component analysis (PCA) and linear regression methods were used to analyze 129 data points from 16 comparative yield trials in eight sites of southeastern Pampas. The rainfed crops were sown between April and July and from 2007 to 2009. Pre- and post-flowering phases were recorded in each experiment; temperature, frost occurrences, rainfall and radiation were measured during each phase. Yield variability (600–3700kgha−1) was slightly lower in spring than in winter genotypes (CV 0.25 versus 0.38). Sixty percent of the winter genotype variability was explained by the first axis which was associated to the pre- and post-flowering durations, while 25% of the variability was explained by the second axis associated to yield. Almost 50% of the spring genotype variability was explained by the first axis associated to pre-flowering and total durations, while 27% of the variability was explained by the second axis in which post-flowering duration was associated to yield. Winter genotypes evidenced vernalization requirements that were either partially or not fulfilled, so, the longer the photoperiod, the longer the pre-flowering phase duration. In the critical period of 30d post-flowering, yield was not associated to the photothermal quotient. In winter genotypes, yield was associated to a linear model which included rainfall during the crop cycle, radiation and pre-flowering temperatures (R2=0.50). The model was adequately validated with independent data (n=116) from official trials. For spring genotypes, only the frost occurrences during the critical period were relevant (R2=0.26) and placing the flowering time after October decreased the risk of late frost damage. Water use efficiency (WUE) values ranged from 1.6 to 6.7kgha−1 per mm of rain without a clear trend between spring and winter genotypes for this trait. In conclusion, winter genotypes did not necessarily yield more than the spring materials. In addition, rainfall during the crop cycle and frost occurrences during flowering were the main limiting factors of the winter and spring genotype yields, respectively, in the southeastern Pampas.

COMPOSITIONAL FIDELITY OF SUBFOSSIL MOLLUSK ASSEMBLAGES IN STREAMS AND LAKES OF THE SOUTHEASTERN PAMPAS, ARGENTINA

The compositional fidelity of dead assemblages—the extent to which subfossil remains reflect the composition, structure, and scale of the original living community—is an indispensable component of studies assessing the quality of the fossil record and the effects of taphonomic biases on species composition and diversity. The aim of the present study is to evaluate (1) how faithfully abundances of subfossil freshwater mollusk shells deposited in dead assemblages correspond to their abundances in the local living assemblages, (2) how reliably diversity of living assemblages is captured by dead assemblages, (3) whether spatial variation in species composition in living assemblages is captured by dead assemblages in the southeastern Pampas, Argentina. Twenty sites corresponding to five lotic and five lentic sites from two geographic areas were analyzed in terms of taxonomic composition and diversity, and homogeneity of multivariate dispersions. We find that (1) living and dead assemblages occupy similar portions of multivariate space and do not significantly differ in composition within an area, (2) living and dead assemblages show similar richness and evenness, and (3) between-environment differences in among-site variation in composition (beta diversity) captured by living assemblages are preserved by dead assemblages. Thus, the results highlight the potential these environments have to enhance the preservation of mollusks and result in fossil assemblages that are suitable for paleoecological and paleoenvironmental studies of freshwater ecosystems.

Diversity, Seasonality and Structure of Bird Assemblages Associated with Three Wetlands in the Southeastern Pampas, Argentina

Los Padres, La Brava and Nahuel Ruca lakes are typical wetlands of the Pampean region. These three shallow lakes share certain limnological features (size, mean depth, littoral macrophytes, among others) and the land usage in their basins (e.g. agriculture, cattle ranching and/or touristic activities). We surveyed the bird community at these three wetlands over three years (May 2006-May 2009), using transects and point counts to characterise their species richness, abundance, community composition and seasonality. We recorded a total of 135 species belonging to 41 families. The Relative Importance Index showed that the neotropic cormorant Phalacrocorax olivaceus, the cattle egret Bubulcus ibis, the white-tufted grebe Rollandia rolland and the white-faced ibis Plegadis chihi are the most representative species. Species richness, species composition and abundance differed between wetlands. Such differences could be attributed to particular factors such as proximity to urban centres or other wetlands, and to the impact of recreational activities. Considering that the Pampean region is currently under strong anthropogenic impacts, the present study contributes to the improvement of management plans that are currently in place or under development.

Alternatives for Nitrogen Diagnosis for Wheat with Different Yield Potentials in the Humid Pampas of Argentina

A correct determination of nitrogen (N) fertilization thresholds in wheat that is based on objective yield produces efficient use of this nutrient. Nitrogen fertilization recommendations for traditional wheat require determination of nitrate (NO3 −)-N availability at 60 cm deep at planting time. However, this methodology is complicated, expensive, and time-consuming; thus, the determination of NO3 −-N level at a lesser depth and at a different time would be desirable. The goals of this work were to determine available N in soil thresholds for traditional and French germplasm wheats and the feasibility of diagnosing N requirements by measuring NO3 −-N at 40 cm deep, at planting or tillering times, in the southeastern Pampas. The experiments were factorial combinations of N rates and fertilization times (planting and tillering) at different sites and years during 2002–2006. Nitrogen fertilization significantly increased grain yield and protein content. French varieties presented greater grain yield (23%), lower protein content (11%), and greater yield per N unit, indicating greater N-use efficiency (NUE) than traditional varieties. A similar relationship was determined between grain yield and available N at both sampling depths. This might be explained by the strong association between NO3 −-N content at 60 and 40 cm deep at both sampling dates. Maximum yield and available N determined at 60 or 40 cm soil deep showed that thresholds were lower for tillering than for planting, regardless of the genotype (152 and 174 kg of available N, respectively). Available N thresholds for 95% of maximum yield were less at 0–40 cm deep than at 0–60 cm deep (10 and 14 kg N ha−1 for traditional and French genotypes, respectively). The results of this experiment suggest the possibility of diagnosing N requirements for wheat by measuring NO3 −-N content at 40 cm deep, instead of the usual 60 cm, for both traditional and French genotypes.

Use of RothC to simulate changes of organic carbon stock in the arable layer of a Mollisol of the southeastern Pampas under continuous cropping

Soil organic matter (SOM) is a key soil component that determines soil quality and health, but its dynamics is driven by complex processes. RothC is a simple model that can be used to simulate SOM content changes in order to plan and recommend soil management practices. The aim of this work was to validate RothC with SOM stock information from two long term experiments carried out at Balcarce, Argentina (37°45′S, 58°18′W) on a complex of fine, mixed, thermic Typic Argiudoll and fine, illitic, thermic Petrocalcic Paleudoll. Yearly soil organic C (SOC) stocks of the arable layer under nineteen different crop sequences with and without N fertilizer and under conventional tillage, were used. Carbon input through crop residues was estimated from grain yield or aboveground biomass production. Model performance was evaluated through several statistical tools: simple least square linear regression of observed vs. simulated values, several indicators based on the difference between observed and simulated values, and the fitting of observed and simulated values to an exponential model as a function of time under cropping. The overall performance of RothC was very good since mean difference between observed and simulated was lower than 2 Mg C ha −1 (2% of the mean observed value), and the root mean square error (RMSE) was lower than RMSE at 95% probability (RMSE95%). Approximately 70% of the relative differences between observed and simulated (RE) were within the range −5% ≤ RE ≤ 5% without identifiable trends associated with the sequences. Besides, all RMSE's corresponding to the crop sequences were lower than RMSE95%. However, RothC tended to underestimate SOC stocks especially when N had been applied and at high levels of SOC stock. Despite there was no statistical difference ( P > 0.05) between the models resulting from fitting observed and simulated values to the exponential model, estimated rates of change of SOC stock along time were different between models. This indicates that simulation was poorer in the short term after a pasture (higher SOC stocks) than in the long run. Inaccurate estimation of C input could have been the cause of most of the differences observed between simulated and observed values. We concluded that RothC can be used as a tool to contribute to make recommendations of crop rotations to manage SOC stock for Mollisols of temperate regions under conventional tillage, provided more careful and precise C input estimations were available.