Pablo Zalba’s research while affiliated with Universidad Nacional del Sur and other places

The introduction of fast-growing coniferous species generates major changes in the edaphic phosphorus (P) dynamics. In general, the literature shows decreases in the organic and total P (Po and Pt) and increases in labile inorganic fractions and contradictory results in the associated enzymatic activity. Therefore, the aim of this study was to quantify the impact produced by the introduction of Pinus radiata on different P pools and to evaluate the enzymatic activity of acid phosphatases with respect to adjacent soils supporting natural grassland vegetation. In the Ventania area, six mature P. radiata stands (Forest treatment: F) and six adjacent natural grassland areas (Grassland treatment: Gr) were selected. On the organic horizons of F and the mineral soil of both treatments (0–90 cm), different P fractions like Pt, Po, inorganic P (Pi), occluded P (Poc), extractable P (Pe), soluble reactive P (SRP) and the acid phosphatase activity (APA) were quantified. The results indicated that the pine afforestation generated a new P reserve constituted by the organic horizons rich in Pt (17.2 g m−2), with a high proportion of Po and high concentration of Pe and SRP. In the mineral soil, no differences were detected between treatments for Po, although there were significant increases in Pi and Poc and consequently Pt, under the pines up to a depth of 90 cm. Labile fractions such as Pe and SRP, showed the major alterations associated with the land-use change, with concentrations up to eight times greater for Pe (0–15 cm) and more than10 times greater for SRP (15–60 cm) in relation with the grassland base line. The APA was similar between both ecosystems, however by calculating the APA/OC ratio, a significant decrease in the enzymatic efficiency was detected under the coniferous vegetation. Overall, the results showed that the afforestation with pines on natural grasslands developed on Mollisols produced a significant alteration of the vertical distribution of different pools of P. The increments of different inorganic fractions of P found under the pines, both labile and non-labile, could not be linked to specific extraction zones within the one-meter radius sampled from the trunk. The changes observed in the vertical distribution of different fractions of P indicates a profound alteration in the cycling of this element in afforestation of P. radiata on Mollisols.

The present study looks at the degree of affinity among the humic substances and each of the chemical nutrients essential for plant growth, taking into account macronutrients as well as micronutrients. Topsoil samples from pine woodlands were compared with agricultural soils in the central-south region of Buenos Aires, Argentina. Two extractions of fulvic acids (FA) and one of humic acids (HA) were carried out. Soils from beneath pine trees had lower concentration of Ca, K, and Cu than those under agricultural use, but the latter were poorer in P, Fe and B. Ca, Mg, K, P and S showed a high affinity for FA and also for Mn and Zn, whereas, Fe and B both had a high affinity for HA. The concentration of nutrients was always higher in the fine fraction than in the whole soil sample, thus validating the study of HA and FA in this fraction.

The objectives of the work were: i) Evaluate the amount and distribution of rye and vetch roots as cover crops (CC); ii) Quantify the contribution of N-roots in the profile and their impact on N-nitrate availability in the soil. The study was carried out in two soils: Petrocalcic Paleustoll and Typic Ustipsamment in long duration trials with rotations including CC and sorghum or maize for silage. Treatments were a control without CC (T), rye

Los objetivos fueron: i) evaluar la cantidad y distribución de raíces de centeno y vicia como cultivos de cobertura (CC) y; ii) cuantificar el aporte de N de raíces en el perfil y su impacto sobre la disponibilidad de N-nitratos en el suelo. La experiencia se desarrolló en dos suelos: Paleustol petrocálcico y Ustipsamment típico y en ensayos de larga duración con rotaciones que incluyen CC y sorgo o maíz para silo. Los tratamientos fueron un testigo sin CC (T), centeno (C), centeno fertilizado con 60 kg N ha-1 (C+N), vicia (V) y la consociación vicia-centeno (VC) dispuestos en un diseño en bloques completamente aleatorizados con cuatro réplicas. Al secado de los CC de la campaña 2015 se determinó cantidad, calidad y distribución de raíces hasta el metro de profundidad estratificando cada 20 cm (BRT), biomasa aérea (BA) y N-nitratos en suelo. En el Ustipsamment la biomasa de raíces hasta el metro de vicia (3165 kg ha-1) resultó inferior al centeno solo o con N (4459 kg ha-1). En este suelo la gramínea presentó mayor acumulación de raíces en los primeros 20 cm del perfil (88%) respecto de la leguminosa (62%). En cambio, en el Paleustol no se observaron diferencias entre V y C o C+N, que en promedio produjeron 4195 kg ha-1 de raíces al metro, aunque inferiores a VC (5014 kg ha-1). La acumulación en el horizonte superficial en este suelo fue similar entre especies (57%). En general, la relación BRT/BA en V fue superior a C (0,98 y 0,49, respectivamente). El aporte de N-raíces varió entre 50 y 75 kg ha-1 y se encontró una relación lineal y positiva entre esta variable y el N mineral a partir de los 15-30 días de secados los CC a 0-20 cm, y a partir de los 45-65 días, para el estrato de 20-100 cm dependiendo del tipo de suelo.

Changes in land use systems such as the introduction of trees on agricultural land can give rise to changes in the physicochemical properties of the soils, also affecting the quantity and quality of organic matter incorporated into the system. The present study assesses humic substances in the soil such as humic (HA) and fulvic (FA) acids by looking at the relationship between the optic densities determined at 465 and 665 nm (E4/E6 ratio). Topsoil samples (0–20 cm) from pine woodlands 60 years of age were compared with agricultural soils of similar age in the central-south region of the Province of Buenos Aires, Argentina. The pH of the topsoil from beneath the pine trees was highly acidic (5.0 vs. 6.2) and a significant increase in the level of organic carbon (OC) was registered. The carbon to nitrogen (C/N) ratio was also higher (by one order of magnitude) beneath the pine trees, though the humification conditions of the soil organic matter (SOM) were good in the soils of both studied land use systems. The E4/E6 ratio was higher in the HA and FA 2º (second extraction) beneath the pines, indicating a smaller molecular size of the humic substances bound to the clay minerals. This fact can be attributed to the higher concentration of hydrogen ions beneath the pines and consequently the loss of polyvalent cations, mainly calcium. The most labile organic molecules (FA 1º—first extraction) were of a larger size in soils beneath the pines, most likely owing to a specific characteristic of the Pinus genus, though the fraction in question constitutes a minority fraction among the humic substances. Clear differences were established between the E4/E6 ratios in HA and FA, making this a highly useful method for determining molecular changes in humic substances as a result of changes in land use.

Afforestation is a recommended practice to mitigate global warming. However, their implementation may generate undesirable impacts, mostly if exotic species are used. Plantations of Pinus radiata D Don in Ventania (Bs. As., Argentina) soils showed notorious increments of extractable P (Pe), which could affect the dynamic of this element as well as the degree of phosphorus saturation (GSPBray). The objectives of this study were: i) to quantify the GSPBray in Mollisols afforested with P. radiata comparing the results with those coming from adjacent, natural grassland areas (base line); ii) to evaluate the potential environmental risk induced by afforestation through the identification of a change point (PC) in the GSPBray indicative of a phosphate leaching increment. Treatments included mature stands of P. radiata (TB) and adjacent areas with natural grassland vegetation (TP). Samples were taken at 0-15; 15-30 and 30-45 cm soil depth, and texture, pH, total organic carbon (COT), Pe, soluble reactive phosphorus (PSR), phosphorus sorption index (ISP) and GSPBray were determined. The results showed a significant acidification in TB and an increase in the COT stock, indicating an additional atmospheric CO2 sequestration by the trees. The Pe and PSR values were notoriously higher in TB, and they were reflected in a significant increment in the GSPBray with respect to TP. The detection of a significant PC in the GSPBray-PSR regression indicates higher chances of phosphate leaching in the forest stands, which could reach water courses, lakes and artificial reservoirs promoting their eutrophication. Because of the potential environmental pollution risk of biologic origin derived from the afforestation with P. radiata in Mollisols areas, their inclusion in clean development practices must be reconsidered.

Soil salinity is a worldwide limitation for agriculture productivity and simplified procedures to assess salinity are needed. In this study, an efficient method for estimating electrical conductivity (EC) on a saturation extract (ECex) from readings in saturated soil-paste to assess soil salinity was improved. EC values measured in a saturation extract (ECex) were compared with those estimated from a saturated paste (ECest) in 36 texturally- different samples from a broad array of soils with halophyte and glycophyte vegetation in the semiarid region of Argentina. The ECest values were very close to ECex values up to 8 dS m-1, at greater values, estimates tended to diverge from observed values. Determination of EC in a saturated soil-paste is a simple and rapid procedure resulting in an accurate estimation of EC in the saturated extract as indicated by the high degree of predictability (r2= 0.95; P< 0.01). In addition, the pH measured in the saturated paste was a good predictor of soil pH, and closely approximated the pH determined in a 1:2.5 soil: water mixture.

Seven phosphate-mobilizing pseudomonads were isolated, identified, and characterized in terms of their biofertilizer potential and root-colonizing properties. Pseudomonas protegens (ex-fluorescens) CHA0 was used for comparative purposes. Four isolates (LF-MB1, LF-P1, LF-P2, and LF-P3) clustered with members of the “Pseudomonas fluorescens complex,” whereas the other three (LF-MB2, LF-V1, and LF-V2) clustered with members of the “Pseudomonas putida/Pseudomonas aeruginosa complex.” Assays in buffered liquid growth medium supplemented with tricalcium phosphate enabled the separation of the isolates into two groups: group A (LF-P1, LF-P2, LF-P3, and LF-V1) solubilized P from 151 up to 182 μg mL−1, and group B (LF-MB1, LF-MB2, and LF-V2) solubilized less than 150 μg P mL−1. All isolates displayed acid and alkaline phosphatase activities. With the exception of LF-MB2, all isolates were able to degrade phospholipids from lecithin. Additionally, all isolates exhibited extracellular protease activity, and four isolates produced hydrogen cyanide, two traits that are related to biocontrol of phytopathogens. To study root colonization in non-sterile soil, isolates were doubly tagged with gfp and a tetracycline resistance cassette. After 15 days of competition with the indigenous bacterial flora, all tagged isolates colonized soybean roots at counts ranging from 7.6 × 105 to 1.7 × 107 CFU g−1. The results indicate that there are already efficient phosphate-mobilizing pseudomonads adapted to agricultural bulk soils under no-till management in Argentina and thus having excellent potential for use as biofertilizers.

Afforestation of grasslands can increase C sequestration and provide additional economic and environmental benefits. Pine plantations, however, have often been found to deplete soil organic C and trigger detrimental effects on soils. We examined soil characteristics under a 45-year-old Pinus radiata stand and under adjacent grassland on maritime dunes in temperate Argentina. Soil under the pine plantation had greater soil organic C (+93%), total N (+55%) and available P (+100%) concentrations than under grassland. Carbon was stored under the pinestand at an estimated mean accretion rate of 0.64 Mg ha y. At 0- to 25-cm depth, soil C amounted to 61 Mg ha under pine and 27 Mg ha under grassland. Soil C accumulated more on dune slopes (35 Mg ha y) than on ridges(29 Mg ha y) and bottoms (12 Mg ha y). Compared with the grassland, soil acidity, cation-exchange capacity, base losses (K > Ca = Mg) and C/N ratio increased under pine. Spatial heterogeneity in soil characteristics was greater under pine than under grassland. Such variability was non-systematic and did not support the ‘single-tree influence circle’ concept. Afforestation increased C in soil, forest floor and tree biomass in dunes with ustic climate regime.