Pablo Zalba

THE IMPACT OF SINGLE TREES ON PROPERTIES OF LOESS‐DERIVED GRASSLAND SOILS IN ARGENTINA

The effect of single-tree influence areas on the physicochemical properties of the soil surface mineral horizon (0–18 cm) was studied in three stands of Pinus radiata D. Don introduced into Sierra de la Ventana, Argentina, grasslands 50 yr ago. Soil samples were taken at distances of 0, 1, and 2 m in transects from the tree to the periphery of the crown. Adjacent grassland soils with mollic epipedons were used as controls. Soil alteration was found to be highest near the trunk, with clear evidence of acid hydrolysis of primary silicates; the epipedon close to the trunk was classified as umbric rather than mollic. Decreasing values of pH, Ca, and exchangeable Mg, and increasing values of exchangeable H and Al, and also of fulvic acid-complexed Al, were registered from the grassland toward the axes of the trees. The soil properties analyzed fall into a distinctive spatial pattern of radial symmetry around each individual tree, with systematic and predictable variations, thus confirming the validity of the concept of “single-tree influence circles” for the study area. Each stand of P. radiata generates a patch of soil alteration within the undisturbed habitat matrix; the internal structure of these patches shows a radial pattern of different polypedons spatially and genetically associated with the inner ring of bark litter and the outer ring of leaf and twig litter. The present work shows that the introduction of P. radiata triggered changes in the evolutionary trend of the soils of such magnitude as to be reflected at the highest taxonomic level in soil taxonomy.

Effect of clay minerals and organic matter on the cation exchange capacity of silt fractions.

The cation exchange capacity (CEC) and specific surface properties were investigated in four particle-size fractions < 50 μm from three loess (one Kastanozem and two Phaeozems), a holocene (Fluvisol) and a basalt soil (Nitisol) before and after destruction of organic matter. Particle-size fractions were separated by sedimentation after chemical and physical dispersion of the soil samples. Illite, amorphous minerals, mixed layers, smectite and kaolinite were the predominant clay minerals. They were detected in all size fractions. The CEC increased with increasing organic matter contents and this effect was more pronounced in coarser fractions. The organic matter content per unit surface area was two or three times larger in coarse silt than in clay, irrespective of the soil type.

Modified Soil‐Test Methods for Extractable Phosphorus in Acidic, Neutral, and Alkaline Soils

Abstract Although numerous soil‐test methods for estimating extractable phosphorus (P) have been developed around the world, their results are difficult to compare because of the very different scale levels used. In the present study, the Bray–Kurtz method (Bray‐P) is used as a reference value. Two other methods [lactate‐P and sodium bicarbonate (NaHCO3)‐P] were modified to facilitate the comparison of extractable‐P determinations, mainly by adjusting the shaking time. These three methods were applied to 101 soil samples from an extensive region of Argentina with soil pH values ranging from 5.5 to 8.5. The results confirm that the Bray‐P and the two modified methods (lactate‐P and NaHCO3‐P) determine similar contents of extractable P but are not applicable to all types of soils and conditions. Equations that minimize the statistical error were selected for soil properties such as organic carbon (OC) content, pH, soluble salts, and calcium carbonate content. Correlation coefficients between Bray‐P and NaHCO3‐P increased to 0.91 and 0.95 in soils with high and low OC levels, respectively. It was also demonstrated that the lactate‐P test is not suitable for soils rich in calcium carbonate or soluble salts. These two modified methods are expected to be useful for testing P values that impact agricultural production.

A rapid method for estimating the carbon concentration of fulvic acids

Abstract A simple method was developed for estimating the carbon concentration of fulvic acid from soil extracts at pH 2. Visible absorbance (410 nm) was measured in 20 samples of fulvic acids from different regions of Argentina. Linear regression analysis of absorbance as a function of carbon concentration yielded a high correlation coefficient (r = 0.983∗∗). The proposed method is expected to be useful in assessing changes in organic matter content as a result of different tillage systems, erosion, plant residues, etc.

Soil Humic and Fulvic Acids from Different Land-Use Systems Evaluated By E4/E6 Ratios

ABSTRACT 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 (HS) in the soil such as humic acid (HA) and fulvic acid (FA) 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, although 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 HS 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, although the fraction in question constitutes a minority fraction among the HS. Clear differences were established between the E4/E6 ratios in HA and FA, making this a highly useful method for determining molecular changes in HS as a result of changes in land use.

Comparative effects of sodium and potassium salts on the physicochemical conditions of soils

Abstract Though it is commonly held that sodium (Na) rather than potassium (K) salts affect soil properties, in some treatmets the opposite has been found to be the case. The present paper describes soil trials carried out to determine changes in organic matter and colloidal content, pH, cation exchange capacity, and exchangeable cations as a consequence of leachings with Na and K salts. It was possible to classify salts into weakly and strongly alkaline according to their effect on soil properties. On the basis of the effect of K on soil properties, it is concluded that this salts should be included in soil salinity classifications.

PHOSPHORUS CONTENT IN SOIL IN RELATION TO FULVIC ACID CARBON FRACTION

The fulvic acid fraction is considered an important constituent of soil organic matter and is sensitive to management regimes and tillage impact. Both humic acids (HA) and fulvic acids (FA) are able to form metal complexes which bind appreciable amounts of phosphate or phosphorus (P) species. The aim of the present study is to evaluate the fulvic acid carbon content of agricultural soils in Argentina and to explore the possible relationship between acid carbon (FC) content and the level of available P in the soil. A total of 74 surface samples of Hapludolls and Haplustolls covering areas with decreasing levels of annual rainfall were analyzed. It was found that soils with FC content >5.5 g kg−1 exhibited available P levels >30 mg kg−1, whereas available P levels <20 mg kg−1 were related to FC contents <4.0 g kg−1. Soils with high FC content thus currently have adequate P without supplementing it in the form of fertilizer. A more meaningful relationship is found between the FC fraction and available P than between P status and total organic carbon of soil.

ORGANIC MATTER MOBILIZATION AS AFFECTED BY SOIL-SOLUTION COMPOSITION AND PREVAILING CLAY MINERALS

Laboratory experiments were conducted to study organic matter (OM) loss from soil columns with different clay mineral composition and treated with high sodium (Na) and potassium (K) saline solutions (SAR and Removal of organic and inorganic colloids was evaluated by determining organic matter content in the soil at the end of the trial and measuring absorbency of the percolated solutions. Though clay dispersion after magnesium (Mg) treatment was marked in the case of illitic and smectitic soils, little effect was detected in kaolinitic soil. For treatments with solutions containing Na, the highest removal of organic colloids was observed in illitic and kaolinitic soils. In smectitic soil this effect was more pronounced with calcium (Ca) in combination with Na and K. These findings have significance to explain differences in OM mobilization in salt affected depressions of different composition.

Coniferous afforestation increases soil carbon in maritime sand dunes

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−1 y−1. At 0- to 25-cm depth, soil C amounted to 61 Mg ha−1 under pine and 27 Mg ha−1 under grassland. Soil C accumulated more on dune slopes (35 Mg ha−1 y−1) than on ridges(29 Mg ha−1 y−1) and bottoms (12 Mg ha−1 y−1). 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.