Ramón Rosell

Correlation of spectroscopic indicators of humification with mean annual rainfall along a temperate grassland climosequence

The stable free radical concentration, as measured by electron spin resonance spectroscopy, and the E4/E6 absorbance ratio were determined for a suite of humic acids extracted from uncultivated surface soils collected along a 620 km climosequence in the Argentine Pampa (mean annual temperature, 14–15°C; mean annual rainfall, 300–1000 mm). These two spectroscopic properties were interpreted as indicators of the degree of humification of soil organic matter, with free radical concentration expected to increase and the E4/E6 ratio to decrease with greater humification. Both the free radical concentration and the E4/E6 ratio were found to be significantly correlated with increasing mean annual rainfall, which was accompanied by an increase in the stable free radical concentration and a decrease in the E4/E6 ratio. Our results indicate that, in the sequence of temperate grassland soils investigated, organic matter humification increased with increasing rainfall.

Long-Term Crop Rotation Effects on Organic Carbon, Nitrogen, and Phosphorus in Haplustoll Soil Fractions

Soil organic matter (SOM) or carbon (SOC) consists of a number of fractions (which can be separated by granulometric wet sieving) having different properties among them. Information on fraction nutrient distribution and long-term crop rotations is lacking for semiarid environments. The objective of this research was to study the agronomic effects on soil OC, N, and P fractions. The humified OC was the largest and least variable fraction of the SOC. Soil under continuous mixed pasture had higher OC contents than under annually tilled treatments. Similarly, soil total nitrogen under the cropped treatments decreased from 1.7 g N kg-¹ in noncultivated soils (reference plots) to 1.0, 0.7 an 0.7 g N kg-¹ under mixed pasture, pasture-crop, and wheat-crop respectively, in the fine soil fraction. The reference plots also showed significantly lower levels of organic phosphorus (P o ) in comparison to the other treatment (from 67.1 w g P o g-¹ to greater than 100 w g P o g-¹ in the fine fraction of the treatments and years). The noncultivated soil showed larger values of P o and inorganic P in the large-size granulometric fraction (0.1-2 mm) than in the soil fine fraction (0-01 mm). However, the rotation treatments had greater concentrations of P in the fine fraction. The P o from the coarse fraction appears to be the most labile and sensitive fraction to tillage and environmental conditions, and may be closely related to P availability.

Humification and nitrogen mineralization of crop residues in semi-arid Argentina

Abstract Crop residues in the soil play a fundamental role in organic matter dynamics and have effects on numerous beneficial soil properties. The humification of residues depends on soil nutrient status and physical and meteorological conditions. The prediction of the humification rate and of the size and decomposition of fresh and humified soil organic matter has been attempted with equations and models which require several constants. To simplify the procedure. Janssen developed an equation which utilizes only one constant, permits calculation of the young (fresh) and old (humified) soil organic matter pools and predicts the production of available or mineralized nitrogen. The validity of Janssens equation was tested in three production systems (a crop rotation and two tillage procedures) in a sandy-loam textured, thermic, entic Haplustoll soil in the pampean semi-arid region of Argentina. The model produced results compatible with actual field data, but information on soil available water was essential for fitting and interpreting the organic matter dynamics.

A comparison of the effectiveness of various extractants for determining humus quality in a calcareous soil

Abstract Humic substances from an Argentinian chesnut (Mollisol) soil were extracted with Na‐Dowex A‐1 resin, 0.1 N NaOH and 0.1 M Na4P2O7 without decalcification and with 0.1 N NaOH after decalcification. Humic to fulvic (Ch/Cf) acid and E4/E6 ratios of the extracts were determined and compared. A simpler and more rapid procedure than the classical ones for extracting and characterizing soil humus is proposed. The procedure is based on the use of a chelating resin as extractant and on the direct determination of analytical properties of humus in aliquots of the extract.

Humus quantity and quality of an entic Haplustoll under different soil‐crop management systems

Abstract The effects of different management systems on the level and composition of humified organic matter in an entic Haplustoll from the semiarid Pampean region were studied. The systems were: TPc, wheat‐mixed pasture; TV, wheat (Triticum aestivum), oat (Avena sativa), corn (Zea mays) and triticale grasses; TP, wheat‐cattle grazing; and V, virgin, non cultivated. Humic acids were extracted, fractionated, and analyzed for their organic carbon (OC) content, elemental composition, and E4:E6 spectral ratios. The infrared (IR), electron spin resonance (ESR). and 13C‐NMR spectra were registered on these humic acids. The TP rotation showed the lowest humic acid‐carbon to fulvic acid‐carbon (HA‐C:FA‐C) ratio. The lower O:C ratio of humic acids from the cropped soils indicates a higher level of oxidation than that of the virgin one. The comparison of the different methodologies allowed us to conclude that crop rotations and conservation tillage were adequate to mantain the level and composition of the soil org...

Soil fixation and availability of phosphate fertilizer

Abstract The addition of low quantity (up to 50 mg/kg) of phosphorus (P) to two calcareous soils in the Argentine semiarid‐subhumid pampean region increased the formation of DCPD (Brushite) and decreased the P (Paer) extractable by an anionic exchange resin. This latter effect was not observed when the quantities of added P were over 50 mg/kg. The pH decreased with P addition up to a point (50 mg/kg) after which it returned to its original value. The percentage in weight of peds smaller than 74 μm increased with the added P up to certain point which decreased to values nearing the original ones. To obtain a positive Availability Index (F1) by incubating for six months the soil with additional P, it is suggested that there are at least two points where the increase of labile P (Paer) takes place.

ANTHROPOGENIC EFFECT ON SOIL ORGANIC PHOSPHORUS FRACTIONS IN TROPICAL ECOSYSTEMS

Total organic phosphorus (P), labile organic P, and total carbon were determined in composite surface soil samples, with three replicates, in a Kandihumult from Misiones province, Argentina. The analysis was performed on <0.1 mm, 2–0.5 mm, and >8 mm aggregate fractions obtained by dry sieving. Representative situations were selected to evaluate changes in P dynamics by soil use: a native subtropical forest, a degraded agroecosystem with a 50-year yerba mate plantation (Ilex paraguariensis Saint Hill.), and a similar plantation with a 6-year associated elephant grass (Pennisetum purpureum Schumach). Native pristine soil from the forest presented the highest content of phosphorus in more humified fractions (<0.1 mm). Fifty-year cultivation of “yerba mate” produced a degraded agroecosystem with 10% decrease of total organic phosphorus forms in humified fractions. Six-year-old elephant grass mulch incorporation increased total organic phosphorus both in more humified fractions and in macroaggregates (>8 mm), due to addition of decomposable organic materials. Labile organic phosphorus was higher under stress conditions.

Phosphorus in the Pampean semi‐arid region of Argentina. I. bray and anionic exchange resin ex‐tractable phosphorus

Abstract There is often a lack response of wheat to phosphorus (P) fertilization in the Pampean semi‐arid area of Argentina. Consequently, the fate of fertilizer P in an entic Haplustoll soil was investigated under three production systems commonly practiced in that region. The following procedures were conducted: i) comparison and linear regression of the values of P obtained by Bray I (Pbray) versus’ the anionic exchange resin (Paer) procedure (Paer = 2.35 + 0.68 Pbray), ii) addition of different amounts of KH2PO4 to the Ap horizon, and after incubation at 25°C for 6 months, extraction of the Paer in order to establish an index of fertilizer availability F1 which was: F1 = ‐ 0.13, and iii) determination by X‐ray diffractometry of the generated brushita (DCPD) in the incubated samples to which P was added. The decrease in labile P could explain the lack of wheat response to the P fertilization in certain years.

Phosphorus sustainability in tropical ecosystems

Highly weathered soils in the tropics display nutrient deficiencies, especially of phosphorus (P) in low-activity clay soils. The objective of this work is to evaluate P distribution in soil fractions (> 8 mm, 2–8 mm, 0.25–2 mm, 0.10–0.25 mm, < 0.10 mm) and to study the effect of management practices on P sustainability and availability. Surface and subsurface soil samples were taken from three different situations: virgin soil, degraded plantation of Ilex paraguariensis Saint Hil. (yerba mate), and a similar plantation consociated with Pennisetum purpureum Schumach (elephant grass), used as a mulch. For virgin soils, the highest P values were present in the 8 mm fractions; macroaggregates P is associated with decomposable material which is related to short-term fertility. Its beneficial action was also observed below the soil surface. Protecting the soil with mulch improved P sustainability in ‘yerba mate’ agroecosystems.