Volumen 22, Número 1 (Julio, 2004)

In order to evaluate the persistence of Sinorhizobium meliloti B399, B58 and B401 strains in the soil, this bacteria population was quantified in seven sites by the NMP method. Conventional microbio- logical protocols and molecular techniques were used for the characterization of S. meliloti isolates. The measured population was 102 in inoculated sites and 103 in a fallow and an undisturbed site.An homology of B401 strain was detected by molecular techniques in one of the studied sites, where a Lucerne crop, implanted three years ago, was present. Moreover, the molecular techniques allowed to detected the presence of a tolerant acid strain type sp.Or 191. Isolates of sites where the strains were introduced six to nine years before the soil tests start and with no Lucerne did not present an homology with the inoculated strain patrons.

The variability in the grain yields of peanut (Arachis hypogaea L.) crops in sandy soils can be attributed to inadequate nitrogen nutrition beacause of a low soil N availability and a poor biological fixation from native rhizobium strains. Our objective was to determine the effects of in-furrow Bradyrhizobium sp. inoculation on nodulation and grain yields of 16 dryland peanut crops in sandy soils from the central and southern part of Córdoba province (Argentina). The study was performed during the 2001-02 and the 2002-03 production seasons on Entic Haplustolls and Typic Ustipsamments. The inoculation improved the number of nodules, both in vegetative and early reproductive growth stages and the leaf nitrogen content in early reproductive stages. The average grain yields response, independently of the studied season, was of 630 kg ha-1 and it increased when the mean productivity of the experimental site increased. In-furrow inoculation with Bradyrhizobium sp. is a recommended practice for achieving high yielding peanut crops in sandy soils.

Tillage practices under continuous agricultural systems modify soil properties and can affect crop productivity. Our objective was to determine long term effects of three tillage systems (moldboard plowing (AR), chisel plowing (AC) and no-tillage (SD)) on maize ( Zea mays L.) and soybean [ Glycine max (L.) Merrill] and on soil properties of a Hapludoll from northwestern part of Buenos Aires province (Argentina). In fall 2003, soil chemical (soil organic matter (SOM), extractable phosphorus (Pe) and water pH) and physical properties (bulk density, soil penetration resistance and fragment size distribution) were determined from a tillage experiment established in 1990 under a fescue (Festuca arundinacea L.) sod (PF). Averaged over the studied period, none of the tillage systems significantly modified soybean or maize grain yields. After twelve years of continuous cropping, SOM in the 0 to 5 cm layer decreased 36% and 32% under AR and AC, respectively. No differences in SOM between SD and PF were observed. Soils Pe levels in the 0 to 5 cm layer were lower under SD and PF than under disruptive tillage practices. Soil pH was not affected by the different tillage systems. The mean size of the soil fragments was greater under SD than under AR or AC. Due to the positive effect of no-tillage practices on SOM conservation and soil aggregation the adoption of no- tillage practices is a recommended practice for establishing sustainable agricultural production systems in Hapludolls from the northwestern part of Buenos Aires province, Argentina.

The use of unscreened biosolids composts or screened by a higher mesh than 0.5 cm may contribute to increase soil water and nutrient storage capacity, and to decrease compaction and the risk of soil erosion. However, the presence of high C/N particles from partially decomposed bulking agents could enhance soil nitrogen immobilization. The objective of this work was to study the effect of different fractions of biosolids composts (menor que 0.5 cm, mayor que 0.5 cm, and unscreened composts), and the addition of low rates of soluble nitrogen (20 mg kg-1 soil) on soil potential nitrogen mineralization and greenhouse ryegrass yields. The application of unscreened composts at different rates (20, 40 and 60 g kg-1 soil) resulted in partial immobilization of the added nitrogen during the first 6-8 weeks of incubation under optimal moisture and temperature conditions. However, the unscreened composts provided higher available nitrogen than the control, with values which would cover the nitrogen requirements of many crops (150-270 mg kg-1 after 28 wk-incubation). It may have been expected that the remnant C would continue decomposing in the soil enhancing nitrogen immobilization, however, the unscreened compost with the addition of urea at low rates (0.5 g N kg-1 compost) did not show any immobilization in incubations conducted after 7 months of ryegrass growth. In the treatment with the compost fraction > 0.5 cm, which showed a higher C/N ratio that the upper limit (C/N = 20) currently regulated by SENASA for organic amendments, a similar rate of soluble nitrogen was also adequate to avoid nitrogen immobilization, and to obtain ryegrass yields significantly higher than the control. These results provide insight about the possible use of different compost fractions as a function of different objectives, and consequently, about the need to establish different compost quality require- ments.

In the Southeastern of Buenos Aires Province little information exists on denitrification losses from soils under no-tillage with and without maize plants and variable soil nitrogen availability. A field experiment was carried out in 1997/98 growing season at Balcarce, Argentina (37° 45′ S, 58° 18 ́ W), on a soil complex of a fine, mixed, thermic Typic Argiudoll and a fine, illitic, thermic Petrocalcic Paleudoll. The treatments were a factorial combination of N rates (0 and 210 kg ha-1 ) and presence or absence of maize plants. Denitrification rates were evaluated during a period of 92 days. Denitrifica- tion rate was significantly lower with presence than in absence of maize plants. For the highest nitrogen rate, maize plants decreased denitrification rate even during the periods of highest soil moisture (393 and 307 g N ha-1 d-1 without and with plants, respectively). Maize plants limited denitrification rate by decreasing water filled pore space (WFPS) and nitrate concentration. A high relationship was found between denitrification rate and soil water availability and nitrate concentra- tion (y= -0.616 + 0.005 x NO –N + 0.0311 x WFPS; r2= 0.58). Cumulative N O-N losses were 11.4 6.5 kg N ha-1 for 0 and 210 kg N ha-1, respectively, for treatments with plants. Results of this experiment show that denitrification losses can be high in soils under no-tillage during the fallow conditions.

We studied modifications in soil pore system analyzing its fractal structure. The study was performed for a natural situation and an agricultural one. Samples were taken from top horizons of typical Argiudoll soils, representative of the middle Luján river basin. Soil blocks were sampled and impregnated with polyester resin in order to obtain sections, which were photographed using a digital camera. Mass fractal dimension of pores and soil (Dmp and Dms), and surface fractal dimension (Ds) were measured from the pictures. The results show thatDms varies between 1.925 and 1.943, Dmp from 0.735 to 1.181, and Ds from 0.910 to 1.277 with the lower values corresponding to more degraded soils. Physical aggression effect is more evident in pore-system related parameters: Dmp and Ds.

Soil subsurface compaction is one of the factors that are frequently related with the reduction in sunflower (Helianthus annus L.) grain production. However, the available information quantifying this effect under field conditions is scarce and contradictory. The objective of this study was to determine the effects of subsurface compaction on sunflower productivity in the subhumid Pampas region of Argentina. A two season field essay was performed on a Typic Hapludoll with a compacted layer at a depth of 0.18 to 0.22 m after different traffic intensity patterns. Soil bulk density (BD) and penetration resistance (PR) after harvest were measured in 25 sampling sites randomly located in the studied area. In one of the growing seasons, rainfall events were enough for normal crop productivity and differences due to compaction treatments were not observed. Only in the season with low rainfall events, subsurface (0.20 to 0.30 m) compaction reduced plant height and crop grain and oil yields. Grain yields decreased at a rate of 2534 kg ha-1 MPa-1 when increasing the PR up to 1.65 MPa. Sunflower grain production also linearly decreased when soil bulk density in the 0.15 to 0.30 m layer increases. These results suggests that in the subhumid Pampas, the risk of sunflower yield reduction after subsurface compaction in tilled soils strongly interacts with environmental conditions during the growing season.

XXX

Biological nitrogen fixation in soybean ( Glycine max (L) Merrill) crops from the Rolling Pampa of Argentina under conventional and no-till production systems were determined in a field experiment using nodulating and non-nodulating isolines by the isotopic dilution methodology. No differences were detected in biomass production or in the nitrogen fixation levels between tillage systems. In average, the nitrogen fixed was 74 kg ha-1 (20 % of total aboveground nitrogen). In both tillage systems the nitrogen budget was negative with a net extraction (harvested N – fixed N) of ca. 130 – 150 kg N ha-1. This amount is equivalent to nitrogen extraction from cereal grain production systems in the region.

Recently no tillage became widespread in the Rolling Pampa. The objective of the present work was to study the effect of this tillage system on the superficial hydrologic soil response under contrasting surface cover conditions and soil water contents. Field experiments employing simulated rainfall on a Vertic Argiudoll under no tillage were carried out. The treatments consisted in: a) soils covered with living plants or residue and b) bare soils. The following parameters were considered: runoff rate, sediment yield, surface cover condition, bulk density, aggregate stability and initial soil water content. A tri-linear regression model was significantly fitted between cumulative rainfall and runoff rate for every simulation event. The obtained parameters were: initial infiltration rate, final infiltration rate, slope of infiltration rate fast decline stage, duration of initial infiltration stage and duration of infiltration rate fast decline stage. The duration of the initial infiltration stage was affected by the soil surface cover condition and also by soil water content. Runoff begun when the cumulative rain was 24,1 mm for the covered soils and when it was 11.6 mm for bare soils. The duration of the infiltration rate fast decline stage did not change between treatments. Final infiltration rate was affected by soil surface condition (20.08 mm h-1 for covered soils and 10.63 mm h-1 for bare soils) but it did not depend on initial soil water content. Also, a significant difference in sediment yield between treatments was observed (536,6 mg l-1 for covered soils and 1983,8 mg l-1 for bare soils). The infiltra- tion process partition into three stages was adequate to determine the relative importance of each of the evaluated factors.