Efectos del pastoreo de los rastrojos y las labranzas sobre la evolucion temporal de la estabilidad de agregados

Abstract

The aim of this paper is to understand the temporary evolution of the soil aggregatestabilization of a typical Hapludol in the experimental field ?Pozo del Carril? of theAgriculture and Veterinary School of the Universidad Nacional de Río Cuarto. The land wascultivated with corn-soy rotation, and in one sector, its stubbles were pastured by cattle. Bothsectors were fertilized with nitrogen and phosphorus. Crops were grown by using threesystems: conventional sowing (CS), reduced sowing (RS) and direct sowing (DS). Acondition with minimum alteration was also used. After the 2003/2004 corn crop, twocompound samples of the first five centimeters of soil were taken in all the farming systemsused and the size distribution of the water-stable aggregates was determined according to thefollowing diameters: 0,1-0,5 mm, 0,5-1 mm, 1-2 mm y 2-4 mm. The results indicate that,according to the pastured lands, the biggest number of soil aggregates having the largestdiameter was identified in non-pastured lands. Among the farming systems, the significantdifferences in soil aggregates are 2-4 mm, being DS>RS>CS and in aggregates whosediameters are 0, 1-0,5 mm, CS equals RS and both are larger than DS. Significant19interactions were found only in the 2-4 mm aggregates with a higher value in the non-pastured direct sowing lands but with a lower value in pastured conventional sowing lands In order to analyze the temporary evolution of the variable, the results were compared withthose obtained in 1999. In this sense, the distribution of the micro-aggregates follows theDS<RS<CS tendency while the macro-aggregates in 1999 the tendency was RS<CS<DS andin 2004, CS<RS<DS. Regarding the minimum alteration case, the tendency is similar to thatin DS though with higher values in the thickest soil aggregates coinciding with the tendencyindicated in 1999. As a conclusion, when all stubbles are left, the conservative farmingsystems show recovering effects of the structure; these effects being more notorious andaccelerated in direct sowing lands. The CS tendency indicated a degrading effect on the soilaggregate stabilization Key Words: soil aggregate stabilization, macro-aggregates, micro-aggregates, farmingsystems.