EFECTO ACUMULATIVO DE LA SIEMBRA DIRECTA SOBRE ALGUNAS CARACTERÍSTICAS DEL SUELO EN LA REGIÓN SEMIÁRIDA CENTRAL DE LA ARGENTINA.

Information about the effects of crop residues on soil parameters in no-tillage systems is not abundant in Argentina because the relative recent adoption this practice. Moreover, most of the available research to date refers to the humid Pampa region. In contrast, information on the semi-arid region is very scarce, despite the fact that an increase in cover stubble is particularly useful in water-limited soils. In this paper we report a long-term evaluation (5 to 10 years) of no-tillage practices on: a) chemical (soil organic matter -SOM, total N and NO3-N) and biological (microbial biomass and activity) soil characteristics, and b) quantity and chemical quality of soybean residues in monoculture and soybean-corn rotation. Research was conducted at Manfredi INTA Experimental Station in semi-arid, central Argentina(31°49' S and 63°46' W). Soils under no-tillage system had higher SOM and total N content compared with control (conservation tillage with chisel plow) in soybean monoculture and soybean-corn rotations. The observed difference increased with time (10 and 20% at 5 and 10 years, respectively). Nitrate content, and microbial activity and biomass showed high variability in the measurement periods, depending on climatic condition at the sampling date. Stubble cover was greater in the corn-soybean rotation with corn as preceding crop (2473.9 g m-2) than in soybean monoculture (1035.7 g m-2)). The stubble non-identifiable fraction was very significant in all treatments (ranging between 2-10 t ha-1), which would favor new surface organic soil formation. The soybean-corn rotation with soybean as preceding crop showed the lowest values in all chemical parameters, particularly in soluble compounds (nitrate, ammonium and soluble carbon). Contrarily to what is usually assumed, our results suggest that nutrient release from long-term stubble may become a significant nutrient source for the crops. Therefore, this nutrient source should be considered when evaluating crop fertilization requirements. [ABSTRACT FROM AUTHOR]