Soil structural behaviour in flooded and agricultural soils of the Argentine pampas

Abstract

Soils of the Argentine Pampas have been developed over same parent material (loess), but evolved under different topography and land use. This led to different soil structural behaviour. In the flooding Pampa of Argentina soils (Solonetzes) are flooded each winter-spring and dried each summer, and are grazed by livestock all year round. Little is known about cattle trampling effects under these environmental conditions. In the nearby rolling Pampa, there are silty loams affected by physical deterioration and water erosion losses after long term conventional tillage (CT). After continuous zero tillage (ZT) these soils often develop shallow compaction. Little is known about abiotic and biotic mechanisms of structural recovery. The general objective of this thesis was to analyze comparatively natural and made-man factors affecting soil structural behaviour in soils with similar parent material (loess) and vegetation (grassland), but later affected by different relief and soil use factors. In the flooding Pampa results showed the occurrence of significant soil volume changes by swelling and shrinking. Soils swell during flooding because of a process of air entrapment. Livestock trampling causes the mechanical destruction of surface macropores in summer when soil dries. The regeneration of damaged pores takes place during flooding, when soils swell at maximum. Droughts -and not floods- cause negative environmental effects in this area. In the rolling Pampa similar soil macropore volumes were determined in pasture, CT and ZT situations, showing none effect from soil management. Topsoil hardening was often found after short term ZT. Results from a greenhouse experiment showed that aggregate stabilization requires a previous fragmentation by short wetting-drying cycles. Clod shrinkage curves and soil cracking studies showed that silty loams do not have the expected poor response to W/D cycles. However, their air filled porosity increases little during drying. Results showed topsoil aggregation to be mainly abiotic in the flooding Pampa, and abiotic and biotic in the rolling Pampa, showing different structural behaviour in soils evolved from same parent material but different relief and land use.