Influence of edaphic and management factors on soils aggregates stability under no-tillage in Mollisols and Vertisols of the Pampa Region, Argentina

In the highly fertile and productive soils of the Pampa Region of Argentina, constraints on soil water regime as a consequence of the decline in aggregates stability and the development of platy structures, are observed. Most of agricultural plots in this region, even under no tillage, are subjected to simplification of crop sequences (low cropping intensity) with predominance of soybean. On the contrary, some farmers are intensifying the crops sequence to avoid soil degradation and equilibrate economical incomes. The aim of this work was to evaluate the effects of intrinsic soil factors and of cropping intensity on aggregates stability in the surface horizon of the main soils in this region. Three Mollisols (an Entic Haplustoll and two Typic Argiudolls) and a Vertisol (Hapludert) located across a west-east transect in the northern part of the Argentinean Pampas were selected for this study. In each site two management treatments under no-till (GAP: Good Agricultural Practices -high cropping intensity and PAP: Poor Agricultural Practices -low cropping intensity-) and a soil without cultivation as a reference (NE: Natural Environment) were compared. In each treatment, aggregates destruction mechanisms were assessed by Le Bissonnais (1996) tests: slaking, microcraking and cohesion loss. Mollisols showed higher aggregates stability than the Vertisol. Differences on aggregates stability depended on management variables and on organic carbon contents in the Mollisols and on both clay content-clay type in the Vertisol, revealing a strong relationship of aggregation mechanisms with soils taxonomic order. In the Mollisols, the labile coarse particulate organic carbon fraction (POCc) determined the shifts on slaking and overall aggregates stability rather than other carbon fractions. In the soils studied, aggregates stability was linked mainly to management variables, best reflected by the cropping intensity index (CI). More intensive agricultural managements (GAP treatments) in Mollisols and Vertisol, resulted in an enhancement of aggregates stability; however, this relationship was stronger in the Mollisols. Surface horizons from both soil orders evidenced a high soil fragility related to slaking process (FW10s and FW tests). Thus, FW test was the best test to discriminate between management treatments. The results obtained in this work allow, on the one hand, to understand the stabilization mechanisms of the structure in the surface horizon of the main Pampas soils and, on the other hand, to highlight the effect of different NT management practice on soil aggregate stability, which highly affects soil health and the sustainability of agricultural systems.