NULL CREATION OF AIR-FILLED STRUCTURAL PORES BY SOIL CRACKING AND SHRINKAGE IN SILTY LOAMY SOILS

Abstract

Information about abiotic regeneration of air-filled porosity in silty soils is scarce. It could be a key mechanism to explain their low physical resilience. In the present work, we aim at evaluating whether changes in intrinsic soil properties (e.g., soil organic carbon, clay content, and clay mineralogy) caused by degradation affected soil volume response to wetting-drying cycles. Volume and size distribution of cracks and clod shrinkage curves were determined in silty loamy soils (Typic Argiudoll) of Argentina under nearby conventionally tilled (CT), eroded CT, and Pasture management. Crack volume increased from 1000 cm³ in CT and Pasture soils to 6000 cm³ in the more clayey and swelling eroded CT soil. Crack size distribution was similar in all studied soils with large cracks (first and second size order) prevailing over small ones (fourth and fifth size order). Clod shrinkage curves had no S-shape, thus showing the lack of structural shrinkage in all studied soil management regimens. Air content in structural pores was as low as 0.03 to 0.10 cm³ g⁻¹ at the air entry point. This little air entry during drying agreed with the lack of small cracks and can be related to the prevalence of plasma (i.e., silt and clay) over sand. Results showed that key intrinsic properties did not drive soil volume changes in the studied silty loamy soils. They change their volume during drying, but the creation of air-filled structural pores is little or null.

ABBREVIATIONS: A_Cr: area of the crack calculated from its width and length; AE: air entry; Airp: air content of the plasma; Airstr (AE): air content of structural porosity at the air entry transition point; Air_str (SAT): air content of structural porosity at saturation; CT: conventionally tilled; Cter: eroded conventionally tilled; D_Cr: depth of the crack; Id: density of cracks; K_Bs: slope of the basic shrinkage; K_R: slope of the residual shrinkage; K_Str: slope of the structural shrinkage; L_Cr: length of the crack; ML: macroporosity limit; MS: aximum swelling; PD: particle density; S: surface of the pot; SC: swelling capacity of the soil from saturation to the shrinkage limit transition point; ShC: shrinkage curve of the soil; SL: shrinkage limit; Subang Bk: subangular blocks; V: bulk specific volume; V_AE: specific volume at the air entry transition point; V_Cr: specific volume of the crack calculated from its area and depth; V_ML: specific volume at the macropore limit transition point; V_MS: specific volume at the maximum swelling transition point; V_P(AE): plasma porosity at the air entry transition point; V_p(ML): plasma porosity at the macropore limit transition point; V_p(MS): plasma porosity at the maximum swelling transition point; V_p(SL): plasma porosity at the shrinkage limit transition point; V_SL: specific volume at the shrinkage limit transition point; V_str(AE): structural porosity at the air entry transition point; V_str(ML): structural porosity at the macropore limit transition point; V_str(MS): structural porosity at the maximum swelling transition point; V_str(SL): structural porosity at the shrinkage limit transition point; W: gravimetric water content; W_AE: gravimetric water content at the air entry transition point; W_Cr: width of the crack; W/D cycles: soil wetting and drying cycles; W_ML: gravimetric water content at the macropore limit transition point; W_MS: gravimetric water content at the maximum swelling transition point; W_p: water content in the plasma porosity; W range (SAT-SL): gravimetric water content range from saturation to the shrinkage limit transition point; W_SL: gravimetric water content at the shrinkage limit transition point; W_Str: water content in the structural porosity; φ_ML: bulk density at the macropore limit transition point; φ_MS: bulk density at the maximum swelling transition point; φ_SAT: bulk density at saturation; φ_SL: bulk density at the shrinkage limit transition point.