Seasonal dynamics of the physical quality of volcanic ash soils under different land uses in southern Chile

Abstract In Chile, volcanic soils were developed under almost all of the diverse rain and temperature regimes, from the Arid Mediterranean to the Wet Zone of the South. Due to their andic properties, volcanic soils exhibit characteristics that are entirely different from other soil types around the world. The soil physical quality (SPQ) is strongly related to the functions of the soil pore system. Thus, soils characterized by a good SPQ have the ability to store and conduct water, air and nutrients promoting both: maximum crop yield and minimum environmental degradation. Many studies and much scientific progress have been made concerning the storing and conducting of water and air functions in volcanic soils. However, soil physical quality indicators and their temporal changes have been less studied. Thus, the objectives of this study were: i) to assess the impact of land use changes on the physical quality of three main volcanic soil groups in Chile, ii) to quantify the magnitude of their temporal changes and iii) to compare these results with threshold values found in the literature. Three soils derived from volcanic materials formed under different conditions and with different degrees of development were sampled (with different intrinsic properties), on five sampling dates, under three land uses (intensities): native forest (NF), prairie (P) and crops (C). Undisturbed samples were taken at two soil depths: 0–15 cm and 15–30 cm, in 230 cm 3 metallic cylinders, and then covered with caps and plastic film to prevent mechanical disturbance and evaporation. We measured and /or calculated from these samples: air capacity (AC), plant available water capacity (PAWC), relative field capacity (RFC), bulk density (dB), air conductivity (kl), pore connectivity indexes (C2 and C3), the coefficient of linear extensibility (COLE) and saturated hydraulic conductivity (Ks). Undisturbed soil blocks were collected to evaluate aggregate stability. To assess differences among land uses and seasons in the SPQ indicators, analyses of variance (ANOVA) were used and an LSD test (p ≤ 0.05) was conducted to separate the means. A principal component analysis (PCA) was performed to associate the SPQ indicators. The results of this study allow us to conclude that the impact on SPQ indicators depended on the soil type, considering their degrees of development, clay content and type, as well as the organic carbon content and the season in which the soil functions were determined. When values of the SPQ indicators were compared with critical and/or threshold values of the mineral soils found in the literature, the indicators of dB and RFC fell completely out of the range considered typical for mineral soils. Thus, several questions remain regarding the real critical values of these soil properties in volcanic soils. More work is necessary to establish the critical value of volcanic soil’s bulk density, since this is a widely used soil property that serves as an estimator of other functional soil quality indicators.