Land-use impact on porosity and water retention of soils rich in rock fragments

Rock fragments are key components of soil systems. However, the role of rock fragments is only sporadically researched and therefore it is essential to establish how they control soil physical, biological and chemical properties and to assess the impacts of land-use on soil functions. Further, rock fragments affect soil water retention and infiltration capacity, and this is relevant for the fate of soils because water determines soil biota and weathering and decomposition processes in the soil system. Herein, we investigated abandoned arable, meadow and forest soils to determine the effect of land-use on soil water retention properties in soils containing rock fragments. Results showed that forest soil had the highest content of rock fragments and that previous ploughing on shallow abandoned arable soil on dolomite bedrock ensured that this had higher rock fragment content than meadows. We then compared coarse rock fragment effects on water retention in three research plots. Measured points in the water retention curves depict the total porosity and water retention determined at 3–1000 hPa suction in the undisturbed soil samples and 1.5 MPa in disturbed samples. Two models with different macro-porosity expressions were applied to the experimental data, and their derivatives provided pore size distribution curves with maxima for textural, structural and macro-pores. However, a new combined model with six independent fitting parameters has proven more suitable to approximate tri-modal soil water retention curves. The textural porosity was highest in the meadows due to the direct effect of organic carbon content and the indirect influence of coarse fragments which reduced the volume of small pores in the soil. Organic carbon content positively affected water retention properties in these soils. This included the permanent wilting point, field capacity and plant available water capacity, but the coarse fragment content negatively correlated with these properties. In addition, the results highlighted the stones’ positive effect on structural pores in forest soils and on macro-pore formation in all soils, while structural pores in all soils were mainly influenced by organic carbon content. The combined structural and macro-pores determine the soil air capacity and this is positively influenced by the increased content of coarse rock fragments. The soil air capacity was therefore highest in the forest soil. Finally, although the irreversible increase in rock fragment content in the abandoned soil has negative impact on agricultural use, the coarse fragment influence on macro-pore volume is positive and this increases soil infiltration capacity and reduces surface runoff and soil erosion, as confirmed in the literature review.