Changes in soil pore network in response to twenty-three years of irrigation in a tropical semiarid pasture from northeast Brazil

Irrigation is a technique in agri-, vini- and horticulture which consists in the controlled supply of water and which has been used for many years in order to ensure agricultural productivity in many regions of the world. In regions where this water application technique is used there is the possibility of changes in soil properties; amongst others, soil structure, a physical attribute that is related to several other soil attributes such as density, porosity and aggregate stability. Soil structure is also greatly affected by wetting and drying cycles, which are provided by irrigation in these regions. This study aims at evaluating changes in the physical attributes of a soil caused by 23 years of irrigation. The study area is located on the Apodi Plateau, Ceará, Brazil. Two areas were evaluated, one irrigated by a central-pivot sprinkler system and a non-irrigated control. For the study, eight disturbed soil samples were collected in each area, in the layers of 0–20, 20–40 and 40–60 cm, and four undisturbed soil samples, using cylinders of 5 cm × 5 cm, in the layers of 0–20 cm and 20–40 cm. The analyzed physical attributes were: particle density, soil bulk density, particle size distribution, total porosity, macroporosity (pores with diameter ≥ 50 μm, applying matric potential of −6 kPa), microporosity (pores with diameter <50 μm, total porosity minus macroporosity), soil air permeability, soil resistance to penetration, aggregate stability and pore continuity indices. The results indicated that irrigation contributed to changes in soil physical attributes directly related to soil structure. Also, the consequent wetting and drying cycles were sufficient to improve aggregate stability; however, these cycles, combined with processes of translocation and compaction, have increased microporosity, causing the formation of less continuous and more tortuous pores and lower soil air permeability.