Seasonal variation of soil physical properties under different water managements in irrigated rice

While soil porosity and soil hydrological properties are key characteristics that define different soil types, they are influenced by many factors: land use, tillage management, and agricultural practices such as irrigation. As expected, water management impacts the physical properties of soil in irrigated rice significantly; however, the importance of seasonal variation on those soil properties requires further consideration, especially given the period of continuous submersion. In this paper, the different soil physical properties have been studied with two goals in mind: (1) to compare the bulk densities, cumulative pore size distribution curves, and near-saturated hydraulic conductivity values associated with seasonal variation induced by submerging water or rainfall on irrigated rice cultivated under two different water managements and in one rain-fed crop, and (2) to describe and parameterize the relationship that links near-saturated hydraulic conductivity to soil porosity in a generic semi-empirical model independent of treatment differences and seasonal variability. The experiment was conducted in the Piedmont Region (NW Italy) in sandy loam soil on three contiguous fields cultivated as follows: (i) continuous rice in submersion, (ii) continuous rice seeded in dry soil submerged one month after the first field, and (iii) maize in rotation with rice (rain-fed treatment). The physical properties of the soil were measured five times over the year at depths of 0–12 cm and 12–25 cm. Results showed a progressive compaction of the soil and a consequent reduction of the near-saturated hydraulic conductivity due to submersion. Macro- and meso-porosity decreased while micro-porosity increased. At the end of submersion, new large porosity was created and the situation reverted to that noted at the start of the year. The non-submerged field showed a different behaviour; in the absence of submersion, bulk density reduced as a result of rainfall but the effect on the different classes of pores was reversed. Finally, a new semi-empirical model is presented that describes near-saturated hydraulic conductivity as a function of soil porosity.