Photosynthesis-driven capillary rise and daily salinity dynamics in an agricultural system of the Po River Lowland

Introduction

In many places, soils and water resources are subject to growing salinity levels (Akter et al., 2019). Soil salinization processes are extremely complex and often related to various factors: climate (Schofield and Kirby, 2003), sea salt spray deposition (Giambastiani et al., 2017), capillary rise of paleosaline groundwater (Colombani et al., 2016), irrigation with poor quality water (Rahman et al., 2022), and salt evapoconcentration triggered by plant activity (Alessandrino et al., 2024). The latter is often overlooked, yet it is a significant additional source of salinity, particularly in agricultural soils. This study investigates the role of plant activity in the daily fluctuation of pore water salinity and soil volumetric water content (VWC) in an agricultural system.

Methods

The study site is located in the Po River lowland (Northern Italy) approximately 24 km from the Adriatic Sea. Soil bulk electrical conductivity (EC_b) and VWC were detected every 30 minutes using 5TE^® probes from Meter. Concurrently, a fixed video camera monitored water level fluctuations in an adjacent irrigation canal.

Results

EC_b in the first 15 cm ranged between 0.76 and 5.75 mS/cm, while the VWC ranged between 0.26 and 0.44 m³/m³. In comparison, groundwater EC at -2.7 m below ground level ranged between 11.16 and 17.01 mS/cm. EC_b in the first 15 cm exhibited the lowest daily peaks in the early afternoon hours. In contrast, the VWC experienced its highest daily peaks during this time interval. At this time of day, plants' photosynthetic activity was at its highest, absorbing more water and triggering an upward capillary flow. The monitoring of the canal water level revealed an exact opposite pattern to that of the soil VWC.

Conclusion

Photosynthesis-driven water uptake in crop systems can trigger significant capillary rise from irrigation canals, modulating soil moisture and salinity before irrigation. Integrating EC, VWC, and canal level monitoring can refine irrigation timing to match plant demand and natural capillary supply.