Soil Salinity Dynamics under Drip Irrigation and Mulch Film and Their Effects on Cotton Root Length

A field experiment based on a monolith method using flooding irrigation under mulch film (FI) as a control was conducted to investigate soil salinity dynamics under drip irrigation with mulch film (DI) and its effects on cotton root length. The average soil salinity increased with duration of irrigation, but salt distribution in the soil profile was uneven and showed strong accumulation in the soil between adjacent mulch films. With advancing growth of cotton plants, the area of salt accumulation gradually expanded, especially from 110 to 125 days after sowing (DAS), when salinity distinctly increased in the 0- to 40-cm soil depth and at distances 30-70 cm from drip lines; the electrical conductivity (EC) under DI in all soil samples was at least 3 mS cm(-1) and in some cases exceeded 5 mS cm(-1). Root length declined significantly by 18.1% from 110 to 125 DAS under DI. The soil area showing the greatest decline in root length under DI coincided with the main site of salt accumulation. Correlation analysis of soil EC and root length density indicated the root length declined when soil salt content exceeded 2.8 mS cm(-1). However, under FI salt accumulation barely exceeded 2.8 mS cm(-1) and no reduction in root length was observed. The results indicated that the main reason for decreased root length in cotton under DI was localized accumulation of salinity.