Irrigation induced deep drainage losses of soil carbon in a cotton farming system of Australia

A field investigation conducted during the 2014-15 summer cropping season in a Vertosol near Narrabri assessed carbon losses in deep drainage. The experimental design was a split plot design with 4 replications where the main plot treatments (in place since 1985) were: cotton monoculture sown either after conventional tillage or on permanent beds, and a cotton-wheat rotation on permanent beds. In the 2014-15 season, the subplot treatments were sown to cotton, however on alternate years these subplots were sown to corn. Individual plots were 190 m long and 8-24 rows wide. Samples were collected (9-12-2014, 16-1-2015, 25-2-2015) from ceramic cup samplers at depths of 60 and 120 cm within a week after the 2nd, 4th, and 7th irrigations. Only 26% of the plots had any drainage measured at the 120 cm depth. When the 120 cm depth was included in analysis, the rotations with corn had higher levels of total organic carbon (TOC) in leachate than rotations without corn (means of 90 and 81 mg/L, p = 0.045), whereas when only the 60 cm depth was analysed corn did not affect the level of TOC (p=0.085). Over the period of sampling there was no trend in the TOC concentrations in deep drainage at either depth with time.