Carbon inputs by irrigated corn roots to a Vertisol

Row crops commonly grown under irrigation in the Vertisols of north-western New South Wales, Australia, include summer crops such as corn (Zea mays L.) and cotton (Gossy- pium hirsutum L.). Soil organic carbon (SOC) and residue (SOR) dynamics in these farming systems have been analysed primarily in terms of inputs of above-ground material and root mass towards the end of a growing season. Addition of root material to SOC and SOR stocks either in the form of roots dying and decaying during and after the crop's growing season may, however, be significant. Carbon inputs by roots of irrigated corn to an irrigated Vertisol were evaluated in an experiment near Narrabri, Australia, where corn grown as a monoculture was compared with corn sown in rotation with cotton. Root growth in the surface 0.10 m was measured with the core-break method, and that in the 0.10 to 1.0 m depth with a minirhizotron and I-CAP image capture system. These measurements were used to derive root length per unit area (LA), root C added to soil through intra-seasonal root death (Clost), C in roots remaining at end of season (Croot) and root C potentially available for addition to soil (Ctotal). Ctotal averaged 5.0 Mg ha -1 with cotton-corn and 9.3 Mg ha -1 with corn monocul- ture, with average Clost accounting for 11%. Intra-seasonal root death from corn made only a small contribution to soil carbon stocks. LA of corn was higher with corn monoculture than with cotton-corn.