Carbon inputs by wheat and vetch roots to an irrigated Vertosol

Abstract

Research on the amounts of carbon that can be added to Vertosols of New South Wales and Queensland by crop roots in irrigated cotton farming systems is sparse. The objective of this study was to determine the amounts of carbon added to soil by roots of wheat (Triticum aestivum L.) and purple vetch (Vicia benghalensis L.) sown in rotation with irrigated cotton (Gossypium hirsutum L.). Measurements were made from 2008 to 2010 in an ongoing experiment near Narrabri, northern NSW, using a combination of soil cores and minirhizotron observations. The experimental treatments were: cotton monoculture; cotton–vetch (CV); cotton–wheat, in which wheat stubble was incorporated into the beds with a disc-hiller (CW); and cotton–wheat–vetch, in which wheat stubble was retained as in-situ mulch (CWV). Vetch was killed by a combination of mowing and contact herbicides, and the residues were retained as in situ mulch. Root length per unit area of vetch in CWV and wheat in both CW and CWV was comparable, although wheat had a higher concentration of roots in surface 0.10 m. Root growth of the CV treatment was sparse. Root carbon available for addition to soil was greater with vetch than with wheat and was in the order: vetch in CWV (5.1 t C/ha.year) > vetch in CV (1.9 t C/ha.year) > wheat in CW (1.6 t C/ha.year) = wheat in CWV (1.7 t C/ha.year). Intra-seasonal root mortality accounted for 12% of total root carbon in vetch and 36% in wheat. The remaining fraction consisted of carbon in the root mass at the end of the growing season. Carbon sequestered by root inputs of the rotation crops was estimated to be ~0.34 t C/ha.year for the vetch and wheat crops in the CWV rotation, 0.10 t C/ha.year for vetch in CV, and 0.08 t C/ha.year for wheat in CW. Rotation CWV was, therefore, the most effective in sequestering carbon from roots.