Fine root production and mortality in irrigated cotton, maize and sorghum sown in vertisols of northern New South Wales, Australia

Abstract

Fine root (<2mm diameter) turnover (production and mortality) drives soil processes such as nutrient fluxes, carbon cycling and sequestration, activity of soil biota and structural stabilisation. Research on fine root dynamics has been focussed primarily on rainfed perennial and annual ecosystems in coarse and medium-textured soils with few studies conducted in irrigated fine-textured soils such as the vertisols. The objective of this study was to quantify the effects of tillage methods and the summer crops of cotton (Gossypium hirsutum L.), maize (Zea mays L.) and sorghum (Sorghum bicolor (L.) Moench.) on fine root number density in the non-sodic sub-surface (0.1–0.5m) and sodic subsoil (0.6–0.9m) of irrigated vertisols (average clay concentration 65g 100g−1 in the surface 1m) in northern New South Wales. It was hypothesised that fine root number density of cotton would be lower than that of maize and sorghum in the subsoil due to cotton's poor tolerance of anaerobic conditions. Fine root production, mortality and root number density of cotton, maize and sorghum, measured with a minirhizotron and a Bartz ICAP® image capture system, were evaluated for the 0.1–0.5m and 0.6–0.9m depth intervals in several irrigated experiments near Narrabri, NSW, Australia. Fine root density of cotton was low, with most occurring in the surface 0.5m. Subsoil (0.6–0.9m) root number densities of cotton were very low in continuous cotton under conventional tillage and on permanent raised beds (PRBs) but were greater with a cotton-wheat (Triticum aestivum L.) rotation sown on PRBs. Averaged over the two seasons, 2004–2005 and 2006–2007, and depth intervals, there were 3.4 roots×100mm−2 in continuous cotton sown after conventional tillage, 1.2 roots×100mm−2 in continuous cotton sown on PRBs and 6.0 roots×100mm−2 in cotton–wheat sown on PRBs. Between cotton genotypes, root number density during boll production and filling of a Bollgard®-Roundup-Ready® cotton variety was less than that of its corresponding non-Bollgard variant. Maize live root numbers did not differ significantly between continuous maize and a cotton-maize rotation but differed between depth intervals. Averaged over years and rotations, during tasselling maize had 21.5 roots×100mm−2 in the 0.1–0.5m depth interval and 5.3 roots×100mm−2 in the 0.6–0.9m depth interval. Root number density of sorghum was greater with no-tillage than with tillage in wetter seasons but generally similar or lower during a drier season. Average root numbers in the 0.1–0.5m depth interval during the “wet” seasons of 2010–2011 and 2011–2012 were 4.4 roots.100mm−2 with tillage and 9.3 roots×100mm−2 with no-tillage, and in the 0.6–0.9m depth interval were 4.5 roots×100mm−2 with tillage and 10.5 roots×100mm−2 with no-tillage. During the “dry” season of 2012–2013, however, root numbers in the 0.1–0.5m depth interval were 16.6 roots×100mm−2 with tillage and 11.8 roots×100mm−2 with no-tillage, and in the 0.6–0.9m depth interval were 13.8 roots×100mm−2 with tillage and 9.0 roots×100mm−2 with no-tillage. Fine root density of cotton was low, whereas those of sorghum and maize were high with relatively large values occurring in the deeper subsoil. Maize and sorghum were able to tolerate the waterlogged conditions in the sodic subsoils of these vertisols and may, therefore, be more suitable rotation crops for irrigated cotton farming systems than the more commonly sown wheat. These results, while relevant to other fine-textured soils that are irrigated or are periodically inundated, should not be extrapolated to medium and coarse-textured soils.