Changes in micronutrient concentrations under minimum tillage and cotton-based crop rotations in irrigated Vertisols

Abstract

Australian irrigated cotton (Gossypium hirsutum L.) yields are among the highest in the world but may deplete soil nutrient reserves faster than in dryland systems. Little is known about changes in long-term micronutrient (Cu, Fe, Mn, Zn) concentrations in these systems. This study investigated changes in soil micronutrient concentrations over time in two long-term tillage and crop rotation experiments under furrow-irrigated cotton systems and a no-till dryland cropping enterprise. The tillage practices that were investigated were maximum (disc to 0.2 m, chisel ploughing to 0.3 m followed by the construction of beds in 1 m spacings) and minimum (mulching cotton residues, followed by root cutting, incorporation of cotton stalks and bed renovation with a disc-hiller) tillage. Soil samples were analysed for diethylenetriamine penta-acetic acid (DTPA) extractable micronutrients, x-ray fluorescence (XRF) total micronutrients, pH, and soil organic carbon (SOC). Both maximum and minimum tillage influenced topsoil distribution of DTPA-extractable Cu, Zn and Mn, with the greatest changes occurring in Mn concentration. Concentrations of Mn in the topsoil (0–0.15 m) during 2015 were higher than those in the subsoil (0.15–0.6 m) by 74 % with maximum tillage and 159 % with minimum tillage, suggesting greater stratification with the latter (28 mg kg−1 in topsoil vs 11 mg kg−1 in subsoil). Including wheat (Triticum aestivum L.) and maize (Zea mays L.) in cotton rotations increased DTPA-extractable Mn concentration. DTPA-extractable Mn was positively correlated with SOC in two experiments (P < 0.01) and DTPA Fe and pH were negatively correlated (P < 0.01). DTPA Zn concentrations under minimum-till cotton systems were stable over 18 years. DTPA Zn concentrations measured at 0.02 m increments suggested that soil below the fertiliser band depth (< 0.04 m) was potentially responsive to Zn application. Sampling in smaller depth increments in the topsoil (0.02 m increments in 0–0.1 m depth) more accurately identified micronutrient stratification and may improve management decisions when sowing. Our results indicated that managing soil pH and SOC in alkaline Vertisols under irrigated cotton systems was a more practical approach to address micronutrient availability than soil application of micronutrients. Future research should consider the implications of current agronomic practices in cotton production, such as the method and timing of nitrogen application as it influences changes in soil pH, which may impact micronutrient availability at critical crop growth stages.