Soil water extraction patterns of lucerne grown on stony soils

Abstract

Background and aims: Lucerne (Medicago sativa L.) is often grown under water-limited production conditions due to its capacity to extract water from deep soil layers through an extensive taproot system. These soils often contain stones which cause the roots to become unevenly distributed due to displacement by rock fragments. To improve the estimation of water use by lucerne grown in stony soils, we investigated the temporal pattern of water supply and demand, and how this influences crop growth. Methods: Naturally occurring stones in the silt loam soil profile were used to reduce the plant available water capacity for three lucerne crops. The Monteith model was fitted against observed soil water to obtain estimates of the extraction rate constant (kl, day ⁻¹) and extraction front velocity (EFV, mm day ⁻¹) of lucerne roots. Crop water demand was represented by transpiration losses driven by intercepted solar radiation based on the “canopy conductance” approach. Results: The Monteith model described the pattern of water extraction for lucerne grown on stony soils. However, a single, constant kl and EFV were inappropriate for characterising water extraction. The main physiological responses to water stress were (i) a reduction in canopy conductance, (ii) a reduction in canopy expansion and (iii) the rearrangement of leaves into a more vertical position. Conclusion: This study validated frameworks to quantify lucerne water extraction and transpiration demand which can be used to improve the estimation of water use by lucerne crops.