Modeling growth of different lucerne cultivars and their effect on soil water dynamics

Summary

Lucerne (Medicago sativa L.) is a perennial legume with high importance as fodder crop under organic farming. Our study analyzes biomass growth and influence on soil water dynamics of three lucerne cultivars in a field experiment and using the simulation model CropSyst. The objective was to evaluate the model for a perennial legume crop, study the relation of cultivar traits with soil water relations and derive a lucerne ideotype for temperate, semi-arid conditions. Two cultivars of European origin (Sitel and Niva)were compared to an Iranian one (Mohajeran). Measurements were made under irrigated (non-water limited) and rain-fed (water limited) conditions. Sitel achieved a cumulative biomass (dry matter yield)of 36.8 t ha−1 (irrigated) and 25.3 t ha−1 (rain-fed) over six cuts during the two experimental years, being significantly higher compared to the other cultivars. It was depicted that for Sitel and Mohajeran biomass growth was determined by the ratio of rainfall-to-evapotranspiration. Niva on the contrary exhibited a distinct response of biomass growth to profile water content changes. Also the more even root distribution over the soil profile of Niva indicated an efficient water uptake for this cultivar. CropSyst simulated biomass growth (RMSE 0.58–3.52 t ha−1) and water content in the soil profile (RMSE 20.9–50.6 mm) satisfactorily. Indices of agreement revealed a better model performance for irrigated conditions compared to water-limited growth. Model deviations from measured biomass and profile water content were highest for Mohajeran and it can be attributed to an inadequate distribution of root water extraction over the soil profile in relation to rooting density. Scenario analysis revealed that for the temperate, semi-arid conditions at the site, a maximum root depth between 1.3 and 1.8 m is sufficient for optimum growth and water use. Optimization of growth is largely determined by leaf traits such as specific leaf area, but our analysis depicted that in a dry year the sustained water supply via the root system becomes decisive.