SEASONAL PATTERNS AND VERTICAL DISTRIBUTIONS OF FINE ROOTS OF ALFALFA (MEDICAGO-SATIVA L)

Seasonal patterns and vertical distributions of alfalfa root systems were investigated with field experiments and a simulation model. We carried out root excavations on an established alfalfa stand at Davis, California, in 1982 and 1983. Thirty-six soil cores to a depth of 150 cm were taken seven times in 1982 and seventy-two soil cores to 30 cm were taken nine times in 1983 over the growth seasons. Live fine roots were recovered through six (in 1982) or seven (in 1983) procedures, including detailed hand sorting for removing debris. Our studies revealed that fine root mass was generally greatest in spring and fall and least during summer. Secondary minima were found within each harvest cycle. At all samplings, root mass declined exponentially with depth. A simulation model was developed for studying dynamics of alfalfa fine roots. The model has two components: root growth and death. Actual growth rate of fine roots was determined by potential growth rate and dependent on soil temperature, water and carbohydrate supply. Root death was controlled by the maximum death rate and influenced by soil temperature and nonstructural carbohydrate in roots. Through differential changes in simulated growth and death rates, the model predicted well the temporal and vertical distributions of fine root mass. A high amount of fine root mass was maintained in spring by rapid growth and moderate death rates. Frequent harvests reduced root mass in summer. Root mass increased in fall again due to a prolonged high growth rate. Sensitivity analyses indicated vertical distributions varied strongly with carbohydrate supply and soil temperature.