The nitrogen dynamics of 1-, 2- and 3-year stands of alfalfa in a cropping system

Alfalfa (Medicago sativa L.) has long been recognized as a source of N for the subsequent crops in a rotation. However, the relationship between symbiotically fixed N and soil mineral N sources, and the way they are partitioned into harvested hay or incorporated biomass, is not well understood. Calculating N budgets (i.e. estimating N input and output in a system) is useful for increasing our understanding of N dynamics in agro-ecosystems. Field experiments were initiated to develop a basic N budget for 1-3-year-old stands of alfalfa in a crop rotation. Changes in total soil N were monitored and N, fixation was measured using the N-15 isotope dilution technique. The total seasonal amount of N, fixed generally increased with each additional year of alfalfa, ranging from 174 kg N ha(-1) for first year alfalfa to 466 kg N ha(-1) for third year alfalfa. Alfalfa was also very effective at accessing deep-leached NO3- (i.e. soil NO3- greater than 1 m below the soil surface). The net soil N balance indicated that an average of 84, 148 and 137 kg N ha(-1) was added to the soil system for 1-, 2- and 3-year-old alfalfa stands. The results of the budgeting process suggest that alfalfa stands as short as 2 years have the potential of making significant benefits to soil N status. (C) 1997 Elsevier Science B.V.