THE NITROGEN DYNAMICS OF 1-YEAR, 2-YEAR 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 bet ween symbiotically fixed N and soil mineral N sources, and the way the y are partitioned into harvested hay or incorporated biomass, is not w ell understood. Calculating N budgets (i.e. estimating N input and out put in a system) is useful for increasing our understanding of N dynam ics 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 us ing the N-15 isotope dilution technique. The total seasonal amount of N, fixed generally increased with each additional year of alfalfa, ran ging from 174 kg N ha(-1) for first year alfalfa to 466 kg N ha(-1) fo r third year alfalfa. Alfalfa was also very effective at accessing dee p-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 al falfa stands. The results of the budgeting process suggest that alfalf a stands as short as 2 years have the potential of making significant benefits to soil N status. (C) 1997 Elsevier Science B.V.