Legumes are commonly used to provide nitrogen (N) for succeeding crops, but the net gain in N to the system is rarely measured. This study developed a N budget for a winter legume production system to estimate sources, sinks, and net system gain in N. Crimson clover ( Trifolium incarnatum L.) reseeded from the preceding year and was killed with herbicide the following spring. A forage sorghum crop ( Sorghum vulgare Pers .) was planted and harvested twice during the summer. Subplots were sampled at frequent intervals throughout the growing seasons for measurements of plant biomass and N content (leaves, steins, seeds, roots, and litter) and soil mineral N. During the clover growing season, soil N mineralization and leaching rates were estimated with an in situ chamber technique, and aerial ammonia (NH 3 ) transport measurements were made biweekly by micrometeorological techniques. Nitrogen in the clover crop increased until anthesis, and then declined slightly prior to desiccation with herbicides. Total N accumulated in the clover at desiccation was 323 kg N ha −1 (28 in leaves, 81 in stems, 40 in seeds, 44 in surface‐layer roots, and 130 in dead leaves and litter). Aerial NH3 absorption by the clover was small during the growing season (0.18 kg N ha −1 ) but NH3 loss from the killed clover was also minimal (0.25 kg N ha −1 ) and occurred during the period shortly after herbicide application. The summer sorghum crop took up 454 kg N ha −1 over the course of two cuttings from the soil mineral and mineralized organic N resource plus atmospheric NHj and other atmospheric N inputs such as wet and dry deposition. Total N accumulated by the clover from N fixation, soil mineral N uptake, and NH 3 absorption provided the equivalent of 70% of N removed by the sorghum crop.
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Harper et al. (1995) studied this question.