Clover management to provide optimum nitrogen and soil water conservation

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, sink, 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, stems, 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 dessiccation with herbicides. Total N accumulated in the clover at dessiccation 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 NH 3 absorption by the clover was small during the growing season (0.18 kg N ha −1 ) but NH 3 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 NH 3 and other atmospheric N inputs such 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