THE FATE OF N-15-LABELED ORGANIC NITROGEN IN SHEEP MANURE APPLIED TO SOILS OF DIFFERENT TEXTURE UNDER FIELD CONDITIONS

The fate of nitrogen from N-15-labelled sheep manure and ammonium sulfate in small lysimeters and plots in the field was studied during two growth seasons. In April 1991, N-15-labelled sheep faeces (87 kg N ha(-1)) plus unlabelled (NH4)(2)SO4 (90 kg N ha(-1)), and ((NH4)-N-15)(2)SO4 (90 kg N ha(-1)) were each applied to three soils; soil 1 (100% soil + 0% quartz sand), soil 2 (50% soil + 50% quartz sand) and soil 3 (25% soil + 75% quartz sand). The lysimeters were cropped with spring barley (Hordeum vulgare L.) and undersown ryegrass (Lolium perenne L.). The barley crop recovered 16-17% of the labelled manure N and 56% of the labelled (NH4)(2)SO4-N. After 18 months 30% of the labelled manure N and 65% of the labelled (NH4)(2)SO4-N were accumulated in barley, the succeeding ryegrass crop and in leachate collected below 45 cm of soil, irrespective of the soil-sand mixture. Calculating the barley uptake of manure N by difference of N uptake between manured and unmanured soils, indicated that 4%, 10% and 14% of the applied manure N was recovered in barley grown on soil-sand mixtures with 16%, 8% and 4% clay, respectively. The results indicated that the mineralization of labelled manure N was similar in the three soil-sand mixtures, but that the manure caused a higher immobilization of unlabelled ammonium-N in the soil with the highest clay content. Some of the immobilized N apparently was remineralized during the autumn and the subsequent growth season. After 18 months, 11-19% of the labelled manure N was found in the subsoil (10-45 cm) of the lysimeters, most of this labelled N probably transported to depth as organic forms by leaching or through the activities of soil fauna. In unplanted soils 67-74% of the labelled manure N was recovered in organic form in the 0-10 cm soil layer after 4 months, declining to 55-64% after 18 months. The lowest recovery of labelled N in top-soil was found in the soil-sand mixture with the lowest clay content. The mass balance of N-15 showed that the total recovery of labelled N was close to 100%. Thus, no significant gaseous losses of labelled N occurred during the experiment.