The fate of radioactive sulfur applied to grazed irrigated lucerne

Abstract

Radioactive gypsum (Ca35S04.2H20) was applied on thirteen 1 m2 plots in each of four 1025 m2 lucerne paddocks, which were rotationally grazed by 15 first-cross wethers. Sampling was from two paddocks only: soil, lucerne and wool samples were collected immediately prior to each grazing of these paddocks over a period of 551 days after the application of radioactive sulfur. Under the conditions of the study sulfur was found to cycle in an equilibrated and almost closed system. During the experiment 8.4% of the applied radioactivity was removed by plants and transferred by sheep to the unlabelled parts of the paddocks. One year after application of the fertilizer, the transferred 35S was the source of up to 94% of the cycling radioactive sulfur. By this time 3.6 % of the applied 35S was still cycling, which was 43.1 % of the total radioactivity extracted by plants. The amount of sulfur involved in the cycle was estimated to be 366 kg ha-1. Sulfur inputs were estimated as 1.6 kg sulfur ha-1 and 11 .9 kg sulfur ha-1 in rain and irrigation water respectively. Sulfur output was estimated as 6.4 sulfur ha-1 in wool and sheep. Soil total sulfur remained relatively constant through the experiment, but extractable sulfates decreased as a fraction of total sulfur, which suggested a big increase in the organic sulfur fraction.