Uptake by grass and transfer to soil of nitrogen from ¹⁵N-labelled legume materials applied to a Rhodes grass pasture

Abstract

Unground legume materials labelled with 15N were applied to the soil surface under a Rhodes grass pasture in south-eastern Queensland and the recovery of the applied 15N was followed over periods of 1-3 years. Comparisons were made between two legumes, Macroptiliurn atropurpureurn cv. Siratro and Desrnodiurn intorturn cv. Greenleaf, between leaf and stem materials of different nitrogen (N) concentrations (0.5-3.8% N), and between fresh and dried materials. After 1 year, 15N in the applied materials had decreased by 25-91%, and 7-25% was recovered in the Rhodes grass. Except for leaf material of Greenleaf, these changes showed a positive, non-linear relationship to the percentage of N (%N) in the applied materials. The changes for leaves of Greenleaf were less than would be predicted from their %N. Drying Siratro leaves and stems before applying them to the soil surface did not significantly affect the above changes. For N-poor materials (0.5-1.8% N) applied at 380-1360 g dry matter m-2, uptake of 15N by Rhodes grass was greater in the second year than in the first year, whereas for N-rich materials (3.8%N) applied at 140 g dry matter m-2 uptake of 15N in the second and third years was only 23 and 12% respectively of that in the first year.