Nitrogen uptake of sorghum (Sorghum bicolor L.) from tree mulch and mineral fertilizer under high leaching conditions estimated by nitrogen-15 enrichment

The effects of applying either inorganic fertilizer or leaf mulch of Acacia saligna (Labill.) H.L. Wend. on yields of Sorghum bicolor (L.) were compar ed with an unfertilized control under the high leaching conditions of runof f irrigation in a dry tropical environment. The N use efficiency and transf er from N-15-labelled (NH4)(2)SO4 or acacia leaves to the sorghum differed in quantity and quality. Only 6% of the applied mulch N was retrieved in th e crop, in contrast to 21% of the fertilizer N. The proportions of N in the crop derived from the fertilizers were small, amounting to 7% and 28%, res pectively, in the mineral fertilizer and mulch treatments. However, the app lication of inorganic fertilizer and mulch significantly increased crop gra in yield (P < 0.05 and P < 0.1, respectively), biomass production and folia r N contents (P < 0.05). The inorganic fertilizer improved crop yields to a larger extent than mulching. At the same time, more N was lost by applying (NH4)(2) SO4 than leaf mulch: only 37% of the N of applied (NH4)(2) SO4 wa s found in the crop and the soil (0-0.3 m), but 99% of the mulched N. High NO3- contents in the topsoil of the inorganic fertilized sorghum treatments indicated the risk of N leaching. However, more important may have been ga seous N losses of surface-applied NH4- From a nutrient conservation point o f view, mulches should be given preferance to inorganic fertilizers under h igh soil pH and leaching conditions, but larger improvements of crop yields could be achieved with mineral fertilizers.