EVOLUTION OF SOIL MINERAL NITROGEN AND YIELD OF SUGARCANE FOLLOWING AGRICULTURAL RE-USE OF SEWAGE SLUDGE IN MAURITIUS

摘要

With the rising costs of chemical fertilisers, alternative nutrient sources must be sought to keep production costs of sugarcane in Mauritius competitive. Sewage sludge may, in this context, represent an appropriate substitute for chemical nitrogen (N) fertilisers in sugarcane. Environmentally sound management of N from sewage sludge, however, requires an understanding of the mineralisation of organic N in sewage sludge, as high application rates may result in nitrate levels in water hazardous to man and the environment. The effect on soil mineral N and on sugarcane yields when sewage sludge was applied to sugarcane was studied in four field trials in Mauritius. Rates of sewage sludge from 0 to 60 t/ha were compared against the current practice of applying chemical N fertiliser at 145 kg N/ha. N uptake was measured in plant-cane and first-ratoon crops at harvest in addition to yield. Soil samples (0 to 45 cm layers) were taken after fertilisation at days 1,3,7, 15,30, 45 and thereafter at monthly intervals for analysis of mineral N (ammonium and nitrate). The 20 t/ha (145 kg N/ha) sewage sludge treatment produced lower yields and N uptake than that observed with chemical fertilisers. The 60 t/ha treatment of sewage sludge did not provide the high concentrations of soil mineral N that was observed with chemical fertiliser treatment (17-8-25) shortly after application. Though this high initial mineral concentration observed with fertiliser declined rapidly, it was essential for high cane yield to be obtained. Inorganic N concentrations became comparable to those observed with sewage sludge one month after sewage was applied. This study showed that mineralisation of N from sewage sludge was too slow to meet the initial N needs of sugarcane. Very high rates ( > 40 t/ha) of sewage sludge would be needed to substitute for chemical fertilisers. The application of sewage sludge to sugarcane would, however, represent little risk of water pollution by nitrate.