EFFECT OF DIGESTED SEWAGE-SLUDGE ON THE EFFICIENCY OF N-FERTILIZER APPLIED TO BARLEY

Enhanced microbial activity following sewage sludge land application m ay affect soil N cycling and, therefore, plant available N. We studied the effect of anaerobically treated sewage sludge on N-fertilizer eff iciency and on some aspects of the soil N cycling. Field plots (3 m x 9 m) sown with barley (Hordeum vulgare L,) in November were amended wi th a) sludge (80 mg ha(-1)) and ammonium nitrate (150 kg N ha(-1)), b) ammonium nitrate (150 kg ha(-1)) only, c) or left unamended. Monthly soil samples were taken from 0 to 20-and 20 to 50-cm depths to determi ne soil inorganic N (NH4+, NO3 (-)). Denitrification in the upper 20-c m horizon was estimated by measuring N2O+N-2 emission from undisturbed soil samples by the acetylene-inhibition technique. Crop yield parame ters were analysed before harvesting, and grain production was recorde d. With respect to the control, the yield increase for the N-fertilize r treatment was 85% and 45% for the sludge + N-fertilizer treatment. T he decrease of N-fertilizer efficiency in sludge amended plots was pre sumably due to a decrease in spring plant available N. Presumably, mic robial immobilisation and denitrification in organic amended treatment s were responsible for the decrease in N-fertilizer efficiency.