The biological and chemical effects of digested sludge as a nitrogenous fertiliser on grass crops using in situ lysimeters

Abstract

This thesis details field and laboratory experiments on the biological and chemical effects of liquid anaerobically digested sewage sludge (LDS) used as a nitrogenous fertiliser on grass crops. The main objectives of the investigation were to ascertain the nitrogen (N) fertiliser value of LDS on grassland and evaluate its leaching losses using monolith lysimeters. Some LDS volatilisation studies were undertaken in the laboratory and a N balance compiled for the field lysimeters. To enable a detailed assessmBnt of crop uptake of LDS N some LDS was prepared with isotopic N15 using laboratory anaerobic digesters. The product LDS contained N15 as organic and inorganic N and was similar to a typical LDS. The field lysimeters were arranged as an integral part of each field plot and some techniques devised for their installation in situ. For laboratory work a scaled-down version of the field lysimeter was used. An assessment of the comparability of lysimeter and field conditions showed a satisfactory correlation with respect to crop productivity. Similar levels of N15 uptake were recorded for the field and lysimeter swards. Crop productivity trials showed that LDS applied as a single and split application was about 70% as effective as a split dressing of ammonium nitrate (Nitram). Evidence from the field trials suggested that LDS organic N contributes less than 10% of its fertiliser value during its year of application. Leaching losses of LDS were found to be similar to those of Nitram. At application rates equivalent to 300 kgN/ha/season typical nitrate-N concentrations in the leachate water were 1 mg/1 and 0.1-6.2% applied N was lost from the field A quick crop response to LDS N was recorded with N15 accumulation by the crop 7 days after LDS application. Crop productivity trials indicated a lack of sustained response by the grass to LDS applications.