Gross N transformation rates after application of household compost or domestic sewage sludge to agricultural soil

Gross N mineralization and immobilization was examined in soil amended with compost and sewage sludge on seven occasions during a year using N-15 pool dilution and enrichment techniques. Gross N mineralization was initially stimulated with both wastes and accelerated through the first 112 days of incubation, peaking at 5 mg N.kg(-1).d(-1) with compost compared with 4 mg N.kg(-1).d(-1) in control and sludge-treated soil. The magnitudes of mineralization rates exceeded those of immobilization by on average 6.3 ( compost) and 11.4 ( sludge) times, leading to a persistent net N mineralization cumulating up to 160 mg N.kg(-1) soil(compost) and 54 mg N.kg(-1) soil (sludge) over the season from May to November. The numerical model FLUAZ comprehensively predicted rates of gross mineralization and immobilization. Sludge exhibited an early season N-release, whereas compost released only 10% of the N during the first two months of incubation. This indicates that compost should be applied well in advance of sowing in order to match crop N demands.